Sishir Bhowmick

High-performance quantum ring detector for the 1-3 terahertz range

Molecular beam epitaxy of InAs/GaAs quantum dots and their subsequent transformation to quantum rings by postepitaxy thermal annealing have been investigated. Photoconductive detectors with multiple quantum ring layers in the active region exhibit dark current density ∼10−8 A/cm2 at a bias of 2 V at 4.2 K. The rings have a single bound state, and emission of photoexcited carriers gives rise to a spectral response peaking at 1.82 THz (165 μm) at 5.2 K. Peak responsivity of 25 A/W, specific detectivity, D∗, of 1×1016 Jones and a total quantum efficiency of 19% are measured with 1 V bias at 5.2 K. At 10 K and 1 V, D∗∼3×1015 Jones is measured.

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}

A rolled-up microtube directional coupler made of twin microtubes is demonstrated. The microtube is made of a InGaAs/GaAs strained bilayer and InAs self-organized quantum dots are inserted in the GaAs layer. The input and coupled microtubes have length and outer diameter of ∼50 and 6 μm, respectively. The coupling characteristics have been analyzed by the three-dimensional finite difference time domain method. The coupling characteristics have also been measured with isopropyl alcohol, instead of air, as the surrounding media to demonstrate the potential of the device as a sensor.A rolled-up microtube directional coupler made of twin microtubes is demonstrated. The microtube is made of a InGaAs/GaAs strained bilayer and InAs self-organized quantum dots are inserted in the GaAs layer. The input and coupled microtubes have length and outer diameter of ∼50 and 6 μm, respectively. The coupling characteristics have been analyzed by the three-dimensional finite difference time domain method. The coupling characteristics have also been measured with isopropyl alcohol, instead of air, as the surrounding media to demonstrate the potential of the device as a sensor.

/InP Quantum Dot 1.55

We have investigated the threshold behavior of GaAs-based InAs self-organized quantum dot microtube lasers. The laser heterostructures were grown by molecular beam epitaxy and the devices were fabricated by standard optical lithography and etching techniques. Measurements have been made on microtube lasers of diameters 6, 9, and 12 . The spectral characteristics were measured by micro-photoluminescence and the data have been analyzed by considering the confinement and mode characteristics of the microtube. The threshold condition was derived after considering radiation, bending, and substrate losses associated with the devices. The threshold modal gain and excitation power are found to depend inversely on the microtube diameter.

\mu{\rm m}

A rolled-up microtube optoelectronic integrated circuit operating as a phototransceiver is demonstrated. The microtube is made of a InGaAs/GaAs strained bilayer with InAs self-organized quantum dots inserted in the GaAs layer. The phototransceiver consists of an optically pumped microtube laser and a microtube photoconductive detector connected by an a-Si/SiO2 waveguide. The loss in the waveguide and responsivity of the entire phototransceiver circuit are 7.96 dB/cm and 34 mA/W, respectively.

Tunnel Injection Laser

The role of polariton-electron scattering on the performance characteristics of an electrically injected GaAs-based quantum well (QW) microcavity diode in the strong coupling regime has been investigated. An electron gas is introduced in the quantum wells by modulation doping with silicon dopants. It is observed that polariton-electron scattering suppresses the relaxation bottleneck in the lower polariton branch. However, it is not adequate to produce a degenerate coherent condensate at k∥ ∼ 0 and coherent emission.

A quantum dot rolled-up microtube directional coupler

The detection of long wavelength and terahertz (THz) radiation is important for a number of applications including molecular spectroscopy, medical diagnostics, security and surveillance, quality control, and astronomy. Semiconductor based quantum dot (QD) and quantum ring (QR) detectors [1, 2] have been used for the detection of long wavelength radiation. While high temperature operation of the devices is desired for some applications, THz detectors operating at low temperatures are also in demand, particularly for astronomy and space applications. Another challenge for semiconductor-based detectors is operation in the 1–3 THz range. We report here a InAs/GaAs quantum ring intersublevel detector (QRID) with spectral response peaking at 1.82 THz (165 µm) and having a peak responsivity Rp of 25 A/W and specific detectivity D* of 1×1016 Jones for 1 V bias at 5.2 K. At 10 K, the spectral response peaks at 2.4 THz (125 µm) with Rp = 3 A/W and D* = 3×1015 Jones. These characteristics compare very favorably with those of bolometers that are currently used.

Threshold Characteristics of Quantum Dot Rolled-Up Microtube Lasers

We report on the observation of confined coherent acoustic plasmon waves in photoexcited GaAs using ultrafast pump probe experiments. Results are in good agreement with theoretical predictions under the random phase approximation.

Quantum dot rolled-up microtube optoelectronic integrated circuit

Electrically injected polariton lasing from a GaAs-based modulation-doped microcavity diode has been demonstrated under an applied magnetic field of 7 Tesla at 30 K. Polariton lasing and condensation characteristics have been measured and analyzed.

Polariton emission characteristics of a modulation-doped multiquantum-well microcavity diode

Suppression of relaxation bottleneck and subsequent polariton lasing is observed in a GaAs-based microcavity under the application of a magnetic field. The threshold injection current density is 0.32 A/cm2 at 7 Tesla.