Chi-Sen Lee

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.

Distinct Lasing Operation From Chirped InAs/InP Quantum-Dash Laser

We study the enhanced inhomogeneity across the InAs quantum-dash (Qdash) layers by incorporating a chirped AlGaInAs barrier thickness in the InAs/InP laser structure. The lasing operation is investigated via Fabry-Pérot ridge-waveguide laser characterization, which shows a peculiar behavior under quasi-continuous-wave (QCW) operation. Continuous energy transfer between different dash ensembles initiated quenching of lasing action among certain dash groups, causing a reduced intensity gap in the lasing spectra. We discuss these characteristics in terms of the quasi-zero-dimensional density of states (DOS) of dashes and the active region inhomogeneity.

Effect of optical waveguiding mechanism on the lasing action of chirped InAs/AlGaInAs/InP quantum dash lasers

We report on the atypical emission dynamics of InAs/AlGaInAs/InP quantum dash (Qdash) lasers employing varying AlGaInAs barrier thickness (multilayer-chirped structure). The analysis is carried out via fabry-perot (FP) ridge (RW) and stripe waveguide (SW) laser characterization corresponding to the index and gain guided waveguiding mechanisms, respectively, and at different current pulse width operations. The laser emissions are found to emerge from the size dispersion of the Qdash ensembles across the four Qdash-barrier stacks, and governed by their overlapping quasi-zero dimensional density of states (DOS). The spectral characteristics demonstrated prominent dependence on the waveguiding mechanism at quasi-continuous wave (QCW) operation (long pulse width). The RW geometry showed unusual spectral split in the emission spectra on increasing current injection while the SW geometry showed typical broadening of lasing spectra. These effects were attributed to the highly inhomogeneous active region, the nonequilibrium carrier distribution and the energy exchange between Qdash groups across the Qdash-barrier stacks. Furthermore, QCW operation showed a progressive red shift of emission spectra with injection current, resulted from active region heating and carrier depopulation, which was observed to be minimal in the short pulse width (SPW) operation. Our investigation sheds light on the device physics of chirped Qdash laser structure and provides guidelines for further optimization in obtaining broad-gain laser diodes.

A quantum ring detector for the 1–3 terahertz range with very high responsivity and specific detectivity

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.

Electrically injected polariton lasing from a GaAs-based microcavity under magnetic field

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.

Photo detectors for multi-spectral sensing

Device concepts of quantum well, dot, and ring for multi-band photodetection are presented in this paper. Results on a preliminary GaAs-based npn-Quantum Well Infrared Photodetector (QWIP) show two combinations of wavelength bands which can be selected using the applied bias. An InP based npn-QWIP structure is proposed to eliminate the cross talk between the bands. As a separate approach, a three band architecture is proposed to obtain response in three bands by combining split-off, interband and intraband transitions which are all bias selectable. Furthermore, in this paper, a dual band Superlattice Quantum Dot Infrared Photodetector (SL-QDIP), providing bias-selectability of the response peaks, is demonstrated. The active region of this detector, consists of two quantum dot super-lattices (SL) which were separated by a graded barrier, enabling photocurrent generation in only one super-lattice for a given bias polarity. Two different response bands, one consisting of three peaks at 4.4, 7.4 and 11 µm, were observed up to 120 K for reverse and forward biases, respectively. Additionally, the intersubband transitions in InAs/GaAs quantum rings have been studied. Quantum ring based photoconductive detectors with multiple quantum ring layers in the active region exhibit dark currents of ∼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 at 5.2 K. This detector exhibits a peak responsivity of 25 A/W and specific detectivity D* of 1016 Jones under 1 V bias at 5.2 K. The detectivity at 10 K, is measured ∼3 × 1015 Jones.