Quantum Interferometry Reveals How Coherent Phonons Are Generated

Scientists at Tokyo Institute of Technology and Keio University wanted to investigate ways to store, move, and process information at exponential speeds using heat and noise (also known as waste vibrations). To do so, they studied the excitation and detection of photogenerated coherent phonons in gallium arsenide (GaAs) semiconductors using an ultrafast dual pump-probe laser.

To better understand the generation process of coherent phonons, the researchers utilized dual pump-probe spectroscopy. An ultrafast laser pulse was split into a “pump” to excite the GaAs sample and a “probe” beam to irradiate the sample. The pump pulse was further split into two collinear pulses, with a slight shift in the wave pattern to produce relative phase-locked pulses. The phonon amplitude was enhanced or suppressed in fringes, depending upon constructive and destructive interference (see Figures 1 and 2).

The probe beam read the interference fringe pattern by reading off changes in optical properties of the sample that arose due to the fringe pattern-dependent vibrations in the lattice. This method — reading off the changes in wave pulses to determine the sample characteristics — is called quantum interferometry.

“Thus, by varying the time delay between the pump pulses in steps shorter than the light cycle and pump-probe pulse, we could detect the interference between electronic states as well as that of optical phonons, which shows temporal characteristics of the generation of coherent phonons via light-electron-phonon interactions during the photo excitation,” said professor Kazutaka Nakamura.

From the quantum mechanical superposition, the researchers could determine that the generation of the phonons was primarily linked to impulsive stimulated Raman scattering.

Figure 1. Double-sided Feynman diagrams for the density matrices corresponding to (a) the impulsive stimulated Raman scattering (ISRS) process and (b) the impulsive absorption (IA) process. The thin and thick solid lines represent the ground and excited states, respectively; the dashed curves represent the one-LO-phonon state; the red and blue Gaussian curves represent the pulse envelope of the first and the second pulses, respectively, with the wavy lines their photon propagators. Courtesy of Physical Review B.

Figure 2. Interference fringes of (a) coherent longitudinal optical (LO) phonons and (b) coherent oscillation of LO phonon-plasmon coupled oscillation in n-type GaAs and (c) optical interference of the pump pulses. Fast oscillations (period of ~2.7 fs) in (a) and (b) are due to interference between electronic states. Courtesy of Physical Review B.

According to the researchers, most information is encoded in the waves and vibrations that propagate in space or solids. These waves and vibrations randomly interact with the particles in solid devices, creating wasteful byproducts (noise). This interaction can occur through absorption of light or through light scattering, and leads to random excitation of the atoms that make up the solid device.

Researchers from professor Nakamura’s laboratory at Tokyo Tech work with the equipment used for the ultrafast dual pump-probe experiments. Courtesy of Tokyo Institute of Technology.

By converting this random excitation of particles into coherent, well-controlled vibrations, the noise (that is, the waste vibration) could be used to transport information. The energy of this lattice vibration is packaged in bundles called phonons. Optical phonons describe a certain mode of vibration that occurs when the neighboring atoms of the lattice move in the opposite direction. Impulsive absorption and impulsive stimulated Raman scattering affect these vibrations in the solid lattice, leading to phonon creation.

The team said that to achieve such a conversion, it is first necessary to understand the generation process for coherent phonons, and the lifetime for which a coherent phonon retains its information-transporting ability.

Through their work, the researchers hope to advance scientific understanding of the waste vibrations in solids so they can be used as building blocks for transistors and electronic devices.

The research was published in Physical Review B (https://doi.org/10.1103/PhysRevB.99.180301).