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-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.

Terahertz light pulses speed up spin switching

Researchers from the Moscow Institute of Physics and Technology and their colleagues from Germany and the Netherlands have achieved material magnetization switching on the shortest timescales, at a minimal energy cost. They have thus developed a prototype of energy-efficient data storage devices. CREDIT: @tsarcyanide/MIPT Press Office

https://physicsworld.com/a/terahertz-light-pulses-speed-up-spin-switching/

A new technique to rapidly reverse a magnet’s polarity in a way that all of its spins coherently rotate could be used to develop more energy-efficient data storage devices and superfast computers in the future. The technique, which works by applying ultrashort pulses of terahertz-frequency light to the magnet, does not produce any waste heat and requires very little energy – just one photon per spin flip.

Modern-day computer hard drives encode data as binary zeros and ones by orienting the spins in magnetic materials using magnetic field pulses created by an electrical current. This process dissipates huge amounts of energy though (and is relatively slow). Indeed, today’s data centres consume between 2 and 5% of the world’s electricity and produce waste heat that, in turn, requires even more power, to cool the servers down.

Researchers in Russia, Germany, the UK and the Netherlands have now exploited a novel, unprecedented strong interaction between the electric field of terahertz light pulses and magnetic spins. The effect, which they discovered in 2016 in the antiferromagnetic material thulium orthoferrite, makes the spins oscillate with large amplitude. This interaction, they found, is still not strong enough to switch the orientations of the spins, however, even using the most powerful THz radiation sources.

To be able to switch spins with THz light, the team designed and fabricated a special nanosized antenna (made of gold) and placed it on top of the thulium orthoferrite magnet. The antenna possesses plasmonic modes (collective oscillations of the metal’s conduction electrons) that increase the coupling between light and the antenna.

Enhancing the local light field

The device collects and focuses light at THz frequencies and enhances the local light electric field by more than 10 times. “This electric field is now strong enough to steer the magnetization of all the spins over a potential energy barrier and into a new orientation, in just picoseconds,” explains team member Rostislav Mikhaylovskiy, formerly of Radboud University in the Netherlands and now at Lancaster University in the UK. “This is because the photon energies of THz radiation are comparable to the energy needed to align the spins in the magnet.”

And that is not all: the temperature of the magnet does not increase at all during switching since the process requires the energy of just one quantum of the terahertz light – a single photon – per spin, he adds.nt

“The speed of purely electrical spin switching is typically limited to the GHz range by capacitances and inductances in electronic circuitry,” explains team member Christoph Lange of Regensburg University in Germany. “Most importantly, however, electronics inherently suffer from Ohmic energy losses, and subsequent heating. The flow of data in modern-day systems is now so intense that this waste heat is already restricting the performance of data centres and supercomputing facilities. Our approach avoids this problem by replacing electric current with light pulses.”

Coherent spin switching

To prove that they had indeed observed coherent switching of all the spins in the magnet, the researchers monitored the spin orientation using the polarization rotation imprinted on a short optical pulse that is delayed relative to the THz pulse. “If the initial spin deflection is not sufficient for synchronous spin switching, we observe a sinusoidal, oscillating signal in the polarization rotation,” explains Mikhaylovskiy. “If, on the other hand, the spins are switched, we observe a ‘beating’ signature on top of the oscillations, which is the characteristic ‘fingerprint’ of the spins deflecting over a potential barrier into a neighbouring local potential energy minimum.”

Our work is a major milestone in the worldwide research effort towards complete control of spins by THz pulses, Mikhaylovskiy tells Physics World. “The technology we have developed could enable highly energy-efficient data storage at greatly increased speeds as compared to existing technology. What is more, the coherent dynamics made possible by the extremely low energy dissipation may even allow for quantum information processing based on solid-state spins at THz clock rates.”

The team, which also includes Stefan Schlauderer and Rupert Huber from Regensburg University, Alexey Kimel of Radboud University and Anatoly Zvezdin from the Russian Academy of Sciences, now plans to continue its research at the new ultrafast laser at Lancaster University and accelerators at the Cockroft Institute. These facilities are able to generate intense pulses of THz light and the new experiments will allow the researchers to determine the practical and fundamental speed and energy limits of magnetic recording using THz light pulses.

The research is detailed in Nature 10.1038/s41586-019-1174-7.

Abstract-Terahertz light-matter interaction beyond unity coupling strength

Andreas Bayer, Marcel Pozimski, Simon Schambeck, Dieter Schuh, Rupert Huber, Dominique Bougeard, Christoph Lange

https://arxiv.org/abs/1807.00533

Achieving control over light-matter interaction in custom-tailored nanostructures is at the core of modern quantum electrodynamics [1-15]. In ultrastrongly coupled systems [5-15], excitation is repeatedly exchanged between a resonator and an electronic transition at a rate known as the vacuum Rabi frequency 
ΩR. For ΩR approaching the resonance frequency ωc, novel quantum phenomena including squeezed states [16], Dicke superradiant phase transitions [17,18], the collapse of the Purcell effect [19], and a population of the ground state with virtual photon pairs [16,20] are predicted. Yet, the experimental realization of optical systems with ΩR/ωc has remained elusive. Here, we introduce a paradigm change in the design of light-matter coupling by treating the electronic and the photonic components of the system as an entity instead of optimizing them separately. Using the electronic excitation to not only boost the oscillator strength but furthermore tailor the shape of the vacuum mode, we push ΩR/ωc of cyclotron resonances ultrastrongly coupled to metamaterials far beyond unity. As one prominent illustration of the unfolding possibilities, we calculate a ground state population of 0.37 virtual photons for our best structure with ΩR/ωc = 1.43, and suggest a realistic experimental scenario for measuring vacuum radiation by cutting-edge terahertz quantum detection [21,22].