Real-time observation of a coherent lattice transformation into a high-symmetry phase

Excursions far from their equilibrium structures can bring crystalline solids through collective transformations including transitions into new phases that may be transient or long-lived. Direct spectroscopic observation of far-from-equilibrium rearrangements provides fundamental mechanistic insight into chemical and structural transformations, and a potential route to practical applications, including ultrafast optical control over material structure and properties. However, in many cases photoinduced transitions are irreversible or only slowly reversible, or the light fluence required exceeds material damage thresholds. This precludes conventional ultrafast spectroscopy in which optical excitation and probe pulses irradiate the sample many times, each measurement providing information about the sample response at just one probe delay time following excitation, with each measurement at a high repetition rate and with the sample fully recovering its initial state in between measurements. Using a single-shot, real-time measurement method, we were able to observe the photoinduced phase transition from the semimetallic, low-symmetry phase of crystalline bismuth into a high-symmetry phase whose existence at high electronic excitation densities was predicted based on earlier measurements at moderate excitation densities below the damage threshold. Our observations indicate that coherent lattice vibrational motion launched upon photoexcitation with an incident fluence above 10 mJ/cm2 in bulk bismuth brings the lattice structure directly into the high-symmetry configuration for tens of picoseconds, after which carrier relaxation and diffusion restore the equilibrium lattice configuration.Comment: 22 pages, 4 figure

Surface-Enhanced Raman Spectroscopy of Graphene Integrated in Plasmonic Silicon Platforms with Three-Dimensional Nanotopography

Integrating graphene with plasmonic nanostructures results in multifunctional hybrid systems with enhanced performance for numerous applications. In this work, we take advantage of the remarkable mechanical properties of graphene to combine it with scalable three-dimensional (3D) plasmonic nanostructured silicon substrates, which enhance the interaction of graphene with electromagnetic radiation. Large areas of femtosecond laser-structured arrays of silicon nanopillars, decorated with gold nanoparticles, are integrated with graphene, which conforms to the substrate nanotopography. We obtain Raman spectra at 488, 514, 633, and 785 nm excitation wavelengths, spanning the entire visible range. For all excitation wavelengths, the Raman signal of graphene is enhanced by 2–3 orders of magnitude, similarly to the highest enhancements measured to date, concerning surface-enhanced Raman spectroscopy of graphene on plasmonic substrates. Moreover, in contrast to traditional deposition and lithographic methods, the fabrication method employed here relies on single-step, maskless, cost-effective, rapid laser processing of silicon in water, amenable to large-scale fabrication. Finite-difference time-domain simulations elucidate the advantages of the 3D topography of the substrate. Conformation of graphene to Au-decorated silicon nanopillars enables graphene to sample near fields from an increased number of nanoparticles. Due to synergistic effects with the nanopillars, different nanoparticles become more active for different wavelengths and locations on the pillars, providing broad-band enhancement. Nanostructured plasmonic silicon is a promising platform for integration with graphene and other 2D materials, for next-generation applications of large-area hybrid nanomaterials in the fields of sensing, photonics, optoelectronics, and medical diagnostics

Carrier confinement and bond softening in photoexcited bismuth films

Femtosecond pump-probe spectroscopy of bismuth thin films has revealed strong dependencies of reflectivity and phonon frequency on film thickness in the range of 25−40 nm. The reflectivity variations are ascribed to distinct electronic structures originating from strongly varying electronic temperatures and proximity of the film thickness to the optical penetration depth of visible light. The phonon frequency is redshifted by an amount that increases with decreasing film thickness under the same excitation fluence, indicating carrier density-dependent bond softening that increases due to suppressed diffusion of carriers away from the photoexcited region in thin films. The results have significant implications for nonthermal melting of bismuth as well as lattice heating due to inelastic electron-phonon scattering.United States. Office of Naval Research (Grant N00014-12-1-0530)National Science Foundation (U.S.) (Grant CHE-1111557

Broadband wavelength-selective isotype heterojunction n(+)-ZnO/n-Si photodetector with variable polarity

An isotype heterojunction n(+)-ZnO/n-Si photodetector is developed, demonstrating wavelength-selective or broadband operation, depending on the applied bias voltage. Additionally, at self-powered (zero bias) operation, it distinguishes between UV, visible, and near IR (NIR) photons by polarity control of the photocurrent. The photodetector is developed by atomic layer deposition (ALD) of ZnO on n-Si, followed by electric contact deposition and annealing. Photoluminescence measurements reveal high optical quality and improved crystallinity of annealed ZnO on silicon. Photocurrent measurements as a function of illumination wavelength and bias voltage show small negative values in the UV-visible spectral range at zero and positive bias voltage and high positive values in the NIR spectral range. For these measurements, we consider the electric contact to ZnO as the anode and the electric contact to silicon as the cathode. At negative bias voltage, the device shows broadband operation with high photocurrent values across the UV-vis-NIR. (C) 2022 Elsevier B.V. All rights reserved