Phonon Reflection from Crystal Interfaces and the Kapitza Problem

PhDT · 2025-10-04

We have used the heat pulse technique to study phonon reflection from sapphire-vacuum and sapphire-liquid helium interfaces. The high resolution data presented here show more structure than has been observed in previous experiments of this type. In order to interpret the complex time-of-flight spectra, the problem of the reflection of elastic waves in an anisotropic medium is analyzed in detail. The analysis shows that there are, in general, nine phonon reflection processes, each with a different time of flight, which transfer energy from heater to detector via a single reflection. Iterative computer calculations are necessary to establish the trajectory of energy flow and the arrival time for each channel. The agreement between calculated and experimentally observed times of flight is very good. Although the sharp features in the reflection signal due to specular (k|| conserved) processes can be explained using anisotropic elastic theory, approximately half the energy which reaches the detector arrives via non-specular channels. The non-specular scattering, which may be due to surface roughness, gives rise to broad features in the signal. The main difference between crystal-vacuum and crystal-helium reflection signals is that the non-specular signal is much smaller for the helium covered surface. In contrast to previous works, we find that the specular signal is not affected by helium. Apparently, the non-specular processes are involved in the anomalous Kapitza conductance. In some crystallographic orientations of heater and bolometer, the non-specular signal is particularly large. The orientational dependence of the diffuse scattering is due to the extreme anisotropy of energy flow in crystals, an effect which is known as phonon focusing. We develop a new method of analyzing phonon focusing based on an asymptotic analysis of the phonon Green's function. Geometric arguments are used to show that certain singularities in the acoustic field called caustics can be expected in most crystals. The general features of caustics can be predicted using results from mathematical catastrophe theory. The caustics in sapphire were located by numerical calculation, and used to explain the results of several experiments.

Laser-induced cavitation in liquid <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>He</mml:mi><mml:mprescripts/><mml:none/><mml:mn>4</mml:mn></mml:mmultiscripts></mml:math> near the liquid-vapor critical point

Physical Review Fluids · 2024-09-09 · 5 citations

Cavitation near the critical point is unusual because the compressibility becomes very high and the density difference between liquid and vapor becomes small and vanishes completely in the single phase supercritical region. We have investigated laser-induced cavitation in this unusual regime with high speed video at up to 5 million frames per second using liquid helium as the working fluid. Our theoretical analysis shows that the pressure in the liquid outside a bubble can be much lower than the ambient pressure. Near the critical point, the low pressure liquid becomes unstable and generates a cloud of microbubbles, which is consistent with predictions of nucleation theory near the spinodal.

Channeling acceleration in crystals and nanostructures and studies of solid plasmas: new opportunities

Journal of Instrumentation · 2023-11-01 · 5 citations

Abstract Plasma wakefield acceleration (PWFA) has shown illustrious progress and resulted in an impressive demonstration of tens of GeV particle acceleration in meter-long single structures. To reach even higher energies in the 1 TeV to 10 TeV range, a promising scheme is channeling acceleration in solid-density plasmas within crystals or nanostructures. The E336 experiment studies the beam-nanotarget interaction with the highly compressed electron bunches available at the FACET-II accelerator. These studies furthermore involve an in-depth research on dynamics of beam-plasma instabilities in ultra-dense plasma, its development and suppression in structured media like carbon nanotubes and crystals, and its potential use to transversely modulate the electron bunch.

Fiber-Optic Based Laser Wakefield Accelerated Electron Beams and Potential Applications in Radiotherapy Cancer Treatments

Photonics · 2022-06-08 · 11 citations

Ultra-compact electron beam technology based on laser wakefield acceleration (LWFA) could have a significant impact on radiotherapy treatments. Recent developments in LWFA high-density regime (HD-LWFA) and low-intensity fiber optically transmitted laser beams could allow for cancer treatments with electron beams from a miniature electronic source. Moreover, an electron beam emitted from a tip of a fiber optic channel could lead to new endoscopy-based radiotherapy, which is not currently available. Low-energy (10 keV–1 MeV) LWFA electron beams can be produced by irradiating high-density nano-materials with a low-intensity laser in the range of ~1014 W/cm2. This energy range could be useful in radiotherapy and, specifically, brachytherapy for treating superficial, interstitial, intravascular, and intracavitary tumors. Furthermore, it could unveil the next generation of high-dose-rate brachytherapy systems that are not dependent on radioactive sources, do not require specially designed radiation-shielded rooms for treatment, could be portable, could provide a selection of treatment energies, and would significantly reduce operating costs to a radiation oncology clinic.

Channeling Acceleration in Crystals and Nanostructures and Studies of Solid Plasmas: New Opportunities

arXiv (Cornell University) · 2022-03-14 · 1 citations

Plasma wakefield acceleration (PWFA) has shown illustrious progress and resulted in an impressive demonstration of tens of GeV particle acceleration in meter-long single structures. To reach even higher energies in the 1 TeV to 10 TeV range, a promising scheme is channeling acceleration in solid-density plasmas within crystals or nanostructures. The E336 experiment studies the beam-nanotarget interaction with the highly compressed electron bunches available at the FACET-II accelerator. These studies furthermore involve an in-depth research on dynamics of beam-plasma instabilities in ultra-dense plasma, its development and suppression in structured media like carbon nanotubes and crystals, and its potential use to transversely modulate the electron bunch.

Channeling Acceleration in Crystals and Nanostructures and Studies of Solid Plasmas: New Opportunities

arXiv (Cornell University) · 2022-03-14

International audience

Approaching PetaVolts per Meter Plasmonics Using Structured Semiconductors

IEEE Access · 2022-12-21 · 2 citations

A newly uncovered class of plasmons in the strongly excited limit opens access to unprecedented Petavolts per meter electromagnetic fields with wide-ranging, transformative impact. Unlike conventional plasmons, such plasmons are constituted by non-perturbative, large-amplitude oscillations of the ultradense, delocalized free electron Fermi gas inherent in conductive media. Here structured semiconductors doped to have an appropriate conduction electron density are introduced to tune the properties of the Fermi gas for matched excitation of large-amplitude plasmons using readily available electron beams which enables immediate experimental validation. Specifically, an electrostatic, surface “crunch-in” plasmon is collisionlessly excited by the beam launched inside a tube. Strong excitation due to matching results in relativistic oscillations of the electron gas and unravels unique phenomena. Relativistically induced ballistic electron transport comes about due to relativistic multifold increase in the mean free path and also leads to unconventional heat deposition beyond Ohm’s law. This explains the absence of observed damage or solid-plasma formation in past experiments on conductive samples interacting with electron bunches shorter than 10−13 seconds. Furthermore, relativistic momentum leads to copious tunneling of electron gas across the surface, which then crunches inside the tube. Relativistic effects along with large, localized electron density variations underlying these modes necessitate kinetic approach to theoretical and computational modeling. Kinetic model presented here demonstrates experimental viability of observing tens of gigavolts per meter plasmonic fields excited by matching readily available electron beams to plasmons in semiconductors with 1018cm−3 free electron density, and paves the way for Petavolts per meter plasmonics.

Nanoplasmonic Accelerators Towards Tens of TeraVolts per Meter Gradients Using Nanomaterials

CERN Document Server (European Organization for Nuclear Research) · 2021-01-01

Ultra-high gradients which are critical for future advances in high-energy physics, have so far relied on plasma and dielectric accelerating structures. While bulk crystals were predicted to offer unparalleled TV/m gradients that are at least two orders of magnitude higher than gaseous plasmas, crystal-based acceleration has not been realized in practice. We have developed the concept of nanoplasmonic crunch-in surface modes which utilizes the tunability of collective oscillations in nanomaterials to open up unprecedented tens of TV/m gradients. Particle beams interacting with nanomaterials that have vacuum-like core regions, experience minimal disruptive effects such as filamentation and collisions, while the beam-driven crunch-in modes sustain tens of TV/m gradients. Moreover, as the effective apertures for transverse and longitudinal crunch-in wakes are different, the limitation of traditional scaling of structure wakefields to smaller dimensions is significantly relaxed. The SLAC FACET-II experiment of the nano2WA collaboration will utilize ultra-short, high-current electron beams to excite nonlinear plasmonic modes and demonstrate this possibility.

Advancing the extreme field frontier using nanostructure nanoplasmonic modes far beyond gaseous plasmas

Bulletin of the American Physical Society · 2021-04-19

Spreading of Normal Liquid Helium Drops

Physical review. E · 2020-10-21 · 1 citations

We have used video imaging and interferometric techniques to investigate the dynamics of spreading of drops of ^{4}He on a solid surface for temperatures ranging from 5.2 K (near the critical point) to 2.2 K (near T_{λ}). After an initial transient, the drops become pancake-shaped with a radius that grows as R(t)≈t^{α}, with α=0.149±0.002. The drops eventually begin to shrink due to evaporation driven by gravitational and curvature effects, which limits their lifetime to about 1000 s. Although helium completely wets the substrate, and the spreading takes place over a pre-existing adsorbed film, a distinct contact line with a contact angle of order one degree is visible throughout this process.