We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz. The beams are generated using a quantum cascade laser (QCL) in combination with a Fourier phase grating. The grating is fully characterized using a third order distributed feedback (DFB) QCL with a single mode emission at 4.7 THz as the input. The measured diffraction efficiency of 74.3% is in an excellent agreement with the calculated result of 75.4% using a 3D simulation. We show that the power distribution among the diffracted beams is uniform enough for pumping an array receiver. To validate the grating bandwidth, we apply a far-infrared (FIR) gas laser emission at 5.3 THz as the input and find a very similar performance in terms of efficiency, power distribution, and spatial configuration of the diffracted beams. Both results represent the highest operating frequencies of THz phase gratings reported in the literature. By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the QCL power value.

We have performed frequency locking of a dual, forward reverse emitting 3rd order distributed feedback quantum cascade laser (QCL) at 3.5 THz. By using both directions of THz emission in combination with two gas cells and two power detectors, we can for the first time perform frequency stabilization, while monitor the frequency locking quality independently. We also characterize how the use of a less sensitive pyroelectric detector can influence the quality of frequency locking, illustrating experimentally that the sensitivity of the detectors is crucial. Using both directions of THz radiation has a particular advantage for the application of a QCL as a local oscillator, where radiation from one side can be used for frequency/phase stabilization, leaving the other side to be fully utilized as a local oscillator to pump a mixer.

We report an extensive study of the effect of an additional dielectric layer on the frequency of terahertz quantum cascade lasers (QCLs). QCLs with third-order distributed feedback structure at frequencies of 3.5 and 4.7 THz are used in our experiment. The dielectric layer applied is either Silicon-dioxide (SiO₂) or Polymethylmethacrylaat (PMMA). We find that both dielectric layers can down shift the lasing frequency up to 6GHz on a 3.5THz QCL, and 13GHz for a 4.7THz QCL. Full 3D FEM simulations suggest that the effect is dominated by the effective thickness of the dielectric on the vertical walls of the laser structure, and also confirm that for a given dielectric layer the effect is stronger in the 4.7THz QCL due to its larger out-distribution of electric-magnetic field. The knowledge provides guideline to shift the frequency of an existing QCL used as a local oscillator in practical applications.

We report an extensive study of the effect of an additional dielectric layer on the frequency of terahertz quantum cascade lasers (QCLs). QCLs with third-order distributed feedback structure at frequencies of 3.5 and 4.7 THz are used in our experiment. The applied dielectric layer is either Silicon dioxide (SiO₂) or Polymethylmethacrylaat (PMMA). We find that both dielectric layers can shift the lasing frequency by up to-6 GHz on a 3.5-THz QCL, and up to-13 GHz for a 4.7-THz QCL. Full 3-D FEM simulations suggest that the effect is dominated by the effective thickness of the dielectric on the side walls of the laser structure, and also confirm that for a given dielectric layer, the effect is stronger in the 4.7-THz QCL due to its different extension of the electromagnetic field to the free space. This study provides a guideline for shifting the frequency of an existing QCL for frequency critical applications such as spectroscopy or use as a local oscillator.

The terahertz region is of great importance for spectroscopy since many molecules have absorption fingerprints there. Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multiheterodyne spectroscopy using two terahertz quantum cascade laser combs. Over a spectral range of 250 GHz, we achieve average signal-to-noise ratios of 34 dB using cryogenic detectors and 24 dB using room-temperature detectors, all in just 100 µs. As a proof of principle, we use these combs to measure the broadband transmission spectrum of etalon samples and show that, with proper signal processing, it is possible to extend the multiheterodyne spectroscopy to quantum cascade laser combs operating in pulsed mode. This greatly expands the range of quantum cascade lasers that could be suitable for these techniques and allows for the creation of completely solid-state terahertz laser spectrometers.