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Atoms in a crystal form a regular lattice, in which they can move over small distances from their equilibrium positions. Such phonon excitations are represented by quantum states. A superposition of phonon states defines a so-called “phonon wavepacket”, which is connected with collective coherent oscillations of the atoms in the crystal. Coherent phonons can be generated by excitation of the crystal with a femtosecond light pulse and their motions in space and time be followed by scattering an ultrashort x-ray pulse from the excited material. The pattern of scattered x-rays gives direct insight in the momentary position of and distances between the atoms. A sequence of such patterns provides a ‘movie’ of the atomic motions.
The physical properties of coherent phonons are determined by the symmetry of the crystal, which represents a periodic arrangement of identical unit cells. Weak optical excitation does not change the symmetry properties of the crystal. In this case, coherent phonons with identical atomic motions in all unit cells are excited (red unit cells in Fig. 1(c) with arrows indicating atomic displacements). In contrast, strong optical excitation can break the symmetry of the crystal and make atoms in adjacent unit cells oscillate differently [Fig. 1(d)]. While this mechanism holds potential for accessing other phonons, it has not been explored so far.
In the journal Physical Review B [below] , researchers from the Max-Born-Institute in Berlin in collaboration with researchers from the University of Duisburg-Essen have demonstrated a novel concept for exciting and probing coherent phonons in crystals of a transiently broken symmetry. The key of this concept lies in reducing the symmetry of a crystal by appropriate optical excitation, as has been shown with the prototypical crystalline semimetal bismuth (Bi).
Ultrafast mid-infrared excitation of electrons in Bi modifies the spatial charge distribution and, thus, reduces the crystal symmetry transiently. In the reduced symmetry, new quantum pathways for the excitation of coherent phonons open up. As illustrated in Fig. 1, the symmetry reduction causes a doubling of the unit-cell size from the red framework with two Bi atoms to the blue framework with four Bi atoms. In addition to the unidirectional atomic motion shown in Fig. 1(c), the unit cell with 4 Bi atoms allows for coherent phonon wave packets with bidirectional atomic motions as sketched in Fig. 1(d).
Probing the transient crystal structure directly by femtosecond x-ray diffraction reveals oscillations of diffracted intensity (Fig. 2), which persist on a picosecond time scale. The oscillations arise from coherent wave packet motions along phonon coordinates in the crystal of reduced symmetry. Their frequency of 2.6 THz is different from that of phonon oscillations at low excitation level. Interestingly, this behavior occurs only above a threshold of the optical pump fluence and reflects the highly nonlinear, so-called non-perturbative character of the optical excitation process.
In summary, optically induced symmetry breaking allows for modifying the excitation spectrum of a crystal on ultrashort time scales. These results may pave the way for steering material properties transiently and, thus, implementing new functions in optoacoustics and optical switching.
Fig. 1. (a) Magenta dashed lines with arrows illustrate diffraction of hard femtosecond x-ray pulses off the lattice planes of the Bi crystal. Red balls connected by red lines: unit cell of an unexcited bismuth crystal containing two Bi atoms with one atom at its origin. The second atom is shown as green balls in panel (b) and is indicated as small balls in panels (c) and (d). Blue balls connected by blue lines: unit cell of the photo-excited crystal with reduced symmetry containing four Bi atoms. (c) Orange curve: electric field of the optical excitation pulse. Weak and/or short-wavelength pulses can only excite coherent phonons with identical motions in all unit cells indicated by light-red balls and arrows. (d) Strong excitation with femtosecond mid-infrared pulses reduces the crystal symmetry and allows for opposite atomic motions (light-blue balls and arrows) in adjacent unit cells.
Fig. 2. (a) Coherent phonon oscillations with a frequency of 2.6 THz observed in optical pump/femtosecond x-ray diffraction probe experiments for different pump fluences of the mid-infrared excitation pulses centered at a wavelength of 5 µm. The phonon wave packets are exclusively observed for strong excitation pulses, i.e., they are absent for pump fluences below 1.9 mJ/cm^2. Thus, the reduction of the symmetry of the unit cell via strong optical pumping is necessary to get access to the phonon motion sketched in Fig. 1(d). (b) Spectrum of the phonon oscillation gained by a Fourier transform of the transient at a fluence of 2.9 mJ/cm^2 shown in panel (a).
See the full article here.
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The Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) is a nonprofit research institution, organized in the legal form of a registered association (Forschungsverbund Berlin e.V.). The MBI is scientifically independent but without legal personality. It is at the same time member of the Leibniz Association and is funded (50% each) by the German federal government (BMBF) and the German states (Länder), in particularly by Berlin.
The MBI maintains close scientific relations with the three Berlin universities. Its directors are jointly appointed by the institute and one of the universities. Marc Vrakking is full professor at FU-Berlin, Stefan Eisebitt at TU-Berlin and Thomas Elsaesser at HU Berlin.
The institute was founded by the end of 1991 and consists presently of about 180 members of staff, among them 90 scientists (including guest scientists and PhD students). The annual budget is typically about € 20 Mio. of which about € 4 Mio. are acquired through third party funding.
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