Ultrafast time resolved reflection high energy electron diffraction with tilted pump pulse fronts

Abstract

Abstract. We present time-resolved RHEED from a laser excited Pb(111) surface using a pulse front tilter for the compensation of the velocity mismatch of electrons and light. The laser pulses with tilted fronts were characterized by a spatially resolving cross correlator. The response of the surface upon excitation was observed to be less than 2 ps.Ultrafast electron diffraction is a promising technique for studying the dynamics of the atomic structure on a sub-picosecond time scale. Time resolved transmission electron diffraction (TR-TED) has been used, for example, to study bulk phenomena such as ultrafast laser-induced structural phase transitions [1], cooperative rearrangement of crystalline structures [2], and dynamics of charge density waves [3]. Time resolution of less than 100 fs [4] has been achieved. Time resolved reflection high energy electron diffraction (TR-RHEED), on the other hand, has been used for the investigation of ultrafast surface dynamics, for example the observation of ultrafast heating of surfaces [5], the heat transfer in hetero structures [6] and laser-induced phase transition of the surface reconstructions [7]. However, the time resolution in these experiments was limited to tens of picoseconds because of the different velocities of electrons and light in combination with the grazing angle of incidence of the electron beam in RHEED geometry.This velocity mismatch can be overcome and the time resolution significantly improved by tilting the laser pulse front with respect to the phase fronts. Baum et al. [8] have demonstrated a configuration for pulse front tilting and achieved an improvement of the temporal resolution to less than one picosecond. Here we report the use of a 4f zero dispersion delay line capable of matching the pulse front of the laser to the electron pulse. The improvement of the temporal resolution is demonstrated by measuring the ultrafast lattice heating of Pb(111) islands on a Si(111) surface.Generally speaking, when a laser pulse encounters angular dispersion, the planes of constant amplitude (intensity) will be tilted with respect to the planes of constant phase, for example when the pulse is reflected from a diffraction grating [9]. The pulse then broadens during the free propagation after the dispersive component. The broadening is reversed and the original pulse width restored by passing the pulse through the two lenses in a 4f configuration. Our pulse tilter was designed to compensate the velocity mismatch for 30 keV electrons. Based on the results of our electron source [10] a temporal resolution of better than 700 fs is expected.In order to independently control and characterize the tilted pulse we used a cross-correlator based on second harmonic generation (SHG). The special feature of this cross-correlator is the possibility to perform spatially resolved measurements of the cross-correlation function between the This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, dis...

Ultrafast electron diffraction from a Bi(111) surface: Impulsive lattice excitation and Debye–Waller analysis at large momentum transfer

“…To compensate this effect and to ensure constant time delays across the sample, the pumping laser pulse intensity front is tilted 30 by an angle of 71° as described in detail in Ref. 31.…”

Non-equilibrium lattice dynamics of one-dimensional In chains on Si(111) upon ultrafast optical excitation

“…For the benefit of an improved signal-to-noise ratio, the number of electrons in the probe pulse was increased at the expense of a slightly reduced temporal resolution of about 1 ps. The incident pump fluence $\Phi$ was adjusted between 0.7 mJ/cm 2 and 6.5 mJ/cm 2 by means of a continuously rotatable λ /2-waveplate in combination with the grating of the pulse front tilter 30 . The pump beam diameter had a width of 8 mm which is much larger than the sample width of 2 mm and ensures a homogeneous excitation of the entire probed sample area.…”

Relaxation of electrons in quantum-confined states in Pb/Si(111) thin films from master equation with first-principles-derived rates

“…The excitation of low-lying Pb modes results in irregular displacements of the atomic positions on medium to large length scale that can be followed in the Debye-Waller factor of a time-resolved electron diffraction experiment. Preliminary experimental data for Pb films [47] indicate that the low-energy phonons are indeed getting excited on the time scale of a few picoseconds. Interestingly, recent diffraction experiments were able demonstrate the mode-selective energy transfer to phonons even for bulk materials such as aluminum [48] or nickel [49].…”