Zunaid Baten

High Performance InAs/

The characteristics of 1.55 μm InAs self-organized quantum-dot lasers, grown on (001) InP substrates by molecular beam epitaxy, have been investigated. Modulation doping of the dots with holes and tunnel injection of electrons have been incorporated in the design of the active (gain) region of the laser heterostructure. Large values of To=227 K (5^°C ≤ T ≤ 45^°C) and 100 K were derived from temperature dependent measurements of the light-current characteristics. The modal gain per dot layer is 14.5 cm^-1 and the differential gain derived from both light-current and small-signal modulation measurements is ~ 0.8×10^-15 cm². The maximum measured -3 dB small-signal modulation bandwidth is 14.4 GHz and the gain compression factor is 5.4×10^-17 cm². The lasers are characterized by a chirp of 0.6 Å for a modulation frequency of 10 GHz and a near zero α-parameter at the peak of the laser emission. These characteristics are amongst the best from any 1.55 μm edge-emitting semiconductor laser.

{\rm In}_{0.53}{\rm Ga}_{0.23}{\rm Al}_{0.24}{\rm As}

Strong coupling effects and polariton lasing are observed at 155 K with an edge-emitting GaAs-based microcavity diode with a single Al0.31Ga0.69As/Al0.41Ga0.59As quantum well as the emitter. The threshold for polariton lasing is observed at 90 A/cm2, accompanied by a reduction of the emission linewidth to 0.85 meV and a blueshift of the emission wavelength by 0.89 meV. Polariton lasing is confirmed by the observation of a polariton population redistribution in momentum space and spatial coherence. Conventional photon lasing is recorded in the same device at higher pump powers.

/InP Quantum Dot 1.55

InGaN/GaN self-organized quantum dots can provide useful advantages over quantum wells for the realization of long-wavelength visible light sources because the dots are formed by strain relaxation. A III–nitride based laser emitting in the red (λ ~ 630 nm), which has not been demonstrated with quantum wells, would be useful for a host of applications. We have investigated the epitaxy and characteristics of self-organized InGaN/GaN multiple layer quantum dots grown by plasma-assisted molecular beam epitaxy and have optimized their properties by tuning the growth parameters. Red-emitting (λ ~ 630 nm) quantum dots have radiative lifetime ~2.5 ns and internal quantum efficiency greater than 50%. Edge-emitting red-lasers with multi-dot layers in the active region exhibit an extremely low threshold current density of 1.6 kA/cm2, a high temperature coefficient T0 = 240 K, and a large differential gain dg/dn = 9 × 10−17 cm2.

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A spin-polarized laser offers inherent control of the output circular polarization. We have investigated the output polarization characteristics of a bulk GaN-based microcavity polariton diode laser at room temperature with electrical injection of spin-polarized electrons via a FeCo/MgO spin injector. Polariton laser operation with a spin-polarized current is characterized by a threshold of ∼69  A/cm^{2} in the light-current characteristics, a significant reduction of the electroluminescence linewidth and blueshift of the emission peak. A degree of output circular polarization of ∼25% is recorded under remanent magnetization. A second threshold, due to conventional photon lasing, is observed at an injection of ∼7.2  kA/cm^{2}. The variation of output circular and linear polarization with spin-polarized injection current has been analyzed with the carrier and exciton rate equations and the Gross-Pitaevskii equations for the condensate and there is good agreement between measured and calculated data.

Tunnel Injection Laser

We report the measurement of diffusive electronic spin transport characteristics in an epitaxial wurtzite GaN lateral spin valve at room temperature. Hanle spin precession and non-local spin accumulation measurements have been performed with the spin valves fabricated with FeCo/MgO spin contacts. Electron spin relaxation length and spin-flip lifetime of 176 nm and 37 ps, respectively, are derived from analysis of results obtained from four-terminal Hanle spin precession measurements at 300 K. The role of dislocations and defects in bulk GaN has also been examined in the context of electronic spin relaxation dynamics.

GaAs-based high temperature electrically pumped polariton laser

Use of large bandgap materials together with electrical injection makes the polariton laser an attractive low-power coherent light source for medical and biomedical applications or short distance plastic fiber communication at short wavelengths (violet and ultra-violet), where a conventional laser is difficult to realize. The dynamic properties of a polariton laser have not been investigated experimentally. We have measured, for the first time, the small signal modulation characteristics of a GaN-based electrically pumped polariton laser operating at room temperature. A maximum −3 dB modulation bandwidth of 1.18 GHz is measured. The experimental results have been analyzed with a theoretical model based on the Boltzmann kinetic equations and the agreement is very good. We have also investigated frequency chirping during such modulation. Gain compression phenomenon in a polariton laser is interpreted and a value is obtained for the gain compression factor.

High performance red-emitting multiple layer InGaN/GaN quantum dot lasers

The role of dislocations and defects in the material constituting the active region of an exciton-polariton laser has been examined and elucidated in the context of dynamic condensation and the temperature of the lower polariton condensate, TLP. For a GaN microcavity diode polariton laser operated at room temperature and characterized in this study, the value of TLP obtained from analysis of measured occupation in momentum space is 270 K, which is lower than the lattice temperature. Similar results from other room temperature GaN devices and GaAs-based polariton lasers operated at cryogenic temperatures are presented and discussed.

Room-Temperature Spin Polariton Diode Laser

Room temperature circularly polarized electroluminescence is observed from bulk GaN-based double-heterostructure edge-emitting light emitting diodes operated with continuous-wave spin-polarized electrical injection. The measured in-plane magnetizing field-dependent output circular polarization in the quasi-Voigt geometry is coincident with the hysteresis characteristics of the spin-injector confirming electron spin injection in remanence from the n-type FeCo/MgO tunnel spin contacts. A peak output circular polarization of ~6 ± 1% is measured at 300 K.

Electrical spin injection and detection of spin precession in room temperature bulk GaN lateral spin valves

Room temperature electrical modulation response of a bulk GaN-based electrically injected polariton laser is reported. The frequency response is derived from the measured time-resolved electroluminescence and a maximum -3 dB bandwidth of 0.65 GHz is obtained.