Abstract-A Broadband Polarization-Rotating Vivaldi Antenna for Beam Focusing of Terahertz Quantum Cascade Lasers

U. Senica, E. Mavrona, T. Olariu, A. Forrer, M. Beck, J. Faist, and G. Scalari

https://www.osapublishing.org/abstract.cfm?uri=CLEO_Europe-2019-cc_2_4

Terahertz (THz) Quantum Cascade Lasers (QCLs) have been of large interest in the context of very broadband and compact frequency combs in the THz spectral range. For THz QCLs, the double metal waveguide proved to be advantageous in terms of bandwidth, compactness and low dispersion. However, as it confines the optical mode to deeply subwavelength dimensions, the output beam is highly divergent. Previous solutions addressing this issue were either impractical or not so broadband. To improve the far-field properties of the double metal waveguide across a very large frequency range, we designed, fabricated and characterized a broadband, narrow beam Vivaldi Antenna .

© 2019 IEEE

Abstract-Terahertz quantum cascade lasers frequency combs: Wide bandwidth operation and dual-comb on a chip

G. Scalari, M. Rösch,   M. Beck,   D. Bachmann,  K. Unterrainer,   J. Faist

http://ieeexplore.ieee.org/document/8086427/

Summary form only given. Terahertz (THz) quantum-cascade lasers (QCLs) constitute a very promising candidate for compact, wide bandwidth, integrated frequency combs [1, 4-7]. QCLs based on heterogeneous cores display the widest spectral coverage reaching more than one octave [2]. The possibility to engineer the gain profile turns out to be fundamental also with respect to dispersion compensation in order to extended the comb spectral bandwidth [2, 7]. In the effort of extending the comb operation to a full-octave to implement the laser self-referencing [3], we present here a new here a new THz QCL active region that allows the generation of a frequency comb with a spectral bandwidth exceeding of 1 THz centered at 3.1 THz. It fully exploits the capability of QCLs to integrate different active region designs within one laser cavity. The used building block is the three-active region design reported in Ref. [2], where a design at 3.4 THz has been added to increase the bandwidth towards higher frequencies. The four designs have central frequencies of 2.3, 2.6, 2.9, and 3.4 THz, the number of periods per design has also been rearranged in order to provide a flat gain resulting in a similar threshold for all the active regions and more dynamic range. Additionally, the doping level has been increased. Laser performance results in a largely extended dynamic range with respect to the original three-stack structure. Peak powers above 8 mW are recorded at 30 K and the lasing spectrum spans over 1.94 THz from 1.88 THz to 3.82 THz covering more than a full octave in frequency (Fig.1(a, b)). Dry-etched lasers with side-absorbers for lateral mode suppression similar as in [5] were fabricated for continuous wave (CW) operation and comb operation is probed through the beatnote analysis. Fig. 1(c, d) shows the beatnote as a function of the injected current with a maximum comb span of 1.1 THz which is the broadest demonstrated so far. Further proof of comb regime comes from the simultaneous measurements of two laser ridges on the same chip that show multiheterodyne spectra working in a dual-comb configuration [6].

Abstract-Negative free carrier absorption in terahertz quantum cascade lasers

C. Ndebeka-Bandou, M. Rösch, K. Ohtani, M. Beck, J. Faist

(Submitted on 28 Nov 2016)

https://arxiv.org/abs/1611.09269

We analyze the peculiar case where the free carrier absorption arising from LO phonon absorption-assisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm−1's at 200K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime.

Abstract-A patch-array antenna single-mode low electrical dissipation continuous wave terahertz quantum cascade laser

L. Bosco1,a), C. Bonzon1, K. Ohtani1, M. Justen2, M. Beck1 and J. Faist1
http://scitation.aip.org/content/aip/journal/apl/109/20/10.1063/1.4967836
1 Institute for Quantum Electronics, ETH Zurich, Auguste-Piccard-Hof 1, 8093 Zurich, Switzerland
2 I. Institute of Physics, University of Cologne, Zulpicher Strasse 77, 50937 Cologne, Germany
a) Electronic mail: lbosco@phys.ethz.ch
Appl. Phys. Lett. 109, 201103 (2016); http://dx.doi.org/10.1063/1.4967836

We introduce a double metal terahertz quantum cascade laser meant for astrophysical heterodyne measurements. The laser ridge is embedded in benzocyclobutene, and the device exhibits single mode, continuous wave operation around 4.745 THz with a peak power of almost 1.8 mW at 10 K and a power consumption of ≈1.6 W. Moreover, thanks to the integration of a top metal contact with a patch array antenna for light out-coupling the beam of the emitted light has a low-divergence single-lobe profile and an FWHM of ≈30°.

Abstract-Terahertz cyclotron resonance spectroscopy of an AlGaN/GaN heterostructure using a high-field pulsed magnet and an asynchronous optical sampling technique

B. F. Spencer1,a), W. F. Smith1, M. T. Hibberd1, P. Dawson1, M. Beck2,A. Bartels2, I. Guiney3, C. J. Humphreys3 and D. M. Graham1

1 School of Physics and Astronomy and the Photon Science Institute, The University of Manchester, Manchester M13 9PL, United Kingdom
2 Laser Quantum GmbH, Max-Stromeyer-Str. 116, 78467 Konstanz, Germany
3 Department of Materials Science and Metallurgy, 27 Charles Babbage Road, University of Cambridge, Cambridge CB3 0FS, United Kingdom
a) Electronic mail: Ben.Spencer@manchester.ac.uk
Appl. Phys. Lett. 108, 212101 (2016); http://dx.doi.org/10.1063/1.4948582

http://scitation.aip.org/content/aip/journal/apl/108/21/10.1063/1.4948582;jsessionid=rXYvggNCFFuhhk8oLtEY-Xhw.x-aip-live-03

The effective mass, sheet carrier concentration, and mobility of electrons within a two-dimensional electron gas in an AlGaN/GaN heterostructure were determined using a laboratory-based terahertz cyclotron resonance spectrometer. The ability to perform terahertz cyclotron resonance spectroscopy with magnetic fields of up to 31 T was enabled by combining a high-field pulsed magnet with a modified asynchronous optical sampling terahertz detection scheme. This scheme allowed around 100 transmitted terahertz waveforms to be recorded over the 14 ms magnetic field pulse duration. The sheet density and mobility were measured to be 8.0 × 1012 cm−2 and 9000 cm2 V−1 s−1 at 77 K. The in-plane electron effective mass at the band edge was determined to be 0.228 ± 0.002m 0

Abstract-Spectral gain profile of a multi-stack terahertz quantum cascade laser

D. Bachmann^1,2,a), M. Rösch³, C. Deutsch^1,2, M. Krall^1,2, G. Scalari³, M. Beck³, J. Faist³, K. Unterrainer^1,2 and J. Darmo^1,2

http://scitation.aip.org/content/aip/journal/apl/105/18/10.1063/1.4901316?TRACK=RSS

a) Author to whom correspondence should be addressed. Electronic mail: dominic.bachmann@tuwien.ac.at
Appl. Phys. Lett. 105, 181118 (2014); http://dx.doi.org/10.1063/1.4901316

The spectral gain of a multi-stack terahertz quantum cascade laser, composed of three active regions with emission frequencies centered at 2.3, 2.7, and 3.0 THz, is studied as a function of driving current and temperature using terahertz time-domain spectroscopy. The optical gain associated with the particular quantum cascade stacks clamps at different driving currents and saturates to different values. We attribute these observations to varying pumping efficiencies of the respective upper laser states and to frequency dependent optical losses. The multi-stack active region exhibits a spectral gain full width at half-maximum of 1.1 THz. Bandwidth and spectral position of the measured gain match with the broadband laser emission. As the laser action ceases with increasing operating temperature, the gain at the dominant lasing frequency of 2.65 THz degrades sharply.

Abstract-Transient increase of the energy gap of superconducting NbN thin-films excited by resonant narrow-band terahertz pulses

M. Beck, I. Rousseau, M. Klammer, P. Leiderer, M. Mittendorff, S. Winnerl, M. Helm, G. N. Gol’tsman, and J. Demsar

http://prl.aps.org/accepted/75075Y06Jb413d3903430035b96813be3c98ad3c4

Observations of radiation-enhanced superconductivity have thus far been limited to a few type-I superconductors (Al, Sn) excited at frequencies between the inelastic scattering rate and the superconducting gap frequency $2\Delta/h$. Utilizing intense, narrowband picosecond THz pulses, tuned to just below and above $2\Delta/h$ of a BCS superconductor NbN, we demonstrate that superconducting gap can be transiently increased also in a type-II dirty-limit superconductor. The effect is particularly pronounced at higher temperatures and is attributed to radiation induced non-thermal electron distribution persisting on a 100 ps timescale.