Abstract-Coulomb forces in THz electromechanical meta-atoms

Allegra Calabrese, Djamal Gacemi, Mathieu Jeannin, Stéphan Suffit, Angela Vasanelli, Carlo Sirtori, Yanko Todorov

https://www.degruyter.com/view/j/nanoph.ahead-of-print/nanoph-2019-0314/nanoph-2019-0314.xml

Thanks to their large sensitivity to electromagnetic fields, microelectromechanical systems are becoming attractive for applications in the THz band (0.1–10 THz). However, up to date all THz electromechanical systems couple electromagnetic fields to mechanical motion only through photothermal dissipative forces: such mechanism allows for sensitive detection but prevents applications that require coherent transfer of information. In this work, we present a THz electromechanical meta-atom where the coupling between an electromagnetic mode and the displacement of a metallic micro-beam is substantially controlled by a conservative Coulomb force due to charge oscillations in the nanometric-size capacitive part of the meta-atom. We present experiments, performed at room temperature, which allow distinguishing and precisely quantifying the contributions of conservative and dissipative forces in the operation of our electromechanical resonator. Our analysis shows that the Coulomb force becomes the dominant contribution of the total driving force for high-order mechanical modes. Such system paves the way for the realization of coherent THz to optical transducers and allows the realization of fundamental optomechanical systems in the THz frequency range.

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-Multi-Terahertz sideband generation on an optical telecom carrier with a Quantum Cascade Laser

Sarah Houver, Armand Lebreton, Alireza Mottaghizadeh, Maria Amanti, Carlo Sirtori, Gregoire Beaudoin, Isabelle Sagnes, Olivier Parillaud, Raffaele Colombelli, Juliette Mangeney, Robson Ferreira, Jérôme Tignon, and Sukhdeep Dhillon

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b01124

Quantum Cascade Lasers (QCLs) are the principal semiconductor light sources for the mid-infrared (MIR) and terahertz (THz) spectral regions. However, up-converting their emission to the technologically mature telecom spectral region (1.3µm to 1.6µm) remains an important goal for applications. This would permit new QCL functionalities to be realized, such as stabilization and injection locking of the QCL emission to precise and low cost telecom frequency combs for spectroscopy. In this work, we demonstrate the up-conversion of the QCL emission to the telecom band at room temperature, using the QCLs’ inherent resonant optical nonlinearities. This is based on the generation of multi-THz sidebands on a telecom carrier, where a low power telecom beam at 1.55 µm is injected into the cavity of an InP-based MIR QCL (9 µm, 33 THz), giving rise to the nonlinear sum frequency at 1.3 µm. The results are supported by a theoretical model that highlights the giant enhancement of the nonlinear interaction through the resonant telecom excitation and the reduced role of phase matching. As well as potentially bringing important spectroscopic capabilities to QCLs, this compact and low power all-optical connection between the low-loss transmission windows of 1.3 µm and 1.55 µm would potentially permit QCLs to be applied as ultrafast wavelength shifters in fiber telecommunication networks.

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-Dynamics of ultra-broadband terahertz quantum cascade lasers for comb operation.

Hua Li, Pierre Laffaille, Djamal Gacemi, Marc Apfel, Carlo Sirtori, Jeremie Leonardon, Giorgio Santarelli, Markus Rösch, Giacomo Scalari, Mattias Beck, Jerome Faist, Wolfgang Hänsel, Ronald Holzwarth, Stefano Barbieri

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http://www.pubfacts.com/detail/26831993/Dynamics-of-ultra-broadband-terahertz-quantum-cascade-lasers-for-comb-operation

We present an experimental investigation of the multimode dynamics and the coherence of terahertz quantum cascade lasers emitting over a spectral bandwidth of ~1THz. The devices are studied in free-running and under direct RF modulation. Depending on the pump current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. We investigate the relation between this phase-noise and the dynamics of the THz modes through the electro-optic sampling of the laser emission. We find that when the phase-noise is large, the laser operates in an unstable regime where the lasing modes are incoherent. Under RF modulation of the laser current such instability can be suppressed and the modes coherence recovered, while, simultaneously, generating a strong broadening of the THz emission spectrum.

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

Terahertz optics taking off

http://www.nature.com/nphoton/journal/v7/n9/full/nphoton.2013.239.html

Advances in terahertz sources and measurement techniques are opening up new opportunities for fundamental material research in the terahertz region.

Terahertz (THz) radiation, widely defined as electromagnetic radiation in the frequency range 0.3–3 THz, is attracting considerable interest owing to potential applications in imaging and spectroscopy for medical diagnostics and biology, high-bandwidth communication, security and defence, and non-destructive evaluation.
THz optics has entered a new phase of research in recent years. The performance of THz sources has been significantly enhanced so that they now have considerably higher output powers, narrower pulses (in both the time and frequency domains) and wider frequency ranges. Furthermore, the increase in their degrees of freedom makes THz sources just as convenient as visible or infrared sources for pump–probe measurements. Finally, THz radiation can be used to obtain important physical data that is not accessible using X-ray or infrared radiation. Several measurement techniques based on THz radiation are now employed for fundamental material research. For example, THz radiation is used to manipulate spin and the superconductivity of materials.
Clearly, the interest in THz science extends beyond the development of light sources and measurement techniques to fundamental material research. This issue of Nature Photonics features a special focus on THz optics with three Reviews, a Commentary, and an Interview, which showcase the exciting state of current research in this area.

MIT

In the Interview on page 670, Xi-Cheng Zhang recalls the early development of THz science in the late 1980s and early 1990s. He identifies two milestones in the development of THz sources — the generation of THz pulses from short laser pulses and the development of the THz quantum cascade laser. He envisages that the next breakthrough in the field could be the development of high-intensity THz sources. He says that non-destructive THz imaging is highly promising for meeting the current demands in the areas of security and quality control. He also considers that combining different technologies, such as electronics and plasmonics, is important for realizing novel THz systems that offer orders of magnitude improvements in performance at a fraction of the cost of existing systems.
In their Review on page 680, Tobias Kampfrath and co-authors describe how intense THz transients interact with molecules in condensed matter and gases, and how they can be used to manipulate spin, electrons, lattice vibrations, molecular orientation and rotation. In addition to resonant manipulation, the Review also considers nonresonant manipulation such as field ionization of impurities, high-field electron transport and impact ionization.
Coincidentally, two of the research papers in this issue also fall in the area of terahertz transients. On page 720, Carlo Vicario and co-workers report off-resonant magnetization dynamics in a cobalt thin film. The strong phase locking between an intense single-cycle THz wave transient and the magnetization permits the magnetization vector of a THz magnetic field to be directly controlled on a femtosecond timescale. This finding promises to open up new avenues for realizing ultrafast data recording in magnetic media. On page 724, Masaaki Sato and co-workers report a new experimental technique for polarization shaping of terahertz pulses to arbitrarily control the chirality, phase, pulse duration and frequency of circularly polarized few-cycle THz pulses. This technique has numerous potential applications in areas such as nonlinear optics, coherent control and coherent multidimensional spectroscopy for which the polarization degree of freedom is important.
THz quantum cascade lasers have evolved dramatically since they were first developed. On page 691, Carlo Sirtori and co-authors review THz engineering using quantum cascade lasers and highlight methods that have been used to control their spectral and beam properties. On the other hand, on page 702, Ulrich Welp and co-authors provide an overview of the state-of-the-art research on superconducting THz emitters based on Bi₂Sr₂CaCu₂O_8+δ. The development of superconducting sources of THz- and sub-THz radiation is of technological importance, because this frequency range remains difficult to cover with compact solid-state sources.
The Commentary by Daniel Mittleman on page 666 notes the catch-22 situation that used to exist in THz research — there was no strong incentive to develop new technologies without first identifying a 'killer application', but there was little motivation to explore applications without establishing a viable technology base. He points out that the development of innovative tools and techniques is vital for improving research capabilities and opening up new research directions in the terahertz regime. He cites the recent progress in THz sources and plasmonics as two examples of exciting advances that contribute to the vibrancy of THz research and technology.
THz radiation shares some characteristics with both visible light and microwaves. This intermediate nature of THz radiation can make it useful for a wide range of applications and give it an advantage over electromagnetic radiation in other frequency regimes. This focus issue provides an overview of the importance and applications of THz radiation.

UK Terahertz Day

UK Terahertz Day - bringing the THz community together

EPSRC Coherent TeraHertz Systems (COTS) Programme Grant (www.terahertzsystems.org)

Tuesday, 16 July 2013 from 10:00 to 16:00 (BST)

Cambridge, United Kingdom

Join professionals from both academia and industry, to discuss this exciting and currently underutilised area of the electromagnetic spectrum. Leading speakers from Universities and Industry will present the latest developments and detail the emerging market areas.

The EPSRC Coherent TeraHertz Systems (COTS) Programme Grant (www.terahertzsystems.org) would like to invite you to THz Day, a THz community discussion event, to be held at the Cavendish Laboratory, University of Cambridge on Tuesday 16 July 2013.

The event will include invited talks from leading THz experts, including Don Arnone(Teraview Ltd) and Carlo Sirtori (University of Paris- Diderot); opportunity to introduce the work and interests of your group and a discussion on opportunities for a UK THz research network.

This event is a great opportunity for the UK Terahertz community to come together and discuss how to open up the THz spectrum for widespread scientific and commercial application, through the use of photonics-enabled coherent techniques. Hosted by insitutions from the new EPSRC funded Programme Grant COTS, our ambition is that this day will lead to THz finally achieving its full scientific and commercial potential. We will work with many academic and commercial partners to achieve this.

Draft Schedule (actual timings to be confirmed) 
10.00 - 16.00

• Presentation of COTS Programme Grant  
• Presentations from other UK THz groups both industry and academic 
• Discussion on opportunities for a UK THz research network

Who should attend?

Terahertz technology offers unique solutions for many strategically important applications in 3D imaging and spectroscopy (e.g. security screening and detection, dental/medical imaging, non-destructive testing, material characterisation and pollution monitoring), as well as TBit/s communications for backbone telecommunication networks and ‘instant’ high-definition video-on-demand optical links. This conference has been designed to bring together the academic and the industrial players in this sector. Anyone interested in the potential of THz wavelengths, whether researcher or industrialist, current user, potential user or just curious will find this a useful and stimulating conference.

Direct surface cyclotron resonance terahertz emission from a quantum cascade structure

http://apl.aip.org/resource/1/applab/v100/i10/p102103_s1?isAuthorized=no

François-Régis Jasnot¹, Louis-Anne de Vaulchier¹, Yves Guldner¹, Gérald Bastard¹, Angela Vasanelli², Christophe Manquest², Carlo Sirtori², Mattias Beck³, and Jérôme Faist³

¹Laboratoire Pierre Aigrain, Ecole Normale Supérieure, CNRS (UMR 8551), 24 rue Lhomond, 75231 Paris Cedex 05, France 
²Laboratoire Matériaux et Phénoménes Quantiques, Université Paris Diderot—Paris 7, CNRS—UMR 7162, Bâtiment Condorcet, 75205 Paris Cedex 13, France 

³Institute of Quantum Electronics, ETH Zürich, CH-8093 Zürich, Switzerland 

A strong magnetic field applied along the growth direction of a semiconductor quantum well gives rise to a spectrum of discrete energy states, the Landau levels. By combining quantum engineering of a quantum cascade structure with a static magnetic field, we can selectively inject electrons into the excited Landau level of a quantum well and realize a tunable surface emitting device based on cyclotron emission. By applying the appropriate magnetic field between 0 and 12 T, we demonstrate emission from a single device over a wide range of frequencies (1-2 THz and 3-5 THz).

© 2012 American Institute of Physics