Observation of terahertz radiation coherently generated by acoustic waves

Abstract

Over the past decade, pioneering and innovative experiments using subpicosecond lasers have demonstrated the generation and detection of acoustic and shock waves in materials with terahertz frequencies, the highest possible frequency acoustic waves 1-5 . In addition to groundbreaking demonstrations of acoustic solitons, these experiments have led to new techniques for probing the structure of thin films 6-8 . Terahertzfrequency electromagnetic radiation has been used in applications as diverse as molecular and material excitations 9,10 , charge transfer 11,12 , imaging 13 and plasma dynamics 14 . However, at present, existing approaches to detect and measure the time dependence of terahertz-frequency strain waves in materials use direct optical probes-time-resolved interferometry or reflectrometry 2,15,16 . Piezoelectric-based strain gauges have been used in acoustic shock and strain wave experiments for decades, but the time resolution of such devices is limited to ∼100 ps and slower, the timescale of electronic recording technology. We have recently predicted that terahertz-frequency acoustic waves can be detected by observing terahertz radiation emitted when the acoustic wave propagates past an interface between materials of differing piezoelectric coefficients 17,18 . Here, we report the first experimental observation of this fundamentally new phenomenon and demonstrate that it can be used to probe structural properties of thin films.As an acoustic wave traverses an interface between materials with differing piezoelectric response, polarization currents and concurrent radiation are predicted to be generated at the boundary. These currents radiate on the timescale of the strain changes 17 . For acoustic waves with characteristic frequencies of terahertz (corresponding to picosecond timescales), terahertz radiation is emitted. This phenomenon bears a close resemblance to the so-called transition radiation phenomenon that occurs when a charged particle propagates past an interface between two dielectric materials, generating polarization currents and radiation from the interface. It is distinct from a narrowband, coherent terahertz emission mechanism that has been predicted to occur when a planar shock wave propagates through an ionic crystal [19][20][21] . When the interface is flat and some propagation properties of the strain wave are known, the time dependence of the strain can be computed from the time dependence of the radiated electric field. This enables the observation of ultrafast strain profiles in regions of a material not accessible to active probes such as those used in interferometryor reflectometry-based methods 2,[22][23][24] . This acoustic transition radiation phenomenon has been predicted theoretically 17 , but not yet observed experimentally. Some experimental evidence exists for other forms of electromagnetic radiation that may

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…This review will be largely limited to THz time-domain spectroscopy using ultrafast lasers generating THz pulses-as opposed to continuous wave farinfrared spectroscopy using Fourier transform infrared spectrometers and THz sources from accelerators such as free-electron lasers. It also excludes works that investigated THz radiation emitted from materials to study charge carrier and lattice dynamics (Dekorsy et al, 1996;Kadoya and Hirakawa, 2005;Armstrong et al, 2009).…”

Controlling femtosecond-laser-driven shock-waves in hot, dense plasma

“…Furthermore, by an independent experiment with external prepulse we have shown a control over the inward motion of the shock wave velocity by manipulating the pre-plasma density scale-length. Therefore by controlling the laser contrast or pre-plasma scale-length, it is possible to manipulate the femtosecond-laser-driven shock wave velocity, which could lead to many potential applications in medicine, engineering, and science 18,19,24,30 .…”

“…In this paper, we introduce the opportunity of manipulating a femtosecond-laser-driven shock wave using a relatively inexpensive approach that is not target specific. This study opens up the opportunity to generate coherent terahertz radiation 18,19,24 at desired frequencies by using the con-trollable shock wave velocity to induce a periodic oscillations in the static polarization inside a crystalline polarizable material.…”

Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

“…The upper limit of this range, a few THz, has drawn the interest of many researchers in the past decade. The generation, analysis (Reed et al, 2008), and creative use (Armstrong et al, 2009) of THz-order acoustic waves has become almost routine since this figure was originally published. From White, 1997. surface acoustic wave sensors, cantilevers, and quartz crystal microbalances that are useful for characterizing fluid viscosity and density, and detecting the presence of monolayers due to binding or growth, biopolymers or biomolecules, or even entire cells and tissue.…”