S. I. Ahn

Detectivity enhancement in quantum well infrared photodetectors utilizing a photonic crystal slab resonator.

We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS) resonator. The strongest resonance of the PCS is designed to coincide with the absorption peak frequency at 7.6 µm of the QWIP. To accurately characterize the detector performance, it is illuminated by using single mode mid-infrared lasers. The strong resonant absorption enhancement yields a detectivity increase of up to 20 times. This enhancement is a combined effect of increased responsivity and noise current reduction. With increasing temperature, we observe a red shift of the PCS-QWIP resonance peak of -0.055 cm(-1)/K. We attribute this effect to a refractive index change and present a model based on the revised plane wave method.

High-power, low-lateral divergence broad area quantum cascade lasers with a tilted front facet

We introduce a simple technique to improve the beam quality of broad area quantum cascade lasers. Moderately tilted front facets of the laser provide suppression of higher order lateral waveguide modes. A device with a width of 60 μm and a front facet angle of 17° shows a nearly diffraction limited beam profile. In addition, the peak output power and the slope efficiency of the device are increased since most of the light inside the cavity is emitted through the tilted front facet by an asymmetric light intensity distribution along the cavity.

Enhanced Electrical Characteristics of AlGaN/GaN Heterostructure Field-Effect Transistor with p-GaN Back Barriers and Si Delta-Doped Layer

We present the electrical characteristics of an AlGaN/GaN/p-GaN heterostructure field-effect transistor (HFET) with a Si delta-doped layer. The p-GaN layer greatly improves buffer isolation (between neighboring mesas) in the AlGaN/GaN HFET and leads to effective carrier confinement. The Si delta-doped layer compensates not only the carrier depletion caused by the formation of a pn junction, but also even causes an increase in two-dimensional electron gas (2DEG) density. The proposed AlGaN/GaN HFET shows greatly improved electrical characteristics such as high drain current density and transconductance and low buffer and gate leakage currents compared with those of conventional AlGaN/GaN HFETs.

Enhanced light output power of quantum cascade lasers from a tilted front facet

We present a technique for enhancing the light output power of quantum cascade lasers (QCLs) by tilting of the front facet, which leads to a change of the modal reflectivity, resulting in an asymmetric light intensity distribution along the laser cavity. This asymmetry provides most of the light being emitted through one facet of the laser. An experimental study of threshold current, slope efficiency and light output power as a function of the front facet angles were performed and compared to conventional QCLs. The lasers with a front facet angle of 8° shows a 20% improved power output from the front facet.

Optimized photonic crystal design for quantum well infrared photodetectors

The performance of quantum well infrared photodetectors (QWIP) can be significantly enhanced combining it with a photonic crystal slab (PCS) resonator. In such a system the chosen PCS mode is designed to coincide with the absorption maximum of the photodetector by adjusting the lattice parameters. However there is a multitude of parameter sets that exhibit the same resonance frequency of the chosen PCS mode. We have investigated how the choice of the PC design can be exploited for a further enhancement of QWIPs. Several sets of lattice parameters that exhibit the chosen PCS mode at the same resonance frequency have been obtained and the finite difference time domain method was used to simulate the absorption spectra of the different PCS. A photonic crystal slab quantum well infrared photodetector with three different photonic crystal lattice designs that exhibit the same resonance frequency of the chosen PCS mode were designed, fabricated and measured. This work shows that the quality factor of a PCS-QWIP and therefore the absorption enhancement can be increased by an optimized PCS design. The improvement is a combined effect of a changed lattice constant, PC normalized radius and normalized slab thickness. An enhancement of the measured photocurrent of more than a factor of two was measured.

Electrical beam steering of Y-coupled quantum cascade lasers

The authors describe electrical beam steering of mid-infrared quantum cascade lasers with a monolithically integrated Y-coupled cavity and electrically separated branches. The deflection of the laser beam is achieved without any additional components such as optic or mechanic systems. This is done by injecting additional direct current into one of the two emitting branches, locally increasing the temperature. We estimate that the required temperature difference between left and right branch is approximately 12 K to achieve 2° of the beam steering. This value is in a good agreement with heat transfer simulations.

Quantum cascade lasers with a tilted facet utilizing the inherent polarization purity

We report on quantum cascade lasers (QCLs) with a tilted facet utilizing their polarization property. Contrary to diode lasers, QCLs generate purely TM polarized light due to the intersubband selection rules. This property enables the utilization of reflectivity in terms of only TM polarized light (TM reflectivity). The TM reflectivity is reduced by tilting the front facet, resulting in enhanced light output power from the tilted facet. The peak output power of a QCL with a facet angle of 12° are increased by 31 %. The slope efficiency of a QCL with a facet angle of 17° are increased by 43 %. Additionally, a peculiar property of TM reflectivity, the Brewster angle, is investigated by using COMSOL simulations to find its availability in QCLs.

Facet reflectivity reduction of quantum cascade lasers by tilted facets

The authors present a technique to reduce the facet reflectivity in quantum cascade lasers (QCLs) by tilted facets. In order to minimize the Fabry-Pérot resonances, the feedback from the laser facets into the cavity must be minimized. Due to intersubband selection rules, the light generated inside QCLs is TM polarized. This polarization purity in QCLs enables the reduction of the facet reflectivity through the angle of light incidence at the laser facet. We observed a maximum threshold current density when the facet is tilted 17° towards the surface normal. This is in agreement with the calculated Brewsters angle for the QCL heterostructure.

Coupling strategies for coherent operation of quantum cascade ring laser arrays

We report the design, fabrication and operation of coherently coupled ring cavity surface emitting quantum cascade lasers, emitting at wavelength around 8 μm. Special emphasis is placed on the evaluation of optimal coupling approaches and corresponding parameters. Evanescent field coupling as well as direct coupling where both devices are physically connected is presented. Furthermore, exploiting the Vernier‐effect was used to obtain enhanced mode selectivity and robust coherent coupling of two ring‐type quantum cascade lasers. Investigations were performed at pulsed room‐temperature operation.

Temperature‐induced beam steering of Y‐coupled quantum cascade lasers

The deflection of the laser beam is achieved without any additional components such as optic or mechanic systems. This is done by injecting additional direct current into one of the two emitting branches, locally increasing the temperature. We estimate that the required temperature difference between left and right branch is approximately 12 K to achieve 2* of beam steering