Spectrally Resolved Specular Reflections of Thermal Phonons from Atomically Rough Surfaces

Abstract

The reflection of waves from rough surfaces is a fundamental process that plays a role in diverse fields such as optics, acoustics, and seismology. While a quantitative understanding of the reflection process has long been established for many types of waves, the precise manner in which thermal phonons of specific wavelengths reflect from atomically rough surfaces remains unclear owing to limited control over terahertzfrequency phonon generation and detection. Knowledge of these processes is critical for many applications, however, and is particularly important for recent attempts to create novel materials by coherently interfering thermal phonons. Here, we report measurements of a key property for these efforts, the phononwavelength-dependent specularity parameter, which describes the probability of specular reflections of thermal phonons at a surface. Our experiments show evidence of specular surface reflections of terahertz thermal phonons in our samples around room temperature and indicate a sensitivity of these reflections to surface imperfections on the scale of just 2-3 atomic planes. Our work demonstrates a general route to probe the microscopic interactions of thermal phonons with surfaces that are typically inaccessible with traditional experiments.

Ballistic thermal phonons traversing nanocrystalline domains in oriented polyethylene

“…Transient Grating Spectroscopy. We used TG (36)(37)(38)(39)(40)(41)(42) to observe heat conduction in the plane of PE films over micrometer length scales. In our implementation of this optical method, a pair of pump laser pulses are interfered on a sample at angle θ, creating a sinusoidal temperature profile with period d as shown in Fig.…”

“…To overcome this difficulty, we use Bayesian inference and a Metropolis-Hastings Markov chain Monte Carlo algorithm to quantify a posterior distribution and credible interval for F (Λ) (42,49). The method finds distributions of F (Λ) that reproduce the measured data while also imposing a prior that incorporates physical knowledge of the solution.…”

Determining phonon mean free path spectrum by ballistic phonon resistance within a nanoslot-patterned thin film

“…30 Experimental studies on the relationship between the surface roughness and the phonon specularity can be found elsewhere. [51][52][53] One major problem for phonon MC simulations lies in its poor computational efficiency, which limits the early frequency-dependent phonon MC studies to nanosized structures. 45 A temperature jump may further occur at the film-reservoir junctions due to strongly ballistic phonon transport within a film.…”

Generalized Fourier's law for nondiffusive thermal transport: Theory and experiment

“…In this regime, the heat flux and temperature fields may differ from the predictions of heat diffusion theory based on Fourier's law. These discrepancies have been observed at a localized hot spot created by a doped resistor thermometer in a suspended silicon membrane [1] and, more recently, in optical pump-probe experiments, including soft x-ray diffraction from nanoline arrays [2,3], transient grating [4,5], and time-domain [6][7][8][9] and frequency-domain [10] thermoreflectance methods. In particular, due to the absence of scattering the transport properties become nonlocal [11][12][13], in contrast to Fourier's law, in which the heat flux at a certain location is determined by the temperature gradient only at that location.…”