Compact Semiconductor Lasers E-Book

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 596

Veröffentlichungsjahr: 2014

Beschreibung

This book brings together in a single volume a unique contribution by the top experts around the world in the field of compact semiconductor lasers to provide a comprehensive description and analysis of the current status as well as future directions in the field of micro- and nano-scale semiconductor lasers.
It is organized according to the various forms of micro- or nano-laser cavity configurations with each chapter discussing key technical issues, including semiconductor carrier recombination processes and optical gain dynamics, photonic confinement behavior and output coupling mechanisms, carrier transport considerations relevant to the injection process, and emission mode control.
Required reading for those working in and researching the area of semiconductors lasers and micro-electronics.

Leseprobe

Table of Contents

Cover

Related Titles

Title Page

Copyright

Preface and Introduction

List of Contributors

1 Nanoscale Metallo-Dielectric Coherent Light Sources

1.1 Introduction

1.2 Composite Metallo-Dielectric-Gain Resonators

1.3 Experimental Validations of Subwavelength Metallo-Dielectric Lasers for Operation at Room-Temperature

1.4 Electrically Pumped Subwavelength Metallo-Dielectric Lasers

1.5 Thresholdless Nanoscale Coaxial Lasers

1.6 Summary, Discussions, and Conclusions

Acknowledgments

References

2 Optically Pumped Semiconductor Photonic Crystal Lasers

2.1 Introduction

2.2 Photonic Crystal Lasers: Design and Fabrication

2.3 Photonic Crystal Laser Characteristics

2.4 The Final Assault: Issues That Have Been Partially Solved and Others That Remain to Be Solved Before Photonic Crystal Lasers Become Ready for Application

2.5 Conclusions

References

3 Electrically Pumped Photonic Crystal Lasers: Laser Diodes and Quantum Cascade Lasers

3.1 Introduction

3.2 Near-Infrared and Visible Laser Diodes

3.3 Mid-Infrared and Terahertz (THz) Quantum Cascade Lasers

3.4 Concluding Remarks and Prospects

References

4 Photonic-Crystal VCSELs

4.1 Introduction

4.2 Numerical Methods for Modeling Photonic-Crystal VCSELs

4.3 Plane-Wave Admittance Method

4.4 Impact of Photonic-Crystal depth on VCSEL Threshold Characteristics

4.5 Top and Bottom-Emitting Photonic-Crystal VCSELs

4.6 Enhanced Fundamental Mode Operation in Photonic-Crystal VCSELs

4.7 Highly Birefringent and Dichroic Photonic-Crystal VCSELs

4.8 Photonic-Crystal VCSELs with True Photonic Bandgap

4.9 Summary and Prospects

References

5 III–V Compact Lasers Integrated onto Silicon (SOI)

5.1 Introduction

5.2 Bonding of III–V Membranes on SOI

5.3 Heterogeneously Integrated Edge-Emitting Laser Diodes

5.4 Microdisk and Microring Lasers

5.5 Summary and Conclusions

References

6 Semiconductor Micro-Ring Lasers

6.1 Introduction

6.2 Historical Review of Major Contributions to Research on SRL Devices

6.3 Waveguide Design of Semiconductor Ring Lasers

6.4 Bending Loss in Semiconductor Ring Lasers

6.5 Nonradiative Carrier Losses

6.6 Semiconductor Microring and Microdisk Lasers with Point Couplers

6.7 Junction Heating in Small SRL Devices

6.8 RIE-Lag Effects in Small SRL Devices

6.9 Racetrack Geometry Microring Lasers

6.10 Chapter Summary

References

7 Nonlinearity in Semiconductor Micro-Ring Lasers

7.1 Introduction

7.2 General Formalism

7.3 Numerical Results for Micro-Ring Lasers

7.4 Numerical Results for Unidirectional Micro-Ring Lasers

7.5 Summary and Conclusions

References

Index

End User License Agreement

Pages

Guide

Table of Contents

List of Illustrations

Figure 1.1

Figure 1.2

Figure 1.3

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Figure 3.1

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Figure 7.25

Figure 7.26

List of Tables

Table 1

Table 7.1

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Compact Semiconductor Lasers

Edited by

Richard M. De La Rue, Siyuan Yu, and Jean-Michel Lourtioz

Editors

Prof. Richard M. De La Rue

Optoelectronics Research Group

School of Engineering

University of Glasgow

Rankine Building

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Glasgow G12 8LT

Scotland, U.K.

Prof. Siyuan Yu

Dept. of Elect. Engineering

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United Kingdom

Dr. (Prof.) Jean-Michel Lourtioz

Directeur de Recherche CNRS

Vice President of Université Paris-Sud

Mission Campus

Bǎtiment 209E

91405 Orsay Cedex

France

Cover

Courtesy of Gabor Mezosi

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Preface and Introduction

The title of this book is “Compact Semiconductor Lasers” and, in accordance with this title, it aims to provide a nearly comprehensive review of efforts in various research laboratories to create semiconductor lasers that are progressively smaller. In our view, efforts to make semiconductor lasers ever more compact are mainly driven by the need to integrate semiconductor lasers in photonic integrated circuits (PICs) or optoelectronic integrated circuits (OEICs). Two major technical advantages can be derived from being small – taking up less space on a chip, and consuming less power per laser – both of which are necessary for higher density and higher device-count integration.

The primary industrial relevance of the compact semiconductor laser possibly comes from its potential for application in advanced PICs aimed at communication applications. PICs, such as those integrating high quality laser sources, modulators, and on-chip monitoring devices, have already been applied commercially for long distance optical communications. While not yet a major constraint on their application, compactness, and power-efficiency are nevertheless high on the wish list for such components. Yet one needs to be aware that compactness may be an attribute that conflicts with other requirements. For instance, the spectral purity, frequency precision, and tunability that are essential for high precision sources such as distributed-feedback (DFB) and distributed Bragg reflector (DBR) waveguide lasers imply the need for device lengths that are specified as several hundred micrometers, if not several millimeters.

Other emerging important information transmission applications may impose very different requirements on laser sources. Optical interconnects have at their base the familiar silicon very-large-scale integration (VLSI) circuit, or more colloquially, the chip. The driving concept of the optical interconnect is that light waves rather than electronic wires should provide the primary means for the transport of information between silicon chips or, more speculatively, from one part of a silicon chip to another. Used properly, optical interconnects could be a vitally important way of bypassing and/or overcoming bottlenecks in data processing and transmission. Therefore, optical interconnects are one form of integrated circuit where photonics might play a role. The compact semiconductor laser could, for instance, simply be organized, in large or small arrays, around the edges of the silicon VLSI chip. Useful performance could mean that each compact laser was both dissipating and delivering sub-microwatt-scale power levels, together with signal bandwidths of 10 Gb s−1 and beyond in each channel. It is obvious that existing communication lasers cannot be suitable, because of their size and power consumption. Here compact semiconductor lasers in the 1300–1600 nm band are the best candidates, since their development is more mature and, being in the transparency window of silicon, they are more complementary metal-oxide semiconductor (CMOS)-compatible than their counterparts at other, shorter, wavelengths.

With an increasing amount of information carried by optical waves, there has also been strong interest in the realization of what we may call “digital” PICs/OEICs. One can envisage the use of compact semiconductor lasers to realize photonic digital logical functionalities that are direct counterparts of several of the electronic logical functionalities. These are routinely built into the silicon VLSI chip, utilizing the nonlinear optical response of compact semiconductor lasers such as optical bistability for binary logic functions including Boolean logic, buffering, and storage. Such nonlinearity stems from the fundamental photon–charge-carrier interaction dynamics of the semiconductor and depends strongly on the structure of the laser cavity. The compact semiconductor laser based PIC/OEICs that might emerge will probably be quite different from the PICs and OEICs that have been most well developed so far for the transmission of information.

Alternative forms of functional PIC/OEIC, such as those being developed for sensing applications, may require different wavelengths and hence different semiconductor materials and structures, yet will still benefit from reduced laser size. For example, quantum cascade lasers (QCLs) that operate in the far infrared can benefit from compactness in applications where low power consumption is required including, for instance, vehicle embarked systems. Finally, it is possible to envisage a number of other situations where compact near-infrared semiconductor lasers could play a role. Examples could include single and paired photon sources that are relevant to various quantum processing applications and novel optomechanical device structures.

The fundamental mechanism that supports both reduced size and low power consumption is the high optical gain provided by direct bandgap semiconductor active materials and the quantum structures (quantum wells, quantum dots, etc.) based on these materials, making it possible for lasing to take place even with a tiny volume of active material. In this perspective, the effort toward smaller size and higher energy efficiency is largely an effort to realize ever-tighter simultaneous confinement of both the photon and the electron in this active volume. The reader may infer that all the structures covered in this book are aimed at strong confinement, which optically benefits from the high refractive index of the semiconductor materials used and at its most extreme is exemplified by the use of metal clad structures to overcome the optical diffraction limit. Truly subwavelength semiconductor lasers are indeed attainable with the incorporation of metal, provided that the resulting absorption losses are sufficiently low as applies, for instance, in the long wavelength region of the spectrum. Whatever may be the structure, much effort must also be dedicated to the electronic confinement, since the reduced active volume typically means a much-increased surface-area to active-volume ratio that leads to higher levels of nonradiative carrier recombination. Making semiconductor lasers more and more compact is, by itself, a worthwhile technological challenge at the level of targeted applications.

A broader commentary on semiconductor lasers and their applications at this point may help our reader to place the contents of this book in the wider context of a field that has been established for 50 years and yet is still evolving rapidly. The compact semiconductor lasers described in the different chapters of this book could enter progressively into the armory of contemporary optoelectronics and photonics technologies, where “conventional” semiconductor lasers are already commonplace items. The latter are routinely manufactured, en masse, with production volumes that are measured in hundreds of millions per year. In fact, almost all of the lasers in the world are semiconductor lasers. Furthermore, semiconductor laser diodes are the primary light source, that is, the primary converters between electrical power and optical power, for a large fraction of all other lasers, for example, where the characteristic gain medium of the laser is a doped fiber, a single-crystal slab, or a rod of glass. In terms of the conversion efficiency between electrical power (current) and optical power, no other laser comes close to the semiconductor laser.

It is salutary to contrast production volume numbers for individual lasers including, for example, the vertical cavity surface-emitting lasers (VCSELs) that are often organized in arrays with financial numbers. Approximately half of the financial volume for the production of “lasers,” around the world, is made up of semiconductor diode lasers. In contrast, the remaining half is almost totally made up of the “big” lasers that are used for applications such as numerically controlled machining and welding. In the case of such large lasers, the actual laser is, of course, built into a physically large machine that is a complete system of electronics, precision mechanisms, and optics.

New applications for semiconductor lasers continue to appear and they may, for instance, even displace the conventional light-emitting diode (LED) in parts of the display technology market. But there is also the possibility that some large-scale applications will disappear or greatly diminish in importance, at least in their most typical form for example, the use of blue diode lasers in compact disk (CD) equipment. The optical spectrum covered by semiconductor lasers is vast, although wavelength tunability remains an important issue. The spectral range of semiconductor lasers that are already being exploited commercially – or will probably be exploited commercially in the near-future – extends from the ultra-violet (UV), through the visible and a long way on into the infra-red (IR). With the inclusion of the quantum-cascade (QC) laser, the IR spectral coverage possible with semiconductor lasers ranges all the way from the shortest IR wavelengths, through the mid-IR to the far-IR, which last spectral region itself extends down in frequency to the terahertz (THz) region.

The list of applications where semiconductor lasers are used is, as already mentioned, extensive and we shall do no more than mention some of the more obvious and important applications. One important area, with financial values (revenue, turnover, and capitalization) measured in tens of billions of dollars, is fiber-optical telecommunications. In this domain, DFB and DBR lasers are the source of choice for large information bandwidth transmission over long distances and seem likely to remain so. On the other hand, the rapidly growing short-reach optical fiber communications market, for example, in access networks (“fiber-to-the-home” or “last mile”) and in the very large-scale data centers that support the Internet “cloud” services, demands large numbers of low-cost semiconductor lasers that are mainly conventional Fabry–Perot (FP) lasers and VCSELs. Newer types of integrated laser, such as those that can emit multiple wavelengths at a low cost, could prove to be vital in the future upgrade of such systems.

Line-of-sight, building-to-building communication using modulated semiconductor lasers, over kilometer scale distances, is of potential importance, although the atmospheric transmission is often impeded by adverse weather conditions. Useful communication in space over thousands of kilometers has already been demonstrated, which is more challenging because of other factors. Although the scale of the market and activity in the domain of free-space optical telecommunications has, so far, been much smaller than for fiber-optics based telecommunications, semiconductor lasers will surely form an important ingredient in this field.

The CD, as used in CD players for audio reproduction (e.g., for music) and in DVD (digital video/versatile disk) players, has been much the largest market in volume for the semiconductor laser, with annual demand amounting to several tens of millions of suitably packaged individual lasers. The semiconductor laser in a CD player is used to read the fine pattern of holes that has been created in the thin sheet of metal deposited on the surface of the plastic substrate. This quite specialized application of the semiconductor laser, despite its historical success, may yet diminish considerably in importance – or even disappear – because of trends in information transmission, storage, and delivery associated with the Internet. More generally, data storage by optical methods that include holographic techniques must compete with electronic and magnetic memory methods that have their own areas of strength, as well as weakness. Our view is that the intrinsic merits of the semiconductor laser are sufficiently strong that it will surely continue to play an important role in major markets, but it is not easy to predict what these will be in, say, 20 years time. For the CD player and DVD player, the optical wavelength used to read the pattern in the disk should in principle be as short as possible, since the resolution-determined hole-packing density (i.e., the information density) increases at least as strongly as the inverse of the wavelength squared. This requirement has led to the use of gallium nitride-based diode lasers that emit at wavelengths in the blue-violet region of the visible spectrum. Laser diodes with even shorter emission wavelengths, some way into the UV, are a credible future possibility.

At the other end of the spectrum from the UV and visible regions, penetrative imaging with electromagnetic waves at long enough wavelengths to be safe for moderate exposure of human beings, that is, THz imaging, since THz radiation is nonionizing, is clearly of emerging importance. But there is a need for sources of THz frequency electromagnetic waves that have much greater efficiency than has so far been possible, as well as other desirable properties such as rapid switching and modulation, compactness, and room temperature operation. This requirement indicates the desirability of a suitably scaled semiconductor-based source for the electromagnetic energy required, but so far this efficient semiconductor source for THz electromagnetic waves has largely been conspicuous by its absence.

For Lidar and optical radar applications in the mid- and near IR, semiconductor lasers, because of their characteristically high efficiency, are of potentially great importance in situations where the output light beam is required to propagate over large distances in “free-space,” in gaseous atmospheres (in particular, the earth's atmosphere), and fluid environments such as the sea. Probing the atmosphere, for example, for gaseous or particle pollutants, using optical radar techniques and sources of coherent short light pulses provides a challenging domain of potential applications for the semiconductor laser. Issues of beam-quality and directional control and the special optical systems that are likely to be required become important. For some of these applications, QCLs that offer wide spectral coverage through much of the IR spectrum have been researched quite intensively. QCLs are still heterostructure semiconductor lasers but have a dramatically different basic epitaxial layer structure, as well as distinctly different physical principles of operation because they are based on unipolar transitions, instead of electron–hole recombination. Polarization of the emission is naturally TM (transverse magnetic), that is, with the predominant magnetic field component of the emitted light parallel to the defining device plane while it is mostly TE (transverse electric) in conventional diode lasers. The range of possible wavelengths obtainable with the QCL extends from the mid-IR, through the far-IR and down to THz frequencies, although the coherent emitted radiation that justifies the epithet “laser” becomes progressively more difficult to generate, as the wavelength increases. The range of emission wavelengths possible with the QCLs overlaps, at near-IR wavelengths, with what is obtainable from conventional diode heterostructure lasers that have a substantial fraction of antimony in the composition of the light emitting III–V semiconductor region.

We return to the important, indeed basic, question: “What is to be gained from the pursuit of compactness in the semiconductor laser?” – a pursuit that is at the center of the research that is analyzed in this compact book. A partial answer to the question of “why compactness?” or “what is compactness for?” comes from considerations that also apply for the classic and central device of modern electronics, the transistor. Transistors are, in their standard format, three-terminal devices that are capable of both switching and amplification and, with feedback, also of oscillation. With appropriate organization, semiconductor lasers are capable of providing the same three functions, and it should also be born in mind that semiconductor lasers are devices that involve both electronic and photonic functionality. For example, when used as an amplifier in the “semiconductor optical amplifier (SOA),” the semiconductor laser takes an optical input and produces a higher power optical output that replicates the input but the amount of amplification of the light depends on electrical control of the gain of the optical amplifier.

In modern integrated electronics, the transistor is firstly a high-speed electronic switch. In a transistor, a controlling data stream goes into the transistor and is processed onto or into another data stream. The maximum speed (i.e., the rate) at which the switching operations involved can be carried out is restricted by the device size, from “large” down to very small sizes. As is well known, the silicon integrated circuit that is at the heart of modern electronics typically has several million interconnected transistors on it, as well as other components such as resistors and capacitors.

Semiconductor lasers can be organized to behave like their electronic counterpart just mentioned, the transistor. In a standard configuration, a heterostructure diode laser has its light output level modulated, in the simplest case, in amplitude by varying the level of the injection current that drives the laser into oscillation. Reducing the dimensions of the laser reduces the lasing threshold and the power consumption needed to modulate the optical output level, just as the reduction of the transistor dimensions implies a smaller energy dissipation per bit in an electronic circuit. For both devices, local heating, crosstalk effects, and low on-state/off-state contrast must be palliated to maintain high performance as the device size becomes progressively smaller, in order to produce a higher integration density. The use of semiconductor lasers to realize logical functionality and their possible integration into multi-stage logic circuits naturally push the semiconductor laser toward compactness. Considerations of speed of operation together with propagation delay, switching energy, and power consumption also provide the pressures that dictate compactness.

The chapters of this book, in order are

Chapter 1: Nano-scale metallo-dielectric coherent light sources.

Chapter 2: Optically pumped semiconductor photonic crystal lasers.

Chapter 3: Electrically pumped photonic crystal lasers: laser diodes and quantum cascade lasers.

Chapter 4: Photonic crystal VCSELs.

Chapter 5: III–V compact lasers integrated onto silicon (SOI).

Chapter 6: Semiconductor microring lasers.

Chapter 7: Nonlinearity in semiconductor microring lasers.

Although we do not claim that the organization of the present contribution to the literature on semiconductor lasers is totally systematic – and even less do we claim that the book is encyclopedic – we believe that what the book contains is organized appropriately, with a logical evolution of topics, and that it provides a good sample of the subtopics that constitute the whole field. Examples of subtopics that might have been included are the pillar geometry VCSEL-like laser and the promising vertical nanowire.

The book begins, in Chapter 1, with the topic of the metal enclosed nanoscale semiconductor laser. Detailed and rigorous electromagnetic analysis, together with creative design, leads to coherent light sources that can be substantially smaller than a free-space wavelength in all three space dimensions, while exploiting essentially the same III–V semiconductor based heterostructure gain medium as that in the conventional “macrolaser.” The combination of the intrinsically high gain available with III–V semiconductor structures and careful minimization of the potentially overwhelming propagation losses that occur for metals at optical frequencies produces the possibility of efficient coherent light sources that could viably be packed in million-scale numbers on a single, modest-area, wafer section.

Chapter 2 shares with the other chapters the basic aspect that the “natural” gain medium for coherent semiconductor light sources is invariably an epitaxial III–V semiconductor heterostructure. As in Chapter 1, the work described is primarily reliant on optical pumping processes for the gain medium. But the device structure is the radically different 2D photonic crystal (PhC) patterned membrane that provides strong confinement in all three space dimensions. The use of optical pumping has allowed a wide-ranging basic (i.e., fundamental) exploration of the characteristics and behavior of very compact lasers that exploit PhC principles for the generation of the feedback required for an optical oscillator.

Chapter 3 shares with Chapter 2 the fact that PhC structures are at the heart of the laser devices that have been investigated and that are described in detail in this chapter. The vital difference between the content of Chapter 3 and that of Chapter 2 is that the optical gain medium is pumped by means of electric charge-carrier injection. The research described in Chapter 3 is important because it directly addresses the technological issues that must be “solved” if the compact PhC-structured semiconductor laser is to be useful in a wide variety of situations – situations where the intrinsic efficiency of electrical pumping and the direct control that it provides are of paramount importance. Both PhC laser diodes and PhC QCLs are considered in this chapter, thereby covering a wide range of emission wavelengths from near infrared to THz waves. It is incidentally shown that the use of microwave- or electronics-inspired resonator structures provides the opportunity to design THz semiconductor lasers that can be much smaller than the emitted wavelength.

Chapter 4 continues with the PhC structuring theme, but with the clear alternative configuration of “vertical” emission that is mediated by the PhC structure. The title of the chapter immediately identifies the compact lasers involved as being a form of VCSEL but one in which the resonant cavity and laser performance stem from the use of a PhC structure on the lateral surface of the laser. The compact PhC VCSEL structure provides control of the single-mode power and polarization, while simultaneously optimizing the output power and coupling through the top surface. Using detailed modeling of the 3D structure, it is also shown that PhC-VCSELs with true photonic bandgap characteristics are feasible.

Chapter 5 departs radically from the previous chapters because it is centrally concerned with a hybrid situation in which the gain medium remains a III–V semiconductor epitaxial heterostructure, but the supporting substrate is a silicon-on-insulator (SOI) wafer section, to which the thinned-down III–V semiconductor laser structure is bonded and the III–V semiconductor substrate on which it has been grown is (almost) completely removed. This chapter suggests the strong plausibility of the transition from compact semiconductor lasers in the research lab to lasers in the real world of optical interconnect applications. The characteristic compact laser geometry of the work in Chapter 5 is the microdisk, which supports the so-called whispering gallery mode (WGM). The crucial – and so far not fully solved – challenge for this hybrid configuration is the need to drive the laser by electrical current injection.

Chapters 6 and 7 are concerned with ring-geometry semiconductor lasers and therefore share, in the simplest limiting case, the same circular geometry as that of the microdisk described in Chapter 5. Furthermore, there is considerable similarity between the mode structure of a disk and a ring with similar diameter and the ring can meaningfully be considered as a limiting case of a disk with the inner wall serving to limit the number of transverse (in this case meaning the radial direction) modes. By demonstrating intrinsically credible electrical pumping in such microring lasers and gaining an in-depth understanding of the photon–charge-carrier interaction dynamics in such structures, Chapters 6 and 7 also demonstrate – both from the experimental and the theoretical points of view – that compact semiconductor lasers could enter the real world of laser applications.

All three chapters – Chapters 5–7 – are substantially concerned with the bi-stable operating characteristics that make such lasers of interest as optical switches with a latching capability. If the electrical pumping and heat-dissipation issues associated with the hybrid configuration of disk lasers mounted on planar silicon waveguides can be addressed satisfactorily, it might eventually be that the compact hybrid semiconductor laser will become the preferred light source and optical switching device in high-density photonic integration based on silicon. Hybrid III–V/SOI integration might therefore win-out over III–V semiconductor monolithic integration.

As already mentioned, this book is by no means encyclopedic. We have not explicitly addressed the micropillar cavity geometry that has received considerable levels of attention for more than a decade. Only part of the research carried out on micropillar cavities was actually concerned with lasers as opposed, for example, to cavity quantum dynamics investigations. Issues such as the need to suppress nonradiative carrier recombination at dry-etch process-exposed surfaces that intersect with the quantum-well active region are arguably of particular importance in this case, as well as in the cases of the microring and microdisk lasers. The situation might well be different for vertical nanowires grown by the vapor–liquid–solid (VLS) method, because this approach generates structures with a much lower number of surface defects. Recent results [1] obtained using optical pumping on these laser structures, – including, for instance, spontaneous emission factors, β, close to unity over very wide bandwidths – have considerable promise for future developments of compact semiconductor lasers. Lasers based on vertical nanowires and micropillars could well be an appropriate additional topic in a future edition of this book.

Reference

1. Claudon, J., Bleuse, J., Malik, N.S., Bazin, M., Jaffrennou, P., Gregersen, N., Sauvan, C., Lalanne, P., and Gerard, J.-M. (2010) A highly efficient single-photon source based on a quantum dot in a photonic nanowire.

Nat. Photonics

,

4

, 174–177.

List of Contributors

Alexandre Bazin

Laboratoire de Photonique et de Nanostructures

CNRS UPR20

Marcoussis

France

Olesya Bondarenko

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla

CA 92093

USA

Xinlun Cai

University of Bristol

Department of Electronic and Electrical Engineering

Bristol

BS8 1UB

UK

Xavier Checoury

Université Paris Sud

Institut d'Electronique Fondamentale

CNRS, UMR 8622

Orsay

France

Yujie Chen

Sun Yat-sen University

State Key Laboratory of Optoelectronic Materials and technologies

School of Physics and Engineering

Guangzhou

China

Raffaele Colombelli

Université Paris Sud

Institut d'Electronique Fondamentale

CNRS, UMR 8622

Orsay

France

Tomasz Czyszanowski

Technical University of Lodz

Institute of Physics, ul. Wolczanska 219,93-005

Lodz

Poland

Maciej Dems

Technical University of Lodz

Institute of Physics, ul.

Wolczanska 219,93-005

Lodz

Poland

Yeshaiahu Fainman

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla

CA 92093

USA

Qing Gu

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla, CA 92093

USA

Michael Kats

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla

CA 92093

USA

Jin-Hyoung Lee

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla

CA 92093

USA

Vitaliy Lomakin

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla, CA 92093

USA

Jean-Michel Lourtioz

Directeur de Recherche CNRS

Vice President of Université Paris-Sud

Mission Campus

Bǎtiment 209E

Orsay Cedex

France

contrname{Gábor Mezosi}

Infineon Technologies Austria AG

High Voltage MOS Technology Development

IFAT PMM DPC HVM TD

Amit Mizrahi

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla, CA 92093

USA

Geert Morthier

Ghent University – imec

Department of Information Technology

Photonics Research Group

Belgium

Maziar P. Nezhad

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla

CA 92093

USA

Krassimir Panajotov

Vrije Universiteit Brussels

Department of Applied Physics and Photonics

Pleinlaan 1050

Brussels

Belgium

and

Institute of Solid State Physics

Tzarigradsko Chaussee blvd

Sofia

Bulgaria

Fabrice Raineri

Laboratoire de Photonique et de Nanostructures

CNRS UPR20

Marcoussis

France

and

Université Paris Diderot

Physics Department

Sorbonne Paris Cité

Paris Cedex 13

France

Rama Raj

Laboratoire de Photonique et de Nanostructures

CNRS UPR20

Marcoussis

France

Gunther Roelkens

Ghent University – imec

Department of Information Technology

Photonics Research Group

Belgium

Aleksandar Simic

University of California, San Diego

Department of ECE

Gilman Drive

La Jolla

CA 92093

USA

Boris Slutsky

University of California, San Diego

Department of ECE

Gilman Drive, La Jolla

CA 92093

USA

Marc Sorel

Optoelectronics Research Group

School of Engineering

University of Glasgow

Rankine Building

Oakfield Avenue

Glasgow G12 8LT

Scotland, U.K.

Dries Van Thourhout

Ghent University – imec

Department of Information Technology

Photonics Research Group

Belgium

Siyuan Yu

University of Bristol

Department of Electronic and Electrical Engineering

Bristol

BS8 1UB

UK

and

Sun Yat-sen University

State Key Laboratory of Optoelectronic Materials and technologies

School of Physics and Engineering

Guangzhou

China

Yanfeng Zhang

Sun Yat-sen University

State Key Laboratory of Optoelectronic Materials and technologies

School of Physics and Engineering

Guangzhou

China

1Nanoscale Metallo-Dielectric Coherent Light Sources

Maziar P. Nezhad, Aleksandar Simic, Amit Mizrahi, Jin-Hyoung Lee, Michael Kats, Olesya Bondarenko, Qing Gu, Vitaliy Lomakin, Boris Slutsky and Yeshaiahu Fainman

1.1 Introduction

Compact photonic components are important for the design and fabrication of integrated optical devices and circuits. In the case of light sources, reducing the size can result in improved metrics such as higher packing density and reduced power consumption and also may enhance cavity–emitter interactions such as the Purcell effect. Until recently it was commonly known that the minimum size for a laser is ultimately determined by the free-space wavelength, λ0. For example, as the size of a conventional Fabry–Perot semiconductor laser is scaled down in all three dimensions toward λ0, three effects adversely influence the lasing process. Firstly, the roundtrip path of the optical wave in the gain medium is shortened. Secondly, radiative losses from the end mirrors have an increased effect. Thirdly, the lateral field confinement in the resonator waveguide is reduced, resulting in a smaller overlap of the optical mode with the gain medium. All these effects lead to a significant increase in the lasing threshold. As a result lasing cannot be achieved below a certain size limit. By allowing the laser size to increase in one or two dimensions, it is possible to reduce the physical size of the laser in the remaining dimension(s) to values below this limit. For example, the disk thickness in whispering-gallery-mode (WGM) lasers [1] can be reduced to a fraction of the free-space wavelength [2] but, to compensate for the small thickness, the disk diameter must be increased. It should be noted that, in addition to the optical mechanisms noted above, nonradiative surface recombination can have a non-negligible negative effect on the emitter efficiency and thus needs to be accounted for in the design and analysis of such sources. The ultimate challenge in this respect is concurrent reduction of the resonator size in all three dimensions, and, at the same time, satisfying the requirements for lasing action.

The size of an optical cavity can be defined using different metrics, for example, the physical dimensions of the cavity or the size of the optical mode. However, if the goal of size reduction is to increase the integration density (for example, in a laser array), the effective cavity size should account for both the overall physical dimensions of the resonator and the spread of the optical mode beyond the physical boundary of the resonator. By this token, most conventional dielectric laser cavities are not amenable to dense integration because they have either a large physical footprint or a large effective mode. For example, distributed Bragg resonators [3] and photonic-crystal cavities [4] (both of which can be designed to have very small mode volumes) have physical footprints that are many wavelengths in size, due to the several Bragg layers or lattice periods that are required for maintaining high finesse. On the other hand, it has been demonstrated that the diameter of thick (λ0/n) micro-disk lasers can be reduced below their free-space emission wavelength [5]; however, the spatial spread of the resultant modes (which have low azimuthal numbers owing to the small disk diameters) into the surrounding space beyond the physical boundaries of the disks may lead to mode coupling and formation of “photonic molecules” in closely spaced disks [6]. For illustration purposes, an M = 4 WGM for a semiconductor disk with radius rc = 460 nm and height hc = 480 nm (Figure 1.1a) is shown in Figure 1.1b, clearly indicating the radiative nature of the mode and its spatial spread, which, as mentioned, can lead to mode coupling with nearby structures. (M is the azimuthal order of the resonance, corresponding to half the number of lobes in the modal plot of |E|.)

Figure 1.1 The M = 4 whispering gallery resonance for a thick semiconductor disk (a) is shown in (b) (rc = 460 nm, hc = 480 nm, and nsemi = 3.4). Note the spatial spread of the mode compared to the actual disk size. (c) The same disk encased in an optically thick (dm = 100 nm) gold shield will have well-confined reflective (d) and plasmonic (e) modes but with much higher mode losses. |E| is shown in all cases and the section plane is horizontal and through the middle of the cylinder. (From [7].). (Please find a color version of this figure on the color plates.)

One approach to alleviate these issues is to incorporate metals into the structure of dielectric cavities, because metals can suppress leaky optical modes and effectively isolate them from their neighboring devices. The modes in these metallo-dielectric cavities can be grouped into two main categories: (i) surface bound (that is, surface plasmon polariton (SPP)) resonant modes and (ii) conventional resonant modes (called photonic modes), resulting purely from reflections within the metal cavity. Although they are highly confined, the disadvantage of plasmonic modes is their high loss, which is caused by the relatively large mode overlap of the optical field with the metal (compared to the reflective case). Owing to the high Joule loss at telecommunication and visible wavelengths, the lasing gain threshold for such cavities can be very large. On the other hand, the negative permittivity of metals not only allows them to support SPP modes, but also enables them to act as efficient mirrors. This leads to the second class of metallo-dielectric cavity modes, which can be viewed as lossy versions of the modes in a perfectly conducting metal cavity. Because the mode volume overlap with the metal is usually smaller than in the plasmonic case, in a cavity supporting this type of mode it is possible to achieve higher resonance quality-factors (Q-factors) and lower lasing gain thresholds, albeit at the expense of reduced mode confinement (compared to plasmonic modes). In general, both types of modes can exist in a metal cavity. Embedding the gain disk mentioned earlier in a gold shield (Figure 1.1c) effectively confines the resonant modes while increasing Joule losses. As discussed, the surface bound plasmonic mode (Figure 1.1e) has both a higher M number and higher losses (M = 6, Q = 36) compared to the non-plasmonic mode (Figure 1.1d, M = 3, Q = 183). It should be noted that even though the metal shield is the source of Joule loss, the large refractive index of the semiconductor core (nsemi ≈ 3.4) aggravates the problem and increases both the plasmonic and Fresnel reflection losses. For SPP propagation on a (planar) semiconductor–metal interface, the threshold gain for lossless propagation is proportional to nsemi3 [8]. This means that, even though plasmonic modes with relatively high Q can exist inside metal cavities with low-index cores (for example, silica, for which n = 1.48), using this approach to create a purely plasmonic, room-temperature semiconductor laser at telecommunication wavelengths becomes challenging, due to the order of magnitude increase in gain threshold. However, plasmonic modes also have an advantage in co-localizing the emitters with the resonant mode volume, thereby leading to a more efficient emission into the lasing mode. This mode of operation is discussed further below, but at this point we focus on novel composite metal-dielectric resonators and the resonant modes that they support.

One possible solution for overcoming the obstacle of metal loss is to reduce the temperature of operation, which will have two coinciding benefits: a reduction of the Joule losses in the metal and an increase in the amount of achievable semiconductor gain. Hill and colleagues [9] have demonstrated cryogenic lasing from gold-coated semiconductor cores with diameters as small as 210 nm. However, in this case the metal is directly deposited on the semiconductor core (with a 10-nm SiN electrical insulation layer between). As a result, owing to the large overlap of the mode with the metal, the estimated room-temperature cavity Q is quite low. The best case is ∼180 for a silver coating (assuming the best reported value for the permittivity of silver [10]) which corresponds to an overall gain threshold of ∼1700 cm−1 and is quite challenging to achieve at room temperature. Even though this device lases when cooled to cryogenic temperatures, it would be challenging to achieve room-temperature lasing with the same approach and a similar sized cavity, owing to the constraints imposed by the amount of available semiconductor gain and the metal losses. The gain coefficient for optically pumped bulk InGaAsP emitting at 1.55 μm is reported to be ∼200 cm−1 [11]. Electrically pumped multiple quantum wells (MQWs), on the other hand, have been reported to have higher material gain coefficients of over 1000 cm−1 [12]. Furthermore, recent results obtained from Fabry–Perot type metallic nanolasers at room temperature indicate that this level of gain is also achievable in bulk InGaAs [13]. However, even if the required gain is achievable at room temperature, efficient operation of the device would still be a challenge because of thermal heating and nonradiative recombination processes (for example, Auger recombination). In particular, to operate a densely packed array of such devices, thermal management would be a major concern, given the requisite intense pumping levels. Consequently, it is extremely important to optimize the resonator design so that the gain threshold is minimized. In this chapter we introduce novel, composite metal-dielectric, three-dimensional resonators, and lasers that are smaller than the wavelength in all three space dimensions (3D), can operate at room temperatures, and can even operate without a threshold [7, 14–18].

1.2 Composite Metallo-Dielectric-Gain Resonators

As indicated in the previous section, the drawback of using metals in optical resonators is their high dissipative loss. In this section, we show that the losses in metal-coated gain waveguides and in 3D laser resonators, can be significantly reduced by introducing a low-index “shield” layer between the gain medium and the metal [7, 14, 17].

Consider a composite gain waveguide (CGW) having a gain medium cylindrical core, a shield layer, and a metallic coating, as shown in Figure 1.2a [14]. For a given CGW cross-section size, the shield layer thickness is then tuned to maximize the confinement of the electric field in the gain medium and reduce the field penetration into the metal. By doing that, we increase the ability of the device to compensate for the dissipated power with power generated in the gain medium. A direct measure of that ability is the threshold gain, that is, the gain required for lossless propagation [8] in the CGW. The field attenuation in the shield layer resembles that of Bragg fibers [19]. The layer adjacent to the core, in particular, is of high importance [20] and has also been used, for example, to reduce losses in infrared hollow metallic waveguides [21].

Figure 1.2 (a) Cross section of the metal-coated composite gain waveguide. (b) Cylindrical closed 3D resonator. (c) Cylindrical open 3D resonator. Rg is the radius of the gain core and Rout is the overall radius of the composite gain/dielectric core. Δ is the thickness of the dielectric shield. εg, εs, and εm are the relative permittivities of the gain, shield, and metal layers, respectively.

(From [14].)

Subsequently, we use the CGW model for the design of subwavelength 3D resonators. To confine the light in the longitudinal direction, the CGW is terminated from both sides by a low-index “plug” region covered with metal, which forms the closed cylindrical structure shown in Figure 1.2b.

A more practical nanolaser configuration from a fabrication point of view is the open structure with a SiO2 substrate shown in Figure 1.2c. The inherent radiation losses into the substrate provide means for collecting the laser light, in contrast to the closed structure, where extracting light requires modification of the metal coating, such as making an aperture in it. The threshold gain for the 3D resonators, defined as the gain required to compensate for the metal losses in the closed structure or to compensate for both the metal and radiation losses in the open structure, is shown in the following sections to be sufficiently low to allow laser action at room temperature.

1.2.1 Composite Gain-Dielectric-Metal Waveguides

We first consider the infinite CGW of Figure 1.2a, with relative permittivities , , and of the gain medium, the shield layer, and the metal, respectively. Assuming a time dependence of , we have > 0, > 0. The radius of the gain medium is Rg, the shield layer thickness is Δ = Rout − Rg, and the metallic coating layer begins at radius Rout. The eigenmodes of the CGW may be derived from the general solution of the longitudinal fields in each layer having the form:

1.1

where or ; and are Bessel functions of the first and second kind, respectively; , ; is the relative permittivity of the layer; and may be expanded by , where the integer m is the azimuthal index. The dispersion relation is found using the transfer matrix method [19]. For the threshold gain , the propagation constant β is real, and the threshold gain

1.2

where the integration in the numerator and denominator is over the cross section of the metal and each propagation mode may be found by imposing Im{β} = 0 in the dispersion relation and then finding the solutions in the plane (Re{β}, ), similarly to [22].

The effect of the shield layer on the TE01 mode threshold gain is demonstrated in Figure 1.3, where is plotted as a function of the shield thickness Δ for a given radius Rout = 300 nm (Figure 1.3a) and Rout = 460 nm (Figure 1.3b). In the simulations we assume a wavelength of λ = 1550 nm, = 12.5 corresponding to an InGaAsP gain medium, = 2.1 for a SiO2 shield layer, and for a gold coating [23]. The rapid field decay in the gold layer permits us to assume that the metal extends to infinity, whereas in reality a coating layer of 100 nm would suffice. As the shield thickness increases, a lower percentage of the field penetrates into the metal, reducing the losses. On the other hand, the gain material occupies less of the CGW volume, which means that a higher gain is required to compensate for the dissipation losses in the metal. The trade-off between these two processes results in an optimal point at which the threshold gain is minimal. This typical behavior of low-order modes is seen in Figure 1.3 for the TE01 mode. For Rout = 300 nm, the improvement of the threshold gain from the Δ = 0 (no shield layer) case is by a factor of 1.7, while for Rout = 460 nm, the improvement is by a factor of 6.1.

Figure 1.3 Threshold gain as a function of the shield thickness Δ for the TE01 mode. (a) Rout = 300 nm. (b) Rout = 460 nm.

(From [14].)

For larger radii, a lower threshold gain may be achieved, as shown in Figure 1.3 and further emphasized in Figure 1.4, where the minimal threshold gain is depicted as a function of Rout for four low-order modes: TM01, TE01, HE11, and HE21. Having the highest confinement around the gain-medium core, the HE11 mode has the lowest threshold gain among the four modes. Generally, for small radii, the shield layer is less effective, since it quickly drives the mode below the cut-off. For large radii, the threshold gain is low, as the field penetration into the metal is small. The optimal shield layer thickness increases monotonically as a function of Rout. For the TE01 mode, it ranges between 80 and 330 nm, for a corresponding range of Rout = 250–650 nm.

Figure 1.4 Minimum threshold gain as a function of Rout. The vertical lines show the cut-off of each mode in the 3D resonator plug region.

(From [14].)

1.2.2 Composite Gain-Dielectric-Metal 3D Resonators

The role of the metal coating, which is important in the infinite CGW model, becomes even more crucial for creating a 3D resonator. As explained above, the CGW facets are terminated by plug regions, which are short metallic waveguides filled with SiO2 as seen in Figure 1.2b,c, in an approach similar to that of Hill et al. [9]. The plug ensures strong confinement of the field in the gain region, provided that the mode residing in it is below the cut-off, that is, decaying exponentially in the z direction.

For the plug region waveguide, the cut-off is not clearly defined since the modes are significantly different from those of the perfectly conducting cylindrical waveguide [24]. A reasonable definition for the cut-off situation is a waveguide with the radius Rout supporting a mode whose β is the closest to the origin on the complex β plane. That cut-off is shown for each one of the modes of Figure 1.4 by the vertical lines, providing a qualitative tool for choosing an operation mode for the entire 3D structure, as the chosen radius needs to be to the left of the vertical line corresponding to the operation mode. The smaller the device radius compared with the cut-off radius, the stronger the decay in the plug; consequently, the threshold gain is lower. While the HE11 mode achieves the lowest threshold gain for a given Rout, its cut-off in the plug region is at a small radius; working below this cut-off entails a relatively high threshold gain. It is therefore seen that the TE01 mode, which has the highest cut-off of the shown modes, is favorable. The result shows that modes corresponding to a larger Rout will have a significantly lower threshold gain. Another advantage of the TE01 mode is that in the gain region it couples only to symmetric TE modes in the plug region, whereas m > 0 modes are hybrid and may couple to all modes with the same azimuthal index.

Using the CGW model at the optimal point of Figure 1.3b as a starting point, a 3D closed resonator with Rout = 460 and a 100 nm thick gold coating was designed for the TE012 mode. 3D finite-element method (FEM) simulation results for the squared electric field magnitude normalized to its maximum value are shown in Figure 1.5. The overall height of the resonator is 1500 nm, and the overall diameter is 1120 nm, making it smaller than the vacuum wavelength in all three dimensions. The resonance was fine-tuned to a wavelength of 1550 nm by setting the gain cylinder height to be about 480 nm and the shield layer thickness to about 200 nm, which is close to the 190 nm predicted by the CGW model. The threshold gain, however, is in less good agreement with the CGW model; the value for the 3D resonator is ≈ 0.011, which corresponds to about 130 cm−1, whereas the CGW model gives about 36 cm−1. This discrepancy is due to the losses occurring in the plug region and the mode deformation at the interfaces between the plug and gain regions, two effects that are not taken into account in the CGW model. It is evident that the longer the resonator, the more accurately the CGW model describes the behavior in the gain region. For instance, a longer resonator with the same radius and designed for the TE013 mode has a threshold gain of about 95 cm−1.

Figure 1.5 Cross section of a closed cylindrical 3D subwavelength laser resonator. The square of the electric field magnitude () normalized to its maximal value of the TE012 mode is shown. The inset shows a similar open structure.

If the structure shown in Figure 1.5 is designed with no shield layer in the gain region, but with the same overall radius and height, then the resulting threshold gain is about 420 cm−1. The gain that may be achieved at room temperature by optical pumping of bulk InGaAsP is about 200 cm−1 [11]. It is therefore evident that a shield layer that lowers the threshold gain from 420 to 130 cm−1 is crucial to enable lasing at room temperature. Slightly modifying the structure for the open configuration, as shown in the inset of Figure 1.5, the field distribution remains nearly unchanged and the threshold gain increases only to about 145 cm−1, owing to the radiation losses. The quality factor of this open resonator without gain is Q = 1125, whereas the values for the other 3D structures with a shield layer discussed above are even higher. Finally, we note that for electrical pumping, considerably higher gains may be reached [12] so that the structure, with appropriate changes, is expected to be even further reduced in size. In the following section we use the optimized thickness of a low-index shield layer between the gain medium and the metal coating of a 3D laser resonator for experimental demonstration of nanolasers.

1.3 Experimental Validations of Subwavelength Metallo-Dielectric Lasers for Operation at Room-Temperature

In continuation of the design methodology for 3D subwavelength lasers presented in the previous section, we now present the steps leading to actual implementation and characterization of the devices. The target device is shown in Figure 1.6, in which a gain core is suspended in a bilayer shell of silica and metal. The device is pumped optically through the bottom aperture and the emitted light is also collected from the same aperture.

Figure 1.6 Schematic view of a practical realization of the laser cavity, compatible with planar fabrication techniques. The air gap at the bottom of the laser is formed after selective etch removal of the InP substrate. In the designed cavity the values for h1, h2, and h3 are 200, 550, and 250 nm, respectively. (From [7].). (Please find a color version of this figure on the color plates.)

1.3.1 Fabrication Processes for Subwavelength Metallo-Dielectric Lasers

The metallo-dielectric laser structure was fabricated from an InGaAsP MQW stack grown on InP. Hydrogen silsesquioxane (HSQ) electron-beam resist was patterned into arrays of dots (Figure 1.7a) using a Raith 50 electron-beam writer, and the size of the dots was varied by changing the pattern size and/or the electron-beam dosage. Cylindrical structures were then etched using CH4/H2/Ar reactive ion etching (RIE) (Figure 1.7b). Using an optimized and calibrated plasma-enhanced chemical vapor deposition (PECVD) process, the silica shield layer was grown to a thickness of ∼200 nm (Figure 1.7c). Note that the outline of the embedded gain core is visible through the silica layer. In practice, the poor adhesion of gold to silica caused separation of the dielectric portion of the structure from the metal layer. Fortunately, aluminum exhibits better adhesion properties, and at the wavelength of interest its optical properties are very close to those of gold. (The cavity Q of the resonator with an aluminum coating (εm = −95.9 − j11) [23] is 1004, which compares with 1030 for gold.) A layer of aluminum with a minimum thickness of 70 nm was sputtered over the silica covered pillars (Figure 1.7