The quantum behavior of atomic vibrations excited in a crystal using light
pulses has much to do with the polarization of the pulses, say materials
scientists from Tokyo Tech. The findings from their latest study offer a new
control parameter for the manipulation of coherently excited vibrations in
solid materials at the quantum level.
To the naked eye, solids may appear perfectly still, but in reality, their
constituent atoms and molecules are anything but. They rotate and vibrate,
respectively defining the so-called "rotational" and "vibrational" energy
states of the system. As these atoms and molecules obey the rules of quantum
physics, their rotation and vibration are, in fact, discretized, with a
discrete "quantum" imagined as the smallest unit of such motion. For
instance, the quantum of atomic vibration is a particle called "phonon."
Atomic vibrations, and therefore phonons, can be generated in a solid by
shining light on it. A common way to do this is by using "ultrashort" light
pulses (pulses that are tens to hundreds of femtoseconds long) to excite and
manipulate phonons, a technique known as "coherent control." While the
phonons are usually controlled by changing the relative phase between
consecutive optical pulses, studies have revealed that light polarization
can also influence the behavior of these "optical phonons."
Dr. Kazutaka Nakamura's team at Tokyo Institute of Technology (Tokyo Tech)
explored the coherent control of longitudinal optical (LO) phonons (i.e.,
phonons corresponding to longitudinal vibrations excited by light) on the
surface of a GaAs (gallium arsenide) single crystal and observed a "quantum
interference" for both electrons and phonons for parallel polarization while
only phonon interference for mutually perpendicular polarization.
"We developed a quantum mechanical model with classical light fields for the
coherent control of the LO phonon amplitude and applied this to GaAs and
diamond crystals. However, we did not study the effects of polarization
correlation between the light pulses in sufficient detail," says Dr.
Nakamura, Associate Professor at Tokyo Tech.
Accordingly, his team focused on this aspect in a new study published in
Physical Review B. They modeled the generation of LO phonons in GaAs with
two relative phase-locked pulses using a simplified band model and "Raman
scattering," the phenomenon underlying the phonon generation, and calculated
the phonon amplitudes for different polarization conditions.
Their model predicted both electron and phonon interference for
parallel-polarized pulses as expected, with no dependence on crystal
orientation or the intensity ratio for allowed and forbidden Raman
scattering. For perpendicularly polarized pulses, the model only predicted
phonon interference at an angle of 45° from the [100] crystal direction.
However, when one of the pulses was directed along [100], electron
interference was excited by allowed Raman scattering.
With such insights, the team looks forward to a better coherent control of
optical phonons in crystals. "Our study demonstrates that polarization plays
quite an important role in the excitation and detection of coherent phonons
and would be especially relevant for materials with asymmetric interaction
modes, such as bismuth, which has more than two optical phonon modes and
electronic states. Our findings are thus extendable to other materials,"
says Nakamura.
Reference:
Itsuki Takagi et al, Theory for coherent control of longitudinal optical
phonons in GaAs using polarized optical pulses with relative phase locking,
Physical Review B (2021).
DOI: 10.1103/PhysRevB.104.134301
Tags:
Physics