Alexander I. Grachev

Adaptive interferometer based on wave mixing in a photorefractive crystal under alternating electric field

Coupling of two waves with different polarization states in a photorefractive crystal under an ac electric field allows us to achieve the linear transformation of a small transient phase shift into the intensity modulation without any output polarization filtering. This can lead to design of an adaptive interferometer with sensitivity that reaches the classical homodyne detection limit.

Fast photogalvanic response of a Bi12SiO20 crystal

A photoelectric response of a Bi12SiO20 crystal grown in an argon atmosphere on a linearly polarized light (which is referred to as the linear photogalvanic effect) is reported for the first time in the nanosecond-time domain. Optimal geometry for detection of the photo-induced current concerning the orientation of the polarization state of the incident light in respect to the crystallographic axes of a sample was determined considering both the natural optical activity and light absorption of sillenite crystals. Spectral dependence of the photogalvanic current was measured in the visible part (410 - 610 nm) of the spectrum. Temporal shape of light-induced electric-current pulses observed at different experimental conditions is discussed. Obtained results are believed to show that sillenite crystals are very prospective for development of different ultra-fast optoelectronics devices.

Electric polarization induced by optical orientation of dipolar centers in non-polar piezoelectrics

We predict new nonlinear optical phenomenon, static electric polarization induced in non-polar piezoelectric by linearly polarized light owing to orientation of the impurity centers with permanent dipole moment. Microscopic model of this effect for the crystals of cubic symmetry is proposed. Estimations of the model show that the induced polarization might be much higher than that caused by optical rectification. It is shown that a transient current Jd generated under non-steady-state illumination could be observed at extremely high modulation frequency. We expect that the effect could be applied for ultrafast optical signal processing.

Origin of transient effects in two-wave mixing experiments in BSO crystals

The transient dynamics of the internal space charge field was observed in two-wave mixing experiments in BSO crystals carried out at the wavelength of 633 nm under the external ac electric field. The effects involve practically complete disappearance of the initially strong space-charge field during continuous illumination of a sample by an interference pattern and the retrieval of the significant part of the initial field after the signal beam is switched off. Moreover, the compensated grating can be retrieved even after homogeneous illumination of whole sample without applying external field during many hours. The observations are explained in the frames of the monopolar free-carriers- transport model using the idea of spatial modulation of a sample photoconductivity. The change of the photoconductivity is due to non-equilibrium population of deep traps participating in photoexcitation of free electrons. The model is in accordance with the data of photoconductivity measurements indicating to the transient behavior of the photocurrent in studied BSO samples at the wavelength of 633 nm.

Experimental observation of optical orientation of dipolar centers

We report discovery of recently predicted phenomenon, optical orientation by polar way of local centers with permanent dipole moment. In a crystal of Bi(12)SiO(20) grown in the argon atmosphere electrical current arises when the polarization of incident light is periodically modulated. Dependence of the current amplitude on the modulation frequency allows us to attribute this current to the predicted effect, which is supported by the data on light-induced dichroism and photoconductivity of the sample. A model of donor-acceptor pairs as dipolar centers is shown to be able to explain main peculiarities of optical orientation of dipolar centers in the crystal.

Amplitude autocorrelation of femtosecond laser pulses using linear photogalvanic effect in sillenite crystals

We demonstrate excitation of the linear photogalvanic current in a Bi12TiO20 crystal by two orthogonally polarized femtosecond laser pulses with detecting the electrical current via charge accumulation on the sample electrodes. Such a setup was used to implement an interferometric autocorrelation technique for characterization of ultrashort light pulses. Integration of the detected current in femtosecond time domain leads to vanishing of a bipolar component of the photogalvanic current which arises due to a pulse chirping. The advantage of the proposed technique is that it produces the electric field correlation function directly without the need for data processing using a compact, robust, and non-expensive detector in the form of a photoconductive cell of a non-centrosymmetric crystal.

Manifestation of long-lived photosensitivity gratings in two-wave mixing experiments with sillenite crystals

Transient recording dynamics and hologram fixing and retrieval were observed in photorefractive Bi12SiO20 crystal in two-wave mixing experiments under an ac electric field. These effects were explained by the light- and space-charge-induced spatial modulation of sample photoconductivity originating from the nonequilibrium initial population of relevant photoactive levels. Such a photosensitivity grating exhibits a relaxation time of several hours even when the crystal is illuminated by a spatially uniform light beam. Thus we have shown for what is the first time to our knowledge that optical information can be stored in a photoconductive crystal as a spatially modulated photosensitivity rather than in the form of a space charge.

Quadrature Mode of Photogalvanic Autocorrelation of Ultra-Short Laser Pulses

A new mode of interferometric autocorrelation based on the linear photogalvanic effect in non-centrosymmetric crystals is proposed. It involves simultaneous measurement of two autocorrelation signals being in quadrature to each other with respect to the phase difference (time delay) of recombined laser pulses. This mode eliminates adverse influence of occasional phase difference, light intensity variations, or current saturation on the accuracy of the measurements. Possibility to obtain frequency chirping coefficients by using the quadrature mode of photogalvanic autocorrelation is discussed.

Optical orientation of dipolar centers: Theory, experiment, application

Recently novel nonlinear optical phenomenon, optical orientation of dipolar centers (OODC), was firstly predicted theoretically and then found experimentally. From the phenomenological point of view the OODC-effect can be considered as a kind of optical rectification and consequently observed in any noncentrosymmetric crystals. However, unlike the later it requires existence in a host crystal matrix some impurities or their associates having permanent dipolar moments and taking participation in electronic transitions induced by illumination of the crystal. In this report three aspects of the OODC-effect are discussed: phenomenology and a simple microscopic model of the effect; results of experimental observation of a macroscopic electric polarization in Bi12SiO20 crystal; and possible application of this phenomenon in different fields of ultrafast optoelectronics in particular for generation of ultrashort electric pulses and ultra-broadband terahertz pulses.

Optical orientation of dipolar centers in Bi12SiO20 crystal grown in an oxygen-free atmosphere

In this paper, we report theoretical description and first experimental observation of recently predicted phenomenon called optical orientation of local centers with permanent dipole moment. An electrical current arising at periodical modulation of the polarization of incident light was observed in a crystal of Bi12SiO20 grown in the argon atmosphere. Modulation frequency-dependence of the current amplitude allows us to attribute this current to the predicted effect. This deduction was additionally supported by our experimental data of light-induced dichroism and photoconductivity of the sample. Using a model of donor-acceptor pairs as dipolar centers we were able to explain features of optical orientation of dipolar centers in the crystal.