COMPACT BLUE-GREEN LASERS This book describes the theory and practical implementation of three techniques for the generation of blue-green light: nonlinear frequency conversion of infrared lasers, upconversion lasers, and wide-bandgap semiconductor diode lasers The book begins with a discussion of the various applications that have driven the development of compact sources of blue-green light Part then describes approaches to blue-green light generation that exploit second-order nonlinear optics, including single-pass, intracavity, resonator-enhanced and guided-wave second harmonic generation Part 2, concerned with upconversion lasers, investigates how the energy of multiple red or infrared photons can be combined to directly pump bluegreen laser transitions The physical basis of this approach is thoroughly discussed and both bulk-optic and fiber-optic implementations are described Part describes wide-bandgap blue-green semiconductor diode lasers, implemented in both II–VI and III–V materials The concluding chapter reflects on the progress in developing these lasers and using them in practical applications such as high-density data storage, color displays, reprographics, and biomedical technology Compact Blue-Green Lasers provides the first comprehensive, unified treatment of this subject and is suitable for use as an introductory textbook for graduate-level courses or as a reference for academics and professionals in optics, applied physics, and electrical engineering william p risk received the PhD degree from Stanford University in 1986 He joined the IBM Corporation in 1986 as a Research Staff Member at the Almaden Research Center in San Jose, CA His work there for several years was concerned with the development of compact blue-green lasers for high-density optical data storage More recently, he has been active in the emerging field of quantum information, and now manages the Quantum Information Group at the Almaden Research Center Dr Risk has authored or coauthored some 70 publications in technical journals and conference proceedings and holds several patents timothy r gosnell has been a technical staff member at Los Alamos National Laboratory since receiving his PhD in physics from Cornell University in 1986 He has pursued research activities in the areas of biophysics, nonlinear optics, ultrafast laser physics and applications, upconversion lasers, and most recently in the laser cooling of solids and applications of magnetic resonance to single-spin detection He is the author of over 40 scientific papers and editor of several books in these fields In addition to his research work in the public sector, Dr Gosnell has recently entered the private sector as a senior scientist for Pixon LLC, an informatics startup company that applies information theory and advanced statistical techniques to image processing and the analysis of complex algebraic systems arto v nurmikko received his PhD degree in electrical engineering from the University of California, Berkeley Following a postdoctoral position at the Massachusetts Institute of Technology, he joined Brown University Faculty of Electrical Engineering in 1975 He is presently the L Herbert Ballou University Professor of Engineering and Physics, as well as the Director of the Center for Advanced Materials Research Professor Nurmikko is an international authority on experimental condensed matter physics and quantum electronics, particularly on the use of laserbased microscopies and advanced spectroscopy for both fundamental and applied purposes His current interests are focused on optoelectronic material nanostructures and their device science Professor Nurmikko is the author of approximately 270 scientific journal publications COM P AC T B LUE - GR EE N L ASE R S W P RISK T R GOSNELL A V NURMIKKO    Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge  , United Kingdom Published in the United States by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521623186 © Cambridge University Press 2003 This book is in copyright Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press First published in print format 2003 ISBN-13 ISBN-10 978-0-511-06604-7 eBook (NetLibrary) 0-511-06604-X eBook (NetLibrary) ISBN-13 978-0-521-62318-6 hardback ISBN-10 0-521-62318-9 hardback ISBN-13 978-0-521-52103-1 paperback ISBN-10 0-521-52103-3 paperback Cambridge University Press has no responsibility for the persistence or accuracy of s for external or third-party internet websites referred to in this book, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate Contents Preface page xi The need for compact blue-green lasers 1.1 A short historical overview 1.2 Applications for compact blue-green lasers 1.2.1 Optical data storage 1.2.2 Reprographics 1.2.3 Color displays 1.2.4 Submarine communications 1.2.5 Spectroscopic applications 1.2.6 Biotechnology 1.3 Blue-green and beyond References Part Blue-green lasers based on nonlinear frequency conversion Fundamentals of nonlinear frequency upconversion 2.1 Introduction 2.2 Basic principles of SHG and SFG 2.2.1 The nature of the nonlinear polarization 2.2.2 Frequencies of the induced polarization 2.2.3 The d coefficient 2.2.4 The generated wave 2.2.5 SHG with monochromatic waves 2.2.6 Multi-longitudinal mode sources 2.2.7 Pump depletion 2.3 Spatial confinement 2.3.1 Boyd–Kleinman analysis for SHG with circular gaussian beams 2.3.2 Guided-wave SHG v 1 3 12 14 17 17 20 20 20 21 21 23 28 30 34 34 38 43 43 51 vi Contents 2.4 Phasematching 2.4.1 Introduction 2.4.2 Birefringent phasematching 2.4.3 Quasi-phasematching (QPM) 2.4.4 Waveguide phasematching 2.4.5 Other phasematching techniques 2.4.6 Summary 2.5 Materials for nonlinear generation of blue-green light 2.5.1 Introduction 2.5.2 Lithium niobate (LN) 2.5.3 Lithium tantalate (LT) 2.5.4 Potassium titanyl phosphate (KTP) 2.5.5 Rubidium titanyl arsenate (RTA) 2.5.6 Other KTP isomorphs 2.5.7 Potassium niobate (KN) 2.5.8 Potassium lithium niobate (KLN) 2.5.9 Lithium iodate 2.5.10 Beta barium borate (BBO) and lithium borate (LBO) 2.5.11 Other materials 2.6 Summary References Single-pass SHG and SFG 3.1 Introduction 3.2 Direct single-pass SHG of diode lasers 3.2.1 Early experiments with gain-guided lasers 3.2.2 Early experiments with index-guided lasers 3.2.3 High-power index-guided narrow-stripe lasers 3.2.4 Multiple-stripe arrays 3.2.5 Broad-area lasers 3.2.6 Master oscillator–power amplifier (MOPA) configurations 3.2.7 Angled-grating distributed feedback (DFB) lasers 3.3 Single-pass SHG of diode-pumped solid-state lasers 3.3.1 Frequency-doubling of 1064-nm Nd:YAG lasers 3.3.2 Frequency-doubling of 946-nm Nd:YAG lasers 3.3.3 Sum-frequency mixing 3.4 Summary References 56 56 57 71 90 97 101 101 101 101 108 110 115 119 119 121 123 124 126 130 130 149 149 151 151 154 156 157 160 161 169 170 177 177 178 178 179 Contents Resonator-enhanced SHG and SFG 4.1 Introduction 4.2 Theory of resonator enhancement 4.2.1 The impact of loss 4.2.2 Impedance matching 4.2.3 Frequency matching 4.2.4 Approaches to frequency locking 4.2.5 Mode matching 4.3 Other considerations 4.3.1 Temperature locking 4.3.2 Modulation 4.3.3 Bireflection in monolithic ring resonators 4.4 Summary References Intracavity SHG and SFG 5.1 Introduction 5.2 Theory of intracavity SHG 5.3 The “green problem” 5.3.1 The problem itself 5.3.2 Solutions to the “green problem” 5.3.3 Single-mode operation 5.4 Blue lasers based on intracavity SHG of 946-nm Nd:YAG lasers 5.5 Intracavity SHG of Cr:LiSAF lasers 5.6 Self-frequency-doubling 5.6.1 Nd:LN 5.6.2 NYAB 5.6.3 Periodically-poled materials 5.6.4 Other materials 5.7 Intracavity sum-frequency mixing 5.8 Summary References Guided-wave SHG 6.1 Introduction 6.2 Fabrication issues 6.3 Integration issues 6.3.1 Feedback and frequency stability 6.3.2 Polarization compatibility 6.3.3 Coupling 6.3.4 Control of the phasematching condition 6.3.5 Extrinsic efficiency enhancement vii 183 183 187 189 191 193 194 207 213 213 214 215 220 220 223 223 224 229 229 231 235 245 249 250 251 252 253 253 253 255 256 263 263 264 269 270 276 282 283 284 viii Contents 6.4 Summary References Part Upconversion lasers: Physics and devices Essentials of upconversion laser physics 7.1 Introduction to upconversion lasers and rare-earth optical physics 7.1.1 Overview of rare-earth spectroscopy 7.1.2 Qualitative features of rare-earth spectroscopy 7.2 Elements of atomic structure 7.2.1 The effective central potential 7.2.2 Electronic structure of the free rare-earth ions 7.3 The Judd–Ofelt expression for optical intensities 7.3.1 Basic formulation 7.3.2 The Judd–Ofelt expression for the oscillator strength 7.3.3 Selection rules for electric dipole transitions 7.4 Nonradiative relaxation 7.5 Radiationless energy transfer 7.6 Mechanisms of upconversion 7.6.1 Resonant multi-photon absorption 7.6.2 Cooperative upconversion 7.6.3 Rate equation formulation of upconversion by radiationless energy transfer 7.6.4 The photon avalanche 7.7 Essentials of laser physics 7.7.1 Qualitative picture 7.7.2 Rate equations for continuous-wave amplification and laser oscillation 7.8 Summary References Upconversion lasers 8.1 Historical introduction 8.2 Bulk upconversion lasers 8.2.1 Upconversion pumped Er3+ infrared lasers 8.2.2 Er3+ visible upconversion lasers 8.2.3 Tm3+ upconversion lasers 8.2.4 Pr3+ upconversion lasers 8.2.5 Nd3+ upconversion lasers 8.3 Upconversion fiber lasers 8.3.1 Er3+ fiber lasers; S3/2 → I15/2 transition at 556 nm 286 287 292 292 292 295 296 303 303 306 324 325 329 336 338 341 345 345 348 357 360 363 364 365 382 383 385 385 397 398 410 420 424 425 427 433 Contents Part 10 11 8.3.2 Tm3+ fiber lasers 8.3.3 Pr3+ fiber lasers 8.3.4 Ho3+ fiber lasers, S2 → I8 transition at ∼550 nm 8.3.5 Nd3+ fiber lasers 8.4 Prospects References Blue-green semiconductor lasers Introduction to blue-green semiconductor lasers 9.1 Overview 9.2 Overview of physical properties of wide-bandgap semiconductors 9.2.1 Lattice matching 9.2.2 Epitaxial lateral overgrowth (ELOG) 9.2.3 Basic physical parameters 9.3 Doping in wide-gap semiconductors 9.4 Ohmic contacts for p-type wide-gap semiconductors 9.4.1 Ohmic contacts to p-AlGaInN 9.4.2 New approaches to p-contacts 9.4.3 Ohmic contacts to p-ZnSe: bandstructure engineering 9.5 Summary References Device design, performance, and physics of optical gain of the InGaN QW violet diode lasers 10.1 Overview of blue and green diode laser device issues 10.2 The InGaN MQW violet diode laser: Design and performance 10.2.1 Layered design and epitaxial growth 10.2.2 Diode laser fabrication and performance 10.3 Physics of optical gain in the InGaN MQW diode laser 10.3.1 On the electronic microstructure of InGaN QWs 10.3.2 Excitonic contributions in green-blue ZnSe-based QW diode lasers 10.4 Summary References Prospects and properties for vertical-cavity blue light emitters 11.1 Background 11.2 Optical resonator design and fabrication: Demonstration of optically-pumped VCSEL operation in the 380–410-nm range ix 436 445 455 457 458 460 468 468 468 470 470 472 474 475 478 479 481 482 484 484 487 487 488 488 496 501 506 509 513 513 517 517 518 526 11 Vertical-cavity blue light emitters Figure 11.7: Emission spectrum of the RCLED device with two dielectric mirrors Figure 11.8: Schematic of an RCLED device with one as-grown GaN/AlGaN and one dielectric DBR mirror, incorporating a nitride tunnel junction as the top current spreading layer on the use of a p ++ /n ++ InGaN/GaN tunnel junction (ITO has been also usefully employed for the hybrid resonator case.) A schematic diagram of the overall vertical-cavity emitter is shown in Figure 11.8 (Diagne et al., 2001) Similar to the optically-pumped VCSELs, AlN strain-relief layers were used in the deposition of a 11.3 Electrical injection 527 60-layer-pair quarter-wave GaN/Al0.25 Ga0.75 N bottom DBR stack Those growths that yielded a root-mean-square average roughness no worse than nm over an area of × mm2 were deemed suitable for continuation of the epitaxy The active p–n junction region was grown directly atop the GaN/(Al,GaN) DBR, composed ˚ with GaN barriers (L B = 60 A), ˚ and typically of In0.08 Ga0.92 N QWs (L w = 40 A) ˚ thick Al0.07 Ga0.93 N current blocking/carrier surrounded by approximately 1000-A confinement layers The tunnel junction in the GaN system (Takeuchi et al., 2001, Jeon et al., 2001) was included in the superstructure of the nitride segment and capped by a n-GaN layer, as shown in Figure 11.8 Note that in this case the positive bias to the device was applied through a contact to this n-layer Lateral current spreading on the scale of ∼100 ␮m was obtained, given the nearly 100 times higher ˚ n-type conductivity of GaN The tunnel junction itself was grown atop the 1000-A ++ ++ thick p-GaN layer as a p /n InGaN/GaN bilayer with the thicknesses of the ˚ and 300 A, ˚ respectively The doping levels were aplayers approximately 150 A 20 −3 proximately × 10 cm Mg and × 1019 cm−3 Si for the junction The vertical cavity was completed by capping the structure with a multi-layer λ/4 stack of SiO2 /HfO2 (R > 0.995), deposited by reactive ion beam sputtering The top dielectric DBR was patterned so that the device had an effective optical aperture varying from 10 to 30 ␮m The current density versus voltage of a typical device is shown in Figure 11.9 up to a high continuous injection level (∼1 kA cm−2 ), which shows evidence of series resistance, assigned in part to the presence of the tunnel junction Nonetheless, Figure 11.9: Current density versus voltage of a hybrid RCLED device with a 20 ␮m diameter mesa defining the vertical current path 528 11 Vertical-cavity blue light emitters Figure 11.10: Emission spectrum of the RCLED hybrid device (From Diagne et al (2001).) lateral current spreading was clearly accomplished so that the far-field light emission from the devices was uniform in its average intensity across the emitting aperture However, when examining the emission under high spatial resolution (∼1 ␮m) evidence was found of some tendency towards “filamental” vertical conduction, reflective perhaps of the influence of local compositional or doping inhomogeneities within the tunnel junction Figure 11.10 shows the output spectrum of a typical device at an operating current density of approximately 0.2 kA cm−2 The emission was observed in the direction normal to the planar device, within an angular view of approximately 10◦ While the optical resonator is rather thick (>10λ), only two vertical cavity modes are seen, demonstrating the restrictive spectral bandwidth of the AlGaN DBR The dominant mode at λ = 413 nm, which coincides with the high-reflectivity region of the DBR and the peak of the QW photoluminescence emission, has a spectral linewidth of approximately 0.6 nm This value is comparable with the linewidths previously measured in the best structures fabricated in the author’s laboratory that were designed for optically pumped VCSEL operation The examples shown above suggest that the implementation of vertical-cavity LEDs has provided the important building blocks for demonstrating a diode VCSEL in the violet We note that the incorporation of an InGaN/GaN p ++ /n ++ tunnel junction has also been exploited in the demonstration of a monolithically integrated two-color blue/green LED (Ozden et al., 2001a) The VCSEL challenge is two-fold: high-quality epitaxy and relatively complex device processing For example, layer 11.3 Electrical injection 529 thickness and composition (Al, In) control over a 2–3-in wafer area is still quite difficult, yet required so that spectral overlap, for example, between the high-reflectivity band of the AlGaN DBR and the maximum of the InGaN MQW gain remain in spectral synchrony Finally we mention that the application of the lateral epitaxial overgrowth techniques discussed in Chapters and 10 may become quite useful also for future vertical cavity emitters The inclusion of ELOG process/growth steps should aid in creating flexibility for designing and implementing blue/violet RCLEDs and VCSELs in at least two different ways Firstly, the patterned growth can be adapted for creating a buried bottom dielectric DBR mirror An illustration is shown in Figure 11.11, where a patterned HfO2 /SiO2 multi-layer dielectric stacks was deposited on a GaN buffer layer prior to subsequent regrowth of GaN The particular dielectric stack was terminated with an SiO2 layer so that the ELOG process would occur normally The second application of the lateral epitaxy pertains to building in a current aperturing scheme in order to alleviate the problem that arises from the competition between vertical and lateral transport in the nitride devices (especially on the p-side) Figure 11.12 shows the schematic of a possible arrangement where an SiO2 -defined current aperture is implemented in conjunction with lateral epitaxial growth Figure 11.11: Illustration of the use of lateral epitaxial GaN overgrowth to “bury” a dielectric DBR The imperfections are due to breaking of the sample so as to gain an edge view in this cross-sectional SEM image 530 11 Vertical-cavity blue light emitters DBR Laser Mirrors Top Electrode Current Blocking p -Type Nitride Layer Active Quantum Well Gain Medium Bottom Electrode n -Type Nitride Buffer Layer Substrate Figure 11.12: Schematic drawing illustrating a possible blue VCSEL device structure which features a buried dielectric DBR and current confining aperture 11.4 SUMMARY As illustrated through specific examples in this chapter, good progress is in evidence in the development of the key building blocks that are necessary for the realization of blue and near-ultraviolet vertical-cavity diode lasers and LEDs We have reviewed the strategies for crafting high-Q-factor resonators, with examples of optically-pumped VCSEL operation, and shown initial examples of vertical-cavity LED emitters While advances in the laboratory are encouraging, one should not underestimate the difficulties that are still faced in efforts to create practical short wavelength VCSELs Yet the payoff for introducing such a new class of optoelectronic devices can be significant, with a wide range of applications If we envision the eventual availability of short-wavelength VCSEL arrays (as matrix addressable LED arrays have now been introduced (Ozden et al., 2001)), the application base expands even further, especially in the areas of future ultracompact chipscale chemical and biological sensing and diagnostic systems The author notes the support of the US National Science Foundation The many contributions by the following individuals are gratefully acknowledged: R L Gunshor, J Ding, Y.-K Song, M Diagne, H Zhou, I Ozden, Y He, and E MaKarona REFERENCES Diagne, M., He, Y., Zhou, H., Makarona, E., Nurmikko, A.V., Han, J., Waldrip, K E., Figiel, J J., Takeuchi, T., and Krames, M (2001) A vertical cavity violet light emitting diode incorporating an AlGaN distributed Bragg mirror and a tunnel junction Appl Phys Lett., 79, 3720 References 531 Han, J., Waldrip, K E., Lee, S R., Figiel, J J., Hearne, S J., Petersen, G A., and Myers, S M (2001) Control and elimination of cracking of AlGaN using low-temperature AlGaN interlayers Appl Phys Lett., 78, 67–69 Honda, T., Katsube, A., Sakaguchi, T., Koyama, F., and Iga, K., (1995) Threshold estimation of GaN-based surface emitting lasers operating in ultraviolet spectral region Jpn J Appl Phys 34(7a), 3527–3532 Jeon, S.-R., Song, Y.-H., Jang, H.-J., Yang, G M., Hwang, S W., and Son, S J (2001) Lateral current spreading in GaN-based light-emitting diodes utilizing tunnel contact junctions Appl Phys Lett., 78, 3265–3267 Kelkar, P V., Kozlov, V G., Nurmikko, A V., Chu, C.-C., Han, J., and Gunshor, R L (1997) Stimulated emission, gain and coherent oscillations in II–VI semiconductor microcavities Phys Rev., B56, 7564–7573 Kelly, M K., Ambacher, O., Dimitrov, R., Handschuh, R., and Stutzmann, M (1997) Optical process for liftoff of group-III nitride films Phys Stat Sol A, 159, R3–R4 Krestnikov, I., Lundin, W., Sakharov, A V., Semenov, V., Usikov, A., Tsatsulnikov, A F., Alferov, Zh., Ledentsov, N., Hoffmann, A., and Bimberg, D (1999) Room-temperature photopumped InGaN/GaN/AlGaN vertical-cavity surface-emitting laser Appl Phys Lett., 75, 1192–1194 Langer, R., Barski, A., Simon, J., Pelekanos, N., Konovalov, O., Andre R., and Dang, L S (1999) High-reflectivity GaN/GaAlN Bragg mirrors at blue/green wavelengths grown by molecular beam epitaxy Appl Phys Lett., 74, 3610–3612 Mackowiak, P., Sarzala, R P., and Nakwaski, W (2001) Novel Design for Nitride VCSELs, in Proc Int Workshop on Nitride Semiconductors, IPAP Conf Series, Vol Tokyo: Institute of Applied Physics, pp 889–891 Nakamura, S (1999) InGaN-based violet laser diodes Semic Sci Technol., 14, R27–R40 Ng, H M., Moustakas, T D., and Chu, S N G (2000) High reflectivity and broad bandwidth AlN/GaN distributed Bragg reflectors grown by molecular-beam epitaxy Appl Phys Lett., 76, 2818–2820 Ozden, I., Diagne, M., Nurmikko, A V., Han, J., and Takeuchi, T (2001) A matrix addressable 1024 element blue light emitting InGaN QW diode array Phys Stat Sol (B), 188(a), 139 Ozden, I., Makarona, E., Nurmikko, A V., Takeuchi, T., and Krames, M (2001a) A dual-wavelength indium gallium nitride quantum well light emitting diode Appl Phys Lett., 79, 3720 Redwing, J M., Loeber, D A S., Anderson, N G., Tischler, M A., and Flynn, J S (1996) An optically pumped GaN–AlGaN vertical cavity surface emitting laser Appl Phys Lett., 69, 1–3 Someya, T., and Arakawa, Y (1998) Highly reflective GaN/Al0.34 Ga0.66 N quarter-wave reflectors grown by metal organic chemical vapor deposition Appl Phys Lett., 73, 3653–3655 Someya, T., Werner, R., Forchel, A., Catalano, M., Cingolani, R., and Arakawa, Y (1999) Room temperature lasing at blue wavelengths in gallium nitride microcavities Science, 285, 1905–1906 Song, Y.-K., Zhou, H., Diagne, M., Odzen, I., Vertikov, A., Nurmikko, A V., Carter-Coman, C., Kern, S., Kish, F A., Krames, M R (1999) A vertical cavity light emitting InGaN QW heterostructure Appl Phys Lett., 74, 3441–3444 Song, Y.-K., Nurmikko, A V., Schneider, R P., Kuo, C P., Krames, M R., Kern, R S., Carter-Coman, C., and Kish, F A (2000) A quasicontinuous wave, optically pumped violet vertical cavity surface emitting laser Appl Phys Lett., 76, 1662–1664 532 11 Vertical-cavity blue light emitters Song, Y.-K., Diagne, M., Zhou, H., Nurmikko, A V., Schneider, R P., and Takeuchi, T (2000a) Resonant cavity InGaN quantum well blue light emitting diodes Appl Phys Lett., 77, 1744–1746 Takeuchi, T., Hasnain, G., Hueschen, M., Kocot, C., Blomqvist, M., Chang, Y.-L., Lefforge, D., Schneider, R., Krames, M R., Cook, L W., and Stockman S A (2001) GaN-based light emitting diodes with tunnel junctions Jpn J Appl Phys in press Taylor, C., Barlett, D., Chason, E., and Floro, J A (1998) A laser-based thin-film growth monitor Ind Physicist, 4, 25–27 Waldrip, K E., Han, J., Figiel, J J., Zhou, H., Makarone, E., and Nurmikko, A V (2001) Stress engineering during metalorganic chemical vapor deposition of AlGaN/GaN distributed Bragg reflectors Appl Phys Lett., 78, 3205–3207 Wong, W S., Sands, T., and Cheung, N W (1998) Damage-free separation of GaN thin films from sapphire substrates Appl Phys Lett., 72, 599–601 Zhou, H., Diagne, M., Makarona, E., Nurmikko, A V., Han, J., Waldrip, K E., and Figiel, J J (2000) Near ultraviolet optically pumped vertical cavity laser Electron Lett., 36, 1777–1779 12 Concluding remarks We began in Chapter by discussing applications that required compact blue-green lasers; hence, it seems only fitting to end by examining the extent to which these requirements have been fulfilled In this final chapter, then, we attempt to gather up some of the diverse topics that this book has treated and establish the current state of the art in the application of compact blue-green lasers The preceding chapters have covered three principal approaches to creating compact blue-green lasers In the first approach, blue-green light is generated through nonlinear frequency conversion of infrared semiconductor diode lasers or diodepumped solid-state lasers We saw that since these nonlinear processes tend to be rather weak, the desire for efficient generation of blue-green light has stimulated the development of high-power infrared lasers as well as the invention of a host of device configurations intended to boost the nonlinear conversion efficiency These configurations include resonator-enhancement schemes, intracavity SHG, and waveguide implementations An alternative approach – the “upconversion laser”– directly excites a blue-green laser transition by combining the energy of two or more lower-energy pump photons through excited state absorption or cooperative energy transfer processes Upconversion lasers using both bulk and fiber-optic media have been demonstrated Finally, we examined semiconductor diode lasers that are pumped by electrical injection and directly produce blue-green photons Two main materials systems have been used to fabricate these devices: GaN and ZnSe We saw that while the GaN system has so far produced more impressive laser devices, work done in both systems has been crucial for an understanding of devices based on these widegap semiconductors We might now ask “Where have the efforts described in this book led? What is the current status and outlook for compact blue-green lasers? Have they been used yet for the applications described in Chapter 1?” We close this treatise by briefly commenting upon these questions 533 534 12 Concluding remarks First, it is probably worth commenting that although some fifteen years have passed since breakthroughs in infrared diode laser technology prompted the first flurry of excitement over new prospects for compact blue-green sources, the need for such sources in a wide variety of applications has not ebbed In fact, it might well be argued that the explosive growth of the internet and the growing convergence of the information and entertainment industries has enhanced interest in certain applications of compact blue-green lasers – for example, higher-density DVDs and large-format, high-resolution color displays Furthermore, the advancement of biomedical technologies continues to sustain interest in compact, efficient, short-wavelength sources for these applications For some applications, the remarkable development of GaN semiconductors lasers has been a major boon The impressive breakthroughs and tremendous progress made in these devices have finally led, at the time of this writing, to the availability of development devices operating in the violet (∼405 nm) at powers up to ∼30 mW in a single transverse mode (according to the website for Nichia Corp.,www.nichia.com) These characteristics are sufficient to allow these devices to be used in applications where the relatively low power level and narrow range of available wavelengths not constitute critical limitations For example, GaN laser diodes have now been used for demonstrations of high-density optical data storage, in which the expected advantages described in Chapter have been realized To cite one example, Akiyama et al (2001) have reported demonstrating a rewritable optical disk with a capacity of 27 GB using a GaN laser Numerous other descriptions of work to develop high-density optical data storage using GaN lasers could also be cited (for example, Ko et al (2001), Ichimura et al (2000)) GaN laser diodes are also well suited to certain biomedical applications, where high power is not essential, and the violet wavelength is very effective for exciting fluorescence, spectral analysis of which reveals information about the sample under observation For example, Gustafsson et al (2000) have used an InGaN laser diode in a compact fluorescence sensor that can be used to monitor the condition of vegetation and to distinguish between healthy and pre-malignant skin tissue; Girkin et al (2000) have reported on the use of a 406-nm InGaN laser diode for confocal microscopy of biological media stained with fluorescent dyes When GaN laser diodes are used with an extended cavity, it becomes possible to tune them sufficiently to make them useful for other spectroscopic applications (Leinen et al., 2000) Gustafsson et al (2000a) have reported using extended-cavity violet laser diodes for spectroscopy of potassium atoms The same group has also mixed the ∼400 nm output of a GaN laser diode with the 688-nm emission from a red diode laser to create a source tunable around 254 nm that can be used for mercury detection (Alnis et al., 2000) 12 Concluding remarks 535 While GaN laser diodes are nearly ideal sources for applications like these, the relatively low power and limited wavelength range limits their use in other important applications Furthermore, the prospects for obtaining significantly higher powers and a broader range of wavelengths from short-wavelength semiconductor diode lasers in the future are unclear at this time Hence, for applications requiring substantially higher powers or wavelengths longer than ∼410 nm, sources based on SHG are more likely to play a key role, at least in the near future Commerciallyavailable, high-power (∼10 W) green lasers based on intracavity SHG are becoming increasingly popular as replacements for argon lasers High-power diode-pumped blue and green lasers based on SHG and SFG continue to be extensively developed for other applications where higher powers or longer blue-green wavelengths are required, such as laser projection displays (Hollemann et al 2000) Upconversion lasers continue to be pursued in the laboratory, but are still more accurately described as “promising” rather than “practical” Fiber upconversion lasers, in particular, remain intriguing because of the attractive features of the fiber geometry, but so far have not progressed to a stage suitable for use in applications However, the pursuit of these lasers has also reinvigorated interest in the spectroscopy and laser characteristics of rare-earth-doped solid-state lasers and has broadened and deepened our understanding of these systems in ways that have had implications for more “conventional” lasers based on these systems Hence, some of the potential benefits of compact blue-green lasers that were anticipated fifteen years ago have begun to be realized, as some of the very diverse technical directions that have been pursued have begun to yield practical devices It is probably true of nearly any technology that as it matures, there is a progressive convergence between what is sought in principle and what can be achieved practically Applications for which there emerges no practical solution may become dormant, while the availability of a new solution may spark the invention of new applications In the case of compact blue-green lasers, fifteen years of intense effort directed at developing sources for specific applications have clarified which approaches are more practical than others, what the limitations and advantages of each implementation are, which approaches “match up” best with preconceived applications, and the ease or difficulty of developing a novel laboratory technology into a product and bringing it to market The current result of this process is that specific types of compact blue-green lasers are beginning to be used in specific applications to which they are especially well suited, as described above On the other hand, the availability of versatile devices like the GaN laser diode are stimulating the conception of new applications in which such components can be used to great advantage For the remainder, there is still more work to be done in finding lasers suitable for existing applications and new applications for existing blue-green lasers 536 12 Concluding remarks REFERENCES Akiyama, T., Uno, M., Kitaura, H., Narumi, K., Kojima, R., Nishiuchi, K., and Yamada, N (2001) Rewritable dual-layer phase-change optical disk utilizing a blue-violet laser Jpn J Appl Phys., 40, 1598–1603 Alnis, J., Gustafsson, U., Somesfalean, G., and Svanberg, S (2000) Sum-frequency generation with a blue diode laser for mercury spectroscopy at 254 nm Appl Phys Lett., 76, 1234–1236 Girkin, J M., Ferguson, A I., Wokosin, D L., and Gurney, A M (2000) Confocal microscopy using an InGaN violet laser diode at 406 nm Opt Express, 7, 336–341 Gustafsson, U., Plsson, S., and Svanberg, S (2000) Compact fiber-optic fluorosensor using a continuous-wave violet diode laser and an integrated spectrometer Rev Sci Instr., 71, 3004–3006 Gustafsson, U., Alnis, J., and Svanberg, S (2000a) Atomic spectroscopy with violet laser diodes Am J Phys., 68, 660–664 Hollemann, G., Braun, B., Dorsch, F., Hennig, P., Heist, P., Krause, U., Kutschki, U., Voelckel, H (2000) RGB lasers for laser projection display Proc SPIE, 3954, 140–151 Ichimura, I., Maeda, F., Osato, K., Yamamoto, K., and Kasami, Y (2000) Optical disk recording using a GaN blue-violet laser diode Jpn J Appl Phys., 39, 937–942 Ko, J., Park, I S., Yoon, D.-S., Chung, C.-S., Kim, Y.-G., Ro, M.-D., Doh, T.-Y., and Shin, D.-H (2001) Optical storage system for 0.4 mm substrate media using 405 nm laser diode and numerical aperture 0.60/0.65 objective lens Jpn J Appl Phys., 40, 1604–1608 Leinen, H., Gl¨aßner, H., Metcalf, H., Wynands, R., Haubrich, D., Meschede, D (2000) GaN blue diode lasers: a spectroscopist’s view Appl Phys B, 70, 567–571 Tieke, B., Dekker, M., Preffer, N., van Woudenberg, R., Zhou, G F., and Urbens, P D (2000) High data rate phase change media for the digital video recording system Jpn J Appl Phys., 39, 762–765 Index ABCD matrices, 209 acceptor binding energies, 477 in GaN, 476 in p-ZnSe, 477 A1GaN/GaN DBRs, 521 AlN strain-relief layers, 526 ambipolar diffusion, 508 amplification rate equations inhomogeneous broadening, 375–378 large-signal gain, 367–369 small-signal gain, 365–367 three- and four-level systems, 378–382, 379f, 380f, 381f angular momentum Ce3+ and, 307–311 quantum mechanics of, 295–296 Yb3+ ions and, 311–312 atomic resonance filter, 10 atomic structure See also electronic structure central potential, 303–306 electron shielding in crystal field, 298–302 elements of, 303–324 ions and electron shells, 296–297f shell model, 304 balanced phasematching, 93–94 biotechnology, 14, 534 bireflection, 215–217, 220, 216f bond energy covalent, 491 Boyd–Kleinman analysis circular Gaussian beams, 43–48 elliptical beams, 49–50 experimental verification, 48, 49f sum-frequency generation, 50 Type-II phasematching, 49 bulk upconversion lasers, 397–427 Ce3+ ions spin-orbit interaction, 307–310 central potential, 303–306 ˇ Cerenkov phasematching, 94 chemical sound energy, 474 cladding layers, 489 collection-mode NSOM, 507 color displays, 6, 535 compositional fluctuations, 490 cooperative upconversion, 348–356 diffusion-limited regime, 351–352 direct-transfer regime, 350–351 hopping or diffusion regime, 353–355 ultrafast-migration regime, 355–356 Coulomb interaction, 313–315 cracks, 490 crystal-field model, 298–302, 301f manifolds and perturbation theory, 325–328, 326f potential, 329–331 diamagnetic shifts, 513 dielectric DBRs, 519 diode lasers advantages for upconversion, 293–299 angled DFB, 169–170, 169f broad-area, 160 gain-guided, 152, 154, 153f high-power, narrow-strip, index-guided, 156 index-guided, 154–157 master-oscillator power-amplified (MOPA), 161–168 multiple-stripe arrays, 158–160, 158f pumping of upconversion lasers, 438–440, 449, 453–454 diode-pumped solid-state lasers 946-nm Nd, 170–171, 245–249 1064-nm Nd, 170–171 end vs side pumping, 173 NPRO (non-planar ring oscillator), 176 single-pass frequency conversion, 177–178 dislocation density, 469 dry-etching, 496 DVD, 469, 534 edge-emitting InGaN lasers, 487 electric dipoles electric field around, 341–343, 342f transition selection rules for, 336–338 537 538 Index electron blocking layer, 489 electronic structure See also atomic structure ions and electron shells, 296–297f multi-electron atom, 305–306 rare-earth ions, 306–311 electrons wave function of, 303–305 energy-gap law, 339–341, 340f, 341 energy transfer radiationless, 341–344 upconversion rate formulation, 357–360 epitaxial lateral overgrowth (ELOG), 472–474, 491–496, 529 Dislocations, 472 Er3+ ions crystal-field effects on manifolds and, 325–328, 326f fiber lasers, 433–436, 434f infrared lasers, 398–410, 408f upconversion pumping of, 404f, 414f visible lasers, 410–420 YLF crystals and, 411–416 exciton binding energy, 475, 513 excitons, 509 lanthanide ions, 293–298 lanthanum, 306–307 large signal gain, 367–369 laser amplification, 364–365, 366f gain saturation, 225–226, 374–375 inhomogeneous broadening, 375–378, 377f oscillating amplifier, 224–228, 369f–375 three-level amplifier, 367f–369 laser oscillation and oscillators, 224–226, 369–375, 370f optimum output coupling, 226, 375, 376f oscillation frequency, 375 quasi-three-level, 245–249 slope efficiency, 246–247, 373–374 threshold condition, 246, 373 lateral injection, 524 leakage current, 490 lifetimes (diode laser), 487, 495 localization, 501, 506 LS coupling, 313–317 fiber-optic upconversion lasers, 427, 432–458 See also optical fiber or ZBLAN fiber ZBLAN and output power of, 441–444 filling of the localized states, 504 form birefringence, 100 Förster–Dexter model/theory, 344, 407 Nd3+ ions upconversion lasers, 425–427, 426f, 457f–458 near-field imaging, 498 near-field optical microscopy, 506 neodymium lasers 946 nm, 170–177 1064 nm, 170–177 NiO, 480 nonlinear coefficient (d), 28–30 nonlinear frequency conversion, focused beams, 43–50 monochromatic waves, 34 multi-longitudinal mode sources, 34–38 pump depletion, 38–42 waveguide confinement, 51–56 waveguide loss, 53 nonlinear materials, 101–130 borates (BBO, LBO), 124–126T isomorphs of KTP, 119 lithium iodate, 123–124T lithium niobate (LN), 101–107T birefringent phasematching, 101–103 quasi-phasematching, 103–104 table of properties, 106–107 waveguides, 104–105 lithium tantalate (LT), 108–110T birefringent phasematching, 108 quasi-phasematching, 108 table of properties, 110 waveguides, 108–109 organic materials, 126–128 poled glasses, 128 potassium lithium niobate (KLN), 121, 123 potassium niobate (KN), 119–121 birefringent phasematching, 120–121 quasi-phasematching, 121 gain saturation, 225–226, 374–375 gain spectra, 502, 503 of GaAs QW, 502 of InGaN QW, 502 of ZnCdSe QW, 502, 508f gain spectroscopy, 507 “Green Problem”, 229–244 Hakki–Paoli method, 502 Hartree–Fock theory, 305–307 historical overview, Ho3+ ions, 395f fiber upconversion lasers, 455–456 Hund’s rules, 324 hydride vapor phase epitaxy, 494 impedance matching, 191–193 indium-tin oxide, 524 infrared upconversion lasers, 407–410 InGaN alloy, 487 InGaN QW diode lasers, 469, 491, 497 inhomogeneous broadening, 375–378, 377f in-situ stress monitoring, 522 Jerlov minimum, 8, 10f Judd–Ofelt intensity parameters, 296, 302 formulation of, 325–328 oscillator strength expression, 329–336 reduced matrix element, 330–336T metal-organic chemical vapor deposition, 471 Mg dopant, 476, 489 multi-longitudinal mode sources, 34 Index table of properties, 122 waveguides, 121 potassium titanyl phosphate (KTP), 110–115 birefringent phasematching, 110–111 quasi-phasematching, 113 table of properties, 116–117 waveguides, 113–115 rubidium titanyl arsenate (RTA), 115 birefringent phasematching, 115 quasi-phasematching, 115 table of properties, 118 waveguides, 119 self-doubling materials, 129–130 semiconductor materials, 129 nonlinear polarization frequencies of, 23 origin, 21 nonradiative relaxation, 338–341 ohmic contacts, 478 to p-AlGaInN, 479 to p-ZnSe, 482 to p-ZnSe/ZnTe superlattice, 482 optical amplifier, 366f, 367f optical data storage, 3–5, 534 optical fiber, 128, 168–169, 394f, 459 See also fiber optic upconversion lasers or ZBLAN fiber optical gain in InGaN diode lasers, 501, 502 and electron–hole pair density, 501 and localization, 501 peak in InGaN QW, 504 optical gain coefficient, 507 patterned growth, 491 p-doping of GaN, 475 of A1GaInN, 475 in superlattices, 476 perturbation theory crystal field and degenerate, 325–326 nondegenerate, 327 phasematching, 56–101 anomalous dispersion, 97–99, 99f birefringent, 57–71 angle tuning, 59–65 basic explanation, 57–59 effective nonlinearity, 64–65 noncritical, 65–66 nonuniformity effects, 70–71 temperature tuning, 59 tolerances, 66–70 walk off angle, 61–64 counterpropagating waves, 99–100 form birefringence, 100 quasi-phasematching (QPM), 71–90 basic explanation, 71–77 fabrication of QPM structures, 77–81 periodic poling, 79–81 theory, 81–85 539 tolerances for imperfect structures, 88–90 tolerances for perfect structures, 85–88 simple explanation, 56–57 total internal reflection, 100 waveguide phasematching, 90–97 balanced phasematching, 93–94 ˇ Cerenkov phasematching, 94–96 modal dispersion, 90–93 noncritical geometry, 96–97 phonon emission, 339–341, 340T photobleaching effect, 443 photodarkening effect, 441–444 photon avalanche, 360–363, 360f population densities 362–363f, 364f piezoelectric coefficients (InGaN QWs), 506 piezoelectric fields, 506 Pr3+ ions eigenvalues, states and vectors, 317–323, 321T multi-wavelength pumping, 446f, 450–455, 451f photon-avalanche pump, 396f Russell–Sanders states, 313–317, 336T spectroscopy and spectrum, 315–317, 338f upconversion lasers, 424–425, 445–455, 446f pumping mechanisms, 292–293 See also upconversion mechanisms diode laser, 174–176, 435, 438 multi-wavelength pumping, 440f, 448f relative pump rate, 381f single-wavelength pumping, 441 two-photon pumping, 346–348, 346f, 347f, 391f vibrational upconversion, 389–390 ZBLAN and output power, 441–444 quantum counters, infrared, 385–386f quantum mechanics of angular momentum, 295–296 radiationless energy transfer processes, 390–391 upconversion rate formulation, 357–360 rare-earth elements Russell–Sanders terms for, 322T rare-earth ions, 293–302 4f shell states, 298–302 crystal field as shielding, 298–302 electronic structure of, 296–297f, 306–311 energy-gap law, 340f, 341 energy-level diagrams, 294f, 299f spectroscopy of, 295–296, 300–301T RCLED, 517 reduced matrix element, 330–336T reprographics, resonant multi-photon absorption, 345–348 two-photon pumping, 346f resonator-enhanced SFG, 217, 218 resonator-enhanced SHG, 183, 218–219 resonators bireflection, 215–217 effect of loss, 189–190 frequency locking, 193–207 540 Index dither locking, 195–196 Hänsch–Couillard locking, 196–198 optical locking, 201–207 Pound–Drever–Hall locking, 198–201 impedance matching, 191–193 mode-matching, 207–213 modulation of SHG output, 214–215 monolithic, 186f ring, 184, 184f standing-wave, 184, 184f temperature-locking, 213–214 theory of resonator enhancement of SHG, 187–190 Russell–Saunders reduced matrix element, 330–336T Russell–Saunders states, 313–317, 334 Russell–Saunders terms for rare-earth elements, 322T, 330, 337 sapphire substrate, 488, 495f semiconductor Bloch equations, 510 separate confinement heterostructure, 488, 489f short-period superlattices, 481, 484 single-pass SHG, 150f using angled-DFB diode lasers, 169 using broad-area diode lasers, 160–161 using diode-pumped 946-nm Nd lasers, 177–178 using diode-pumped 1064-nm Nd lasers, 177 using gain-guided diode lasers, 152 using high-power index-guided narrow-stripe diode lasers, 156–157 using index-guided diode lasers, 154 using MOFAs, 168–169 using MOPAs, 161, 168 using multiple-stripe arrays, 157–160 single-pass sum-frequency mixing, 178 SiC substrate, 488 slope efficiency, 246, 373–374, 497 small-signal gain amplification rate equations, 365–367 spectroscopic applications, 12, 534 spectroscopy notation, 311, 313 rare-earth, 295–296 submarine communications, thermal conductivity, 495 GaN, 495 Sapphire, 495 SiC, 495 threshold current density, 497, 498 threshold gain of blue VCSEL, 521 Tm3+ ions fiber lasers, 436–445, 437f optical fiber, 394f upconversion lasers, 420–424, 421f, 438f–440f, 444–445 transparency condition, 504 tunnel junction, 481, 524 upconversion lasers, bulk, 397–427 cavity configurations, 399f fiber optic, 427, 432–458 history of, 385–387 infrared, 407–410 introduction to, 292–295, 293f kinetics, 405–406 multicolor output, 447–448f output power, 415 room-temperature operation, 437–438f upconversion laser experiments Bloembergen, 385 CNET group – Allain et al., 393–394 Gosnell and Xie, 447–448 Grubb, 437–438f Hughes group, 392, 396, 398, 404, 405, 407, 412 IBM group, 390, 392, 393, 396, 411, 412, 418, 419, 422, 426 Johnson and Guggenheim, 387–388 Laperle, 442–443 Lenth, 391, 397, 412, 427 Macfarlane, 391, 397, 419, 426, 426f, 427 McFarlane, 392, 412–413, 416–417 Pollack and Chang, 392, 398, 404–407 Rand and Xie, 407–410, 416 tables of, 400–403, 428–431 Tohmon, 438–441, 439f upconversion mechanisms, 345f See also pumping mechanisms cooperative upconversion, 348–356 photon avalanche, 360–363, 360f, 448f radiationless energy transfer formulation, 357–360, 358f resonant multi-photon absorption, 345–348 VCSEL (vertical cavity surface emitting laser), 517 vertical cavity, 517 vertical cavity LED, 525 vibrational upconversion pumping, 389–390 waveguides confinement for SHG, 51–56 loss, 53–54, 54f nonuniform nonlinearity, 54f, 55 phasematching, 90–97 waveguide layers (quantum well), 489 Wigner–Eckart theorem, 331 Yb3+ ions infrared pumped, 414f upconversion lasers, 311–313, 446f ZBLAN fiber, 393–395 See also fiber optic upconversion lasers or optical fiber development of, 433 output power losses and photodarkening, 441–444 Pr3+ ion use with, 338f, 451f–455 ZnCdSe QW lasers, 469, 488