Pasi Vahimaa

Partially coherent Gaussian pulses

The concept of a plane-wave pulse with a Gaussian spectrum and a Gaussian distribution of correlations between different frequency components is introduced. The temporal coherence properties of such a pulse are related to the spectral coherence properties and equivalence relations between pulses with different spectral and temporal correlation parameters are established.

Theory of spatially and spectrally partially coherent pulses

A coherent-mode representation for spatially and spectrally partially coherent pulses is derived both in the space-frequency domain and in the space-time domain. It is shown that both the cross-spectral density and the mutual coherence function of partially coherent pulses can be expressed as a sum of spatially and spectrally and temporally completely coherent modes. The concept of the effective degree of coherence for nonstationary fields is introduced. As an application of the theory, the propagation of Gaussian Schell-model pulsed beams in the space-frequency domain is considered and their coherent-mode representation is presented.

Optical activity in subwavelength-period arrays of chiral metallic particles

We report on a chirality-induced polarization effect in a planar subwavelength metallic nanograting. We demonstrate that the grating rotates the polarization at normal incidence. Because of the fourfold rotation symmetry, the effect does not depend on the incident beam polarization, but resembles optical activity in isotropic media. We use rigorous diffraction theory to show that polarization effects in the zeroth diffraction order take place in the presence of waveguide resonances with subwavelength-period arrays of chiral metallic particles.

Spectral coherence properties of temporally modulated stationary light sources

It is shown that partially spectrally coherent pulses of light with controlled spectral coherence properties can be generated by temporal modulation of beams emitted by stationary light sources. A method for generation of spectrally Gaussian Schell-model-type pulses is presented.

LINEAR AND SECOND-ORDER NONLINEAR OPTICAL PROPERTIES OF ARRAYS OF NONCENTROSYMMETRIC GOLD NANOPARTICLES

The optical properties of nanoscopic arrays of metal particles are dominated by plasmon resonances and electromagnetic interaction between the particles. We use electron-beam lithography to prepare arrays of noncentrosymmetric gold particles and study their linear and second-order nonlinear optical properties. By varying the orientation of the particles in a fixed lattice, we observe shifts in the polarized linear extinction spectra. The second-harmonic generation efficiencies of the two types of samples differ by up to 60%. The results show that the properties of the samples are sensitive to the smallest details of their structure.

Refractive index and extinction coefficient dependence of thin Al and Ir films on deposition technique and thickness

We show that the optical properties of thin metallic films depend on the thickness of the film as well as on the deposition technique. Several thicknesses of electron-beam-gun-evaporated aluminium films were measured and the refractive index and the extinction coefficient defined using ellipsometry. In addition, the refractive indexes and the extinction coefficients of atomic-layer-deposited iridium were compared with those of evaporated iridium samples.

Finite-elementary-source model for partially coherent radiation.

An efficient method is presented for propagating light emitted by a wide class of spatially partially coherent sources. This class includes all quasi homogeneous sources with slowly varying intensity distributions in comparison to their spatial coherence areas. The method is based on the construction of a set of individually coherent but mutually uncorrelated, identical, laterally shifted elementary sources of finite extent.

Overall coherence and coherent-mode expansion of spectrally partially coherent plane-wave pulses.

The modal theory for spectrally partially coherent nonstationary plane waves is introduced. The theory is first developed in the space-frequency domain and then extended to the space-time domain. Propagation properties of the coherent modes are analyzed. The concept of the overall degree of coherence is extended to the domain of nonstationary fields, and it is shown that the overall degree of coherence of partially coherent plane-wave pulses is the same in the space-frequency and space-time domains. The theory is applied to the recently introduced concept of spectrally Gaussian Schell-model plane-wave pulses.

Strong enhancement of second-harmonic generation in all-dielectric resonant waveguide grating

The authors demonstrate that a dielectric resonant waveguide grating can enhance optical second-harmonic generation by a factor of 550 compared to a similar flat surface. Their structure, which consists of purely dielectric and thereby transparent materials, has a low index silicon dioxide grating covered by a high index titanium dioxide layer and it is optimized for the fundamental wavelength of 1064nm. Polarization dependence of the second-harmonic response suggests that the enhancement arises from the favorable interaction of the resonant waveguide mode and its strong local field with the inherent surface nonlinearity of the dielectric materials.

Unified measures for optical fields: degree of polarization and effective degree of coherence

The connection between the degree of polarization and the effective degree of coherence is established. It is shown that from the formal point of view, the degree of polarization is a quantity analogous to the degree of coherence for scalar fields, provided that the coherence properties are examined as an average in some given domain. It is also demonstrated that the effective degree of coherence has a geometrical interpretation as the distance between the given mixed state and the incoherent state, normalized by the maximum distance in the geometry examined. The meaning of an analogous quantity in quantum optics is briefly discussed.