Vyacheslav N. Gorshkov

Beam wandering in the atmosphere : The effect of partial coherence

The effect of a random phase screen on laser beam wander in a turbulent atmosphere is studied theoretically. The photon distribution function method is used to describe the photon kinetics of both weak and strong turbulence. By bringing together analytical and numerical calculations, we have obtained the variance of beam centroid deflections caused by scattering on turbulent eddies. It is shown that an artificial distortion of the initial coherence of the radiation can be used to decrease the wandering effect. The physical mechanism responsible for this reduction and the applicability of our approach are discussed.

Suppression of intensity fluctuations in free space high-speed optical communication based on spectral encoding of a partially coherent beam

A new concept of a free space, high-speed (Gbps) optical communication system based on spectral encoding of radiation from a broadband pulsed laser is developed. It is shown that, in combination with the use of partially coherent laser beams and a relatively slow photosensor, scintillations can be suppressed by orders of magnitude for distances of more than 10 km.

Semiclassical Monte-Carlo approach for modelling non-adiabatic dynamics in extended molecules

Modelling of non-adiabatic dynamics in extended molecular systems and solids is a next frontier of atomistic electronic structure theory. The underlying numerical algorithms should operate only with a few quantities (that can be efficiently obtained from quantum chemistry), provide a controlled approximation (which can be systematically improved) and capture important phenomena such as branching (multiple products), detailed balance and evolution of electronic coherences. Here we propose a new algorithm based on Monte-Carlo sampling of classical trajectories, which satisfies the above requirements and provides a general framework for existing surface hopping methods for non-adiabatic dynamics simulations. In particular, our algorithm can be viewed as a post-processing technique for analysing numerical results obtained from the conventional surface hopping approaches. Presented numerical tests for several model problems demonstrate efficiency and accuracy of the new method.

Magnetic resonance force microscopy and a single-spin measurement

Spin Dynamics -- Quasiclassical Description Spin Dynamics -- Quantum Description Mechanical Vibrations of the Cantilever Single-Spin Detection in Magnetic Force Microscopy (MFM) Transient Process in MFM -- The Exact Solution of the Master Equation Periodic Spin Reversals in Magnetic Resonance Force Microscopy (MRFM) Driven by -Pulses Oscillating Adiabatic Spin Reversals Driven by the Frequency Modulated rf Field Oscillating Cantilever-Driven Adiabatic Reversals (OSCAR) Technique in MRFM CT-Spin Dynamics in the OSCAR Technique Magnetic Noise and Spin Relaxation in OSCAR MRFM MRFM Applications: Measurement of an Entangled Spin State and Quantum Computation MRFM Techniques and Spin Diffusion.

Spin relaxation caused by thermal excitations of high-frequency modes of cantilever vibrations

We consider the process of spin relaxation in the oscillating cantilever-driven adiabatic reversals technique in magnetic-resonance force microscopy. We simulated the spin relaxation caused by thermal excitations of the high-frequency cantilever modes in the region of the Rabi frequency of the spin subsystem. The minimum relaxation time obtained in our simulations is greater than but of the same order of magnitude as one measured in recent experiments. We demonstrated that using a cantilever with nonuniform cross-sectional area may significantly increase spin-relaxation time.

Scintillation reduction for laser beams propagating through turbulent atmosphere

We numerically examine the spatial evolution of the structure of coherent and partially coherent laser beams, including the optical vortices, propagating in turbulent atmospheres. The influence of beam fragmentation and wandering relative to the axis of propagation (z-axis) on the value of the scintillation index (SI) of the signal at the detector is analyzed. These studies were performed for different dimensions of the detector, distances of propagation, and strengths of the atmospheric turbulence. Methods for significantly reducing the scintillation index are described. These methods utilize averaging of the signal at the detector over a set of partially coherent beams (PCBs). It is demonstrated that the most effective approach is using a set of PCBs with definite initial directions of propagation relative to the z-axis. This approach results in a significant compensation of the beam wandering which in many cases is the main contributor to the SI. A novel method is to generate the PCBs by combining two laser beams - Gaussian and vortex beams, with different frequencies (the difference between these two frequencies being significantly smaller than the frequencies themselves). In this case, the effective suppression of the SI does not require high-frequency modulators. This result is important for achieving gigabit data-rates in long-distance laser communication through turbulent atmospheres.

Semiclassical Monte Carlo: A first principles approach to non-adiabatic molecular dynamics

Modeling the dynamics of photophysical and (photo)chemical reactions in extended molecular systems is a new frontier for quantum chemistry. Many dynamical phenomena, such as intersystem crossing, non-radiative relaxation, and charge and energy transfer, require a non-adiabatic description which incorporate transitions between electronic states. Additionally, these dynamics are often highly sensitive to quantum coherences and interference effects. Several methods exist to simulate non-adiabatic dynamics; however, they are typically either too expensive to be applied to large molecular systems (10s-100s of atoms), or they are based on ad hoc schemes which may include severe approximations due to inconsistencies in classical and quantum mechanics. We present, in detail, an algorithm based on Monte Carlo sampling of the semiclassical time-dependent wavefunction that involves running simple surface hopping dynamics, followed by a post-processing step which adds little cost. The method requires only a few quantities from quantum chemistry calculations, can systematically be improved, and provides excellent agreement with exact quantum mechanical results. Here we show excellent agreement with exact solutions for scattering results of standard test problems. Additionally, we find that convergence of the wavefunction is controlled by complex valued phase factors, the size of the non-adiabatic coupling region, and the choice of sampling function. These results help in determining the range of applicability of the method, and provide a starting point for further improvement.

Reduction of laser intensity scintillations in turbulent atmospheres using time averaging of a partially coherent beam

We demonstrate experimentally and numerically that the application of a partially coherent beam (PCB) in combination with time averaging leads to a significant reduction in the scintillation index. We use a simplified experimental approach in which the atmospheric turbulence is simulated by a phase diffuser. The role of the speckle size, the amplitude of the phase modulation, and the strength of the atmospheric turbulence are examined. We obtain good agreement between our numerical simulations and our experimental results. This study provides a useful foundation for future applications of PCB-based methods of scintillation reduction in physical atmospheres.

Random spin signal in magnetic resonance force microscopy

Abstract We study a random magnetic resonance force microscopy (MRFM) signal caused by the thermal vibrations of high frequency cantilever modes in the oscillating cantilever-driven adiabatic reversals (OSCAR) technique. We show that the regular MRFM signal with a characteristic decay time, τm, is followed by a non-dissipative random signal with a characteristic time τr. We present the estimates for the values of τm and τr. We argue that this random MRFM signal can be used for spin detection. It has a “signature” of a sharp peak in its Fourier spectrum.

Uncooled infrared and terahertz detectors based on micromechanical mirror as a radiation pressure sensor

We consider midinfrared (5 - 25 μm) and terahertz (100 - 1000 μm), room-temperature detectors based on a microcantilever/micromirror sensor of the radiation pressure. The significant enhancement of sensitivity is due the combination of non-absorption detection and a high quality optical microcavity. Applications for spectrometry and imaging are analyzed. It is shown that the radiation pressure sensor potentially has sensitivity at the level of or better than the best conventional uncooled detectors.