V. N. Gridnev

Spontaneous non-reciprocal reflection of light from antiferromagnetic Cr2O3

The reflection of light from a medium with ordered spin structure characterized by the breakdown of time-reversal and parity symmetry is expected to be non-reciprocal even if the net magnetic moment of the medium equals zero. We report on the first experimental observation of spontaneous non-reciprocal rotation and circular dichroism in spin magnetoelectric Cr2O3. Non-reciprocal effects were observed below the antiferromagnetic transition temperature TN=307 K and their temperature behaviour roughly corresponds to that of the order parameter. Observed values of (1-4)*10-4 for the magnetoelectric susceptibility in the optical range are several orders of magnitude higher than predicted earlier. This increase of the susceptibility is presumably attributable to electronic dipole transitions in the optical range.

Theory of Faraday rotation beats in quantum wells with large spin splitting

Spin dynamics of conduction electrons in a quantum well with a zinc blende structure is considered theoretically for the case where spin splitting exceeds the collisional broadening of energy levels. It is shown that, under certain conditions, the spin density component normal to the quantum well plane may oscillate with time even in the absence of an external magnetic field. These oscillations can be excited and detected using nonlinear two-pulse spectroscopy. Contrary to the case of small spin splitting, the external transverse magnetic field strongly affects spin dynamics in this regime.

Magnetization-odd nonreciprocal reflection of light from the magnetoelectric—ferromagnet LiFe5O8

Circular dichroism, which changes sign under magnetization reversal and exhibits a 120° periodicity, is observed in the reflection of light from the (111) plane of a noncentrosymmetric LiFe5O8 crystal in the transverse geometry k⊥M, where the linear Kerr effect is forbidden. It is shown that this phenomenon is due to the manifestation of optical magnetoelectric susceptibility and a Kerr effect of third order in the magnetization. The spectral dependences of the circular dichroism in the range 1.4–3.1 eV show that this phenomenon is of a resonance character.

Spin dynamics of semiconductor electrons in a hybrid ferromagnet/semiconductor structure

We analyze the recent experimental study by R.J. Epstein et al. [Phys. Rev. B 65, 121202 (2002)] on the spin dynamics of semiconductor electrons in a hybrid ferromagnet/semiconductor structure by using a simple model based on the Bloch equations. A comparison between the model calculations and the experimental observations shows that the spin relaxation rate is strongly anisotropic. We interpret this anisotropy as a manifestation of the exchange interaction between metallic and semiconductor electrons at the ferromagnet/semiconductor interface.

Dispersion of the Voigt effect in the magnetic semiconductors Cd1−xMnxTe

Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd1−xMnxTe (0≤x≤0.52) in order to investigate how the exchange interaction of Mn2+ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x2 is independent of x and the magnetic field direction, i.e., the effect is due to the Mn2+ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(Eg−ℏω) −3.5. This can be explained by a decrease of the exchange interaction of Mn2+ ions with itinerant electrons with increasing distance from the center of the Brillouin zone.

Coherence-mediated laser control of exciton and trion spins in CdTe/CdMgTe quantum wells studied by the magneto-optical Kerr effect

Two temporally non-overlapping linearly cross-polarized 140 fs laser pulses are shown to control the spin polarization in a three-level system. Simultaneous excitation of the two excited states triggers quantum beatings originating from the interference of the wavefunctions corresponding to different spin sublevels of the states. Although the beatings are not seen in the spin densities of the excited states they are clearly observed in the magneto-optical Kerr effect. An analytical expression for the description of the beatings is obtained. Experimental results are in good agreement with theoretical predictions and demonstrate the control of beatings with attosecond resolution.

Anisotropic spin diffusion in a semiconductor quantum well

We show that spin diffusion of an inhomogeneous spin-density distribution in an asymmetric zinc-blende semiconductor quantum well is anisotropic in coordinate space, if the D’yakonov–Perel’spin-relaxation mechanism is dominant. This anisotropy depends on the relation between the Dresselhaus and Rashba contributions to the spin splitting and reaches its maximum when both contributions are equal in magnitude. Under this condition, the temporal behavior of spin density strongly depends on the relation between the initial spatial extent of the spin packet and spin diffusion length.

Optical control of the coherent dynamics of excitons in a semiconductor quantum well

The dynamics of the coherent superposition of exciton states in a semiconductor quantum well excited by the joint action of two successive light pulses with the time interval between them t12 has been theoretically studied. The dependences of the amplitude and initial phase of the quantum bits on t12 have been found. It has been shown that the time dependence of the Kerr rotation of the reflected probe pulse is determined not only by the dynamics of the excited state, but also by the optical properties of the heterostructure containing the quantum well. The dependence of the decay rate of the excited state on t12 has been predicted.

Low-frequency behavior of optical spatial dispersion effects

A theoretical explanation is given for the frequency independence of the nonreciprocal birefringence of light, which was recently observed in the semiconductors Cd1−xMnxTe, Zn1−xMnxTe, and GaAs in the frequency range below the frequency corresponding to the interband absorption edge. It is shown that the symmetry of the effect becomes higher at such frequencies if the light-induced excitation energy ℏωn(k) only slightly depends on the photon momentum k. In this case, the nonreciprocal birefringence is completely determined by the second-rank magnetoelectric tensor. It is shown that the nonreciprocal birefringence of light can be observed in magnetic media with a tensor order parameter.

Sub-Picosecond Dynamics of the Photo-Induced Magneto-Optical Kerr Effect in CdTe at Room Temperature

A time-resolved, pump and probe technique based on the photo-induced magneto-optical Kerr effect is applied to the spectral study of the room temperature spin dynamics in CdTe between 1.44 and 1.63 eV. Two relaxation processes with respective decay times of up to 200 fs and 2.5 ps are distinguished. A change of the spectral dependence regime of the Kerr effect during the faster relaxation is observed and discussed.