Phonon-Polaritons in Hexagonal Boron Nitride Induced by Evanescent Radiative Coupling

As electronic devices continue to reduce towards nanoscale dimensions, un-

derstanding heat transport mechanisms at these scales becomes vital. In most

non-metallic materials heat dissipates according to the movements of phonons

or phonon-like waves. The efficiency of this phonon-mediated heat transport is

dictated by various characteristics of these modes including their lifetime, scat-

tering rate, and velocity. Thus, the high group velocity of phonon-polarions can

facilitate more efficent heat transport in devices. From an experimental perspec-

tive, the majority of recent advances in such characteristics stems from ther-

moreflectance measurements, namely TDTR and FDTR, where phonon trans-

port is inferred from trends in thermal conductivity of materials. In other words,

these methods measure the average temperature of a material following pulsed

excitation rather than directly monitoring the activity of individual modes of

interest. In this work, we use a novel ultrafast pump-probe technique that can

observe specific vibrational modes with sub-picosecond time resolution, thus

characterizing the aforementioned phonon-polariton dynamics.

Our technique relies on a wavelength tunable mid-infrared probe pulse, al-

lowing us to directly resonate with vibrational heat carriers, thus providing

a direct measure of phonon dynamics in nanoscale material systems. In par-

ticular, we investigate the phonon-polariton dynamics in hexagonal boron ni-

tride (h-BN); the high frequency optical modes in this material lend themselves

to the potential for manipulation of thermal radiation in near room temper-

ature conditions. Thus we investigate the ultrafast thermal dynamics of the

transverse optical (TO) modes, the high reflectivity Reststrahlen band, and a

unique regime of polaritonic heat transfer: the generation of a thermal radiation-

induced phonon polariton. For the first time, we experimentally demonstrate

the ability to stimulate phonon polaritons in h-BN through remote heating of a

metal contact. Our results open the door for future experiments to manipulate

and guide phonons via heterostructures, which could increase thermal efficiency

of microelectronics and photonics.