Sylvain R. Laframboise

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.

Terahertz quantum-cascade lasers based on a three-well active module

The authors report on a design of terahertz quantum-cascade lasers based on three-well active modules. Each module consists of two tunnel-coupled wells for the two lasing states and another well for both resonant-phonon depopulation and carrier injection. This design is the simplest so far among the various published working devices. The test device has a lasing frequency of 3.4THz and maximum operating temperature of 142K.

Passivation of InGaAs surfaces and InGaAs/InP heterojunction bipolar transistors by sulfur treatment

The surface properties of InGaAs(100) after ex situ treatment with (NH4)2S solution were investigated by photoluminescence (PL) and high-energy resolution x-ray photoelectron spectroscopy. The As 3d, Ga 2p3/2, and In 3d5/2 core level studies show that the surface is free of native oxides and is terminated by S after treatment. A dramatic increase (∼40 times) in the PL efficiency was observed on undoped InGaAs(100) surfaces after sulfur passivation. This S treatment has also been applied to the passivation of the extrinsic base of InGaAs/InP heterojunction bipolar transistors (HBTs). The effectiveness of the sulfur passivation treatment was confirmed by the resulting devices which exhibited dc current gain values of up to 200 at very low collector currents (nA). Further, the sulfur passivated HBTs do not show any dependence on the perimeter-to-area (P/A) ratio of the emitter junction which is of interest for high frequency characteristics while maintaining high current gain.

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.

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

A novel silicon patch‐clamp chip permits high‐fidelity recording of ion channel activity from functionally defined neurons

We report on a simple and high‐yield manufacturing process for silicon planar patch‐clamp chips, which allow low capacitance and series resistance from individually identified cultured neurons. Apertures are etched in a high‐quality silicon nitride film on a silicon wafer; wells are opened on the backside of the wafer by wet etching and passivated by a thick deposited silicon dioxide film to reduce the capacitance of the chip and to facilitate the formation of a high‐impedance cell to aperture seal. The chip surface is suitable for culture of neurons over a small orifice in the substrate with minimal leak current. Collectively, these features enable high‐fidelity electrophysiological recording of transmembrane currents resulting from ion channel activity in cultured neurons. Using cultured Lymnaea neurons we demonstrate whole‐cell current recordings obtained from a voltage‐clamp stimulation protocol, and in current‐clamp mode we report action potentials stimulated by membrane depolarization steps. Despite the relatively large size of these neurons, good temporal and spatial control of cell membrane voltage was evident. To our knowledge this is the first report of recording of ion channel activity and action potentials from neurons cultured directly on a planar patch‐clamp chip. This interrogation platform has enormous potential as a novel tool to readily provide high‐information content during pharmaceutical assays to investigate in vitro models of disease, as well as neuronal physiology and synaptic plasticity. Biotechnol. Bioeng. 2010;107:593–600.

An indirectly pumped terahertz quantum cascade laser with low injection coupling strength operating above 150 K

We designed and demonstrated a terahertz quantum cascade laser based on indirect pump injection to the upper lasing state and phonon scattering extraction from the lower lasing state. By employing a rate equation formalism and a genetic algorithm, an optimized active region design with four-well GaAs/Al0.25Ga0.75As cascade module was obtained and epitaxially grown. A figure of merit which is defined as the ratio of modal gain versus injection current was maximized at 150 K. A fabricated device with a Au metal-metal waveguide and a top n+ GaAs contact layer lased at 2.4 THz up to 128.5 K, while another one without the top n+ GaAs lased up to 152.5 K (1.3ℏω/kB). The experimental results have been analyzed with rate equation and nonequilibrium Greens function models. A high population inversion is achieved at high temperature using a small oscillator strength of 0.28, while its combination with the low injection coupling strength of 0.85 meV results in a low current. The carefully engineered wavefunctions e...

Simultaneous detection of ultraviolet and infrared radiation in a single GaN/GaAlN heterojunction

Results are presented for a dual-band detector that simultaneously detects UV radiation in the 250-360 nm and IR radiation in the 5-14 microm regions with near zero spectral cross talk. In this detector having separate UV- and IR-active regions with three contacts (one common contact for both regions) allows the separation of the UV and IR generated photocurrent components, identifying the relative strength of each component. This will be an important development in UV-IR dual-band applications such as fire-flame detection, solar astronomy, and military sensing, eliminating the difficulties of employing several individual detectors with separate electronics-cooling mechanisms.

Thermal Behavior Investigation of Terahertz Quantum-Cascade Lasers

This paper investigates the heat conduction behavior of a terahertz (THz) quantum-cascade laser (QCL) active region by measuring its temperature using in-situ microprobe band-to-band photoluminescence (PL) technique. The heat resistance of different regions in QCL structure is derived from the temperature measurement. Experimental results show that thinning the substrate from 300 mum thick to 140 mum lowers thermal resistance of the device by 21%, which helps achieving continuous-wave operation. A thermodynamic differential equation was numerically solved and the temperature profiles and thermal behavior of various regions within actively biased QCL devices under various conditions were obtained. The simulation confirms the measured results that the substrate accounts for 59% of the total device thermal resistance.

A study of terahertz quantum cascade lasers: Experiment versus simulation

The authors present the testing method and electrical and optical characteristics of a 4.1 THz quantum cascade laser with a four-well resonant-phonon design. The mode evolution in pulsed and continuous wave modes as a function of drive current has been investigated. An ensemble Monte Carlo model is used to analyze the carrier transport and output characteristics of the device. The calculated current density, lasing domain, and lasing frequency as a function of electric field are consistent with the measured results.