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. (C) 2008 American Institute of Physics.

Quantum-confined Stark effects in a single GaN quantum dot

Using analytical expressions for the polarization field in GaN quantum dot, and an approximation by separating the potential into a radial and an axial, we investigate theoretically the quantum-confined Stark effects. The electron and hole energy levels and optical transition energies are calculated in the presence of an electric field in different directions. The results show that the electron and hole energy levels and the optical transition energies can cause redshifts for the lateral electric field and blueshifts for the vertical field. The rotational direction of electric field can also change the energy shift.

Realization of extremely broadband quantum-dot superluminescent light-emitting diodes by rapid thermal-annealing process

The first demonstration, to our knowledge, of the creation of ultrabroadband superluminescent light-emitting diodes using multiple quantum-dot layer structure by rapid thermal-annealing process is reported. The device exhibits a 3 dB emission bandwidth of 146 nm centered at 984 mm with cw output power as high as 15 mW at room temperature corresponding to an extremely small coherence length of 6.6 mu m. (C) 2008 Optical Society of America.

Comparative study of optical properties between quantum dot chains band quantum dots

A comparative study of the steady-state and transient optical properties was made between InGaAs/GaAs quantum do chains (QDCs) and quantum dots (QDs). It was found that the photoluminescence (PL) decay time of QDCs exhibited a strong photon energy dependence, while it was less sensitive in QDs. The PL decay time increased much faster with the excitation power in the QDCs than that in QDs. When the excitation power was large enough, the PL decay time tended to be saturated. In addition, it was also found that the PL rise time was much shorter in QDCs than in QDs. All these experimental results show that there is a strong carrier coupling along the chain direction in the QD chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.

High-power quantum-dot superluminescent LED with broadband drive current insensitive emission spectra using a tapered active region

High-power and broadband quantum-dot (QD) superluminescent light-emitting diodes are realized by using a combination of self-assembled QDs with a high density, large inhomogeneous broadening, a tapered angled pump region, and etched V groove structure. This broad-area device exhibits greater than 70-nm 3-dB bandwidth and drive current insensitive emission spectra with 100-mW output power under continuous-wave operation. For pulsed operation, greater than 200-mW output power is obtained.

Numerical analysis of gain saturation, noise figure, and carrier distribution for quantum-dot semiconductor-optical amplifiers

The gain saturation behaviors and noise figure are numerically analyzed for quantum-dot semiconductor optical amplifiers (QD-SOAs). The carrier and photon distributions in the longitudinal direction as well as the photon energy dependent facet reflectivity are accounted in the rate equations, which are solved with output amplified spontaneous emission spectrum as iterative variables. The longitudinal distributions of the occupation probabilities and spectral-hole burning are presented for electrons in the excited and ground states of quantum dots. The saturation output power 19.7 dBm and device gain 20.6 dB are obtained for a QD-SOA with the cavity length of 6 rum at the bias current of 500 mA. The influences of them electron intradot relaxation time and the QD capture time on the gain spectrum are simulated with the relaxation time of 1, 30, and 60 ps and capture time of 1, 5, and 10 ps. The noise figure as low as 3.5 dB is expected due to the strong polarization sensitive spontaneous emission. The characteristics of gain saturation and noise figure versus input signal power for QD-SOAs are similar to that of semiconductor. linear optical amplifiers with gain clamping by vertical laser fields.

Effects of accumulated strain on the surface and optical properties of stacked 1.3 mu m InAs/GaAs quantum dot structures

We report the effects of accumulated strain by stacking on the surface and optical properties of stacked 1.3 mu m InAs/GaAs quantum dot (QD) structures grown by MOCVD. It is found that the surface of the stacked QD structures becomes more and more undulated with stacking, due to the increased strain in the stacked QD structures with stacking. The photoluminescence intensity from the QD structures first increases as the stacking number increases from 1 to 3 and then dramatically decreases as it further increases, implying a significant increase in the density of crystal defects in the stacked QD structures due to the accumulated strain. Furthermore, we demonstrate that the strain can be reduced by simply introducing annealing steps just after growing the GaAs spacers during the deposition of the stacked QD structures, leading to significant improvement in the surface and optical properties of the structures. (C) 2007 Elsevier B.V. All rights reserved.

Single-photon emission from a single InAs quantum dot

Excitation power-dependent micro-photoluminescence spectra and photon-correlation measurement are used to study the optical properties and photon statistics of single InAs quantum dots. Exciton and biexciton emissions, whose photoluminescence intensities have linear and quadratic excitation power dependences, respectively, are identified. Under pulsed laser excitation, the zero time delay peak of second order correlation function corresponding to exciton emission is well suppressed, which is a clear evidence of single photon emission.

High-power operation of uncoated strain-compensated quantum cascade lasers at 4.8 mu m

High-power operation of uncoated 22-mu m-wide quantum cascade lasers (QCLs) emitting at lambda approximate to 4.8 mu m is reported. The emitting region of the QCL structure consists of a 30-period strain-compensated In0.68Ga0.32As/In0.37Al0.63As superlattice. For a 4-mm-long laser in pulsed mode, a peak output power is achieved in excess of 2240mW per facet at 81K with a threshold current density of 0.64kA/cm(2). The effects of varying the cavity lengths from 1 to 4mm on the performances of the QCLs are analysed in detail and the low waveguide loss of only about 1.4 cm(-1) is extracted.

High responsivity resonant-cavity-enhanced InGaAs/GaAs quantum-dot photodetector for wavelength of similar to 1 mu m at room temperature

The characteristics of a resonant cavity-enhanced InGaAs/GaAs quantum-dot n-i-n photodiode with only a bottom distributed Bragg reflector used as the cavity mirror, are reported. To suppress the dark current, an AlAs layer is inserted into the device structure as the blocking layer. It turns out that the structure still possesses the resonant coupling nature, and makes Rabi splitting discernible in the photoluminescence spectra. The measured responsivity spectrum of the photocurrent shows a peak at lambda = 1030 nm, and increases rapidly as the bias voltage increases. A peak responsivity of 0.75 A/W, or equivalently an external quantum efficiency of 90.3%, is obtained at V-bias = -1.4 V.

Investigation of Electronic Energy Levels in InAs Quantum Dot with Shape of Lens by Using B-Spline Technique

The dependence of the electronic energy levels on the size of quantum dots (QDs) with the shape of spherical lens is studied by using the B-spline technique for the first time. Within the framework of the effective-mass theory, the values of electronic energy levels are obtained as a function of the height, radius and volume of QDs, respectively. When the height or radius of QDs increases, all the electronic energy levels lower, and the separations between the energy levels decrease. For lens-shape QDs, height is the key factor in dominating the energy levels comparing with the effect of radius, especially in dominating the ground-state level. These computational results are compared with that of other theoretical calculation ways. The B-spline technique is proved to be an effective way in calculating the electronic structure in QDs with the shape of spherical lens.

Electrically Driven InAs Quantum-Dot Single-Photon Sources

Electrically driven single photon source based on single InAs quantum dot (QDs) is demonstrated. The device contains InAs QDs within a planar cavity formed between a bottom AlGaAs/GaAs distributed Bragg reflector (DBR) and a surface GaAs-air interface. The device is characterized by I-V curve and electroluminescence, and a single sharp exciton emission line at 966nm is observed. Hanbury Brown and Twiss (HBT) correlation measurements demonstrate single photon emission with suppression of multiphoton emission to below 45% at 80K

Correlations between antibunching and blinking of photoluminescence from a single CdSe quantum dot

The antibunching and blinking from a single CdSe/ZnS nanocrystal with an emission wavelength of 655 nm were investigated under different excitation powers. The decay process of the photoluminescence from nanocrystal was fitted into a stretched exponential, and the small lifetime and the small stretching exponent under a high excitation power were explained by using nonradiative multi-channel model. The probability of distributions for off-times from photoluminescence intermittence was fitted into the power law, and the power exponents were explained by using a tunneling model. For higher excitation power, the Auger-assisted tunneling model takes effect, where the tunneling rate increases and the observed lifetime decreases. For weak excitation power, the electron directly tunnels between the nanocrystal and trapping state without Auger assistance. The correlation between antibunching and blinking from the same nanocrystal was analyzed.

Solid source MBE growth of quantum cascade lasers

High material quality is the basis of quantum cascade lasers (QCLs). Here we report the solid source molecular beam epitaxy (MBE) growth details of realizing high quality of InGaAs/InAlAs QCL structures. Accurate control of material compositions, layer thickness, doping profile, and interface smoothness can be realized by optimizing the growth conditions. Double crystal x-ray diffraction discloses that our grown QCL structures possess excellent periodicity and sharp interfaces. High quality laser wafers are grown in a single epitaxial run. Room temperature continuous-wave (cw) operation of QCLs is demonstrated.

ac gate-induced Fano resonance and peak splitting of conductance in a quantum dot

We have investigated the conductance of a quantum dot system suffering an anti-symmetric ac gate voltage which induces the transition between dot levels in the linear regime at zero temperature in the rotating wave approximation. Interesting Fano resonances appear on one side of the displaced resonant tunnelling peaks for the nonresonant case or the peak splitting for the resonant case. The line shape of conductance (vs Fermi energy) near each level of the quantum dot can be decomposed into two profiles: a Breit-Wigner peak and a Fano profile, or a Breit-Wigner peak and a dip in both cases.