Sergei M. Pimenov

Investigation of charge transfer in Au nanoparticle–ZnO nanosheet composite photocatalysts

Ohmic contact formation at the interface of the Au nanoparticle (NP)-ZnO nanosheet (NS), which facilitates photoelectron transfer from ZnO NSs to Au NPs, is determined by scanning Kelvin microscopy for the first time. Reduction of charge recombination in the ZnO NSs confirmed by the quench of green band emission results in the enhancement of photocatalytic activity of the Au NP-ZnO NS composite.

Picosecond-laser bulk modification induced enhancement of nitrogen-vacancy luminescence in diamond

We report on the enhancement of nitrogen-vacancy (NV) luminescence induced by the bulk structure modification of a type IIa single-crystal diamond with a visible picosecond laser. Using online monitoring of picosecond-laser-induced photoluminescence (PL) in the bulk regions we found that the integrated intensity of the NV PL is significantly increased after the bulk microstructure formation. The confocal PL spectroscopy investigations of the bulk microstructures have evidenced the enhanced NV luminescence and the splitting of the NV− emission. The increased concentration of the NV defects during picosecond-laser bulk modification correlates with the formation of sp3 carbon allotropic structures and other defect centers revealed in PL/Raman spectra of high-stress regions near the fabricated microstructures.

Picosecond-laser bulk modification, luminescence and Raman lasing in single-crystal diamond

Bulk modification and micro-structuring of diamonds using ultra-short laser pulses is of great interest due to its potential in photonic applications, radiation detectors and diamond gem marking. We report on bulk micro-structuring and stimulated Raman scattering (SRS) in type IIa single crystal diamond with multi pulse irradiation by picosecond-laser pulses at the wavelength 532nm (10ps & 44ps). The experiment was expanded by additional setups for on-line video imaging and spectroscopic measurements during laser irradiation and structure growth in the bulk diamond from the backside of the crystal. We discuss the influence of the crystal orientation ({100} and {110}) relative to the laser beam onto (i) the optical breakdown threshold, (ii) the character of the structural modifications and (iii) generation of SRS during irradiation. We show that the formation of bulk microstructures dramatically influences the behavior of the SRS and that the structure growth and the laser-induced breakdown in the bulk are governed by the dielectric breakdown mechanism. We will further present the conditions for efficient SRS lasing depending on the different pulse durations. Based on the Stokes-to-anti-Stokes intensity ratio in the recorded SRS spectra we will finally propose a method of local temperature measurements in the bulk of diamond to determine the “pre-breakdown” temperature.

Machining of semiconductors and dielectrics with ultra-short pulses: Influence of the wavelength and pulse bursts (Conference Presentation)

In the burst mode the reported removal rates were often higher than the ones achieved with single pulses at identical repetition rate and average power. But this effect is mainly caused by the reduced energy per single pulse in the burst and the corresponding fluence which is then nearer its optimum value showing highest specific removal rate. But there exist special situations where the burst mode shows a higher efficiency and therefore an increased specific removal rate. For copper e.g. it was found that a 3-pulse burst with a time spacing of 12 ns at a wavelength of 1064 nm leads to an about 15% higher specific removal rate. We extended the burst investigations to semiconductors and isolators and measured the specific removal rate as a function of the applied peak fluence for different materials, number of pulses in the burst and time spacing. For 1064 nm silicon e.g. shows a maximum specific removal rate which amounts about 1.7 µm3/µJ for single pulses and a 2 pulse burst as well. Then it almost linearly increases up to about 5 µm3/µJ when the number of pulses in the burst is raised to 8. A similar effect is found for machining grooves into diamond-like nanocomposite films with single pulses and a 2- and 3-pulse burst, respectively. In contrast, for silicon and 532 nm wavelength where the photon energy exceeds the bandgap, only a small difference in the maximum specific removal was observed. Heat accumulation is assumed to cause the higher specific removal rates but further experiments are needed gain a clearer picture.

Single cycle THz pulses in 1D and 2D photonic crystal structures

We present coherent time-resolved near-field imaging of single-cycle THz pulses in 1D and 2D photonic crystals. The results agree well with simulations and reveal the bandgaps and the dispersive properties of the photonic structures.