Abstract-Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

 

Shovon Pal, Nives Strkalj, Chia-Jung Yang, Mads C. Weber, Morgan Trassin, Michael Woerner, and Manfred Fiebig

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021023

Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric 

${\mathrm{SrTiO}}_3$ thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.

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Abstract-Origin of Terahertz Soft-Mode Nonlinearities in Ferroelectric Perovskites

 

Shovon Pal, Nives Strkalj, Chia-Jung Yang, Mads C. Weber, Morgan Trassin, Michael Woerner, and Manfred Fiebig

https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.021023

Soft modes are intimately linked to structural instabilities and are key for the understanding of phase transitions. The soft modes in ferroelectrics, for example, map directly the polar order parameter of a crystal lattice. Driving these modes into the nonlinear, frequency-changing regime with intense terahertz (THz) light fields is an efficient way to alter the lattice and, with it, the physical properties. However, recent studies show that the THz electric-field amplitudes triggering a nonlinear soft-mode response are surprisingly low, which raises the question on the microscopic picture behind the origin of this nonlinear response. Here, we use linear and two-dimensional terahertz (2D THz) spectroscopy to unravel the origin of the soft-mode nonlinearities in a strain-engineered epitaxial ferroelectric ${\mathrm{SrTiO}}_3$ thin film. We find that the linear dielectric function of this mode is quantitatively incompatible with pure ionic or pure electronic motions. Instead, 2D THz spectroscopy reveals a pronounced coupling of electronic and ionic-displacement dipoles. Hence, the soft mode is a hybrid mode of lattice (ionic) motions and electronic interband transitions. We confirm this conclusion with model calculations based on a simplified pseudopotential concept of the electronic band structure. It reveals that the entire THz nonlinearity is caused by the off-resonant nonlinear response of the electronic interband transitions of the lattice-electronic hybrid mode. With this work, we provide fundamental insights into the microscopic processes that govern the softness that any material assumes near a ferroic phase transition. This knowledge will allow us to gain an efficient all-optical control over the associated large nonlinear effects.

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Abstract-Two-dimensional coherent spectroscopy of a THz quantum cascade laser: observation of multiple harmonics

Sergej Markmann, Hanond Nong, Shovon Pal, Tobias Fobbe, Negar Hekmat, Reshma A. Mohandas, Paul Dean, Lianhe Li, Edmund Linfield, A. G. Davies, Andreas D. Wieck, Nathan Jukam,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-18-21753&origin=search

Two-dimensional spectroscopy is performed on a terahertz (THz) frequency quantum cascade laser (QCL) with two broadband THz pulses. Gain switching is used to amplify the first THz pulse and the second THz pulse is used to probe the system. Fourier transforms are taken with respect to the delay time between the two THz pulses and the sampling time of the THz probe pulse. The two-dimensional spectrum consists of three peaks at (ωτ = 0, ωt = ω0), (ωτ = ω0, ωt = ω0), and (ωτ = 2ω0, ωt = ω0) where ω0 denotes the lasing frequency. The peak at ωτ = 0 represents the response of the probe to the zero-frequency (rectified) component of the instantaneous intensity and can be used to measure the gain recovery.

© 2017 Optical Society of America

Abstract-Spectral purity and tunability of terahertz quantum cascade laser sources based on intracavity difference-frequency generation

Luigi Consolino.  Seungyong Jung,  Annamaria Campa, Michele De Regis, Shovon Pal, Jae Hyun Kim, Kazuue Fujita, Akio Ito, Masahiro Hitaka, Saverio Bartalini, Paolo De Natale, Mikhail A. Belkin, Miriam Serena Vitiello,

http://advances.sciencemag.org/content/3/9/e1603317


Terahertz sources based on intracavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically pumped semiconductor sources capable of operating at room temperature across the 1- to 6-THz range. Despite tremendous progress in power output, which now exceeds 1 mW in pulsed and 10 μW in continuous-wave regimes at room temperature, knowledge of the major figure of merits of these devices for high-precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. By exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute center frequency of individual emission lines of these sources with an uncertainty of 4 × 10−10. The unveiled emission LW (400 kHz at 1-ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications that require narrow-LW THz sources.

Abstract-Broadband terahertz dispersion control in hybrid waveguides

Tobias Fobbe, Sergej Markmann, Felix Fobbe, Negar Hekmat, Hanond Nong, Shovon Pal, Patrick Balzerwoski, Janne Savolainen, Martina Havenith, Andreas D. Wieck, and Nathan Jukam
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-24-19-22319

Dispersion control is a key objective in the field of photonics and spectroscopy, since it enhances non-linear effects by both enabling phase matching and offering slow light generation. In addition, it is essential for frequency comb generation, which requires a phase-lock mechanism that is provided by broadband compensation of group velocity dispersion (GVD). At optical frequencies, there are several well-established concepts for dispersion control such as prism or grating pairs. However, terahertz dispersion control is still a challenge, thus hindering further progress in the field of terahertz science and technology. In this work, we present a hybrid waveguide with both broadband, tuneable positive and more than octave-spanning negative terahertz GVD on the order of 10−22 s2/m, which is suitable for either intra- or extra cavity operation. This new terahertz device will enable ultra-short pulse compression, allow soliton propagation, improve frequency comb operation and foster the development of novel non-linear applications.

New access to the interior of electronic components

http://www.spacedaily.com/reports/New_access_to_the_interior_of_electronic_components_999.html

An interdisciplinary team at the Ruhr-Universitat Bochum has found a way of accessing the interior of transistors. The researchers have manipulated the electron gas contained within by applying resonators to generate rhythmic oscillation in the terahertz range inside. They shared their findings in the journal Scientific Reports.

Used for switching and amplifying, transistors are fundamental elements of modern electronics. By applying a specific voltage externally to a transistor, an electric current is controlled inside, which, in turn, generates a new voltage.

Compared with the externally applied voltage, the new voltage may be amplified, may oscillate or be logically connected to it. In order to interact with their surroundings via electric current and voltage, transistors contain ultra-thin electron layers, so-called 2D electron gases. The RUB team demonstrated that these gases can be controlled not only via DC and radio-frequency voltages.

"A 2D electron gas is like jelly," explains Prof Dr Andreas Wieck from the Chair for Applied Solid State Physics. "If pressure is electrically applied to the gas from above with a characteristic frequency, thickness and density oscillations are generated."

Accordingly, the gas can be manipulated via electric forces, which oscillates much more rapidly than any radio or microwave frequency. As it has a thickness of just about ten nanometres, the oscillations follow the laws of quantum mechanics. This means: all occurring oscillations have a specific frequency, namely in the terahertz range, i.e. in the range of 1012 Hertz.

"Pressure to the electron gas must be applied in that rapid change," elaborates Wieck. Andreas Wieck, Dr Shovon Pal, Dr Nathan Jukam and other colleagues from the workgroup Terahertz Spectroscopy and Technology as well as from the Chair of Electronic Materials and Nanoelectronics have found a way to trigger the required oscillations. Thus, a new method of accessing the interior of a transistor has been created.

One hundred nanometres above the electron gas, the RUB researchers evaporated an array of identical metallic resonators which can oscillate with the required fixed frequency.

The electron gas was embedded in a semiconductor and could be modified via external DC voltage, namely it could be made a bit thicker or thinner. The thickness determines the frequency which makes the gas oscillate optimally.

Deploying external voltage, the researchers were able to fine-tune the electron gas to the resonators, i.e. adjust the gas so that the alternating electric pressure of the resonators excites it optimally to oscillate in the terahertz range.

This method could be of interest for sensors in chemical and environmental applications, as the researchers suggest. This is because molecule oscillations typically happen in the terahertz range. With modified transistors, such oscillations can be recorded and sensors can be developed that react to the frequencies of certain gases or liquids.

Pal et al. (2015): Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure, Scientific Reports, DOI: 10.1038/srep16812

Manipulating transistors at terahertz frequencies

http://phys.org/news/2015-11-transistors-terahertz-frequencies.html#jCp

An interdisciplinary team at the Ruhr-Universität Bochum has found a way of accessing the interior of transistors. The researchers have manipulated the electron gas contained within by applying resonators to generate rhythmic oscillation in the terahertz range inside. They shared their findings in the magazine Scientific Reports.


Transistors can be manipulated not only with voltages

Used for switching and amplifying, transistors are fundamental elements of modern electronics. By applying a specific voltage externally to a transistor, an electric current is controlled inside, which, in turn, generates a new voltage. Compared with the externally applied voltage, the new voltage may be amplified, may oscillate or be logically connected to it. In order to interact with their surroundings via electric current and voltage, transistors contain ultra-thin electron layers, so-called 2D electron gases. The RUB team demonstrated that these gases can be controlled not only via DC and radio-frequency voltages.

Electron gas can be oscillated like jelly

"A 2D electron gas is like jelly," explains Prof Dr Andreas Wieck from the Chair for Applied Solid State Physics. "If pressure is electrically applied to the gas from above with a characteristic frequency, thickness and density oscillations are generated." Accordingly, the gas can be manipulated via electric forces, which oscillates much more rapidly than any radio or microwave frequency. As it has a thickness of just about ten nanometres, the oscillations follow the laws of quantum mechanics. This means: all occurring oscillations have a specific frequency, namely in the terahertz range, i.e. in the range of 1012 Hertz. "Pressure to the electron gas must be applied in that rapid change," elaborates Wieck. Andreas Wieck, Dr Shovon Pal, Dr Nathan Jukam and other colleagues from the workgroup Terahertz Spectroscopy and Technology as well as from the Chair of Electronic Materials and Nanoelectronics have found a way to trigger the required oscillations. Thus, a new method of accessing the interior of a transistor has been created.

Resonators generate thickness oscillations

One hundred nanometres above the electron gas, the RUB researchers evaporated an array of identical metallic resonators which can oscillate with the required fixed frequency. The electron gas was embedded in a semiconductor and could be modified via external DC voltage, namely it could be made a bit thicker or thinner. The thickness determines the frequency which makes the gas oscillate optimally. Deploying external voltage, the researchers were able to fine-tune the electron gas to the resonators, i.e. adjust the gas so that the alternating electric pressure of the resonators excites it optimally to oscillate in the terahertz range.

Sensors for chemical and environmental technology

This method could be of interest for sensors in chemical and environmental applications, as the researchers suggest. This is because molecule oscillations typically happen in the terahertz range. With modified transistors, such oscillations can be recorded and sensors can be developed that react to the frequencies of certain gases or liquids.

 Explore further: Technique makes it possible to measure the intrinsic properties of quantum dot transistors

More information: Shovon Pal et al. Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure, Scientific Reports (2015). DOI: 10.1038/srep16812