Optical physics

Coherent-population-trapping resonances within the degenerate two-level system of the F=2→F'=1 transition of the 87 Rb D1 line were investigated in an uncoated Rb vapor cell by means of level-crossing-type experiments. Tuning over the two-photon resonance is achieved sweeping a magnetic field around zero value. The influence of transverse magnetic fields on the amplitude and the width of the resonances, recorded in fluorescence and absorption, were investigated in the cases of excitation with linear, circular, and elliptical laser light polarization. A theoretical analysis was performed for the case of linearly polarized excitation, the results of which are in good agreement with the experiment.

We describe in detail a general strategy for implementing a conditional geometric phase between two spins. Combined with single-spin operations, this simple operation is a universal gate for quantum computation, in that any unitary transformation can be implemented with arbitrary precision using only single-spin operations and conditional phase shifts. Thus quantum geometrical phases can form the basis of any quantum computation. Moreover, as the induced conditional phase depends only on the geometry of the paths executed by the spins it is resilient to certain types of errors and offers the potential of a naturally fault-tolerant way of performing quantum computation.

We present both a computational and an experimental approach to the problem of biological aerosol characterization, joining the expertises reached in the field of theoretical optical scattering by complex, arbitrary shaped particles (multipole expansion of the electromagnetic fields ...

In this paper the two-photon absorption by a molecule is studied when photons are prepared in a high-dimension entangled state. The light field is prepared in a spatial two-photon qudit state and its interaction with a molecule shows new interference effects observed in the calculated absorption cross-section. Oscillations in the absorption cross-section demonstrate its dependence on the path phases of the two-qudit state. The two-photon absorption cross-section is dependent on the dimension of the two-qudit photonic state.

We have studied the effects of random laser speckle and self-mixing interference on TDLS based gas measurements made using integrating spheres. Details of the theory and TDLS apparatus are given in Part 1 of this paper and applied here to integrating spheres. Experiments have been performed using two commercial integrating spheres with diameters of 50 mm and 100 mm for the detection of methane at 1651 nm. We have calculated the expected levels of laser speckle related uncertainty, considered to be the fundamental limiting noise, and imaged subjective laser speckle in a sphere using different sized apertures. For wavelength modulation spectroscopy, noise equivalent absorbances (NEAs) of around 5×10−5 were demonstrated in both cases, corresponding to limits of detection of 1.2 ppm methane and 0.4 ppm methane respectively. Longer-term drift was found to be at an NEA of 4×10−4. This lies within our broad range of expectations. For a direct spectral scan with no wavelength dither, a limit of detection of 75 ppm or fractional measured power uncertainty of 3×10−3 corresponded well with our prediction for the objective speckle uncertainty.

A nonlinear image technique for characterization of the optical nonlinearities is used to investigate the solid semiconductor ZnSe at 600 nm. The method based on a 4f nonlinear image technique with a phase object is used to obtain the diffraction pattern of the nonlinear filter in solid ZnSe located at the Fourier plane by a CCD camera. The nonlinear absorption coefficient and nonlinear refraction index were both obtained by fitting the nonlinear image. Good agreement between the experiment data and the simulated result are obtained indicating a sensitive and powerful method for nonlinear optical measurements.

This paper deals with the theoretical analysis of ultra refraction properties of slab photonic crystals. Strong angular dispersions induced by wavelength variations have been experimentally observed near the Γ–M and Γ–K directions around λ = 1320 nm with negative and positive signs, respectively. An interpretation of these phenomena using three-dimensional plane wave method (3D-PWM) and finite-difference time-domain (3D-FDTD) calculations is presented. Equifrequency surfaces are obtained from PWM calculations and group velocities of Bloch waves excited in the patterned medium are predicted as a function of the incident wave wavelength and wavevector orientation. FDTD calculations have been performed to evaluate the influence of the finite size of the structure and to estimate the photonic crystal coupling losses for various configurations. This theoretical approach allows identifying Bloch waves responsible for the observed properties.