Abstract
We report on a quasi-nondegenerate pump–probe technique that is based on spectral-filtration of femtosecond laser pulses by a pair of mutually-spectrally-disjunctive commercially available interference filters. The described technique enables to obtain pump and probe pulses with wavelengths that are spectrally close but distinct. These contradictory requirements, which are dictated, for example, by a suppression of stray pump photons from the probe beam in spin-sensitive magneto-optical experiments in non-magnetic semiconductors, can be fulfilled at very low cost and basically no requirement on space. Especially the second feature is important in pump–probe microscopy where collinear propagation of pump and probe pulses is dictated by utilization of a microscopic objective and where the setups are typically quite complex but suffer from a limited size of optical breadboards. Importantly, this spectral-filtration of 100 fs long laser pulses does not affect considerably the resulting time-resolution, which remains well below 500 fs. We demonstrate the practical applicability of this technique by performing spin-sensitive magnetooptical Kerr effect (MOKE) experiment in GaAs/AlGaAs heterostructure, where a high-mobility spin system is formed after optical injection of electrons at wavelengths close to the MOKE resonance. In particular, we studied the time- and spatial-evolutions of spin-related (MOKE) and charge-related (reflectivity) signals. We revealed that they evolve in a similar but not exactly the same way which we attributed to interplay of several electron many-body effects in GaAs.
Highlights
The pump-probe technique is well established stroboscopic optical method allowing to measure ultrafast dynamical response of various materials [1]
In this paper we show that sub-picosecond time resolution can be achieved in a very simple quasi-nondegenerate pump-probe experiment where an output of a femtosecond oscillator is filtered by a pair of well-chosen mutually-spectrally-disjunctive interference filters
For initial test experiment we used the experimental setup with a non-collinear incidence of pump and probe beams [see Fig. 1(a)] where the separation of pump photons from probe beam can 12 be achieved both by a spatial- and spectral-filtration of the probe beam reflected from the sample. (Note that if we used the setup with a collinear incidence of pump and probe beams, Fig. 1(b), the pump-probe signals without the spectral-filtration could not be measured at all because it would be completely masked by scattered pump photons.) In Fig. 4(b) we show the experimental data measured by these two filtration methods, which demonstrate the very slow damping of the electron spin precession in the studied sample [15,24]
Summary
The pump-probe technique is well established stroboscopic optical method allowing to measure ultrafast dynamical response of various materials [1]. Because nonlinear optical effects are used in a photonic crystal fiber, OPO and OPA, in all these cases the resulting signal-to-noise ratio is usually considerably worse than the one obtained when the fundamental frequency output from the laser is used in the degenerate pump-probe experiment. Another convenient option to obtain synchronized pump and probe pulses at different wavelengths is to select two distinct spectral regions within a spectrum of the femtosecond laser pulse. We demonstrate the utilization of this technique for a measurement of electron spin dynamics at the heterointerface GaAs/AlGaAs
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