We began in Chapter 1 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 diode-pumped 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.

The organizing idea of the previous chapters is that nonlinear, nonresonant, properties of insulating materials can be exploited to convert long-wavelength coherent radiation into short-wavelength radiation. As was discussed at length, this compels the device designer to simultaneously satisfy two demanding operational constraints: (1) Optical intensities at a first-harmonic frequency must be sufficiently high that the electromagnetic response of dielectric media is pushed into the nonlinear regime. (2) A travelling wave thus generated at the second-harmonic frequency must propagate at the same phase velocity as the fundamental wave lest the second harmonic switch roles from receiver to donor of optical power in the device. However, nonlinear frequency generation hardly requires that the operative interactions take place off resonance. Photon adding functions can be accomplished equally well with the aid of resonant optical processes in insulating materials, in turn wholly eliminating the two challenging constraints just named. We are, of course, referring to upconversion lasers. In this chapter and the next, we present this second approach to the nonlinear generation of short visible wavelengths and discuss what different operational challenges arise in creating a practical upconversion device.

INTRODUCTION TO UPCONVERSION LASERS AND RARE-EARTH OPTICAL PHYSICS

Upconversion lasers function just as ordinary lasers do, at least insofar as the mechanism by which their output beams are generated: A population inversion is created between two widely separated states thus making possible optical gain and laser oscillation in whatever media play host to the atoms, ions, or molecules possessing those states. The difference comes in the pumping mechanism.

The science of upconversion laser development can lay claim to many critical achievements on the path to a practical device. Among these are broad spectral coverage, room-temperature operation, single-wavelength pumping, and all-solid-state design. Arguable remaining milestones to overtake are high power, low cost, and easy manufacture. In this chapter we present a discussion of the origins and development of this subject with an eye toward conferring on researchers, who are new to the field, a comprehensive grasp of the published literature on upconversion laser experiments. After an account of the early history of upconversion laser research, we divide the topic into parts: the first comprises work involving bulk laser gain media – exclusively rare-earth-doped crystals – whereas the second comprises work on optical fiber gain media – primarily doped fluorozirconate glasses. Following a similar organization, nearly all the publications discussed in this chapter are reprinted under one cover in the anthology by Gosnell (2000). The chapter concludes with a brief discussion of potential directions for future research.

HISTORICAL INTRODUCTION

The notion of multi-photon upconversion in the solid state was first discussed by Bloembergen (1959) in the context of microwave and infrared quantum counters. The idea, depicted in Figure 8.1, is to exploit the photon counting capabilities of photomultiplier tubes sensitive only in the visible and near-infrared spectral range to detect upconversion emission. As seen in the figure, a single low-energy “signal” photon is converted to a single high-energy output photon when an upconverting pump photon supplied by an external source is simultaneously absorbed by the system. Transparent host materials doped with rare-earth and transition-metal ions were specifically named as potential detectors.

OVERVIEW OF BLUE AND GREEN DIODE LASER DEVICE ISSUES

In this chapter we focus chiefly on the device science and engineering features of the violet edge-emitting InGaN MQW diode lasers. The extraordinary progress made with these devices since 1999, spearheaded by Nakamura and coworkers, seems to assure them an important place in future optoelectronics technology (Nakamura, 1999). By 2001, approximately half a dozen research groups reported achieving lifetimes of hundreds to a thousand hours for cw room-temperature operation, although the extrapolated lifetime of 15 000 hours at Nichia remained unequalled. Among the other groups we mention those at the laboratories of Sony, Toyoda Gosei, NEC, and Sharp in Japan, Samsung in Korea, and Xerox PARC, Cree Lighting, and Agilent Technologies in the USA.

A number of the core issues that intertwine the design, performance, and the physics of operation of the nitride lasers will be discussed in this chapter. We will focus on representative heterostructures that encompass the requirements of joint electronic and optical confinement, comment on some fabrication techniques, and highlight continuing challenges. The last include questions concerning the high threshold current density and the continued efforts to create artificial substrate templates for reducing the misfit (threading) dislocation density for improved device performance and lifetime. At a more fundamental level, there is evidence that the InGaN alloy which forms the optically-active QW medium has characteristic compositional disorder that impacts the gain spectrum of the laser. This feature, which increases in seriousness with the indium concentration, may restrict the operation of the devices at practical threshold current densities to the violet, leaving the longer blue and green regions to await future developments, perhaps involving complementary material approaches.

INTRODUCTION

Blue-green light can be generated by using nonlinear crystals to “upconvert” the infrared wavelengths produced by high-power semiconductor diode lasers. In secondharmonic generation (SHG), a single infrared laser with frequency ω1 is passed through a nonlinear crystal and blue-green light emerges with frequency 2ω1. In sum-frequency generation (SFG), two infrared lasers with frequencies ω1 and ω2 are combined in the crystal; the generated blue-green beam then has frequency ω1 + ω2. These “second-order” nonlinear effects are relatively weak, yet it is still possible to use them to generate blue-green radiation at power levels suitable for the applications described in Chapter 1. In fact, of the three basic approaches to blue-green light generation discussed in this book, nonlinear frequency upconversion has so far been the most extensively developed and the most prolific in spawning commercial blue-green laser products.

The inherent weakness of these nonlinear effects has forced researchers and laser engineers to explore a variety of techniques for enhancing the efficiency of these interactions. In Chapters 3–6, we will discuss these different approaches, which include such things as intracavity frequency-doubling, resonant enhancement, and guided-wave interactions. However, all of these different embodiments exploit the same basic nonlinear interactions, and this chapter is devoted to explaining the essential nature of those processes. In it, we will give a qualitative explanation of the physical process underlying SHG and SFG, we will present some of the basic equations necessary for understanding and designing blue-green lasers based on these effects, we will discuss techniques for providing “phasematching”, which we will see is a crucial requirement for efficient generation of blue-green light, and we will examine some of the nonlinear materials that can be used for frequency conversion of near-infrared light.

OVERVIEW

From its inception in 1962, the semiconductor laser based on GaAs and related III–V compounds has evolved into an extraordinary and versatile optoelectronic device. Today we find this ubiquitous, relatively low-cost, high-efficiency coherent light source used in a wide spectrum of applications ranging from long-haul optical communications systems to high-density optical data storage. The evolution of these devices to their present state of sophistication – with wall-plug efficiencies of 50% and greater in the conversion of DC electric current to coherent photons (as one illustrative characteristic) – did not happen overnight, however. Nearly two decades of research and development were spent before the edge-emitting GaAs-based diode laser emerged as a mature optoelectronic technology suited for applications and integration. Once the basic guiding scientific principles were established in the design of these devices, making specific use of optical and electronic confinement in heterostructures, a major challenge remained at the basic materials science level. This challenge concerned the role of crystal defects, both point and extended-state in nature, which were found to lead to degradation and early burnout of the lasers. However, advances in epitaxial techniques, in the III–V arsenide and phosphide systems, together with improvements in the device fabrication techniques, led to the development of low threshold current density quantum well (QW) heterostructure lasers, which are now contributing to the revolution in optical network technology.

The history of semiconductor light emitters at the short visible wavelengths stands in sharp contrast.

Introduction

We were able to show in Chapter 3 that a medium in which we can obtain a population inversion (i.e. a situation in which the population density in the excited state is greater than that in the fundamental level) allows for optical gain of an electromagnetic wave having a frequency near to the resonant frequency of the system. By introducing feedback of the amplified signal into the medium, the system can be made to oscillate naturally, resulting in laser oscillations. To obtain this population inversion, we must introduce at least a third (and perhaps even a fourth) energy level into the system. (We saw how a two-level system under the influence of an intense pump beam will saturate with no resulting population inversion.) The aim then of this chapter is to introduce the concepts necessary to extend our two-level system into a working model capable of illustrating the phenomenon of laser oscillation. We will not spend too much time discussing atomic transition lasers as they do not figure readily in our treatment of quantum electronic properties of semiconductors. An exception will be made, however; we brush upon the particular topics of a diode pumped laser in Complement 4.E and a quantum cascade laser in Complement 13.H.

Population inversion and optical amplification

Population inversion

We will show how population inversion can be achieved by carrier transfer from higher lying levels to the upper level of a two-level subsystem of interest.