Abstract-Up to 70 THz bandwidth from an implanted Ge photoconductive antenna excited by a femtosecond Er:fibre laser

• Abhishek Singh, 
• Alexej Pashkin,  
• Stephan Winnerl, 
• Malte Welsch, 
• Cornelius Beckh, 
• Philipp Sulzer, 
• Alfred Leitenstorfer, 
• Manfred Helm,  
• Harald Schneider, 

Ultrabroadband THz emission from a Ge:Au antenna pumped at 1100 nm.

https://www.nature.com/articles/s41377-020-0265-4

Phase-stable electromagnetic pulses in the THz frequency range offer several unique capabilities in time-resolved spectroscopy. However, the diversity of their application is limited by the covered spectral bandwidth. In particular, the upper frequency limit of photoconductive emitters - the most widespread technique in THz spectroscopy – reaches only up to 7 THz in the regular transmission mode due to absorption by infrared-active optical phonons. Here, we present ultrabroadband (extending up to 70 THz) THz emission from an Au-implanted Ge emitter that is compatible with mode-locked fibre lasers operating at wavelengths of 1.1 and 1.55 μm with pulse repetition rates of 10 and 20 MHz, respectively. This result opens up the possibility for the development of compact THz photonic devices operating up to multi-THz frequencies that are compatible with Si CMOS technology.

Extreme conditions in semiconductors

This is a close-up of the experimental setup in the University of Konstanz's high-field Terahertz lab. Under the extreme conditions of the experiment, a bright red glow can be seen to emanate from the gallium arsenide crystal used as a semiconductor. This is due to the system's extremely high optical nonlinearity, which occurs when Wannier-Stark localization sets in.CREDIT Leitenstorfer research team

Physicists from the Universities of Konstanz, Paderborn and ETH Zurich have succeeded in experimentally demonstrating Wannier-Stark localization

UNIVERSITY OF KONSTANZ

https://www.eurekalert.org/pub_releases/2018-07/uok-eci073018.php

Scientists from the University of Konstanz and Paderborn University have succeeded in producing and demonstrating what is known as Wannier-Stark localization for the first time. In doing so, the physicists managed to overcome obstacles that had so far been considered insurmountable in the field of optoelectronics and photonics. Wannier-Stark localization causes extreme imbalance within the electric system of crystalline solids. "This fundamental effect was predicted more than 80 years ago. But it has remained unclear ever since whether this state can be realized in a bulk crystal, that is, on the level of chemical bonds between atoms", says Professor Alfred Leitenstorfer, Professor of Experimental Physics at the University of Konstanz. Analogues of the effect have so far been demonstrated only in artificial systems like semiconductor superlattices or ultracold atomic gases. In a bulk solid, Wannier-Stark localization can only be maintained for an extremely short period of time, shorter than a single oscillation of infrared light. Using the ultrafast laser systems at the University of Konstanz, Wannier-Stark localization has now been demonstrated for the first time. The experiment was conducted in a high-purity gallium arsenide crystal grown at ETH Zurich using epitaxial growth. The research results were published in the scientific journal Nature Communications on 23 July 2018.

What is Wannier-Stark localization?

If we tried to picture the atoms of a crystal, it would have to be as a three-dimensional grid composed of small beads that repel each other and are only kept together by rubber bands. The system remains stable as long as the rubber band is as strong as the repulsion is. If this is the case, the beads neither move closer to each other, nor do they move away from each other - the distance between them remains about the same. Wannier-Stark localization occurs when the rubber bands are removed abruptly. It is the electronic state that happens at the precise moment in time when the rubber bands have already gone but the beads still remain in place: The chemical bonds that hold the crystal together have been suspended.

If this state is maintained for too long, the beads will break apart and the crystal dissolves. To analyze Wannier-Stark localization, the physicists had to remove the stabilizing structures, capture the system within a fraction of a light oscillation using light pulses, and finally to stabilize it again to prevent the atoms from breaking apart. The experiment was made possible through the highly intense electric field of an ultrashort infrared light pulse, which is present in the crystal for a few femtoseconds only. "This is what we specialize in: studying phenomena that only exist on very short time scales", explains Alfred Leitenstorfer.

"In perfect insulators and semiconductors, electronic states expand throughout the entire crystal. According to an 80-year-old prediction, this changes as soon as electrical voltage is applied", says Professor Torsten Meier from Paderborn University. "If the electric field inside the crystal is strong enough, the electronic states can be localized to a few atoms. This state is called the Wannier-Stark ladder", explains the physicist, who is also Vice-President for International Relations at Paderborn University.

New electronic characteristics

"A system that deviates so extremely from its equilibrium has completely new characteristics", says Alfred Leitenstorfer about why this state is so interesting from a scientific perspective. The short-lived Wannier-Stark localization correlates with drastic changes to the electronic structure of the crystal and results, for example, in extremely high optical nonlinearity. The scientists also assume that this state is chemically particularly reactive.

The first-ever experimental realization of Wannier-Stark localization in a gallium arsenide crystal was made possible through highly intense Terahertz radiation with field intensities of more than ten million volts per centimetre. The application of more ultrashort optical light pulses resulted in changes to the crystal's optical characteristics, which was instrumental to proving this state. "If we use suitably intense light pulses consisting of a few oscillations lasting some ten femtoseconds only, we can realize the Wannier-Stark localization for a short period of time", says Alfred Leitenstorfer. "Our readings match the theoretical considerations and simulations carried out both by my own research team and by that of my colleague, Professor Wolf Gero Schmidt", adds Torsten Meier. The researchers are planning to study the extreme state of Wannier-Stark localization on the atomic scale in more detail in the future and intend to make its particular characteristics usable.

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Facts:

• Scientists from the Universities of Konstanz, Paderborn and ETH Zurich succeed in experimentally demonstrating Wannier-Stark localization in a high-purity gallium arsenide crystal grown at ETH Zurich.
• Original publication: C. Schmidt, J. Bühler, A.-C. Heinrich, J. Allerbeck, R. Podzimski, D. Berghoff, T. Meier, W. G. Schmidt, C. Reichl, W. Wegscheider, D. Brida, A. Leitenstorfer: Signatures of transient Wannier-Stark localization in bulk gallium arsenide, Nature Communications 9, 2890 (2018), direct link: https://rdcu.be/3ntP
• Wannier-Stark localization occurs in extremely high electric fields in solid-state crystals and results in new characteristics such as optical nonlinearity. The scientists also assume that this state exhibits extremely high chemical reactivity.
• Implementation of Wannier-Stark localization through highly intense Terahertz radiation with field intensities of more than ten million volts per centimetre. Demonstration through ultrashort light pulses in the femtosecond range.
• Research funded through: ERC Advanced Grant 290876 "UltraPhase" of the European Research Council (ERC, A. Leitenstorfer, Konstanz), Emmy Noether Programme of the German Research Foundation (DFG, D. Brida, Konstanz), Collaborative Research Centre SFB-TRR 142 (DFG, T. Meier and W. G. Schmidt, Paderborn), Carl Zeiss Foundation (J. Bühler, Konstanz), Swiss National Science Foundation (C. Reichl and W. Wegscheider, Zurich).

Note to editors:

You can download photos here:

https://cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Extreme_Zustaende.jpg

Caption: Close-up of the experimental setup in the University of Konstanz's high-field Terahertz lab. Under the extreme conditions of the experiment, a bright red glow can be seen to emanate from the gallium arsenide crystal used as a semiconductor. This is due to the system's extremely high optical nonlinearity, which occurs when Wannier-Stark localization sets in.

Photo: Leitenstorfer research team

https://cms.uni-konstanz.de/fileadmin/pi/fileserver/2018/Bilder/Extreme_Zustaende_Leitenstorfer.jpg

Caption: Professor Alfred Leitenstorfer, Professor of Experimental Physics at the University of Konstanz

Photo: University of Konstanz

Abstract-Terahertz shockwaves generated by a precise subcycle cut of the electric field

Johannes Bühler, Jonas Allerbeck, Gabriel Fitzky, Daniele Brida, and Alfred Leitenstorfer

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-5-7-821

Electromagnetic transients with a steep onset of the electric field represent the optical analog to acoustic shockwaves. Impulsive excitation of an electron-hole plasma with 8 fs pulses activates the reflection of high-field terahertz transients from a semiconductor surface on a deeply subcycle timescale. The resulting waveforms display a few-femtosecond rise of the electric field, equivalent to a broadening of their spectral content by several octaves. Such synthetic waveforms with subcycle shaping can be used, for example, as a tool to study extreme transport phenomena in condensed matter.

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

Thin-disk laser technology to power-scale ultrashort pulses

Alfred Leitenstorfer, Jonathan Fischer and Daniele Brida

http://spie.org/newsroom/6388-thin-disk-laser-technology-to-power-scale-ultrashort-pulses

A hybrid high-energy thin-disk laser enables a new family of driving sources for ultrafast experiments.

5 May 2016, SPIE Newsroom. DOI: 10.1117/2.1201603.006388

Energy scaling of light sources has always been a driving force in the development of novel laser technologies. High optical power in a beam of coherent radiation is particularly appealing, since it maximizes the nonlinear interaction between light and matter. This fact allows for a broad portfolio of applications, ranging from industrial tasks such as nonthermal cutting and welding to scientific experiments that exploit advanced frequency conversion and compression schemes. Ultimately, the goal is to achieve maximum peak power with optimally stable, compact, and reliable systems.

For two decades titanium:sapphire has been the gain medium of choice for the construction of mode-locked oscillators (complemented by high-power amplifiers). The remarkable gain bandwidth of this material enables direct generation of intense pulses that have duration as short as 25fs at kilohertz repetition rates. However, in terms of power scalability, this class of lasers is limited by thermal management within the gain medium, and by the complexity of the Q-switched green pump sources for the amplifiers. Typically, pulse energies are limited to a few millijoules at a 1kHz repetition rate, and the only capacity for improvement is by aggressive cryocooling of the rod-type crystals used for amplification.

The search for alternatives focuses on two aspects: finding an active material that supports ultrashort pulses while limiting power dissipation into heat, and selecting a smart geometry of the gain medium that enables efficient cooling while ideally preserving unlimited power scalability. The introduction of ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk lasers efficiently targeted both issues, providing record-breaking average powers and the possibility of pulsed operation. The geometry of this gain medium—which was pioneered by Adolf Giesen at the University of Stuttgart—exploits effective cooling by means of a large contact area with a heat sink to enable a longitudinal heat flow parallel to the laser beam direction that does not degrade the transverse mode quality. In addition, Yb:YAG can be pumped by diode lasers at 976nm while lasing at 1030nm. This small quantum defect ensures ideal conditions for the scaling of the output power.

One challenge left for Yb:YAG thin-disk technology is the possibility of achieving sub-picosecond pulse duration at the multi-millijoule energy level. Yb:YAG displays a limited bandwidth, resulting in severe gain narrowing at any amplifier stage. To overcome this restriction, we implemented a system that combines erbium:fiber lasers with ytterbium:fibers and Yb:YAG thin-disk technology for high-power amplifications (see Figure 1). The key to our success is the capability of the erbium:fiber front end to tailor-make ultrabroadband spectra of high brilliance and coherence for the seeding of the ytterbium amplifier chain.1 This driving source is extremely reliable and robust, since it requires only standard telecom components. The broadband character of the seed pulses is preserved in a single-pass ytterbium:fiber pre-amplifier that operates in a chirped pulse scheme using a grating stretcher. At this stage, the energy is boosted to 100nJ, while the bandwidth still supports sub-100fs temporal duration. The excess spectral width of the seed source minimizes gain narrowing in the thin-disk regenerative section. In this way, we obtain 615fs pulses with 17mJ of energy at a 3kHz repetition rate after compression.2

Figure 1. Illustration of the laser system, comprising an ultrafast erbium:fiber seed system followed by an ytterbium:fiber pre-amplifier with a chirped-pulse scheme in a thin-disk regenerative cavity. The output pulses centered at 1030nm are 615fs long and contain 17mJ of energy at a repetition rate of 3kHz. This output may be exploited for experiments testing the behavior of condensed matter under extreme sub-cycle electrical bias (which can only be applied optically). I: Intensity.

In summary, our approach combines broadband and intense fiber seeding with thin-disk laser technology. It qualifies this class of hybrid high-energy lasers as a new family of driving sources for ultrafast experiments. The sub-picosecond pulse duration is sufficient to access fundamental nonlinear interactions such as white-light generation in bulk sapphire plates. This is a crucial benchmark for the design of further nonlinear conversion stages based, for example, on optical parametric amplification. Generation of ultra-intense few-cycle pulses in a broad range that spans frequencies from UV to terahertz is now within reach.

One motivation for the development of such high-power lasers is the aim of conducting terahertz experiments with extreme field amplitudes that reach a non-perturbative regime of light-matter interaction.3 There are already examples of electric fields exceeding 100MV/cm being implemented based on an erbium:fiber–titanium:sapphire hybrid setup.4 In future work, our system will likely go beyond this, while preserving the capabilities for sub-cycle sampling of the optical oscillations, as a direct gateway to elementary quantum dynamics and its precise control. As an example, we may demonstrate new transient states of condensed matter that are far from thermal equilibrium. The rich variety of physical phenomena that can be accessed in the terahertz domain comprises the interplay of electronic charges, spins, and the ionic backbone. We may therefore directly study elementary degrees of freedom, such as phonons, magnons, or energy gaps induced by strong electronic correlations.5, 6 In addition, intense terahertz transients open up the possibility of investigating condensed matter systems under extreme electrical bias (which cannot be applied under stationary conditions). For example, chemical bonds may be transiently broken under high fields, thus unveiling the atomic origin of an electronic band structure.

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Alfred Leitenstorfer, Jonathan Fischer, Daniele Brida

University of Konstanz

Konstanz, Germany

Alfred Leitenstorfer is a professor and is chair of Ultrafast Phenomena and Photonics. His scientific interests are in subcycle quantum photonics, multi-terahertz physics, femtosecond technologies, and their applications.

Jonathan Fischer is pursuing his PhD, which is aimed at developing ultrafast light sources and their application in multi-terahertz experiments.

Daniele Brida is Emmy Noether group leader and Zukunftskolleg Fellow. His research targets the development of advanced optical technologies and their use in the investigation of ultrafast phenomena in condensed matter.

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References:

1. D. Brida, G. Krauss, A. Sell, A. Leitenstorfer, Ultrabroadband Er:fiber lasers, Laser Photon. Rev. 8, p. 409, 2014.

2. J. Fischer, A.-C. Heinrich, S. Maier, J. Jungwirth, D. Brida, A. Leitenstorfer, 615fs pulses with 17mJ energy generated by an Yb:thin-disk amplifier at 3kHz repetition rate, Opt. Lett.41, 246.

3. F. Junginger, B. Mayer, C. Schmidt, O. Schubert, S. Mährlein, A. Leitenstorfer, R. Huber, A. Pashkin, Nonperturbative interband response of a bulk InSb semiconductor driven off resonantly by terahertz electromagnetic few-cycle pulses, PRL 109, p. 147403, 2012.

4. A. Sell, A. Leitenstorfer, R. Huber, Phase-locked generation and field-resolved detection of widely tunable terahertz pulses with amplitudes exceeding 100MV/cm, Opt. Lett. 33, p. 2767, 2008.

5. T. Kampfrath, A. Sell, G. Klatt, A. Pashkin, S. Mährlein, T. Dekorsy, M. Wolf, M. Fiebig, A. Leitenstorfer, R. Huber, Coherent terahertz control of antiferromagnetic spin waves, Nat. Photon. 5, p. 31, 2010.

6. A. Pashkin, M. Porer, M. Beyer, K. W. Kim, A. Dubroka, C. Bernhard, X. Yao, et al., Femtosecond response of quasiparticles and phonons in superconducting YBa2Cu3O7−δstudied by wideband terahertz spectroscopy, PRL 105, p. 067001, 2010.

Abstract-Direct Detection of Vacuum Fluctuations of the Multi-Terahertz Electric Field

Alfred Leitenstorfer, Claudius Riek, and Denis V. Seletskiy
https://www.osapublishing.org/abstract.cfm?uri=nlo-2015-NM3A.7

We show that ultrabroadband electro-optic sampling in free space is able to directly detect the vacuum fluctuations of the electric field, opening up a time-domain approach to quantum statistics that operates on the sub-cycle scale.

© 2015 OSA

PDF Article

Abstract-Direct Detection of Vacuum Fluctuations of the Multi-Terahertz Electric Field

Alfred Leitenstorfer, Claudius Riek, and Denis V. Seletskiy
https://www.osapublishing.org/abstract.cfm?uri=NLO-2015-NM3A.7

We show that ultrabroadband electro-optic sampling in free space is able to directly detect the vacuum fluctuations of the electric field, opening up a time-domain approach to quantum statistics that operates on the sub-cycle scale.

© 2015 OSA

PDF Article