Tabletop Deep Ultraviolet Transient Grating Measurements of Diamond and Silicon at Record Sub-300nm Length Scales

Transient grating (TG) experiments provide unique insight into microscopic electron and phonon transport dynamics by interfering two laser beams on a sample surface. This results in periodic charge and phonon excitations, and is a non-contact approach – without the need to coat or pattern the sample. Visible TG experiments have observed the onset of non-diffusive heat conduction in semiconductors such as silicon [1] – however, they are limited to opaque materials and length scales set by the visible diffraction limit (&gt;500nm). Recent extreme ultraviolet (EUV) TG implementations can probe transport at much shorter length scales but face practical limitations in manipulating the EUV light. To date, a large free electron laser facility with limited access was required to generate sufficient pump fluence [2]. Here we present a tabletop deep ultraviolet (DUV) TG beamline, which utilizes the 6.3eV / 196nm fourth harmonic of a Ti:Sapphire amplifier to achieve record tabletop excitation periodicities &lt;300nm for the first time. The high DUV photon energy enables the investigation of wide-bandgap materials at high carrier densities, including diamond and boron nitride. Most importantly, the short DUV wavelength accesses transport on length scales that are below the visible diffraction limit, while the &lt;300fs DUV pulse duration is critical for resolving very fast nanoscale electron and phonon dynamics. We first investigate in-plane electron diffusion in diamond, bridging the micron- and nanoscale measurements in Refs. [2] and [3] and show that comparisons between differing pump wavelengths may provide insight into the physical mechanisms modifying the electron diffusion coefficient at high carrier concentrations. We also extend the non-diffusive phonon transport measurements in silicon thin films presented in Ref. [1] to length scales almost an order of magnitude shorter, and compare to the predictions of state-of-the-art hydrodynamic and ballistic phonon transport models. This novel DUV-TG beamline provides a non-contact, tabletop route to rapidly investigate nanoscale carrier dynamics and addresses challenges associated with interfaces, defects and doping in a greatly expanded range of materials.<br/><br/><br/>[1] Johnson et al., <i>Phys. Rev. Lett. </i><b>10</b>, 025901 (2013)<br/>[2] Maznev et al., <i>Appl. Phys. Lett. </i> <b>5</b>, eaaw5805 (2019)<br/>[3] Malinauskas et al., <i>Phys. Status Solidi A</i> <b>207</b>, 2058-2063 (2010)