Dayan Ban

Terahertz quantum cascade lasers operating up to ~200 K with optimized oscillator strength and improved injection tunneling

A new temperature performance record of 199.5 K for terahertz quantum cascade lasers is achieved by optimizing the lasing transition oscillator strength of the resonant phonon based three-well design. The optimum oscillator strength of 0.58 was found to be larger than that of the previous record (0.41) by Kumar et al. [Appl. Phys. Lett. 94, 131105 (2009)]. The choice of tunneling barrier thicknesses was determined with a simplified density matrix model, which converged towards higher tunneling coupling strengths than previously explored and nearly perfect alignment of the states across the injection and extraction barriers at the design electric field. At 8 K, the device showed a threshold current density of 1 kA/cm2, with a peak output power of ∼ 38 mW, and lasing frequency blue-shifting from 2.6 THz to 2.85 THz with increasing bias. The wavelength blue-shifted to 3.22 THz closer to the maximum operating temperature of 199.5 K, which corresponds to ∼ 1.28ħω/κB. The voltage dependence of laser frequency is related to the Stark effect of two intersubband transitions and is compared with the simulated gain spectra obtained by a Monte Carlo approach.

Effect of doping concentration on the performance of terahertz quantum-cascade lasers

We characterized a set of terahertz quantum-cascade lasers with identical device parameters except for the doping concentration. The δ-doping density was varied from 3.2×1010to4.8×1010cm−2. We observed that the threshold current density increased monotonically with doping. Moreover, the measured results on devices with different cavity lengths provided evidence that the free carrier absorption caused waveguide loss also increased monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping density of 3.6×1010cm−2.

Near-infrared to visible light optical upconversion by direct tandem integration of organic light-emitting diode and inorganic photodetector

The authors report a hybrid organic/inorganic optical upconversion device that converts 1.5μm infrared light to ∼520nm visible light. The device was made by direct tandem integration of an inorganic InGaAs∕InP photodetector with an organic light-emitting diode (OLED). Optical upconversion with an external efficiency of 0.7% W/W at room temperature has been achieved. Interfacial structure at the inorganic-organic interface was found to play a vital role in enabling the integration of the hybrid tandem upconverter. Both sulfur-terminated InP surface and nanocarbon fullerene interlayer were found crucial to form a good interface contact, permitting continuous flow of photocarriers from the inorganic detector into the OLED.

A phonon scattering assisted injection and extraction based terahertz quantum cascade laser

A lasing scheme for terahertz quantum cascade lasers, based on consecutive phonon-photon-phonon emissions per module, is proposed and experimentally demonstrated. The charge transport of the proposed structure is modeled using a rate equation formalism. An optimization code based on a genetic algorithm was developed to find a four-well design in the GaAs/Al0.25Ga0.75As material system that maximizes the product of population inversion and oscillator strength at 150 K. The fabricated devices using Au double-metal waveguides show lasing at 3.2 THz up to 138 K. The electrical characteristics display no sign of differential resistance drop at lasing threshold, which, in conjunction with the low optical power of the device, suggest—thanks to the rate equation model—a slow depopulation rate of the lower lasing state, a hypothesis confirmed by non-equilibrium Green’s function calculations.

A terahertz pulse emitter monolithically integrated with a quantum cascade laser

A terahertz pulse emitter monolithically integrated with a quantum cascade laser (QCL) is demonstrated. The emitter facet is excited by near-infrared pulses from a mode-locked Ti:sapphire laser, and the resulting current transients generate terahertz pulses that are coupled into an electrically isolated QCL in proximity. These pulses are used to measure the gain of the laser transition at ∼2.2 THz, which clamps above threshold at ∼18 cm−1 and has a full width at half-maximum linewidth of ∼0.67 THz. The measurement also shows the existence of absorption features at different biases that correspond to misalignment of the band structure and to absorption within the two injector states. The simplicity of this scheme allows it to be implemented alongside standard QCL ridge processing and to be used as a versatile tool for characterizing QCL gain media.

Transparent organic light-emitting devices using a MoO3/Ag/MoO3 cathode

MoO3/Ag/MoO3 (MAM) stacks are investigated for utilization as transparent cathodes in organic light-emitting devices (OLEDs). The stacks can be fabricated using simple thermal evaporation at relatively low temperatures, and can be readily and safely utilized as top electrodes, without causing deposition damage to the organic layers. Results show that it is possible to achieve efficient electron injection from the MAM electrode into organic layers by means of incorporating a suitable electron injection layer (EIL) at the interface. Results also show that a MAM stack can exhibit high optical transmittance, amounting to about 65–80% in the 400–700 nm, and a low sheet resistance (9 Ω/□). A transparent OLED with a MAM cathode incorporated with a 10 nm Bphen: Cs2CO3 (10%) EIL is studied. By fine tuning the MAM structure, optimal OLED performance has been achieved: a total luminance of 1300 cd/m2 (representing ∼1000 cd/m2 and ∼300 cd/m2 from the bottom and the top, respectively) at 20 mA/cm2 and a corresponding driving voltage of 7.2 V. The OLED exhibits a peak transmittance of ∼90% in the 450–475 nm range, and a transmittance above 45% over the entire visible (i.e., 400–700 nm) range.

Two-dimensional profiling of carriers in a buried heterostructure multi-quantum-well laser: Calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

We report results of a scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) study of the distribution of charge carriers inside multi-quantum-well (MQW) buried heterostructure (BH) lasers. We demonstrate that individual quantum-well–barrier layers can be resolved using high-resolution SSRM. Calibrated SSRM and SCM measurements were performed on the MQW BH laser structure, by utilizing known InP dopant staircase samples to calibrate the instrumentation. Doping concentrations derived from SSRM and SCM measurements were compared with the nominal values of both p- and n-doped regions in the MQW BH lasers. For n-type materials, the accuracy was bias dependent with SSRM, while for SCM, excellent quantitative agreement between measured and nominal dopant values was obtained. The SSRM was able to measure the dopant concentration in the p-type materials with ∼30% accuracy, but quantitative measurements could not be obtained with the SCM. Our results demonstrate the utility of c...

Effect of oscillator strength and intermediate resonance on the performance of resonant phonon-based terahertz quantum cascade lasers

We experimentally investigated the effect of oscillator strength (radiative transition diagonality) on the performance of resonant phonon-based terahertz quantum cascade lasers that have been optimized using a simplified density matrix formalism. Our results show that the maximum lasing temperature (Tmax) is roughly independent of laser transition diagonality within the lasing frequency range of the devices under test (3.2‐3.7THz) when cavity loss is kept low. Furthermore, the threshold current can be lowered by employing more diagonal transition designs, which can effectively suppress parasitic leakage caused by intermediate resonance between the injection and the downstream extraction levels. Nevertheless, the current carrying capacity through the designed lasing channel in more diagonal designs may sacrifice even more, leading to electrical instability and, potentially, complete inhibition of the device’s lasing operation. We propose a hypothesis based on electric-field domain formation and competition/switching of different current-carrying channels to explain observed electrical instability in devices with lower oscillator strengths. The study indicates that not only should designers maximize Tmax during device optimization but also they should always consider the risk of electrical instability in device operation. V C 2013 American

Optimized GaAs∕AlGaAs light-emitting diodes and high efficiency wafer-fused optical up-conversion devices

Achieving a high internal quantum efficiency in GaAs∕AlGaAs based light-emitting diodes (LEDs) for room-temperature operation at low current-density injection is crucial for applications such as optical up-converters based on the integration of LEDs and photodetectros. We report the experimental results as well as the theoretical analyses of the internal quantum efficiency of GaAs∕AlGaAs LEDs as a function of the p-doping concentration of the active region for low current injection operation. By optimizing the doping concentration, we have achieved a close to 100% internal quantum efficiency for room-temperature operation of LEDs in the low injection current-density range, i.e., around 0.1A∕cm2. An optical up-converter was fabricated using wafer-fusion technology by integrating the optimized GaAs∕AlGaAs LED with an InGaAs∕InP photodetector. The internal up-conversion quantum efficiency was measured to be 76%.

Near-Infrared Inorganic/Organic Optical Upconverter with an External Power Efficiency of >100%

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