Seyed Ghasem Razavipour

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

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...

Direct Nanoscale Imaging of Evolving Electric Field Domains in Quantum Structures

The external performance of quantum optoelectronic devices is governed by the spatial profiles of electrons and potentials within the active regions of these devices. For example, in quantum cascade lasers (QCLs), the electric field domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially nonuniform and temporally unstable. Unfortunately, there exists no prior means of probing the inner potential profile directly. Here we report the nanoscale measured electric potential distribution inside operating QCLs by using scanning voltage microscopy at a cryogenic temperature. We prove that, per the EFD hypothesis, the multi-quantum-well active region is indeed divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules does not continuously increase in proportion to gradual increases in the applied device bias, but rather hops between discrete values that are related to tunneling resonances. We also report the evolution of EFDs, finding that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region.

Electrically switching transverse modes in high power THz quantum cascade lasers

The design and fabrication of a high power THz quantum cascade laser (QCL), with electrically controllable transverse mode is presented. The switching of the beam pattern results in dynamic beam switching using a symmetric side current injection scheme. The angular-resolved L-I curves measurements, near-field and far-field patterns and angular-resolved lasing spectra are presented. The measurement results confirm that the quasi-TM(01) transverse mode lases first and dominates the lasing operation at lower current injection, while the quasi-TM(00) mode lases at a higher threshold current density and becomes dominant at high current injection. The near-field and far-field measurements confirm that the lasing THz beam is maneuvered by 25 degrees in emission angle, when the current density changes from 1.9 kA/cm(2) to 2.3 kA/cm(2). A two-dimension (2D) current and mode calculation provides a simple model to explain the behavior of each mode under different bias conditions.

A high carrier injection terahertz quantum cascade laser based on indirectly pumped scheme

A Terahertz quantum cascade laser with a rather high injection coupling strength based on an indirectly pumped scheme is designed and experimentally implemented. To effectively suppress leakage current, the chosen quantum cascade module of the device is based on a five-well GaAs/Al0.25Ga0.75As structure. The device lases up to 151 K with a lasing frequency of 2.67 THz. This study shows that the effect of higher energy states in carrier transport and the long-range tunnel coupling between states that belong to non-neighbouring modules have to be considered in quantum design of structures with a narrow injector barrier. Moreover, the effect of interface roughness scattering between the lasing states on threshold current is crucial.

Effects of interface roughness scattering on device performance of indirectly pumped terahertz quantum cascade lasers

The impacts of interface roughness (IR) scattering on device performance of indirectly-pumped (IDP) terahertz quantum cascade lasers are studied. Three different active region designs with almost the same lasing frequency at threshold and comparable oscillator strength are experimentally investigated and the measurement data are analyzed and compared with numerical simulation. The simulation results show that all structures suffer from the detrimental effect of intersubband roughness scattering in terms of threshold current density, and probably operating temperature. The intrasubband IR scattering time could also to be a limiting factor in the IDP structures due to the employed high energetic barrier.

An investigation on optimum ridge width and exposed side strips width of terahertz quantum cascade lasers with metal-metal waveguides

The impacts of side exposed side strips (for high order modes suppression) and ridge width on terahertz (THz) quantum cascade laser (QCL) performance are investigated through numerical modeling and verified experimentally. Our results show that shrinking ridge width of THz QCLs with metal-metal waveguides leads to a substantial degradation of device performance due to higher optical loss resulting from the side-exposed strips in the highly-doped top contact layer. Nevertheless, the side-exposed strips facilitate single mode operation by strongly suppressing higher-order modes. An optimal width of the side exposed strips is obtained for achieving adequate higher-order mode suppression and maintaining sufficiently low fundamental mode loss.

Terahertz quantum well photodetectors with improved designs by exploiting many-body effects

A systematic study on many-body effects on Terahertz Quantum Well Photodetectors (THZ QWPs) is reported. Peak absorption frequency differs by more than 20% when taking many-body effects into account. The phenomenon is shown to be critical in designs with a small barrier height and a high doping density. In order to exploit them and minimize their adverse impacts, a doping profile symmetrically split in the barrier layers, resembling a double-barrier QWP, is proposed. Simulation results show the design reduces dark current by one order of magnitude compared against conventional designs with a uniform doping profile in the quantum well.

Electrical scanning probe microscopy of electronic and photonic devices: connecting internal mechanisms with external measures

Abstract The inner workings of semiconductor electronic and photonic devices, such as dopants, free charge carriers, electric potential, and electric field, are playing a crucial role in the function and performance of the devices. Electrical scanning probe microscopy (SPM) techniques have been developed and deployed to measure, with nanometric spatial resolution and high quantitative accuracy, the two-dimensional profiles of dopant, potential, electric field, and free carrier distribution, within unbiased and/or operating electronic and photonic devices. In this review paper, we summarize our latest SPM experimental results, including the scanning spreading resistance microscopy and scanning capacitance microscopy of terahertz quantum cascade lasers, scanning capacitance microscopy of non-volatile memory devices, scanning voltage microscopy of terahertz quantum cascade lasers, and scanning voltage microscopy of interband cascade lasers. Interpretation of the measured quantities are presented and calibrated, demonstrating that important internal physical quantities and inner mechanisms of device operation can be uncovered. It reveals that the novel SPM techniques would find more applications to the emerging semiconductor quantum devices and nanoelectronics.