The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.

http://science.sciencemag.org/content/sci/290/5490/314.full.pdf

Optical gain and stimulated emission in nanocrystal quantum dots.

Optical gain and stimulated emission in nanocrystal quantum dots

Room-temperature lasing at the wavelength of 1.31 μm is achieved from the ground state of an InGaAs/GaAs quantum-dot ensemble. At 79 K, a very low threshold current density of 11.5 A/cm2 is obtained at a wavelength of 1.23 μm. The room-temperature lasing at 1.31 μm is obtained with a threshold current density of 270 A/cm2 using high-reflectivity facet coatings. The temperature-dependent threshold with and without high-reflectivity end mirrors is studied, and ground-state lasing is obtained up to the highest temperature investigated of 324 K.

1.3 μm room-temperature GaAs-based quantum-dot laser

The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminesce...

Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

We demonstrate that very few (2\char21{}4) quantum dots as a gain medium are sufficient to realize a photonic-crystal laser based on a high-quality nanocavity. Photon correlation measurements show a transition from a thermal to a coherent light state proving that lasing action occurs at ultralow thresholds. Observation of lasing is unexpected since the cavity mode is in general not resonant with the discrete quantum dot states and emission at those frequencies is suppressed. In this situation, the quasicontinuous quantum dot states become crucial since they provide an energy-transfer channel into the lasing mode, effectively leading to a self-tuned resonance for the gain medium.

https://web.physics.ucsb.edu/~quopt/self_tun.pdf

Self-tuned quantum dot gain in photonic crystal lasers.

Low threshold, large T/sub o/ injection laser emission via zero-dimensional states in (InGa)As quantum dots is demonstrated. The dots are formed due to a morphological transformation of a pseudomorphic In/sub 0.5/Ga/sub 0.5/As layer. Laser diodes are fabricated with a shallow mesa stripe geometry.<
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Low threshold, large To injection laser emission from (InGa)As quantum dots

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A/spl middot/cm/sup -2/ at 77 K and of 62 A/spl middot/cm/sup -2/ at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of /spl sim/80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to /spl sim/50 meV, ultrahigh material gain of /spl sim/10/sup 5/ cm/sup -1/, differential gain of /spl sim/10/sup -13/ cm/sup 2/ and strong nonlinear gain effects with a gain compression coefficient of /spl sim/10/sup -16/ cm/sup 3/. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is /spl sim/0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 /spl mu/m and 1.37 /spl mu/m at 300 K.

InGaAs-GaAs quantum-dot lasers

Low threshold current density (AlInGa)As/GaAs lasers based on InGaAs quantum dots (QDs) are grown by metal organic chemical vapour deposition (MOCVD). Quantum dots deposited at 490° C and covered with GaAs are directly revealed in the active region. On a transmission electron microscopy (TEM) image of the laser structure no large clusters or dislocations are found over a macroscopic distance. We show that the properties of QD lasers can be strongly improved if the QDs are confined by Al0.3Ga0.7As barriers and the cladding layers are grown at high temperature. Optimisation of the laser structure geometry allows extension of the range of ultrahigh temperature stability (T0=385 K) of the threshold current to 50° C.

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T 0= 385 K) Grown by Metal Organic Chemical Vapour Deposition

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T_0 = 385 K) Grown by Metal Organic Chemical Vapour Deposition ( Quantum Dot Structures)

We report the observation of suppression of the dc current through an Esaki–Tsu GaAs/AlAs superlattice (that shows a negative differential conductance due to Bloch oscillations of miniband electrons) caused by irradiation with millimeter wave radiation (frequency 78 GHz). A theoretical analysis of the dc current indicated that the high‐frequency electric current followed the high‐frequency field with a response time smaller than the period (∼10−11 s) of the field. Our experiment, with the superlattice at room temperature, demonstrates that the Esaki–Tsu superlattice is suitable for ultrafast millimeter wave detection and other high‐frequency applications.

Petr S. Kop'ev

Papers

Low threshold, large To injection laser emission from (InGa)As quantum dots

InGaAs-GaAs quantum-dot lasers

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T 0= 385 K) Grown by Metal Organic Chemical Vapour Deposition

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T_0 = 385 K) Grown by Metal Organic Chemical Vapour Deposition ( Quantum Dot Structures)

Suppression of current through an Esaki–Tsu GaAs/AlAs superlattice by millimeter wave irradiation