An ultrafast dual pump-probe laser for quantum interferometry has allowed scientists in Japan to excite and detect photogenerated coherent phonons in the polar semiconductor gallium arsenide. The study overcomes the problem of impulsive absorption (IA) and impulsive stimulated Raman scattering (ISRS) affecting the requisite vibrations in a solid lattice leading to phonon creation.
The team’s motivation is to find ways to use “waste” vibrations – heat and noise – in semiconductor materials that might be exploited in faster computer processors and memory. Current computers essentially encode information in electromagnetic fields but the propagation of all those bits and bytes as electrons in the actual solids from which the chips are made randomly interact with the immediate environment generating unwanted vibrations that represent a loss of efficiency and a heating problem for keep the circuitry cool. There are two ways in which the waste is propagated: absorption of light or scattering by light, both processes lead to random excitation of atoms that make up the solid. A collaboration between researchers at the Tokyo Institute of Technology and the Quantum Computing Center, at Keio University, hopes that by converting this random excitation of particles into coherent, well-controlled vibrations of the solid, they can overcome the problem of heat and noise and use light or even sound to carry information; the energy of the lattice vibrations packaged in well-defined phonons would be the information currency.
Success will hinge on whether or not we can fully understand two fundamental phenomena – the generation of the necessary coherent phonons and their lifetime during which they retain their information-transporting capacity. Optical phonons can be used to describe a certain mode of vibration, which occurs when the neighbouring atoms within a lattice move in the opposite direction. However, “Because impulsive absorption (IA) and impulsive stimulated Raman scattering (ISRS) cause zapping of such vibrations in the solid lattice leading to phonon creation, our aim was to shed light on narrowing down this dichotomy,” Tokyo Tech’s Kazutaka Nakamura explains.
The researchers utilized dual pump-probe spectroscopy, where an ultrafast laser pulse is split into a stronger “pump” to excite atoms in the gallium arsenide sample and a weaker “probe” beam on the excited sample to detect the effects. The team explains that they split the pump pulse into two collinear pulses but with a slight shift in their wave pattern to produce relative phase-locked pulses. The phonon amplitude is thus enhanced or suppressed in fringes because of constructive and destructive interference. The researchers report that the probe beam then reads the interference fringes.
“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,” the researchers write. Because of quantum mechanical superposition, it is then possible to sieve out the information to reveal that the phonons were predominantly linked to scattering (ISRS).
Phys Rev B 2019, online: “Ultrafast quantum-path interferometry revealing the generation process of coherent phonons”
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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