We describe an improved two-dimensional optical scanning technique combined with an ultrafast Sagnac interferometer for delayed-probe imaging of surface wave propagation. We demonstrate the operation of this system, which involves the use of a single focusing objective, by monitoring surface acoustic wave propagation on opaque substrates with picosecond temporal and micron lateral resolutions. An improvement in the lateral resolution by a factor of 3 is achieved in comparison with previous setups for similar samples.
The recent development of optical technology has enabled the practical use of a carrier-envelope phase-controlled monocycle electric field in the terahertz (THz) regime. By combining this technique with metal nanostructures such as nanotips, which induce near-field enhancement, the development of novel applications is anticipated. In particular, THz scanning tunneling microscopy (THz-STM) is a promising technique for probing ultrafast dynamics with the spatial resolution of STM. However, the modulation of the THz waveform is generally accompanied by an enhancement of the electric field, which is unknown in actual measurement environments. Here, we present a method enabling direct evaluation of the enhanced near field in the tunnel junction in THz-STM in the femtosecond range, which is essential for the use of the THz near field. In the tunneling regime, it was also demonstrated that the transient electronic state excited by an optical pulse can be evaluated using the THz-STM, and the ultrafast carrier dynamics in 2H-MoTe2 excited by an optical pulse was reproducibly probed.
Studying the microscopic behavior of free carriers in materials at an ultrashort time scale is critical to developing semiconductor, optoelectronic, and other technologies satisfying the ever-increasing requirements for smaller sizes and higher speeds. Understanding the effect of local potential modulations and localized states due to nanoscale microstructures on carrier dynamics is essential to realize these requirements. Here, we used time-resolved scanning tunneling microscopy/spectroscopy (STM/STS) combined with a carrier-envelope phase (CEP)-controlled subcycle THz electric field, THz-STM, to probe the ultrafast motion of electrons photoinjected into C 60 multilayer structures grown on Au substrate. We have succeeded in demonstrating the time-resolved measurement of ultrafast electron dynamics with sub-nanoscale spatial resolution and subcycle time resolution for the first time and successfully visualized the electron motion triggered by the spatial variation in the lowest unoccupied molecular orbital (LUMO). The difference in the effects of molecular defects, such as a molecular vacancy and orientational disorder, was also clearly distinguished with single-molecular-level spatial resolution. This method is expected to play an important role in the precise evaluation of local electronic structures and dynamics for the future development of new functional materials and device elements.
Picosecond acoustic-phonon pulse generation and detection is investigated in a sample containing three GaAs/ Al 0.3 Ga 0.7 As quantum wells of different thickness with an interferometric optical pump and probe technique. The pump photon energy is tuned through the hh1-e1 transitions of each well and the probe photon energy is chosen to allow the detection of the phonon pulses at the sample surface. The phonon pulse shapes are explained with a model that relates the carrier wave functions to the acoustic strain, and the acoustic strain to the detected optical reflectance and phase changes.
Electron excitation and relaxation in chromium are probed with 20-fs time resolution using an ultrafast optical technique. We obtain good fits to the data for the transient reflectivity and transmittivity changes in a thin film using a simple model of electron relaxation, suggesting the existence of an efficient electron-electron thermalization process on ultrashort-time scales. Quantitative analysis allows the extraction of thermo-optic coefficients and dielectric constant variations related to both the electron and the lattice temperatures. DOI: 10.1103/PhysRevB.68.113102 PACS number͑s͒: 78.47.ϩp, 78.20.Nv, 78.20.Ci Nonequilibrium electron distributions can be excited in metals with an ultrashort light pulse.1 The subsequent energy exchange between the electrons and the lattice is governed predominantly by the electron-phonon ͑e-p͒ scattering time, but is also affected by the electron-electron ͑e-e͒ scattering time. The noble metals, possessing simple band structures, have provided a fertile testing ground for theories of nonequilibrium electron relaxation and diffusion. [2][3][4][5][6][7][8] It is now possible to investigate the electron dynamics on time scales of the order of the e-e scattering time, typically 10-50 fs for excess electron energies ϳ1 eV. Although in the noble metals and in the alloy CoPt 3 , where the e-p interaction is relatively weak, the evolution of such transient athermal electron distributions was investigated with ϳ20-fs time resolution, 7-9 in other transition metals no studies have been made on these time scales, to the best of our knowledge.The group-VIB transition metals ͑Cr, Mo, W͒ are interesting because they have large values of the e-p coupling constant, resulting in short electron energy relaxation times ϳ200 fs compared to the noble metals ͑ϳ1 ps͒.10 Their band structure is complicated, there being a significant density of states due to d electrons in the region around the Fermi level.11 Understanding the short-time electron dynamics in such metals with strong e-p coupling should become essential for applications in future ultrafast devices with ultrahigh switching speeds. The electron relaxation in thin films of chromium and tungsten under spatially homogeneous conditions was measured by Brorson et al. with optical pulses of duration 60 fs.10 But only the reflectivity change was probed, thus preventing access to the transient dielectric constant. In this report we monitor both the reflectivity and transmittivity changes in a thin polycrystalline film of chromium with 20-fs optical pulses to elucidate the ultrafast dynamics of the electrons therein.The film of Cr on a crown glass substrate is excited and probed from the front side with near Fourier-transformlimited optical pulses from a Ti:sapphire laser ͑Kapteyn-Murmane Labs͒ of central wavelength 790 nm, repetition rate 87 MHz, pulse duration L ϭ20 fs ͓full width at half maximum ͑FWHM͒ intensity͔, and spectral width ϳ50 nm ͑FWHM͒. A schematic diagram of the apparatus is shown in Fig. 1. Interband transitions of electro...
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