Wenquan Ma

Resonance Fluorescence from a Coherently Driven Semiconductor Quantum Dot in a Cavity

We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, g(tau), as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3 microeV. Second-order correlation measurements further confirm nonclassical light emission.

InGaAs/GaAs three-dimensionally-ordered array of quantum dots

We report on the first fabrication of (In,Ga)As/GaAs quantum dots with both vertical and lateral ordering forming a three-dimensional array. An investigation of the photoluminescence spectra from the ordered array of quantum dots, as a function of both temperature and optical excitation intensity, reveals both a lateral and vertical transfer of excitation.

Anisotropic photoconductivity of InGaAs quantum dot chains measured by terahertz pulse spectroscopy

We report results of time-resolved terahertz (THz) pulse spectroscopy experiments on laterally ordered chains of self-assembled InGaAs quantum dots photoexcited with 400nm, 100fs laser pulses. A large anisotropy in the transient photoconductive response is observed depending on the polarization of the THz probe pulse with respect to the orientation of the dot chains. Fast (3.5–5ps) and efficient carrier capture into the dots and one-dimensional wetting layers underneath the dot chains is observed below 90K. At higher temperatures, thermionic emission into the two-dimensional wetting layers and barriers becomes significant and the anisotropy in the photoconductive signal is reduced.

Voltage tunable two-color InAs∕GaAs quantum dot infrared photodetector

We report a bias voltage tunable two-color InAs/GaAs quantum dot infrared photodetector working under the normal incidence infared irradiation. The two-color detection of our device is realized by combining a photovoltaic and a photoconductive response by bias voltage tuning. The photovoltaic response is attributed to the transition of electron from the ground state to a high continuum state. The photoconductive response arises from the transition of electron from the ground state to the wetting layer state through the barrier via Fowler-Nordheim tunneling evidenced by a broad feature of the photocurrent peak on the high energy side

Long Wavelength Infrared InAs/GaSb Superlattice Photodetectors With InSb-Like and Mixed Interfaces

We report on long wavelength infrared photodetectors using InAs/GaSb superlattices (SLs) with InSb-like and mixed interfaces (IFs). X-ray diffraction (XRD) measurements indicate that the SLs with mixed IFs have a narrower linewidth. The full-width at half-maximum of the XRD satellite peak is 24 arcsec for the sample with InSb-like IFs and is only 17 arcsec for the sample with mixed IFs. However, in terms of infrared photodetection, InSb-like IFs are superior to the mixed ones. Stronger photoluminescence and photoresponse signals are observed for the sample with InSb-like IFs.

High Structural Quality of Type II InAs/GaSb Superlattices for Very Long Wavelength Infrared Detection by Interface Control

We investigate the interface control for very long wavelength infrared InAs/GaSb superlattice (SL) structures. An InAs/GaSb SL photodetector with very high structural quality has been demonstrated by precisely controlling the Sb-soak, the growth stop time and the InSb layer thickness at the interfaces. The full width at half maximum of the X-ray diffraction satellite peaks of a p-i-n device structure is only 21 arcsec. The 50% cutoff wavelength of the detector is 14.5 μm at 77 K. At 14.5 μm, the quantum efficiency is 14%, while at the photoresponse maximum position of 7.7 μm it is 50%.

Spontaneous quasi-periodic current self-oscillations in a weakly coupled GaAs/(Al,Ga)As superlattice at room temperature

We have experimentally observed spontaneous quasi-periodic current self-oscillations at room temperature in a doped, weakly coupled GaAs/(Al,Ga)As superlattice (SL) with 50 periods, 7 nm well width, and 4 nm barrier width. The mole fraction of the aluminum in the barrier has been chosen to be 0.45 so that the direct barrier at the Γ point is as high as possible and thermal carrier leakage through the X valley is as small as possible. A spectral analysis of the current self-oscillations, which are observed under DC voltage bias alone, demonstrates that spontaneous quasi-periodic oscillation modes coexist with periodic ones.

Mid Wavelength Type II InAs/GaSb Superlattice Photodetector Using SiOxNy Passivation

We report on a mid wavelength (MW) type II InAs/GaSb superlattice (SL) photodetector structure using SiOxNy as the passivation material. The 50% cutoff wavelength of the photoresponse is 4.8 µm at 77 K. R0A, the resistance-and-area product at zero bias, is 2.1×103 Ωcm2 for the device with the SiOxNy passivation, which is about 13 times larger than that without the passivation. Our result indicates SiOxNy passivation is an effective way to reduce the shunt current for MW InAs/GaSb SL photodetector.

Narrow-band long-/very-long wavelength two-color type-II InAs/GaSb superlattice photodetector by changing the bias polarity

We report on a narrow-band two-color photodetector using type-II InAs/GaSb superlattices (SLs) in the long-/very-long wavelength infrared (VLWIR) ranges by changing the polarity of the bias. The narrow-band photoresponse is achieved by sequentially growing the doped SL structure that has a shorter cutoff wavelength as a low-pass filter for the absorption layers that has a longer cutoff wavelength. At 77 K, the 50% cutoff wavelength of the photodiode is 10 μm when the applied bias voltage is –0.1 V and is 16 μm at +40 mV. The δλ/λ is 44% for the LWIR band and is 46% for the VLWIR band.

Three-region characteristic temperature in p-doped quantum dot lasers

We have investigated the temperature dependence of threshold in p-doped 1.3 μm InAs/GaAs quantum dot (QD) lasers with ten layers of QDs in the active region. It is found that the dependence of threshold current density on the temperature within the temperature range from 10 to 90 °C can be divided into three regions by its characteristic temperature (T0): negative, infinite, and positive T0 regions. Furthermore, the T0 region width is dependent on the cavity length: the longer cavity length of the QD lasers correspondingly the wider T0 region. Additionally, for the broad area laser, the threshold modal gains of the lasers with different cavity lengths can be fitted by an empirical expression as a function of the threshold current density, when at the temperatures of 30, 50, and 70 °C. We find that the transparency current density (Jtr) remains almost unchanged under different temperatures according to the extracted parameters from these fitted results, which indicates that Jtr plays an important role in b...