Robert W. Adams

Terahertz sources based on Čerenkov difference-frequency generation in quantum cascade lasers

We report room-temperature terahertz sources based on Cerenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers with giant resonant optical nonlinearities originating from intersubband transitions. A Cerenkov difference-frequency generation scheme allows for extraction of THz radiation along the whole length of the laser waveguide and provides directional terahertz emission. Experimentally, our sources demonstrate a conversion efficiency of up to 70 μW/W2 approximately an order of magnitude improvement over the previous reports.

GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation operating up to 172 K

We report the design and performance of GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers with double-phonon resonant depopulation and a vertical lasing transition. Lasing at 3 THz was observed up to a heat-sink temperature of 172 K.

Room-temperature operation of 3.6 μm In0.53Ga0.47As/Al0.48In0.52As quantum cascade laser sources based on intracavity second harmonic generation

We discuss a design of short-wavelength quantum cascade laser sources based on intracavity second harmonic generation. A passive heterostructure tailored for giant optical nonlinearity is integrated on top of an active region and patterned for quasiphase matching. We demonstrate operation of λ≈3.6 μm lattice-matched InGaAs/AlInAs/InP sources with approximately 6 μW of second-harmonic output at room temperature and conversion efficiency of approximately 130 μW/W2. Threshold current densities of devices with a nonlinear section were similar to that of the reference lasers without the nonlinear section.

Terahertz sources based on intracavity frequency mixing in mid-infrared quantum cascade lasers with passive nonlinear sections

We report the design and performance of terahertz quantum cascade laser sources based on intracavity difference frequency generation in dual-wavelength mid-infrared quantum cascade lasers with a passive nonlinear section at the exit facet, designed for giant second-order nonlinear susceptibility. These devices operate in the mid-infrared at 1 = 8.4 m and 2 = 9.5 m, with terahertz output at the difference frequency, 3 73 m. Terahertz output of approximately 100 nW was observed up to a heat sink temperature of 210 K.

Terahertz quantum cascade laser sources based on Čerenkov difference-frequency generation

Room-temperature terahertz (THz) quantum cascade laser (QCL) sources based on intra-cavity difference-frequency generation (DFG) with record THz conversion efficiencies is reported. THz DFG QCLs reported previously are highly inefficient since THz radiation produced more than ~100 μm away from the exit facet is fully absorbed due to high THz losses in the QCL waveguide. Our lasers use a non-collinear Čerenkov DFG scheme to extract THz radiation from the active region. Dual-color mid-infrared quantum cascade lasers with integrated giant optical nonlinearity are grown on semi-insulating (S.I.) InP substrates. A lateral current extraction scheme is used. THz radiation is emitted at an angle into the substrate with respect to the mid-infrared pumps. Since S.I. InP is virtually lossless to THz radiation, this scheme allows for efficient extraction of THz radiation along the whole waveguide length. As a result, our sources demonstrate large mid-infrared-to-THz conversion efficiency and directional THz output. Experimentally, proof-of-principle devices demonstrate a conversion efficiency up to 70 μW/W2 and provide output across a 1.2 - 4.5 THz spectral range.

Nonlinear GaInAs/AlInAs/InP quantum cascade laser sources for wavelength generation in the 2.7-70 μm wavelength range

In this paper we describe the design and performance of nonlinear quantum cascade laser sources for near-infrared and terahertz applications. Our devices are based on monolithically integrated mid-infrared quantum cascade lasers and passive nonlinear structures which provide a giant nonlinear response for the pumping frequency. Such design concept can be applied for both short-wavelength and long-wavelength generation. In our work, short-wavelength devices were based on the concept of second-harmonic generation whereas long-wavelength devices utilized difference-frequency generation. With this approach we demonstrate room-temperature operation down to 2.7 μm for near-infrared devices and up to 70 μm at 210 K for terahertz devices. The performance of our nonlinear devices is very sensitive to the resonance condition for the pump and nonlinearity. We demonstrate that, once resonant, nonlinear powers in the mW range are available.

Terahertz quantum cascade laser sources based on difference-frequency generation: from passive nonlinearity to leaky THz waveguide device concept

GaInAs/AlInAs/InP quantum cascade lasers have established themselves as reliable laser sources in the mid-infrared region (3.8-10) μm, where they operate at room-temperature in continuous-wave with Watt-level output powers. However, wavelengths above this wavelength region are difficult to generate. At long wavelengths, devices suffer from increased free-carrier absorption and poor population inversion due to the short upper laser state lifetime, thus limiting their operation to cryogenic temperatures. An alternative way to generate new frequencies is the by means of nonlinear frequency mixing. For long-wavelengths, the process of difference frequency mixing is of particular interest, as it is possible to utilize the good performance of the mid-infrared QCLs, acting as pump sources, together with the giant nonlinear properties that can be realized in the intersubband transitions of the quantum wells. Moreover, the giant nonlinearity can be monolithically integrated with the pump sources, leading to a compact, electrically pumped room-temperature semiconductor laser source, emitting at terahertz frequencies. In our work, we present several different concepts of monolithic nonlinear quantum cascade laser sources, designed to emit in the THz range: devices with passive giant nonlinearities, active nonlinearities and, finally, devices with active nonlinearities, combined with novel THz waveguiding techniques. We will demonstrate how application of novel THz waveguiding techniques avoids the efficiency suppression the large free-carrier absorption at THz frequencies in the doped semiconductor layers enabling room-temperature operation up to 1.2 THz.

InGaAs/AlInAs quantum cascade laser sources based on intra-cavity second harmonic generation emitting in 2.6-3.6 micron range

We discuss the design and performance of quantum cascade laser sources based on intra-cavity second harmonic generation operating in at wavelengths shorter than 3.7μm. A passive heterostructure tailored for giant optical nonlinearity is integrated on top of an active region and patterned for quasi-phasematching. We demonstrate operation of λ≈3.6μm, λ≈3.0μm, and λ≈2.6m devices based on lattice-matched and strain-compensated InGaAs/AlInAs/InP materials. Threshold current densities of typical devices with nonlinear sections are only 10-20% higher than that of the reference lasers without the nonlinear section. Our best devices have threshold current density of 2.2kA/cm2 and provide approximately 35μW of second-harmonic output at 2.95μm at room temperature. The second-harmonic conversion efficiency is approximately 100μW/W2. Up to two orders of magnitude higher conversion efficiencies are expected in fully-optimized devices.

Room-temperature operation of λ≈3.7µm Ga0.47In0.53As/ Al0.48In0.52As quantum cascade laser sources

We report room-temperature operation of λ≈3.7µm lattice-matched InGaAs/AlInAs/InP quantum cascade lasers based on frequency doubling with ∼2mW/W² conversion efficiencies. Similar devices based on 1% strain-compensated materials can operate at λ=3–3.7µm.