Abstract-Quantum cascade laser-pumped terahertz molecular lasers: frequency noise and phase-locking using a 1560 nm frequency comb

Jean-Francois Lampin, Antoine Pagies, Giorgio Santarelli, Jeffrey Hesler, Wolfgang Hansel, Ronald Holzwarth,  Stefano Barbieri,

(a) Energy diagram of the levels relevant for laser operation. (b) Schematic of the QCL-pumped ML. The laser cavity is closed at one end (left) by an output coupler, obtained by depositing a metallic grid on a Silicon substrate, and on the other end (right) by a plane metallic mirror, with a 1.2 mm hole drilled in its center. An isolator (∼30 dB isolation, and ∼60% transmission) is used to reduce the optical feedback on the DFB QCL. (c) THz ouput power vs MIR pump power, measured without the optical isolator displayed in panel (b). Inset: intensity plot of the optical beam collected with a microbolometer camera positioned at about 4 cm from the laser output coupler.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-2-2091

We report the measurement of the frequency noise power spectral density (PSD) of a Terahertz (THz) molecular laser (ML) pumped by a mid-infrared (MIR) quantum cascade laser (QCL), and emitting 1 mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080th harmonic of the repetition rate of a 1560 nm frequency comb (FC). We find a frequency noise PSD < 10Hz2/Hz (-95dBc/Hz) at 100kHz from the carrier. To demonstrate the effect of the stability of the pump laser on the spectral purity of the THz emission we also measure the frequency noise PSD of a CO2-laser-pumped 2.5THz ML, reaching 0.1Hz2/Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the FC repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Quantum cascade laser-pumped terahertz molecular lasers: frequency noise and phase-locking using a 1560nm frequency comb

Jean-Francois Lampin, Antoine Pagies, Giorgio Santarelli, Jeffrey Hesler, Wolfgang Hänsel, Ronald Holzwarth, Stefano Barbieri

https://arxiv.org/abs/1911.06568

The recent demonstration of a terahertz (THz) molecular gas laser pumped by a mid-infrared quantum cascade laser (QCL) has opened up new perspectives for this family of sources, traditionally relying on CO2-laser pumping. A so far open question concerning QCL-pumped THz molecular lasers (MLs) is related to their spectral purity. Indeed, assessing their frequency/phase noise is crucial for a number of applications potentially exploiting these sources as local oscillators. Here this question is addressed by reporting the measurement of the frequency noise power spectral density (PSD) of a THz ML pumped by a 10.3{\mu}m-wavelength QCL, and emitting 1mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080th harmonic of the repetition rate of a 1560nm frequency comb. We find a frequency noise PSD < 10Hz2/Hz (-95dBc/Hz) at 100kHz from the carrier. To demonstrate the effect of the stability of the pump laser on the spectral purity of the THz emission we also measure the frequency noise PSD of a CO2-laser-pumped 2.5THz ML, reaching 0.1Hz2/Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the frequency comb harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the frequency comb repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth.

Abstract-Unambiguous real-time terahertz frequency metrology using dual 10 GHz femtosecond frequency combs

Oliver Kliebisch, Dirk C. Heinecke, Stefano Barbieri, Giorgio Santarelli, Hua Li, Carlo Sirtori, Thomas Dekorsy,

Fig. 1. (a) Schematic overview of the dual-comb sampling setup. The two femtosecond lasers have the same wavelength but are depicted in red and orange for visual clarity. The terahertz beam path is indicated in gray. The dashed black line marks the common 10 MHz reference clock shared among all synthesizers. The dashed light blue path matches the stabilization feedback loop, which is shown in more detail in Fig. 1(b). A full description is given in the text. ZnTe, 2 mm thick ZnTe crystal; QWP, quarter-wave plate; PBSC, polarizing beam-splitter cube; BP, electronic bandpass filter; (b) detailed view of the signal conditioning of the first photodiode for frequency stabilization and in-loop characterization. After the longitudinal beat-mode spectrum is amplified, and a single mode is filtered, the signal is split into a feedback loop branch and an in-loop characterization branch [not shown in Fig. 1(a)].

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-11-1431

Terahertz frequency metrology by radio frequency downconversion using femtosecond optical sampling relies on the harmonic factor retrieval between the terahertz frequency and the optical sampling rate. At typical femtosecond laser repetition rates, this imposes an ambiguity for frequency metrology. We report on a dual-comb sampling system for the unambiguous frequency measurement of terahertz quantum cascade lasers with hertz-level precision. Two Ti:sapphire oscillators with 10 GHz repetition rate are used for the electro-optic sampling of terahertz radiation at 2.5 THz emitted by actively mode-locked terahertz quantum cascade lasers with 9.7 GHz and 19.6 GHz repetition rates. By coherent downconversion, the emitted terahertz waveforms are measured in the radio frequency domain. The terahertz frequency comb is stabilized by employing a phase-locked loop on a radio frequency beat-note signal. A second infrared sampling comb is used to measure the absolute frequencies of the terahertz radiation. This method, which is based on the detuning of the sampling repetition rates, allows the direct retrieval of the quantum cascade laser’s absolute frequency in real time without using additional optical frequency references for calibration. In order to demonstrate the feasibility of the stabilization and readout technique, a high-resolution spectroscopy measurement on gaseous methanol is presented.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Optomechanical terahertz detection with single meta-atom resonator

Cherif Belacel, Yanko Todorov, Stefano Barbieri, Djamal Gacemi, Ivan Favero, Carlo Sirtor

https://www.nature.com/articles/s41467-017-01840-6


Most of the common technologies for detecting terahertz photons (>1 THz) at room temperature rely on slow thermal devices. The realization of fast and sensitive detectors in this frequency range is indeed a notoriously difficult task. Here we propose a novel device consisting of a subwavelength terahertz meta-atom resonator, which integrates a nanomechanical element and allows energy exchange between the mechanical motion and the electromagnetic degrees of freedom. An incident terahertz wave thus produces a nanomechanical signal that can be read out optically with high precision. We exploit this concept to demonstrate a terahertz detector that operates at room temperature with high sensitivity and a much higher frequency response compared to standard detectors. Beyond the technological issue of terahertz detection, our architecture opens up new perspectives for fundamental science of light–matter interaction at terahertz frequencies, combining optomechanical approaches with semiconductor quantum heterostructures.

Abstract-Beat note analysis and spectral modulation of terahertz quantum cascade lasers with radio frequency injection

Yonghao Zhu, Hua Li, Wenjian Wan, Li Gu, Tao Zhou, Stefano Barbieri, and Juncheng Cao

https://www.osapublishing.org/col/abstract.cfm?uri=col-15-1-011404

We demonstrate the electrical beat note analysis and radio frequency (RF) injection locking of a continuous wave (cw) terahertz quantum cascade laser (QCL) emitting around 3 THz (∼100 μm). In free running the beat note frequency of the QCL shows a shift of ∼180 MHz with increasing drive current. The beat note, modulation response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II, and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect and spectral modulation measurements verify that in the current regime I the RF injection locking is more efficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.

© 2016 Chinese Laser Press

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Abstract-Wave engineering with THz quantum cascade lasers

                                          Photonic-crystal structures for high power extraction
Carlo Sirtori, Stefano Barbieri¹ Raffaele Colombelli²
http://www.nature.com/nphoton/journal/v7/n9/full/nphoton.2013.208.html

Quantum cascade lasers are compact devices based on mature compound semiconductors such as GaAs that take advantage of highly developed optoelectronic fabrication techniques to integrate linear and nonlinear functions. This Review discusses terahertz-wave engineering using quantum cascade lasers with a particular focus on techniques that have been implemented to control their spectral and output beam properties. After briefly introducing the types of active regions and surveying present maximum operating temperatures, we review several photonic structures used for frequency and beam engineering, ranging from distributed feedback lasers to photonic crystals. We then describe techniques that allow the upconversion of terahertz quantum cascade laser radiation in the near-infrared region using nonlinear intracavity mixing. Finally, we review frequency stabilization of terahertz quantum cascade lasers with a special emphasis on phase locking to near-infrared frequency combs