P. Malý

Experimental observation of the optical spin transfer torque

Spin transfer torque—the transfer of angular momentum from a spin-polarized current to a ferromagnet’s magnetization—has already found commercial application in memory devices, but the underlying physics is still not fully understood. Researchers now demonstrate the crucial role played by the polarization of the laser light that generates the current; a subtle effect only evident when isolated from other influences such as heating.

Temperature study of trap-related photoluminescence decay in CdSxSe1−x nanocrystals in glass

The trap-related photoluminescence dynamics in CdSxSe1−x nanocrystals (for x=0.24 and x=0.74) in glass were studied in the temperature interval 10–300 K. A close link between the temperature behavior of the photoluminescence decay rate and that of the photoluminescence efficiency was found, which indicates a dominant role of nonradiative recombination. The trap-related photoluminescence was interpreted as the recombination of a shallowly trapped electron with a hole in a deep trap. An exponential decrease (with a constant in the range 0.2–0.4 eV) in the density of the deep traps with increasing energy above the valence band was found. Apart from the Arrhenius type of temperature behavior of decay rate with the activation energy ≈50 meV, the Berthelot type (characteristic temperature ≈160 K) was also observed. The latter was modeled by carrier tunneling between localized sites and the energetic distribution of the tunneling distances was obtained.

Ammonia-free method for preparation of CdS nanocrystalline films by chemical bath deposition technique

The preparation of thin films of CdS by chemical bath deposition is mostly based on the utilisation of ammonia as a complexing agent for cadmium ions. Here we report on a new method based on potassium nitrilotriacetate that eliminates the problems of ammonia volatility and toxicity. We prepared high quality films of closely packed nanocrystals with a thickness of up to 300 nm. The radius of the nanocrystals can be tuned from 2.6 to more than 10 nm by post-preparation heat treatment of the as-prepared films.

The essential role of carefully optimized synthesis for elucidating intrinsic material properties of (Ga,Mn)As

(Ga,Mn)As is at the forefront of spintronics research exploring the synergy of ferromagnetism with the physics and the technology of semiconductors. However, the electronic structure of this model spintronics material has been debated and the systematic and reproducible control of the basic micromagnetic parameters and semiconducting doping trends has not been established. Here we show that seemingly small departures from the individually optimized synthesis protocols yield non-systematic doping trends, extrinsic charge and moment compensation, and inhomogeneities that conceal intrinsic properties of (Ga,Mn)As. On the other hand, we demonstrate reproducible, well controlled and microscopically understood semiconducting doping trends and micromagnetic parameters in our series of carefully optimized epilayers. Hand-in-hand with the optimization of the material synthesis, we have developed experimental capabilities based on the magneto-optical pump-and-probe method that allowed us to simultaneously determine the magnetic anisotropy, Gilbert damping and spin stiffness constants from one consistent set of measured data.

Ultrafast photoluminescence in silicon nanocrystals studied by femtosecond up-conversion technique

Photoluminescence dynamics in silicon nanocrystals measured by a femtosecond up-conversion technique are reported. The samples were prepared by embedding porous silicon grains in a sol-gel derived SiO2 matrix. Efficient initial relaxation of the excess energy of photoexcited carriers with the effective rate ⩾3.8eV∕ps was observed. A fast decay component (400fs) of the photoluminescence signal was found and interpreted in terms of quenching the interior exciton radiative recombination by carrier trapping on the nanocrystal surface. The ultrafast photoluminescence dynamics are followed by a microsecond decay of the stretched-exponential type.

Light-controlled growth of CdSe nanocrystalline films prepared by chemical deposition

Abstract We report on preparation of high quality thin films of CdSe nanocrystals by chemical solution deposition technique. The samples characterized by optical spectroscopy have the nanocrystal radii 1.9–10 nm. We demonstrated that the sizes of nanocrystals could be controlled by changing the intensity and/or the spectrum of light illumination of growing film.

Nanocrystalline titanium dioxide films: Influence of ambient conditions on surface- and volume-related photoluminescence

We report on systematic study of photoluminescence properties of nanocrystalline titanium dioxide films consisting of predominantly anatase nanoparticles with diameters larger than 13 nm. We measured photoluminescence under two selected excitation wavelengths (325 nm/442 nm, i.e., above/below band gap excitation), under different values of ambient air pressure (5–105 Pa), and in the temperature interval 10–300 K. On basis of our results, we are able to distinguish two different processes leading to photoluminescence: the exciton recombination and the recombination of carriers in the energy states related to the surface of nanocrystals. We propose microscopic models describing well the effects of temperature and ambient conditions on photoluminescence of nanocrystalline titanium dioxide films.

Light-induced magnetization precession in GaMnAs

We report the dynamics of the transient polar Kerr rotation (KR) and of the transient reflectivity induced by femtosecond laser pulses in ferromagnetic (Ga,Mn)As with no external magnetic field applied. It is shown that the measured KR signal consists of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in (Ga,Mn)As. The origin of the light-induced magnetization precession is discussed and the magnetization precession damping (Gilbert damping) is found to be strongly influenced by annealing of the sample.

Systematic Study of Mn-Doping Trends in Optical Properties of (Ga,Mn)As

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible Mn(Ga) dopings. The material synthesis was optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blueshift for increasing Mn doping. In the visible range, a peak in the magnetic circular dichroism also shifts with increasing Mn doping. The results are consistent with the description of ferromagnetic (Ga,Mn)As based on the microscopic valence band theory. They also imply that opposite trends seen previously in the optical data on a limited number of samples are not generic and cannot serve as an experimental basis for postulating the impurity band model of ferromagnetic (Ga,Mn)As.

Transmission study of picosecond photocarrier dynamics in free-standing porous silicon

Abstract Pump and probe technique has been used to study time-resolved differential absorbance of free-standing porous silicon at room temperature. The second (532 nm) harmonics of a mode-locked Nd 3+ :YAG picosecond laser served as pump pulses and the fundamental frequency (1.06 μm) were used as probe pulses. Photocarrier dynamics shows a fast decay in time domain of hundred picoseconds. A model of bimolecular radiative recombination is suggested, the values of bimolecular radiative recombination coefficient B and excited carrier cross section σ have been found to be B = 4 × 10 −10 cm 3 s −1 and σ = 9 × 10 −18 cm 2 , very close to the values reported for direct gap semiconductors.