Spatiotemporal Coherent Control of Thermal Excitations in Solids

Abstract

X-ray reflectivity (XRR) measurements of femtosecond laser-induced transient gratings (TG) are applied to demonstrate the spatio-temporal coherent control of thermally induced surface deformations on ultrafast timescales. Using grazing incidence x-ray diffraction we unambiguously measure the amplitude of transient surface deformations with sub-Å resolution. Understanding the dynamics of femtosecond TG excitations in terms of superposition of acoustic and thermal gratings makes it possible to develop new ways of coherent control in x-ray diffraction experiments. Being the dominant source of TG signal, the long-living thermal grating with spatial period Λ can be canceled by a second, time-delayed TG excitation shifted by Λ/2. The ultimate speed limits of such an ultrafast x-ray shutter are inferred from the detailed analysis of thermal and acoustic dynamics in TG experiments. [5][6][7] or plasmonic [8,9] degrees of freedom. Strain-induced phenomena may be used to discover new material properties and develop new applications, for example the modification of optical and electronic properties in semiconductor nanostructures [10]. Surface acoustic waves (SAWs) are often employed as a source of lattice strain. They can be generated [11] and controlled [12] optically via the excitation of transient gratings (TGs) [13,14]. Recently, these TG-excitations heave been used to probe heat transport in suspended thin films [15] and magneto-elastic coupling in thin nickel films [16][17][18]. Optical excitation of a solid generates not only coherent sound waves but also incoherent thermal strain. Coherent excitations can be controlled in amplitude and phase by series of light pulses in time domain, which is labeled temporal coherent control [19]. The main fraction of the deposited optical energy is stored in incoherent excitations of the lattice, i.e., heat [20,21] which can consequently not be controlled by a temporal sequence of light pulses. This thermal lattice excitation often generates a background which makes is difficult to precisely observe the coherent acoustic signal in purely optical experiments.In this letter we demonstrate, for the first time, the coherent control of incoherent, thermal transient gratings. We apply spatio-temporal coherent control showing that the spatial part of coherent control adds a new degree of freedom to control the amplitude and the phase of a thermally deformed surface. This is clearly a new approach that introduces the concept of spatial coherent control to the dynamics of incoherent excitations on ultrafast time scales, a phenomenon impossible to achieve with temporal coherent control only. We also demonstrate the control of a transient thermal grating on a timescale faster than the oscillation of the simultaneously excited coherent acoustic modes. Our new quantitative method allows for decomposing the coherent and incoherent dynamics in the sample by measuring the amplitude of the surface excursion with sub-Å precision and ≈ 70 ps temporal resolution. The modification of x-ray diffracti...

Generation of coherent phonons by coherent extreme ultraviolet radiation in a transient grating experiment

“…An effective approach to generate tunable coherent phonons by light is the transient grating (TG) technique, [18][19][20] which can be regarded as a third order wave-mixing process. In this method, widely used in both fundamental and applied research, [21][22][23][24] two coherent pulses crossed in the sample form an optical interference pattern with a spatial period K ¼ k=2 sin ðhÞ, where k is the wavelength and 2h is the angle between the two beams. The interaction of the radiation with the medium, via various mechanisms, generates coherent phonons at the wavelength equal to K, which are detected via diffraction of a probe laser beam.…”

Full Spatiotemporal Control of Laser-Excited Periodic Surface Deformations

“…15. The concept of spatiotemporal coherent control is generally applicable to an arbitrary number of coherent modes [14,15,41], but in the present case the data only exhibit a single Rayleigh-like SAW mode. We can thus restrict our model to only include this single coherent mode.…”

“…The main challenge for this device is to achieve high diffraction efficiencies. Our previous studies suggest that a maximum efficiency of more than 30% could be reached [14,15,67].…”

“…A detailed view of our optical experimental setup is shown in Fig 2a-e). The general layout of the TG setup is described elsewhere [14,33]. Our particular setup was designed for optical pump-TR-XRR probe measurements at the ID09 beamline at the European Synchrotron Radiation Facility (ESRF).…”

“…Alternatively, PSDs can be detected using x-ray diffraction and photoemission electron microscopy techniques [30][31][32]. We have recently shown that the PSD associated with the excited quasi-static and transient modes may also be probed by TR-XRR [14,15]. This method is exclusively sensitive to the surface displacement and can detect deformations of only few nanometers [14].…”