Abstract-Frequency and power stabilization of a terahertz quantum-cascade laser using near-infrared optical excitation

T. Alam, M. Wienold, X. Lü, K. Biermann, L. Schrottke, H. T. Grahn, and H.-W. Hübers

 The schematic diagram of the setup used to stabilize the frequency and output power. The QCL is operated in a mechanical cryocooler. The combination of the absorption cell and the Ge:Ga detector A is used to lock the frequency, and the second Ge:Ga detector B is used as a reference for the output power stabilization. (b) Rear-facet illumination with a low-NA single-mode fiber.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-25-36846

We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL). This technique exploits frequency and power variations upon near-infrared illumination of the QCL with a diode laser. It does not require an external terahertz optical modulator. By locking the frequency to a molecular absorption line, we obtain a long-term (one-hour) linewidth of 260 kHz (full width at half maximum) and a root-mean-square power stability below 0.03%. With respect to the free-running case, this stabilization scheme improves the frequency stability by nearly two orders of magnitude and the power stability by a factor of three.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Hamamatsu Photonics successfully produced terahertz waves at a wavelength of 450 μm which is the world’s longest wavelength available from a single semiconductor laser

Terahertz nonlinear QCL designed to emit terahertz waves at a wavelength of 450 μm

https://www.hamamatsu.com/us/en/news/product_technology/2019/20191115000000.html

Hamamatsu Photonics has succeeded in producing terahertz waves at a wavelength of 450 micrometers (μm: micro or μ is one millionth) in the terahertz range which is the world’s longest wavelength available from a single semiconductor laser operating at room temperature. To achieve this breakthrough, we have developed long-wavelength mid-infrared quantum cascade laser, in which we designed the laser structure based on research and analysis results of the terahertz wave generation principle. Results from this research will be useful in applications such as quality testing and non-destructive inspection of drugs and foods containing components that absorb electromagnetic waves in the sub-terahertz range as well as submillimeter astronomy and high-speed and high-capacity communication over short distances.

Results of this research were published on November 2 (Thu) in the electronic version of the European Scientific Journal “Nanophotonics.” This research and development work was supported by the MIC/SCOPE #195006001.

Abstract-Bow-Tie Cavity for Terahertz Radiation

Luigi Consolino, Annamaria Campa, Davide Mazzotti, Miriam Serena Vitiello, Paolo De Natale, Saverio Bartalini

https://arxiv.org/abs/1904.13143

We report on the development, testing, and performance analysis of a bow-tie resonant cavity for terahertz (THz) radiation, injected with a continuous-wave 2.55 THz quantum cascade laser. The bow-tie cavity employs a wire-grid polarizer as input/output coupler and a pair of copper spherical mirrors coated with an unprotected 500 nm thick gold layer. The improvements with respect to previous setups have led to a measured finesse value F=123, and a quality factor Q = 5.1x10^5. The resonator performances and the relevant parameters are theoretically predicted and discussed, and a comparison among simulated and experimental spectra is given.

Abstract-Amplitude Stabilization of a Terahertz Quantum Cascade Laser with an External Metamaterial Amplitude Modulator

B. Wei, S. J. Kindness, N. W. Almond, R. Wallis, Y. Wu, Y. Ren, P. Braeuninger-Weimer, S. Hofmann, H. E. Beere, D. A. Ritchie, and R. Degl'lnnocenti

https://www.osapublishing.org/abstract.cfm?uri=cleo_si-2018-STu4D.5&origin=search

Terahertz laser sources with stable power levels are requested for astronomical, communication and spectroscopic applications. Here we demonstrate the amplitude stabilization of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array. This integrated amplitude modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and stabilizing the power output via a PID loop. The laser power fluctuation was reduced from 1.82% to 0.19% of the total power.

© 2018 The Author(s)

Abstract-Low-threshold terahertz molecular laser optically pumped by a quantum cascade laser

A. Pagies, G. Ducournau,  J.-F. Lampina,

https://aip.scitation.org/doi/abs/10.1063/1.4945355

We demonstrate a low-threshold, compact, room temperature, and continuous-wave terahertz molecular laser optically pumped by a mid-infrared quantum cascade laser. These characteristics are obtained, thanks to large dipole transitions of the active medium: NH3 (ammonia) in gas state. The low-power (<60 mW) laser pumping excites the molecules, thanks to intense mid-infrared transitions around 10.3 μm. The molecules de-excite by stimulated emission on pure inversion “umbrella-mode” quantum transitions allowed by the tunnel effect. The tunability of the quantum cascade laser gives access to several pure inversion transitions with different rotation states: we demonstrate the continuous-wave generation of ten laser lines around 1 THz. At 1.07 THz, we measure a power of 34 μW with a very low-threshold of 2 mW and a high differential efficiency of 0.82 mW/W. The spectrum was measured showing that the linewidth is lower than 1 MHz. To our knowledge, this is the first THz molecular laser pumped by a solid-state source and this result opens the way for compact, simple, and efficient THz source at room temperature for imaging applications.

Abstract-Amplitude stabilization and active control of a terahertz quantum cascade laser with a graphene loaded split-ring-resonator array

B. Wei, N. W. Almond, S. J. Kindness,R. Degl'lnnocenti, 

https://www.researchgate.net/publication/325163880_Amplitude_stabilization_and_active_control_of_a_terahertz_quantum_cascade_laser_with_a_graphene_loaded_split-ring-resonator_array

We demonstrate the amplitude stabilization of a 2.85 THz quantum cascade laser with a graphene loaded split-ring-resonator array acting as an external amplitude modulator. The transmittance of the modulator can be actively changed by modifying the graphene conductivity via electrostatic back-gating. The modulator operates at room temperature and is capable of actively modulating the quantum cascade laser power level and thus stabilizing the power output via a proportional-integral-derivative feedback control loop. The stability was enhanced by more than 10 times through actively tuning the modulation. Furthermore, this approach can be used to externally control the laser power with a high level of stability.

Abstract-Doppler-free spectroscopy with a terahertz quantum-cascade laser

M. Wienold, T. Alam, L. Schrottke, H. T. Grahn, and H.-W. Hübers

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-6-6692

We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Coherent detection of THz laser signals in optical fiber systems

Thomas G. Folland, Owen P. Marshall, Harvey E. Beere, David A. Ritchie, and Subhasish Chakraborty


https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-21-25566&origin=search

Terahertz (THz) coherent detectors are crucial for the stabilization and measurement of the properties of quantum cascade lasers (QCLs). This paper describes the exploitation of intra-cavity sum frequency generation to up-convert the emission of a THz QCL to the near infrared for detection with fiber optic coupled components alone. Specifically, a low cost infrared photodiode is used to detect a radio frequency (RF) signal with a signal-to-noise ratio of approximately 20dB, generated by beating the up-converted THz wave and a near infrared local oscillator. This RF beat note allows direct analysis of the THz QCL emission in time and frequency domains. The application of this technique for QCL characterization is demonstrated by analyzing the continuous tuning of the RF signal over 2 GHz, which arises from mode tuning across the QCL’s operational current range.

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.

US Patent-Quantum cascade external cavity laser with metasurfaces

United States Patent 9728930
Inventors:
Williams, Benjamin Stanford (Los Angeles, CA, US) 
Xu, Luyao (Los Angeles, CA, US) 
Chen, Daguan (Los Angeles, CA, US)

http://www.freepatentsonline.com/9728930.html

A metasurface reflector for quantum cascade lasing is disclosed. The metasurface reflector uses an array of subcavities disposed on a substrate and spaced with a sub-wavelength period. Each of the subcavities includes a layer of quantum-cascade-laser-active material sandwiched between two metallic layers. The array of subcavities reflect an incident light of a resonant frequency with amplification. When used with an output coupler, a quantum cascade laser beam can be generated.

Abstract-Measurement of the emission spectrum of a semiconductor laser using laser-feedback interferometry

• James Keeley, 
• Joshua Freeman, 
• Karl Bertling, 
• Yah L. Lim, 
• Reshma A. Mohandas, 
• Thomas Taimre, 
• Lianhe H. Li, 
• Dragan Indjin, 
• Aleksandar D. Rakić, 
• Edmund H. Linfield, 
• A. Giles Davies & 
• Paul Dean

https://www.nature.com/articles/s41598-017-07432-0?WT.feed_name=subjects_chemistry

The effects of optical feedback (OF) in lasers have been observed since the early days of laser development. While OF can result in undesirable and unpredictable operation in laser systems, it can also cause measurable perturbations to the operating parameters, which can be harnessed for metrological purposes. In this work we exploit this ‘self-mixing’ effect to infer the emission spectrum of a semiconductor laser using a laser-feedback interferometer, in which the terminal voltage of the laser is used to coherently sample the reinjected field. We demonstrate this approach using a terahertz frequency quantum cascade laser operating in both single- and multiple-longitudinal mode regimes, and are able to resolve spectral features not reliably resolved using traditional Fourier transform spectroscopy. We also investigate quantitatively the frequency perturbation of individual laser modes under OF, and find excellent agreement with predictions of the excess phase equation central to the theory of lasers under OF.

US Patent- Terahertz quantum cascade laser implementing a {hacek over (C)}erenkov difference-frequency generation scheme

United States Patent 9711948
Inventors

Belkin, Mikhail (Austin, TX, US) 
Adams, Robert (Austin, TX, US)
Amann, Markus Christian (Garching, DE)
Vizbaras, Augustinas (Garching, DE)

http://www.freepatentsonline.com/9711948.html

A terahertz source implementing a {hacek over (C)}erenkov difference-frequency generation scheme in a quantum cascade laser. The laser includes an undoped or semi-insulating InP substrate with an exit facet that is polished at an angle between 10° to 40°. The laser further includes a first waveguide cladding layer(s) in contact with an active layer (arranged as a multiple quantum well structure) and a current extraction layer on top of the substrate. Furthermore, the laser includes a second waveguide cladding layer(s) on top of the active layer, where the first and second waveguide cladding layers are disposed to form a waveguide structure by which terahertz radiation generated in the active layer is guided inside the laser. The terahertz radiation is emitted into the substrate at a {hacek over (C)}erenkov angle relative to a direction of the nonlinear polarization wave in the active layer, and once in the substrate, propagates towards the exit facet.

Abstract-Frequency Tunability and Spectral Control in Terahertz Quantum Cascade Lasers With Phase-Adjusted Finite-Defect-Site Photonic Lattices

http://ieeexplore.ieee.org/document/7948783/

We report on the effect of finite-defect-site photonic lattices (PLs) on the spectral emission of terahertz frequency quantum cascade lasers, both theoretically and experimentally. A central π-phase adjusted defect incorporated in the PL is shown to favor emission selectively within the photonic bandgap. The effect of the duty cycle and the longitudinal position of such PLs is investigated, and used to demonstrate three distinct spectral behaviors: single-mode emission from devices in the range 2.2−5 THz, with a side-mode suppression ratio of 40 dB and exhibiting continuous frequency tuning over >8 GHz; discrete tuning between two engineered emission modes separated by ∼40 GHz; and multiple-mode emission with an engineered frequency spacing between emission lines.

Abstract-Quasi-continuous frequency tunable terahertz quantum cascade lasers with coupled cavity and integrated photonic lattice

Iman Kundu, Paul Dean, Alexander Valavanis, Li Chen, Lianhe Li, John E. Cunningham, Edmund H. Linfield, and A. Giles Davies

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-1-486

We demonstrate quasi-continuous tuning of the emission frequency from coupled cavity terahertz frequency quantum cascade lasers. Such coupled cavity lasers comprise a lasing cavity and a tuning cavity which are optically coupled through a narrow air slit and are operated above and below the lasing threshold current, respectively. The emission frequency of these devices is determined by the Vernier resonance of longitudinal modes in the lasing and the tuning cavities, and can be tuned by applying an index perturbation in the tuning cavity. The spectral coverage of the coupled cavity devices have been increased by reducing the repetition frequency of the Vernier resonance and increasing the ratio of the free spectral ranges of the two cavities. A continuous tuning of the coupled cavity modes has been realized through an index perturbation of the lasing cavity itself by using wide electrical heating pulses at the tuning cavity and exploiting thermal conduction through the monolithic substrate. Single mode emission and discrete frequency tuning over a bandwidth of 100 GHz and a quasi-continuous frequency coverage of 7 GHz at 2.25 THz is demonstrated. An improvement in the side mode suppression and a continuous spectral coverage of 3 GHz is achieved without any degradation of output power by integrating a π-phase shifted photonic lattice in the laser cavity.

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.

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Abstract-Potential-inserted quantum well design for quantum cascade terahertz lasers

• 
  

http://onlinelibrary.wiley.com/doi/10.1002/pssr.201600334/full

• We report on a new design of terahertz quantum cascade laser based on a single, potential-inserted quantum well active region. The quantum well properties are engineered through single monolayer InAs inserts. The modeling is based on atomistic, spds* tight-binding calculations, and performances are compared to that of the classical three-well design. We obtain a 100% increase of the oscillator strength per unit length, while maintaining a high, nearly temperature-independent contrast between phonon-induced relaxation times of the upper and lower lasing states. The improved performances are expected to allow THz lasing at room temperature.

Abstract-Measuring intensity correlations of a THz quantum cascade laser around its threshold at sub-cycle timescales

Ileana Cristina Benea Chelmus, Christopher Bonzon, Curdin Maissen, Giacomo Scalari, Mattias Beck, Jerome Faist

(Submitted on 29 Nov 2016)

https://arxiv.org/abs/1611.09562

The quantum nature of photonic systems is reflected in the photon statistics of the light they emit. Therefore, the development of quantum optics tools with single photon sensitivity and excellent temporal resolution is paramount to the development of exotic sources, and is particularly challenging in the THz range where photon energies approach kbT at T=300 K. Here, we report on the first room temperature measurement of field g1({\tau}) and intensity correlations g2({\tau}) in the THz range with sub-cycle temporal resolution (146 fs) over the bandwidth 0.3-3 THz, based on electro-optic sampling. With this system, we are able to measure the photon statistics at threshold of a THz Quantum Cascade Laser.

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-High-spectral-resolution terahertz imaging with a quantum-cascade laser

Till Hagelschuer, Nick Rothbart, Heiko Richter, Martin Wienold, Lutz Schrottke, Holger T. Grahn, and Heinz-Wilhelm Hübers

https://www.osapublishing.org/oe/abstract.cfm?URI=oe-24-13-13839

We report on a high-spectral-resolution terahertz imaging system operating with a multi-mode quantum-cascade laser (QCL), a fast scanning mirror, and a sensitive Ge:Ga detector. By tuning the frequency of the QCL, several spectra can be recorded in 1.5 s during the scan through a gas cell filled with methanol (CH3OH). These experiments yield information about the local absorption and the linewidth. Measurements with a faster frame rate of up to 3 Hz allow for the dynamic observation of CH3OH gas leaking from a terahertz-transparent tube into the evacuated cell. In addition to the relative absorption, the local pressure is mapped by exploiting the effect of pressure broadening.

© 2016 Optical Society of America

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Abstract-20 Mbps wireless communication demonstration using terahertz quantum devices

Li Gu, Zhiyong Tan, Qingzhao Wu,  Chang Wang,  Juncheng Cao, 
http://www.opticsjournal.net/abstract.htm?aid=OJ150729000104w3y6B8

A wireless terahertz (THz) communication link is demonstrated, in which a THz quantum cascade laser and a THz quantum-well photo-detector (QWP) serve as the emitter and receiver, respectively. With the help of the well-matched THz QWP, the optical collection efficiency has greatly been improved. A data signal transmitted over 2.2 m with a low bit error rate (<=1 \times 10-8) and data rate as high as 20 Mbps is achieved, which are almost 1 order of magnitude higher than that previously reported.

Abstract- Three-dimensional terahertz imaging using swept-frequency feedback interferometry with a quantum cascade laser

Three-dimensional terahertz imaging using swept-frequency feedback interferometry with a quantum cascade laser

J. Keeley, P. Dean, A. Valavanis, K. Bertling, Y. L. Lim, R. Alhathlool, T. Taimre, L. H. Li, D. Indjin, A. D. Rakić, E. H. Linfield, and A. G. Davies  »View Author Affiliations

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-40-6-994

Optics Letters, Vol. 40, Issue 6, pp. 994-997 (2015)
http://dx.doi.org/10.1364/OL.40.000994

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We demonstrate coherent three-dimensional terahertz imaging by frequency modulation of a quantum cascade laser in a compact and experimentally simple self-mixing scheme. Through this approach, we can realize significantly faster acquisition rates compared to previous schemes employing longitudinal mechanical scanning of a sample. We achieve a depth resolution of better than 0.1 μm with a power noise spectral density below −50  dB/Hz, for a sampling time of 10  ms/pixel.

© 2015 Optical Society of America