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.

Music goes terahertz: Scientists achieve breakthrough for pulsed terahertz lasers

https://www.nanowerk.com/nanotechnology-news2/newsid=56003.php

(Nanowerk News) An international research team from Germany, Italy, and the UK has developed a key photonics component for the intriguing terahertz spectral range. By mixing electronic resonances in semiconductor nanostructures with the photon field of microresonators, they designed a stained mirror that bleaches more easily than ever and could make terahertz lasers ultrafast.

The results are published in Nature Communications ("Ultrafast terahertz saturable absorbers using tailored intersubband polaritons").

A strong light pulse (white) can turn the saturable absorber (gold grating) into a nearly perfect mirror. Background photo: magnified view of a quantum cascade laser (center part of the silver area). (Image: Juergen Raab, Universität Regensburg)

Terahertz radiation – often dubbed T-rays – marks one of the last frontiers in photonics. Located in the spectral gap between microwave electronics and infrared optics, T-rays offer enormous application potential, but they have been expensive to generate. First broadly available terahertz applications range from body scanners at airports and rapid gas sensing to ultrafast communication.

Many more ideas could hit the market if ultrashort pulses could be directly generated in so-called quantum cascade lasers, special types of electrically driven, compact terahertz lasers. These sources typically operate in continuous wave mode, but it has been widely predicted that they might change into pulsed operation if a key photonics element was incorporated into the laser – a so-called saturable absorber.

A saturable absorber works like a foggy mirror that transiently turns clear if the incident light becomes too bright. If all the power inside a laser concentrates in a short pulse it would easily saturate the absorber and suffer less loss than a continuous wave beam.

Such elements are readily available in optics, whereas in the terahertz domain they have only existed for impracticably intense radiation, not achievable with quantum cascade lasers.

A European consortium formed by the research groups of Miriam S. Vitiello, Pisa, Edmund Linfield, Leeds, and Rupert Huber, University of Regensburg, have now joined forces to develop a new class of saturable absorbers operating at much lower saturation intensities.

Their novel idea is inspired by a strategy well-known in music: resonators. Where does a Steinway piano get its unique sound from? The secret is less in the strings than in the resonating body. This is where the exact sound is defined and its dynamical response to a forte keystroke.

“We essentially transfer this idea into terahertz optics”, says Jürgen Raab, lead author of the manuscript.

Miriam Vitiello’s group designed a microstructured assembly of a gold mirror and a gold grating that jointly work like a resonating body for terahertz radiation. These resonances can be coupled strongly with electrons that can hop between two quantum states defined by an atomically precise sequence of semiconducting nanostructures, designed and grown in the group of Edmund Linfield.

The pivot: The strong coupling between the electrons and the terahertz microcavity results in an excitation that is half electron, half terahertz photon. This situation not only shapes the “tone” of the resonance, but it also dramatically changes the way the system reacts to a “forte keystroke”, corresponding to an intense terahertz pulse.

The group put the new terahertz Steinway to its ultimate test. In a specially designed setup in Regensburg, they focused an ultrashort terahertz pulse onto the saturable absorber and developed an extreme slow-motion camera to follow its saturation dynamics on the femtosecond time scale – the millionth part of a billionth of a second.

The amazing result: The absorber was not only much easier to saturate than the electronic transition alone, by approximately an order of magnitude. It also saturates faster than a single oscillation cycle of the terahertz pulse, and the “tone” of the resonator morphs so well during the saturation process that essentially no absorption remains while the intense THz pulse is applied. These are the best possible genes of saturable absorbers.

Miriam Vitiello is convinced: “Now we have all components at hand to build ultrafast terahertz quantum cascade lasers with saturable absorbers”.

Such a source could dramatically extend the scope of terahertz photonics. Surpassing the frequency of modern computers by a staggering factor of 1000, ultrashort terahertz pulses could form the backbone of revolutionary next-generation telecommunication links. Compact quantum cascade lasers, emitting ultrashort T-rays, may allow also boost chemical analytics and enable an enormous variety of applications in diagnostics and medicine. With the current results, an important milestone towards these bold goals has been reached.

Abstract-Ultrafast terahertz saturable absorbers using tailored intersubband polaritons

Jürgen Raab, Francesco P. Mezzapesa, Leonardo Viti, Nils Dessmann, Laura K. Diebel, Lianhe Li, A. Giles Davies, Edmund H. Linfield, Christoph Lange, Rupert Huber, Miriam S. Vitiello,

         Polaritonic saturable absorber structure.

https://www.nature.com/articles/s41467-020-18004-8

Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field amplitudes owing to the weak light-matter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic single-chip design.

Abstract-High-speed modulation of a terahertz quantum cascade laser by coherent acoustic phonon pulses

• Aniela Dunn, 
• Caroline Poyser, 
• Paul Dean, 
• Aleksandar Demić, 
• Alexander Valavanis, 
• Dragan Indjin, 
• Mohammed Salih, 
• Iman Kundu, 
• Lianhe Li, 
• Andrey Akimov, 
• Alexander Giles Davies, 
• Edmund Linfield, 
• John Cunningham & 
• Anthony Kent 
• 
  

https://www.nature.com/articles/s41467-020-14662-w#Abs1

The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast amplitude modulation that can be controlled using the QCL drive current or strain pulse amplitude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.

Abstract-Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers

Jürgen Raab, Christoph Lange, Jessica L. Boland, Ignaz Laepple, Martin Furthmeier, Enrico Dardanis, Nils Dessmann, Lianhe Li, Edmund Linfield, A. Giles Davies, Miriam S. Vitiello, and Rupert Huber

(a) Schematic diagram of the THz saturable absorber structure showing the grating and the MQW stack. 𝛿$\delta$-Si: Silicon delta-doping layers. (b) Electron envelope functions of the first (𝛹1${\Psi }_1$, red) and second (𝛹2${\Psi }_2$, blue) subbands, and the conduction band edge (CB, black), in the MQW structure. (c) Cross section of the sample, showing the simulated field enhancement of the z-component ℰ𝑧${\mathcal{E}}_z$at 2.7 THz underneath one period of the gold grating, normalized to the incident electric field. Dashed horizontal lines indicate a GaAs layer, separating the MQW section from the metal grating. Lower panel: magnified view of the marked part of the upper panel. (d) Electric field waveform of the THz pulses used to excite the ISB system. (e) Amplitude spectrum of the THz transient shown in (d) along with the measured field transmission of the sample. The blue arrow indicates the expected ISB transition frequency. (f) Experimental principle showing the two identical THz pulses with fieldsℰA${\mathcal{E}}_{\text{A}}$andℰB${\mathcal{E}}_{\text{B}}$ delayed by a time 𝜏$\tau$, which prepare and interrogate the structure’s nonlinear response.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-3-2248

Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure. Clear signatures of incoherent pump-probe and coherent four-wave mixing signals are recorded as a function of the peak electric field of the single-cycle THz pulses. All nonlinear signals reach a pronounced maximum for a THz electric field amplitude of 11 kV/cm and decrease for higher fields. We demonstrate that this behavior is a fingerprint of THz-driven carrier-wave Rabi flopping. A numerical solution of the Maxwell-Bloch equations reproduces our experimental findings quantitatively and traces the trajectory of the Bloch vector. This microscopic model allows us to design tailored MQW structures with optimized dynamical properties for saturable absorbers that could be used in future compact semiconductor-based single-cycle THz sources.

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-Optomechanical response with nanometer resolution in the self-mixing signal of a terahertz quantum cascade laser

Andrea Ottomaniello, James Keeley, Pierluigi Rubino, Lianhe Li, Marco Cecchini, Edmund H. Linfield, A. Giles Davies, Paul Dean, Alessandro Pitanti, Alessandro Tredicucci,

(a) Sketch of the two configurations of the SM apparatus. (b) Calculated Δ𝑁$\mathrm{\Delta }N$ (blue curve), and measured 𝑉SM$V_{\mathrm{S}\mathrm{M}}$ (red points) as a function of Δ𝐿$\mathrm{\Delta }L$ using configuration 1

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-23-5663

Owing to their intrinsic stability against optical feedback (OF), quantum cascade lasers (QCLs) represent a uniquely versatile source to further improve self-mixing interferometry at mid-infrared and terahertz (THz) frequencies. Here, we show the feasibility of detecting with nanometer precision, the deeply subwavelength (<𝜆/6000$<\lambda /6000$) mechanical vibrations of a suspended Si3N4${\mathrm{S}\mathrm{i}}_3{\mathrm{N}}_4$ membrane used as the external element of a THz QCL feedback interferometer. Besides representing an extension of the applicability of vibrometric characterization at THz frequencies, our system can be exploited for the realization of optomechanical applications, such as dynamical switching between different OF regimes and a still-lacking THz master-slave configuration.

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.