5. Controlling coherent phonons

    When a single crystal is irradiated by a pulsed laser beam with a time width shorter than the period of its phonon oscillation, the phonon motion of the crystal can be excited. Unlike thermally excited phonons, the phases of the atomic (molecular) motions are aligned in the region where the laser beam is irradiated, and such phonon oscillations are called coherent phonons. Coherent phonons are generally measured as changes in reflectance by pump-probe spectroscopy.

5-1. Manipulation and visualization of two-dimensional atomic motion in Bismuth

    Figure 5-1 shows the unit cell of bismuth, which has two phonon modes: an A_1g mode oscillating in the z-axis direction and a doubly degenerate E_g mode in the xy-plane. If the excitation amplitudes of these modes can be controlled by light, it will lead to control the motion of atoms in the crystal lattice. In our experiment, we used the optical system shown in Figure 5-2 to measure the change in reflectance of the probe light due to irradiation of the pump light.

Fig. 5-1 Unit cell of bismuth and  two phonon modes

Fig. 5-2 Optical setup for  coherent phonon measurement

The amplitude of each phonon is controlled by using a pair of chirped pulse as the excitation pulse, and by controlling the delay time between each chirped pulse. This results in the modulation of spectrum in the THz region (Fig. 5-3). Furthermore, we have calculated the proportionality constant between the change in reflectivity and the displacement of atoms by ab initio calculations, and succeeded in visualizing the displacement of atoms in the plane illuminated by the light from the change in reflectivity. For more details, please refer to the  references listed below.

Fig. 5-3 Amplitude manipulation for A1g and Eg modes

5-2. Amplitude manipulation of low frequency vibrational modes in Rubrene crystal

    Experiments on coherent phonons have been reported not only in inorganic crystals but also in organic molecular crystals. In organic molecules, there are intramolecular vibrations in addition to the usual phonon motions, and the two are intricately intertwined in the THz region. In organic molecular crystals, where multiple modes exist, phonon motions are known to affect electronic properties in the crystal as a result of electron-phonon interactions. In pursuing such research, we believe that selective excitation of specific phonon vibrational modes is an important technique to induce molecular vibration in a specific direction, and thus to control thermal and electronic properties.

    In our experiments, we used a single crystal of rubrene, a well-known organic semiconductor crystal, and X-ray structural analysis confirmed that the a-b axis is oriented in-plane (Fig. 5-4). Using the same reflective optics layout as in Figure 5-2, we measured coherent phonons using a rubrene crystal cooled down to 90K.

Fig. 5-4 Crystal structure of Rubrene

    Since the excitation wavelength in this experiment was 830 nm and no electronic excitation was performed, the phonons are considered to be generated by Raman transitions in the electronic ground state. Figure 5-5 shows the actually observed coherent phonon signal. The Fourier transform identified multiple modes, and we focus on three modes at 3.2, 3.7, and 4.2 THz, which we refer to as ν₁, ν₂, and ν₃ below. Past studies have shown that these modes are mostly composed of intramolecular vibrational motion. The time corresponding to one period of each mode is calculated to be 312fs(T₁), 271fs(T₂), and 238fs(T₃), respectively.

Fig. 5-5 Coherent phonon signal of rubrene single crystal

    Next, individual phonon amplitude was controlled by double-pulse excitation. The phonon signals and their Fourier transforms measured with delay times of 1T₁, 1.5T₁, 1T₂, 1.5T₂, 1T₃, and 1.5T₃ between double pulses are shown in Fig.5-6(a) and (b). Amplification (by a factor of 2) and reduction (by a factor of 0) of the amplitude of each mode is achieved in accordance with the selected double pulse delay. The reason why the signals fluctuate between 0 and 2 times, unlike the example of para-hydrogen, is that the observed quantities are linearly proportional to the displacement of the atoms, not the square. This result indicates that the mode distribution in the THz region can be controlled with a high degree of freedom.

Fig. 5-6 Coherent phonon manipulation in rubrene single crystal. (a) temporal, and (b) frequency domain signal. (c) Auto-correlation of the pump pulse.

【Future plan】

    Since the experiment described above is controlling the phonon amplitude of the electron ground state, it cannot be directly expanded to controlling the electron motion in the conduction band by selective phonon excitation. Although it is possible to tune the wavelength of the oscillator output pulses into the visible region, the energy per pulse is insufficient with our current setup.  

We are currently planning experiments from a different approach.

【Related papers】

1. Optical manipulation of coherent phonons in superconducting YBa2Cu3O7-δ thin films
   Y. Okano, H. Katsuki, Y. Nakagawa, H. Takahashi, K. G. Nakamura and K. Ohmori, Faraday Discussions 153, 375-382 (2011).
2. All-Optical Control and Visualization of Ultrafast 2D Atomic Motions in a Single Crystal of Bismuth
    H. Katsuki, J. C. Delagnes, K. Hosaka, K. Ishioka, H. Chiba, E. S. Zijlstra, M. E. Garcia, H. Takahashi, K. Watanabe, M. Kitajima, Y. Matsumoto, K. G. Nakamura, and K. Ohmori, Nature Communications 4:2801 doi:10.1038/ncomms3801 (2013).
3. Mode Selective Excitation of THz vibrations in Single Crystalline Rubrene
   K. Yano, H. Katsuki, and H. Yanagi,
   J. Chem. Phys. 150, 054503 (2019).