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Quantum Dot Laser Seminar Report 2004 TABLE OF CONTENTS INTRODUCTION 1 LASER 2 • PRICIPLE OF OPERATION 3 • LASER DIODE 4 PRINCIPLE OF OPERATION 5 TYPES OF LASER DIODE 6 SEMICONDUCTOR LASER 7 QUANTUM WELL LASER 7 QUANTUM DOT LASER 8 QUANTUM DOT 9 GROWTH TECHNIQUES 12 CHALLENGES 13 SPECTRAL ANALYSIS OF QUANTUM DOT 14 LASERS HIGH TEMPERATURE PROPERTIES 21 CONCLUSION 24 Dept of Electronics & Communication GEC Thrissur Quantum Dot Laser Seminar Report 2004 INTRODUCTION Since the invention of semiconductor lasers in 1962, significant progress has been made in terms of high performance in many applications including telecommunications, optical storage, and instrumentation. Most modern semiconductor lasers operate based on quantum mechanical effects. Quantum well lasers have been used with impressive performance, while novel quantum dot lasers, a subject of intense research, show a great promise Lasers come in many sizes and can be made from a variety of resonant cavities and active laser materials. Generally, increasing confinement enforces an increasing quantization in the energy of electrons. Therefore quantum dots, essentially zero-dimensional bits of material, will (once excited) re-emit light at nearly a single wavelength. Quantum dots are therefore a good starting point for producing laser light Some existing quantum dot lasers employ dots made epitaxially: the atoms in the dots are laid down meticulously using beams of atoms or molecules In the MIT laser the gain medium consists of nm-sized particles of CdSe coated with a layer of organic molecules and then immersed in a glassy film. The medium sits in a waveguide atop a grating. The fabrication advantage in this case derives from the fact that one uses simple solution processing rather than the more exacting technique of epitaxy usually needed for semiconductors. Furthermore, the color of the output laser light can be varied by changing the size of the CdSe particles, the grating spacing, or the refractive index of the waveguide, giving great flexibility to the design and application of the laser Dept of Electronics & Communication 1 GEC Thrissur Quantum Dot Laser Seminar Report 2004 LASER A Laser (light amplification by stimulated emission of radiation) is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light. Light from a laser is often very collimated and monochromatic, but this is not true of all laser types. Common light sources, such as the electric light bulb, emit photons in all directions, usually over a wide spectrum of wavelengths. Most light sources are also incoherent; i.e., there is no fixed phase relationship between the photons emitted by the light source. By contrast, a laser generally emits photons in a narrow, well-defined beam of light. The light is often near-monochromatic, consisting of a single wavelength or color, is highly coherent and is often polarised. Some types of laser, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for the generation of extremely short pulses of light, on the order of a femtosecond (10-15 seconds). Laser light can be highly intense — able to cut steel and other metals. The beam emitted by a laser often has a very small divergence (highly collimated). A perfectly collimated beam cannot be created, due to the effect of diffraction, but a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some lasers, especially semiconductor lasers due to their small size, produce very Dept of Electronics & Communication 2 GEC Thrissur Quantum Dot Laser Seminar Report 2004 divergent beams. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources can generally not be collimated. A laser medium can also function as an optical amplifier when seeded with light from another source. The amplified signal can be very similar to the input signal in terms wavelength, phase and polarisation; this is particularly important in optical communications. The output of a laser may be a continuous, constant-amplitude output (known as c.w. or continuous wave), or pulsed, by using the techniques of Q-switching, modelocking or Gain-switching. In pulsed operation, much higher peak powers can be achieved. Principle Of Operation The basic physics of lasers centres around the idea of producing a population inversion in a laser medium by 'pumping' the medium; i.e., by supplying energy in the form of light or electricity, for example. The medium may then amplify light by the process of stimulated emission. If the light is circulating through the medium by means of a cavity resonator, and the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. Eventually it will get so strong that the gain is saturated (reduced). In continuous operation, the intracavity laser power finds an equilibrium value which is saturating the gain exactly to the level of the cavity losses. If the pump power is chosen too small (below the 'laser threshold'), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. Dept of Electronics & Communication 3 GEC Thrissur Quantum Dot Laser Seminar Report 2004 A great deal of quantum mechanics and thermodynamics theory can be applied to laser action (see laser science), though in fact many laser types were discovered by trial and error. Population inversion is also the concept behind the maser, which is similar in principle to a laser but works with microwaves. The first maser was built by Charles H. Townes in 1953. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was then known. The word laser was coined in 1957 by Gordon Gould, who was also credited with lucrative patent rights in the 1970s, following a protracted legal battle Even low-power lasers with only a few milliwatts of output power can be hazardous to a person's eyesight. The coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localised burning and permanent damage in seconds. Lasers are classified by wavelength and maximum output power into safety classes numbered I (inherently safe) to IV (even scattered light can cause eye and/or skin damage). Laser products available for consumers, such as CD players and laser pointers are usually in class I or II . LASER DIODE A laser diode is a laser where the active medium is a semiconductor p-n junction similar to that found in a light-emitting diode. Laser diodes are sometimes referred to (somewhat redundantly) as injection laser diodes or by the acronyms LD or ILD. Dept of Electronics & Communication 4 GEC Thrissur Quantum Dot Laser Seminar Report 2004 Principle of operation When a diode is forward biased, holes from the p-region are injected into the n-region, and electrons from the n-region are injected into the p- region. If electrons and holes are present in the same region, they may radiatively recombine—that is, the electron "falls into" the hole and emits a photon with the energy of the bandgap. This is called spontaneous emission, and is the main source of light in a light-emitting diode. Under suitable conditions, the electron and the hole may coexist in the same area for quite some time (on the order of microseconds) before they recombine. If a photon of exactly the right frequency happens along within this time period, recombination may be stimulated by the photon. This causes another photon of the same frequency to be emitted, with exactly the same direction, polarization and phase as the first photon. In a laser diode, the semiconductor crystal is fashioned into a shape somewhat like a piece of paper—very thin in one direction and rectangular in the other two. The top of the crystal is n-doped, and the bottom is p- doped, resulting in a large, flat p-n junction. The two ends of the crystal are cleaved so as to form perfectly smooth, parallel edges; two reflective parallel edges are called a Fabry-Perot cavity. Photons emitted in precisely the right direction will be reflected several times from each end face before they are emitted. Each time they pass through the cavity, the light is amplified by stimulated emission. Hence, if there is more amplification than loss, the diode begins to "lase". Dept of Electronics & Communication 5 GEC Thrissur Quantum Dot Laser Seminar Report 2004 Types of laser diode The type of laser diode just described is called a homojunction laser diode, for reasons which should soon become clear. Unfortunately, they are extremely inefficient. They require so much power that they can only be operated in short "pulses;" otherwise the semiconductor would melt. Although historically important and easy to explain, such devices are not practical. Double heterostructure lasers In these devices, a layer of low bandgap material is sandwiched between two high bandgap layers. One commonly-used pair of materials is GaAs with AlGaAs. Each of the junctions between different bandgap materials is called a heterostructure, hence the name "double heterostructure laser" or DH laser. The kind of laser diode described in the first part of the article is referred to as a "homojunction" laser, for contrast with these more popular devices.