Abstract-Exact frequency and phase control of a terahertz laser

Reshma A. Mohandas, Lalitha Ponnampalam, Lianhe Li, Paul Dean, Alwyn J. Seeds, Edmund H. Linfield, A. Giles Davies, and Joshua R. Freeman

Schematic diagram of the experimental arrangement. EDFA, erbium doped fibre amplifier; Tx, photomixer emitter; Rx1 and Rx2, photomixer receivers; PLL, phase lock loop; Δ𝜑$\mathrm{\Delta }\phi$, variable delay line. Electrical connections are shown in black, optical fiber and IR connections in red, and terahertz connections in green.

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1143

The accuracy of high-resolution spectroscopy depends critically on the stability, frequency control, and traceability available from laser sources. In this work, we report exact tunable frequency synthesis and phase control of a terahertz laser. The terahertz laser is locked by a terahertz injection phase lock loop for the first time, with the terahertz signal generated by heterodyning selected lines from an all-fiber infrared frequency comb generator in an ultrafast photodetector. The comb line frequency separation is exactly determined by a Global Positioning System-locked microwave frequency synthesizer, providing traceability of the terahertz laser frequency to primary standards. The locking technique reduced the heterodyne linewidth of the terahertz laser to a measurement instrument-limited linewidth of <1Hz$<1\mathrm{H}\mathrm{z}$, robust against short- and long-term environmental fluctuations. The terahertz laser frequency can be tuned in increments determined only by the microwave synthesizer resolution, and the phase of the laser, relative to the reference, is independently and precisely controlled within a range ±0.3𝜋$\pm 0.3\pi$. These findings are expected to enable applications in phase-resolved high-precision terahertz gas spectroscopy and radiometry.

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-Continuous frequency tuning with near constant output power in coupled Y-branched terahertz quantum cascade lasers with photonic lattice

Iman Kundu, Paul Dean, Alex Valavanis, Joshua R. Freeman, Mark C. Rosamond, Lianhe H. Li, Yingjun Han, Edmund H. Linfield, and Alexander Giles Davies

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00251?journalCode=apchd5

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modelling and the spectral characteristics are modelled using a transfer matrix model. Continuous frequency tuning of ~19 GHz is demonstrated while maintaining an output power of ~4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically-controlled continuous tuning performance from a THz QCL without significant change in output power

Abstract- Injection locking of a terahertz quantum cascade laser to a telecommunications wavelength frequency comb

Joshua R. Freeman, Lalitha Ponnampalam, Haymen Shams, Reshma A. Mohandas, Cyril C. Renaud, Paul Dean, Lianhe Li, A. Giles Davies, Alwyn J. Seeds, and Edmund H. Linfield

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-4-9-1059

High-resolution spectroscopy not only can identify atoms and molecules but also can provide detailed information on their chemical and physical environment and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications, and its frequency is not directly referenced to primary frequency standards. In this work, we injection lock a QCL operating at 2 THz to a compact fiber-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, <100  Hz$<100\mathrm{Hz}$. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be −75  dBc/Hz$-75\mathrm{dBc}/\mathrm{Hz}$ at 10 kHz offset from the 2 THz carrier.

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-Terahertz generation mechanism in nano-grating electrode photomixers on Fe-doped InGaAsP

Reshma A. Mohandas, Joshua R. Freeman, Michele Natrella, Mark C. Rosamond, Lalitha Ponnampalam, Martyn J. Fice, Alwyn J. Seeds, Paul. J. Cannard, Michael. J. Robertson, David. G. Moodie, A. Giles Davies, Edmund H. Linfield, and Paul Dean

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-9-10177

We report the generation mechanism associated with nano-grating electrode photomixers fabricated on Fe-doped InGaAsP substrates. Two different emitter designs incorporating nano-gratings coupled to the same broadband antenna were characterized in a continuous-wave terahertz (THz) frequency system employing telecommunications wavelength lasers for generation and coherent detection. The current-voltage characteristics and THz emission bandwidth of the emitters is compared for different bias polarities and optical polarisations. The THz output from the emitters is also mapped as a function of the position of the laser excitation spot for both continuous-wave and pulsed excitation. This mapping, together with full-wave simulations of the structures, confirms the generation mechanism to be due to an enhanced optical electric field at the grating tips resulting in increased optical absorption, coinciding with a concentration of the electrostatic field.

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-Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter

David R. Bacon, Andrew D. Burnett, Matthew Swithenbank, Christopher Russell, Lianhe Li, Christopher D. Wood, John Cunningham, Edmund H. Linfield, A. Giles Davies, Paul Dean, and Joshua R. Freeman

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-23-26986

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate. Through these improvements, we demonstrate a factor of ~8 increase in emitted THz field strength at the maximum operating voltage. In addition we find improved performance when these devices are used for photoconductive detection, which we explain through a combination of reduced parasitic substrate currents and reduced space-charge build-up in the device.

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-Gain recovery time in a terahertz quantum cascade laser

David R. Bacon1, Joshua R. Freeman1,a), Reshma A. Mohandas1, Lianhe Li1, Edmund H. Linfield1, A. Giles Davies1 and Paul Dean1
 1 School of Electronic and Electrical Engineering, University of Leeds, Woodhouse Lane, Leeds LS9 2JT, United Kingdom
a) Electronic mail: j.r.freeman@leeds.ac.uk
Appl. Phys. Lett. 108, 081104 (2016); http://dx.doi.org/10.1063/1.4942452

The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, attaining a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns.

Abstract-Generating ultrafast pulses of light from quantum cascade lasers

Feihu Wang, Kenneth Maussang, Souad Moumdji, Raffaele Colombelli, Joshua R. Freeman, Iman Kundu, Lianhe Li, Edmund H. Linfield, A. Giles Davies, Juliette Mangeney, Jérôme Tignon, and Sukhdeep S. Dhillon

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-2-11-944

The generation of ultrashort pulses from quantum cascade lasers (QCLs) has proved to be challenging. It has been suggested that the ultrafast electron dynamics of these devices is the limiting factor for mode locking and, hence, pulse formation. Even so, the clear mode locking of terahertz (THz) QCLs has been demonstrated recently, but the exact mechanism for pulse generation is not fully understood. Here we demonstrate that the dominant factor necessary for active pulse generation is in fact the synchronization between the propagating electronic modulation and the generated THz pulse in the QCL. By using the phase-resolved detection of the electric field in QCLs embedded in metal–metal waveguides, we demonstrate that active mode locking requires the phase velocity of the microwave round-trip modulation to equal the group velocity of the THz pulse. This allows the THz pulse to propagate in phase with the microwave modulation along the gain medium, permitting short-pulse generation. Mode locking was performed on QCLs employing phonon depopulation active regions, permitting the coherent detection of large gain bandwidths (500 GHz) and the generation of 11 ps pulses centered around 2.6 THz when the above “phase-matching” condition is satisfied. This work brings an enhanced understanding of QCL mode locking and will permit new concepts to be explored to generate shorter and more intense pulses from mid-infrared, as well as THz, QCLs.

© 2015 Optical Society of America

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Abstract-Laser-seeding dynamics with few-cycle pulses: Maxwell-Bloch finite-difference time-domain simulations of terahertz quantum cascade lasers

Joshua R. Freeman, Jean Maysonnave, Suraj Khanna, Edmund Linfield, A. Giles Davies, Sukhdeep S. Dhillon, and Jérôme Tignon

http://pra.aps.org/accepted/c107eNceP701751e604f1373d9f2f77c5fdeba29b

We implement a Maxwell-Bloch simulation for a two-level system within the FDTD method to simulate the seeding of lasers by broadband pulse injection. The model does not make the slowly varying envelope approximation and the full electromagnetic field is simulated so that we are able to obtain time resolved seeding by few-cycle pulses. The model is compared to recent results on seeding of THz quantum cascade lasers to aid interpretation of their complex signals. The simulations are found to be in good agreement with the data when gain recovery times of 15ps are used. Furthermore we find that the emission from the laser depends only weakly on the seed used to initiate laser action. The model is readily applicable to any seeded laser system where few-cycle seed pulses are used.