M. Kozák

Optical gating and streaking of free electrons with sub-optical cycle precision

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

Nonlinear optical properties of nanocrystalline diamond

We report on investigation of nonlinear optical phenomena in nanocrystalline diamond prepared by microwave plasma enhanced chemical vapour deposition. We observed the upconverted photoluminescence, the second and the third harmonic generation and Z-scan signal. The value of the third order nonlinear susceptibility was estimated. Our results show that nonlinear optical properties of nanocrystalline diamond have many features of the bulk diamond affected to some extent by the presence of grain boundaries.

A miniaturized electron source based on dielectric laser accelerator operation at higher spatial harmonics and a nanotip photoemitter

Here we propose a miniaturized electron source driven by recent experimental results of laser-triggered electron emission from tungsten nanotips and dielectric laser acceleration of sub relativistic electrons with velocities as low as or energies as low as 9.6 keV, less than 20% of the speed of light. The recently observed laser-triggered emission of coherent low-emittance electron pulses from tungsten nanotips naturally lends itself towards incorporation with subrelativistic dielectric laser accelerators (DLAs). These structures have previously been shown to accelerate 28 keV electrons and here we report on the utilization of the 4th and 5th spatial harmonics of near fields in the single grating DLA to achieve acceleration of electrons with kinetic energies of 15.2 and 9.6 keV. We then propose the combination of needle tip emitters with subrelativistic accelerators to form a mm-scale device capable of producing electrons with arbitrary energies.

Large prolongation of free-exciton photoluminescence decay in diamond by two-photon excitation

We report on time-resolved photoluminescence of a free-exciton in IIa chemical vapor deposition diamond crystal. Large difference between decay times for one- and two-photon excitation processes was observed. The longest room-temperature exciton photoluminescence lifetime τ(FE)=220 ns was obtained under two-photon excitation with a photon energy of 4.7 eV. The role of diffusion and surface recombination velocity in exciton photoluminescence dynamics was studied using a new optical method based on two-photon excited time-resolved photoluminescence. The measured room-temperature value of diffusion coefficient in diamond was D=40 cm(2)/s.

Coherent phonon dynamics in micro- and nanocrystalline diamond

We report on the time-resolved coherent anti-Stokes Raman spectroscopy of phonon dephasing in micro- and nanocrystalline diamond films. The dephasing times T(2) were found to be dependent on the morphology of diamond films (average size of crystals and content of nondiamond carbon phase) and changed from 0.7 to 1.72 ps. The dephasing times were found to be temperature independent in the range 10-295 K. In addition to diamond Raman active phonon mode at 1332 cm(-1), we investigated also the dynamics of a broad Raman peak at 1530 cm(-1) which is present in samples with higher content of nondiamond sp(2) hybridized carbon phase. This peak was found to be homogenously broadened with very fast dephasing (T(2)~50 fs).

Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface

We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration elements surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

Influence of boron doping and hydrogen passivation on recombination of photoexcited charge carriers in silicon nanocrystal/SiC multilayers

The influence of boron (B)-doping and remote plasma hydrogen passivation on the photoexcited charge carrier recombination in silicon nanocrystal/SiC multilayers was investigated in detail. The samples were prepared by high temperature annealing of amorphous (intrinsic and B-doped) Si1−xCx/SiC superlattices. The photoluminescence (PL) intensity of samples with B-doped silicon rich carbide layers was found to be up to two orders of magnitude larger and spectrally red shifted in comparison with that of the other samples. Hydrogen passivation leads to an additional increase in PL intensities. The PL decay can be described well by a mono-exponential function with a characteristic decay time of a few microseconds. This behavior agrees well with the picture of localized PL centers (surface states) together with the passivation of non-radiative defects by boron. The samples with B-doped SiC layers exhibit an additional PL band in the green spectral region that is quenched by hydrogen passivation. Its origin is at...

Transverse and longitudinal characterization of electron beams using interaction with optical near-fields

We demonstrate an experimental technique for both transverse and longitudinal characterization of bunched femtosecond free electron beams. The operation principle is based on monitoring of the current of electrons that obtained an energy gain during the interaction with the synchronized optical near-field wave excited by femtosecond laser pulses. The synchronous accelerating/decelerating fields confined to the surface of a silicon nanostructure are characterized using a highly focused sub-relativistic electron beam. Here the transverse spatial resolution of 450 nm and femtosecond temporal resolution of 480 fs (sub-optical-cycle temporal regime is briefly discussed) achievable by this technique are demonstrated.

Hot-carrier transport in diamond controlled by femtosecond laser pulses

A laser-induced transient grating technique with femtosecond temporal resolution was used for the study of hot-carrier diffusion and anisotropy of an ambipolar diffusion coefficient in monocrystalline diamond. A hot-carrier transport regime observed at temperatures below 200 K and excited carrier densities lower than 1016 cm−3 persist in the sample 20–30 ps after photoexcitation. Measured drift velocity of hot carriers was approximately 4–8 times higher compared to thermalized carriers. At low sample temperatures and excited carrier densities, the ambipolar diffusion coefficient was found to be anisotropic between 〈100〉 and 〈110〉 crystallographic directions. We demonstrated experimentally that the carrier energy distribution can be controlled on a sub-picosecond timescale by an additional laser pulse with photon energy below the width of a diamond band gap absorbed by the excited carrier system. Our experimental data were reproduced well by Monte Carlo simulations that confirm the presence of a hot-carrier diffusion regime in diamond.

Hot-phonon-induced indirect absorption in silicon nanocrystals

We studied the nonlinear transient absorption dynamics in silicon nanocrystals/SiO2 superlattices. A different dependence of the measured dynamics on excitation intensity was observed depending on the relative position of the probe photon energy and the absorption edge of nanocrystals. At low photon energies, the dynamics changed with excitation fluence and the signal was ascribed to excited state absorption. However, at photon energies above the absorption edge, the phonon enhancement of indirect absorption prevails over excited state absorption what is reflected in excitation fluence independent dynamics of transient absorption.