Terahertz difference-frequency generation in quantum cascade lasers with high conversion efficiency

We report room-temperature terahertz sources based on Čerenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers with giant resonant optical nonlinearities originating from intersubband transitions. A Čerenkov 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.

We present THz surface emission by difference frequency generation in nonlinear quantum cascade lasers operating at room temperature. The device comprises two separate, transversely superimposed gratings, one for selective feedback of the mid-infrared modes and one for surface emission of the generated THz radiation. The mid-infrared grating is realized as a higher order, loss-coupled distributed feedback grating for single mode emission at two distinct wavelengths. It defines the spatial distribution of the nonlinear polarization wave, which allows for precise placement of the second order top grating for surface emission.

Room-temperature terahertz (THz) quantum cascade laser sources with intracavity difference-frequency nonlinear mixing are electrically pumped monolithic semiconductor laser sources operating in the 0.6–6 THz spectral range. We report widely tunable, low-frequency THz quantum cascade laser sources using a lens-coupled Cherenkov waveguide scheme. Based on a watt-class high-power, λ ∼ 13.7 μm quantum cascade laser, the monolithic THz source is strongly coupled with a high-resistivity silicon lens, which causes a major increase in the THz coupling efficiency and demonstrates significant performance improvements. A room-temperature 1.5 THz device produces a 0.2 mW peak output power with a high-quality beam pattern. Improved THz outcoupling efficiency using the lens-coupled scheme enabled the demonstration of a high-performance external-cavity semiconductor THz source that is tunable from 420 GHz to 2 THz. The external-cavity, lens-coupled device configuration can technically be assembled into a butterfly-style package for a thumb-sized, widely frequency tunable THz semiconductor source.

Terahertz generation in quantum-cascade lasers through down-conversion optical nonlinearity: Design and modeling

We discuss the design and operation of widely-tunable terahertz sources based on Cherenkov intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers. Laser chips are integrated into a Littrow-type external cavity system. Devices demonstrate continuous terahertz emission tuning at room temperature with a record tuning range from 1.2 THz to 5.9 THz and peak power output varying between 5 and 90 μW, depending on the operating frequency. Beam steering of terahertz Cherenkov emission with frequency is suppressed and mid-infrared-to-terahertz conversion efficiency is improved by bonding devices onto high-resistivity silicon substrates that have virtually no refractive index dispersion and vanishingly-small optical loss in 1–6 THz range.

This paper aims to integrate AlInAs/InGaAs quantum well structures with high second-order nonlinear susceptibility into a dual-wavelength quantum cascade laser (QCL) for difference frequency generation (DFG) of THz radiation. The QCL active region contains two sections: a section with 30 cascades of bound-to-continuum design with integrated optical nonlinearity emitting at 8.7 mum and a section with 20 cascades of two-phonon design emitting at 7.6 mum. The nonlinear susceptibility for the DFG process is found to have a value of chi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> ~4times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> pm/V.

A monolithic tunable single mode terahertz quantum cascade laser source based on intra-cavity difference frequency generation with a tuning range of ~580 GHz is demonstrated at room temperature.

Terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically pumped semiconductor sources capable of operating at room temperature across the 1- to 6-THz range. Despite tremendous progress in power output, which now exceeds 1 mW in pulsed and 10 μW in continuous-wave regimes at room temperature, knowledge of the major figure of merits of these devices for high-precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. By exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute center frequency of individual emission lines of these sources with an uncertainty of 4 × 10-10. The unveiled emission LW (400 kHz at 1-ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications that require narrow-LW THz sources.

A systematic optimization study of quantum cascade lasers with integrated nonlinearity for third-harmonic generation is performed. To model current transport the Pauli master equation is solved using a Monte Carlo approach. A multi-objective particle swarm optimization algorithm is applied to obtain the Pareto front. Our theoretical analysis indicates an optimized structure with five orders of magnitude increase in the generated third-harmonic power with respect to the reference design. This striking performance comes with a low threshold current density of about 1.6 kA/cm2 and is attributed to double resonant phonon scattering assisted extraction and injection scheme of the laser.

Milliwatt second harmonic power at 4.45 µm was achieved in InP lattice-matched quantum cascade lasers with monolithically-integrated resonant nonlinear cascades and modal phase matching. The use of an InP top-cladding layer and high-reflectance coating on the laser back facet enabled high linear laser power and consequently record nonlinear power.

In this paper, we consider terahertz sources based on Čerenkov difference frequency generation in a dual-wavelength mid-infrared quantum cascade laser and calculate analytically the optical external efficiency of the out-going terahertz wave. The mid-infrared pumps operate simultaneously and have a common upper level. For short cavity lengths and low terahertz losses in the substrate, we find that implementation of the Čerenkov emission scheme in terahertz difference frequency generation quantum cascade lasers improves the optical external efficiency dramatically. The maximum value of the latter is close to 19%, but is only attainable if the laser exhibits negligible mid-infrared transmission losses.

Terahertz (THz) frequency sources such as quantum cascade lasers (QCLs) offer a variety of new applications in far-infrared frequency bands. For better understanding of their behavior, a device-specific modeling approach is used to predict the optical output power under current modulation and chip temperature. This letter reports the numerical simulation of gain switching to evaluate temperature dependence for short pulse generation in QCLs. Modified rate equations are used for self-consistent computation of emitted THz optical power from QCLs. The device emits at 116 µm and is driven by various inputs such as haversine, tangential hyperbolic and square electrical pulses. From the simulation it is evident that the square input pulse requires the least magnitude current (0.83 A) at 15 K to generate pulses with minimum full width half maximum (FWHM). Moreover, when the peak amplitude of the square input signal is 1.01 A, it generates optical pulses with the least FWHM of 19.42 ps at 45 K. In the case of the haversine input with peak amplitude of 1.8A, a peak power of 27.86 mW is generated at 15 K.

Terahertz (THz) quantum cascade laser sources based on optical nonlinearity are the only electrically pumped monolithic semiconductor sources operable at room temperature in the 0.6–6 THz range. We investigated the temperature dependence of the imaging characteristics of a broadband THz nonlinear quantum cascade laser and evaluated several important properties: the spectrum, far-field pattern and THz imaging results. Consequently, we found that the far-field patterns were single-lobed Gaussian-like, and THz images were well-resolved despite the lower operating temperature of the device. The stable temperature-performance indicates that this broadband THz source is promising for THz imaging applications.

Terahertz quantum cascade laser sources based on intra-cavity nonlinear frequency mixing are currently the only electrically pumped monolithic semiconductor light source operating in THz frequency range at room temperature. We demonstrated the power enhancement of the devices in 0.4-3 THz range by using a lens-coupled Cherenkov radiation scheme. Adopting this scheme, the device produces ∼1 milli-watt peak power at a frequency of 2.38 THz at room temperature, which was increased by a factor of approximately four compared to that of a conventional Cherenkov extraction scheme.

We report terahertz quantum cascade laser sources based on intra-cavity difference-frequency generation processed into double-metal waveguides with surface-grating outcouplers. Over 112 μW of peak power output is produced at room temperature at 1.9 THz.