Hans Callebaut

3.4-THz quantum cascade laser based on longitudinal-optical-phonon scattering for depopulation

We report the development of a quantum cascade laser, at λ=87.2 μm, corresponding to 3.44 THz or 14.2 meV photon energy. The GaAs/Al0.15Ga0.85As laser structure utilizes longitudinal-optical (LO) phonon scattering for electron depopulation. Laser action is obtained in pulsed mode at temperatures up to 65 K, and at 50% duty cycle up to 29 K. Operating at 5 K in pulsed mode, the threshold current density is 840 A/cm2, and the peak power is approximately 2.5 mW. Based on the relatively high operating temperatures and duty cycles, we propose that direct LO-phonon-based depopulation is a robust method for achieving quantum cascade lasers at long-wavelength THz frequencies.

Terahertz quantum-cascade laser at λ≈100 μm using metal waveguide for mode confinement

We report lasing at ∼3.0 THz (λ≈98–102 μm) in a quantum-cascade structure in which mode confinement is provided by a double-sided metal waveguide. The depopulation mechanism is based on resonant phonon scattering, as in our previous work. Lasing takes place in pulsed mode up to a heat-sink temperature of 77 K. The waveguide consists of metallic films placed above and below the 10-μm-thick multiple-quantum-well gain region, which gives low losses and a modal confinement factor of nearly unity. Fabrication takes place via low-temperature metallic wafer bonding and subsequent substrate removal using selective etching. This type of waveguide is expected to be increasingly advantageous at even longer wavelengths.

Importance of coherence for electron transport in terahertz quantum cascade lasers

A density matrix approach is used in combination with a tight-binding model to describe electron transport in terahertz quantum cascade lasers and is incorporated into a Monte Carlo simulation. Scattering events, including LO-phonon, electron-electron, and ionized impurity scattering, are treated semiclassically but contribute to dephasing scattering. In addition, a phenomenological “pure dephasing rate” was introduced to take into account dephasing caused by interface roughness scattering. This model was used to investigate the influence of dephasing on electron transport through a barrier. Additionally, current densities, populations and electron temperatures were calculated for a simple three-level structure and a five-level structure that achieved lasing at 3.2THz, and the results were compared to a semiclassical simulation. We find that the inclusion of coherent transport and dephasing in the calculations is essential when transport is dominated by transitions between weakly coupled states.

Terahertz quantum-cascade laser operating up to 137 K

We report operation of a terahertz quantum-cascade laser at 3.8 THz (λ≈79 μm) up to a heat-sink temperature of 137 K. A resonant phonon depopulation design was used with a low-loss metal–metal waveguide, which provided a confinement factor of nearly unity. A threshold current density of 625 A/cm2 was obtained in pulsed mode at 5 K. Devices fabricated using a conventional semi-insulating surface-plasmon waveguide lased up to 92 K with a threshold current density of 670 A/cm2 at 5 K.

Importance of electron-impurity scattering for electron transport in terahertz quantum-cascade lasers

Using an ensemble Monte Carlo simulation, including both electron–electron and electron–phonon scattering as well as electron-impurity scattering, the current density, population inversion, electron temperature, and gain in two THz quantum-cascade structures are investigated and compared to measurements. We find that the inclusion of electron-impurity scattering in the calculations is crucial when modeling the intersubband transport dynamics in these devices. However, the calculated gain is higher than inferred from experiments. This can be attributed to wavefunction localization caused by dephasing scattering, which is unaccounted for in the present model.

Analysis of transport properties of tetrahertz quantum cascade lasers

We present a self-consistent modeling of a 3.4-THz intersubband laser device. An ensemble Monte Carlo simulation, including both carrier–carrier and carrier-phonon scattering, is used to predict current density, population inversion, gain, and electron temperature. However, these two scattering mechanisms alone appear to be insufficient to explain the observed current density. In addition, the insufficient scattering yields a gain that is slightly higher than inferred from experiments. This suggests the presence of a non-negligible scattering mechanism which is unaccounted for in the present calculations.

Resonant-phonon-assisted THz quantum-cascade lasers with metal–metal waveguides

We report our development of terahertz (THz) quantum-cascade lasers (QCLs) based on two novel features. First, the depopulation of the lower radiative level is achieved through resonant longitudinal optical (LO-)phonon scattering. This depopulation mechanism is robust at high temperatures and high injection levels. In contrast to infrared QCLs that also use LO-phonon scattering for depopulation, in our THz lasers the selectivity of the depopulation scattering is achieved through a combination of resonant tunnelling and LO-phonon scattering, hence the term resonant phonon. This resonant-phonon scheme allows a highly selective depopulation of the lower radiative level with a sub-picosecond lifetime, while maintaining a relatively long upper level lifetime (>5 ps) that is due to upper-to-ground-state scattering. The second feature of our lasers is that mode confinement is achieved by using a novel double-sided metal–metal waveguide, which yields an essentially unity mode confinement factor and therefore a low total cavity loss at THz frequencies. Based on these two unique features, we have achieved some record performance, including, but not limited to, the highest pulsed operating temperature of 137 K, the highest continuous-wave operating temperature of 97 K, and the longest wavelength of 141 µm (corresponding to 2.1 THz) without the assistance of a magnetic field.

Magnetotunneling spectroscopy of resonant anticrossing in terahertz intersubband emitters

Intersubband transport and electroluminescence has been investigated in a double-quantum-well structure based on an intrawell (vertical) THz radiative transition. Magnetotunneling spectroscopy was used to determine subband energies, including the minimum energy separation (∼1.7 meV) between two anticrossed levels. The presence of this anticrossing indicates that in this structure, electron removal from the lower radiative state should be modeled by coherent resonant tunneling rather than incoherent sequential tunneling.

Terahertz quantum cascade lasers based on resonant phonon scattering for depopulation

We report our development of terahertz (THz) quantum cascade lasers (QCLs), in which the depopulation of the lower radiative level is achieved through resonant longitudinal optical (LO) phonon scattering. This depopulation mechanism, similar to that implemented in all the QCLs operating at mid–infrared frequencies, is robust at high temperatures and high injection levels. The unique feature of resonant LO–phonon scattering in our THz QCL structures allows a highly selective depopulation of the lower radiative level with a sub–picosecond lifetime, while maintaining a relatively long upper level lifetime (more than 5 ps) that is due to upper–to–ground–state scattering. The first QCL based on this mechanism achieved lasing at 3.4 THz (λ ≈ 87 μm) up to 87 K for pulsed operations, with peak power levels exceeding 10 mW at ca. 40 K. Using a novel double–sided metal waveguide for mode confinement, which yields a unity mode confinement factor and therefore a low total cavity loss at THz frequencies, we have also achieved lasing at wavelengths longer than 100 μm.

Terahertz quantum cascade lasers

Based on a robust THz gain medium and low-loss waveguide structures, we have developed THz quantum-cascade lasers that operate up to 137 K in pulsed mode, 97 K in CW mode, and 141-/spl mu/m wavelength.We report our successful development of THz quantum cascade lasers with two unique features. The intersubband gain medium is based on resonant optical phonon scattering to depopulate the lower lasing level, and the THz mode confinement is achieved using a metal-semiconductor-metal waveguide. To date, lasing has been achieved at wavelengths longer than 100 mW and above liquid nitrogen temperature. Maximum peak power levels exceed 10 mW and more than 4 mW of power is still available at liquid nitrogen temperature.