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-High-Resolution Terahertz Spectrometer

Gregor Klatt,  Raphael Gebs,   Hanjo Schäfer,   Michael Nagel,  Christof Janke,  Albrecht Bartels, Thomas Dekorsy


https://ieeexplore.ieee.org/document/5487397/

Terahertz time-domain spectroscopy (THz-TDS) based on high-speed asynchronous optical sampling (ASOPS) with two offset-locked GHz femtosecond lasers requires no mechanical time-delay scanner. Consequently, measurements with 1-GHz frequency resolution are performed at intrinsically high scan rates in the multikilohertz range. This is at least one order, in most cases several orders of magnitude faster than conventional approaches employing mechanical time-delay scanners. We report a system offering a unique combination of high-frequency resolution (1 GHz) and high scan rate (2 kHz) with a spectral coverage of more than 6 THz. Its capabilities for high-precision spectroscopy are demonstrated by measuring the absorption spectrum of a mixture of H 2 O, D 2 O, and hydrogen deuterium oxide (HDO) vapor. H 2 O and HDO vapor absorption spectra are accurately tabulated in databases. However, D 2 O absorption data are rare, because of residual H 2 O and HDO often present when measuring pure D 2 O. Here, we present a high-resolution absorption spectrum of D 2 O vapor numerically extracted from the absorption spectrum of the three-component mixture. In addition, we show that the high spectral resolution of the ASOPS THz-TDS system provides benefits in the analysis of frequency-selective surface sensors, which are promising candidates for biosensing applications in the THz regime.

Konstanz researcher receives research fellowship in the field of acoustics

Dr Yuning Guo is awarded prestigious F.V. Hunt Postdoctoral Research Fellowship in Acoustics

https://www.eurekalert.org/pub_releases/2018-03/uok-krr032118.php

The Acoustical Society of America has selected Dr Yuning Guo of the University of Konstanz as its 2018-2019 Frederick V. Hunt Postdoctoral Research Fellow in Acoustics. Her successful research proposal entitled "Terahertz coherent phonons in thermal conduction modulation" will help advance research in the field of physical acoustics by providing useful insights into how to manipulate heat in non-metallic materials. The postdoctoral research fellowship, established in 1978 to fulfill Professor Frederick V. Hunt's wish that his estate be used to advance both science and education in acoustics, is awarded annually to one postdoctoral researcher. As the 42nd Hunt Fellow, Dr Guo will receive a research stipend in excess of EUR 40,000 and will carry out her research project at the University of Colorado Boulder, USA in collaboration with Professor Baowen Li, a pioneer in the field of phononics.

After completing her undergraduate and graduate degrees in China, Dr Guo carried out her doctoral studies in phononics at the University of Konstanz under the supervision of Professor Thomas Dekorsy in the Department of Physics. Phononics is a branch of physics concerned with the understanding and manipulation of quantized acoustic waves - phonons - in a broad frequency range, from sound to heat. Whereas Dr Guo's recent research as part of her doctoral thesis focused on acoustic waves that oscillate a billion times per second (gigahertz) in nanomaterial, she will now focus on phonons that vibrate a trillion times per second (terahertz) in semiconducting materials. More specifically, Dr Guo intends to help address a heat-related research gap in the emerging field of phononics: How do phonons in the terahertz (THz) range affect the conduction of heat within nanostructures? "High-frequency acoustic waves in solids, particularly in semiconductors, are of great interest for both fundamental research and industrial applications because phonons are the main heat carriers in these materials", explains Dr Guo.

Compared to optical waves (photons), phonons have more degrees of freedom in terms of movement. As a result, phonons can behave much more erratically, which makes it more challenging to predict and control their behavior. Based on the extensive experience with and knowledge of high-frequency phonon dynamics and thermal phonons that she accumulated during her time at the University of Konstanz, Dr Guo's future research, funded in part through the F.V. Hunt Postdoctoral Research Fellowship, will take on this challenge by helping to identify how THz coherent phonons contribute to the conduction of heat in nanoscale semiconducting materials. She anticipates that such heat conduction can eventually be modulated by controlling the movement of these phonons: "I expect that strategies can be developed to both suppress and promote thermal conduction in nanostructured materials, thereby enhancing phonon-based applications and devices in the near future".

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Facts:

• University of Konstanz early career researcher, Dr Yuning Guo, is awarded 2018-2019 F.V. Hunt Postdoctoral Research Fellowship in Acoustics
• One fellowship is awarded per year to a researcher who advances science and education in acoustics
• Dr Guo will carry out her research on Terahertz coherent phonons in thermal conduction modulation at the University of Colorado
• Fellowship includes a research stipend in excess of EUR 40,000