S.P. Merkulova

From two-beam surface plasmon interaction to femtosecond surface optics and spectroscopy

Abstract New surface-sensitive method of time-resolved studies is proposed. Frequency mixing is demonstrated in degenerate three-wave mixing with two noncollinear beams of femtosecond laser pulses on a metal grating. The observed enhancement of the nonlinear optical response is connected with the synchronised excitation of two surface plasmon polaritons (SPP) waves, their nonlinear interaction and photon generation with the sum frequency. The dependence of this process versus the temporal delay between two laser pulses and the mutual spatial overlapping of the beams on the grating surface are studied. The possibility of direct measurement of the SPP temporal properties using the suggested method is discussed. The possibility of enhancement of a weak signal from SPP is demonstrated. Experiments of this type opens a possibility of development of femtosecond surface plasmon optics and spectroscopy.

Ultrafast relaxation in YBa2Cu3O7−δ on the femtosecond scale: Luttinger or two-dimensional Fermi liquid picture?

Abstract A method for probing Fermi liquid (FL) versus non-Fermi liquid (NFL) behavior in the normal state of high Tc oxide-superconductors YBa2Cu3O7−δ by femtosecond pump-supercontinuum probe spectroscopy is presented. The method is based on the determination of the spectral dependence of non-equilibrium charge carriers relaxation rate γ(ωprobe) in a wide spectral range of probing, ℏω probe =1.6–3.2 eV . It is shown that the relaxation rate decreases sharply for the optical transitions in the vicinity of the Fermi level EF, the position of the rate minimum pointing to the location of EF. The probe energy dependence of the relaxation rate around the minimum gives unique information on the damping rate of quasi-particles near the Fermi level and hence can resolve the FL versus NFL behavior. Deviations from the FL behavior in YBa2Cu3O7−δ are observed and discussed.

Observation of coherent phonon states in porous silicon films

The dynamics of differential transmission and reflectance spectra of porous silicon films was studied using the femtosecond excitation technique (τ≈50 fs, ℏωpump=2.34 eV) with supercontinuum probing (ℏωprobe=1.6–3.2 eV) and controlled time delay with a step of Δt=7 fs between the pump and probe pulses. A short-lived region of photoinduced bleaching was observed in the differential transmission spectra at wavelengths shorter than the pump wavelength. The excitation of coherent phonon states with a spectrum corresponding to nanocrystalline silicon with an admixture of a disordered phase was observed. The relaxation of electronic excitation was found to slow down in the spectral region where the amplitude of excited coherent vibrations was maximal.

Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells

Abstract Two schemes for steady stimulated phonon generation (saser, i.e., phonon laser ) are proposed. The first scheme exploits a narrow-gap indirect semiconductor or analogous indirect gap semiconductor heterostructure where the tuning into resonance of one-phonon transition of electron–hole recombination can be carried out by external pressure, magnetic or electric fields. The second scheme uses one-phonon transition between direct and indirect exciton levels in coupled quantum wells. The tuning into the resonance of this transition can be accomplished by engineering of dispersion of indirect exciton by external in-plane magnetic and normal electric fields. In the second scheme the magnitude of phonon wave vector is determined by magnitude of in-plane magnetic field and, therefore, such a saser is tunable. Both schemes are analyzed and estimated numerically.

Time-resolved nonlinear surface plasmon optics

A new surface-sensitive method of time-resolved optical studies is proposed. The method consists in the independent excitation of several surface electromagnetic waves (SEW) by two laser femtosecond pulse beams with varied time delay Δτ and distance Δr between corresponding excitation regions on the surface. To fulfill the phase-matching condition for plasmon-photon coupling, metal grating is used. Due to nonlinear plasmon interaction, the optical radiation with ω1 + ω2 and 2ω1 − ω2 (where ω1, ω2 are corresponding laser beam frequencies) is generated. The intensity of this nonlinear response versus Δτ and Δr are studied. The direct measurements of the SEW temporal properties are presented. Experiments of this type are important for the development of femtosecond surface plasmon optics.

Femtosecond spectroscopy of relaxation processes in metals and high-Tc superconductors

Spectral dependences of charge carrier relaxation rates, γe-e(ħω) and γ e-ph(ħω), were observed in Au and Cu films and YBa2Cu3O7-δ high-Tc superconductor films. The relaxation rates decreased substantially in the spectral region corresponding to interband transitions to the Fermi level region (ħωAu = 2.45 eV, ħωCu = 2.15 eV, and ħ ω1 = 1.89 eV and ħω2 = 2.08 eV for YBa2Cu3O7-δ). This relaxation deceleration opens up possibilities for developing a new method, based on the spectral dependences of relaxation rates, for the determination of the Fermi level position and the parameters of electron-electron and electron-phonon interactions on the one hand and for studying deviations from the Fermi-liquid behavior in strongly correlated electronic systems. The linear γe-e (ħω) ∝ |ħω− EF| spectral dependence was observed for a YBa2Cu3O7-δ film near ħω2 = 2.08 eV, which may be evidence of a non-Fermi-liquid behavior of the electronic subsystem.

The possibility of nanolocal reactions on surfaces

Abstract The possibility is considered of modifying the scanning tunnel microscope to bring about local chemical reactions between samples and hydrogen. The microscope uses needles made of metals (Pd, Ti, Nb, etc.) capable of absorbing large amounts of hydrogen. The hydrogen emission rate is estimated to be sufficient for practical applications of the instrument.