Only recently, a novel type of intensity autocorrelator in the time-domain has been reported for the Terahertz frequency range. The technique is based on fast electro-optic sampling in a double beam configuration and its temporal resolution is ultra-fast, as short as only few hundreds of femtoseconds. In particular, the self-referencing character of the technique is suitable for any type of source, including free-running sources. These unique characteristics enable therefore the investigation of the output profile of Terhertz Quantum Cascade Laser based Frequency Combs, with typical roundtrip times of few tens of picoseconds. The output dynamics of such devices have been investigated theoretically by Maxwell-Bloch equations and experimentally using Shifted Wave Interference Fourier Transform Spectroscopy. In this work, we present the results of the direct measurement of intensity autocorrelations of a Terahertz comb around 2.5 THz, when operated in the comb and high-noise regime, with radio-frequency beatnotes of 800 Hz and few MHz, respectively. We find the laser to be both amplitude- and frequency-modulated in both regimes, with a modulation ratio of the intensity of roughly 90 percent.The technique might come to use in future for the measurement of free-running pulses at Terahertz frequencies with high temporal resolution.
Terahertz quantum cascade lasers (THz QCL) are a very promising source for efficient frequency comb generation at terahertz frequencies. They do not only provide an output power of the order of milliwatts but are also covering a large spectral bandwidth. Octave spanning devices have recently been reported by our group. They provide a very low intrinsic dispersion due to the flat gain curve and the flat losses of the resonator. This allows frequency comb operation up to more than 600 GHz bandwidth with standard broadband metal-metal waveguide Fabry-Pérot QCLs. Frequency combs at terahertz frequencies are especially interesting for spectroscopic applications employing the powerful dual-comb setup. Such a setup requires a fast detector which is difficult to get with a sufficient sensitivity at terahertz frequencies. We present here an alternative approach, which does not need a fast detector but rather uses one of the two THz QCL frequency combs as an ultrafast multiheterodyne detector integrating local oscillator (LO) and detector in one single device. Two laser ridges are fabricated on the same chip at a distance of 500 um. Part of the light from the sample laser is coupled into the LO laser via the metallic ground plane. The downconverted multiheterodyne beatnote can be measured through the laser power supply line with a bias Tee. The obtained dual-comb covers a bandwidth of 630 GHz with a central frequency of 2.5 THz. The frequency comb spacing was analysed using frequency counting techniques revealing an accuracy down to _frep=fcarrier 10^(-12) at the carrier frequency of 2.5 THz.
Recent work has been showing the possibility of generating frequency combs at terahertz frequencies using terahertz quantum cascade lasers. The main efforts so far were on getting the laser to work in a stable comb operation over an as broad as possible spectral bandwidth. Another issue is the scattered farfield of such combs due to their subwavelength facets of the used metal-metal waveguide. In contrast to single mode lasers the monolithic approaches of distributed feedback lasers or photonic crystals cannot be used. We present here a monolithic broadband extractor compatible with frequency comb operation based on the concept of an end-fire antenna. The antenna can be fabricated using standard fabrication techniques. It has been designed to support a bandwidth of up to 600 GHz at a central frequency of 2.5 THz. The fabricated devices show single lobed farfields with only minor asymmetries, increased output power along an increased dynamical range of frequency comb operation. A side-absorber schematics using a thin film of Nickel has been used to suppress any higher-order lateral modes in the laser. The reported frequency combs with monolithic extractors are ideal candidates for spectroscopic applications at terahertz frequencies using a self-detected dual-comb spectroscopy setup due to the increased dynamical range along with the improved farfield leading to more output power of the frequency combs.
We present a THZ quantum cascade laser operating in continuous wave with an emission covering more than one octave
in frequency, and displaying homogeneous power distribution among the lasing modes. The gain medium is based on a
heterogeneous quantum cascade structure operating in the THz range. Laser emission takes place from 1.64 THz to 3.35
THz with optical powers of 3 mW and 84 modes above threshold. For narrow waveguides a collapse of the free-running
beatnote to linewidths of 980 Hz, limited by jitter, indicate frequency comb operation on a spectral bandwidth as wide as