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On CHip terahertz frequency Combs

Periodic Reporting for period 4 - CHIC (On CHip terahertz frequency Combs)

Okres sprawozdawczy: 2021-09-01 do 2022-08-31

The terahertz (THz) portion of the electromagnetic spectrum is at the junction between optics and electronics. As the fundamental vibration modes of many molecules lie in this spectral range, THz is a gate to sensing applications in fields as varied as medical, environmental and process control. Material inspection and non-invasive imaging for safety and medical applications greatly benefit from the non-ionizing, high resolution features of T-rays. The next major step for short-range high data rate wireless communication is the extension to higher frequencies entering the THz range.
Optical frequency combs have dominated the scene of laser physics in the last 10 years revolutionizing many fields of optics from metrology to high precision spectroscopy. Optical frequency combs act as rulers in the frequency domain and are characterized by their perfectly equally spaced and coherent modes. An extremely appealing application of optical frequency combs is the so-called dual-comb spectroscopy where multi-heterodyne detection is performed allowing broadband Fourier transform spectroscopy with high resolution, high sensitivity and no moving parts. Nowadays the THz range is still lacking bandwidth coverage and high power from frequency comb sources.
The objective of this proposal is to create on-chip, self-referenced frequency combs operating in the spectral region from 1.5-5-5 THz and room temperature on-chip dual band Mid-IR and THz combs.
We significantly extended the frequency bandwidth of quantum cascade laser THz harmonic frequency combs to more than 1.1 THz, with still more than 800 GHz at high temperatures on fundamental comb state (90 K).In order to achieve this objective we optimised the technology for the fabrication of low-loss double-metal waveguides. We worked on the refinement of the wafer bonding process as well as on the copper deposition.
Exploiting such high-performance devices and optimised double-metal technology we pioneered a new planarized waveguide platform for integrated THz photonics that allowed the complete control of the laser dynamics with RF injection , switching from FM combs to AM combs, obtaining pulses as short as 4.4 ps.
The planarized approach allowed integration of optical elements as y-split, passive waveguides and antennas. We engineered planarized waveguides that allow control of the dispersion and realize broadband combs with them. The possibility to introduce anomalous dispersion proved essential for teh generation of THz solitons in coupled rings where we demonstrated THz optical solitons with pulse lengths as short as 12 ps.
Still exploiting the planarized devices and the integrated antennas we demonstrated a new way to detect THz radiation exploiting the regenerative amplification approach. THz detection up to 70 K has been demonstrated.
In the context of dual-comb spectroscopy, a topic that has been pursued since the beginning of the ERC grant was finally applied for patenting at the European patent office. The patent covers a new, fast delay line that can be used in different context for spectroscopy using laser systems. The delay line key component is a rotating, multi-faceted mirror inserted in a multi-reflecting cage. The PI G. Scalari is the main inventor and A. Forrer also significantly contributed to the invention. A publication exploiting the delay line as a core component in a single.comb high resolution high speed spectrometer has been recently accepted for publication in Communication Physics.
Concerning the high power and broadband extraction we implemented the concept of a broadband extractor by designing and fabricating a Vivaldi antenna for THz QCLs. The antenna is based on a polymer technology and successive metallization and mounted on the laser facet. Considerable improvement in the far field and polarization control have been achieved. By using the planarized approach already mentioned before we where able to ultimately obtain high power (> 15 mW) in a narrow beam from THZ QCL combs.
We demonstrated THZ quantum cascade lasers frequency combs spanning more than 1.1 THz both on fundamental and harmonic state and more than 800 GHz when operated at 80 K.
We demonstrated self-starting harmonic comb operation in THz quantum cascade lasers as well as RF injection locking with record-low power.
We demonstrated pulsed operation up to 210 K of THz quantum cascade lasers, enabling operation of thermoelectric cooler.
We demonstrated a new planarized waveguide approach that allows integration of passive and active elements and opens the path to integrarted THz photonics. By exploiting such planarized waveguide approach we demonstrate RF control of THz QCL combs switching from FM mode locking to AM operation with 4.4 ps pulses
Dual comb spaced by one octave from a monolithic laser ridge
THZ QCL operating up to 210 on a Peltier cooler