Hanna Lajunen

Propagation characteristics of partially coherent beams with spatially varying correlations.

We introduce a class of partially coherent beams with spatially varying correlation properties. It is shown that mathematically simple modifications in the coherence function of conventional Gaussian Schell-model beams lead to partially coherent fields with extraordinary free-space propagation characteristics, such as locally sharpened and laterally shifted intensity maxima. We study the properties of such fields based on an elementary-mode interpretation and by numerical simulations. The results demonstrate the potential of coherence modulation for beam shaping applications.

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.

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.

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.

Non-uniformly correlated partially coherent pulses

We consider partially coherent plane-wave pulses with non-uniform correlation distributions and study their propagation in linear second-order dispersive media. Particular models for coherence functions are introduced both in time and frequency domains. It is shown that the maximum peak of the pulse energy can be accelerating or decelerating and also self-focusing effects are possible due to coherence-induced propagation effects.

Design of polarization gratings for broadband illumination.

Design of broadband diffractive elements is studied. It is shown that dielectric polarization gratings can be made to perform the same optical function over a broad band of wavelengths. Any design of paraxial-domain diffractive elements can be realized as such broadband elements that may, e.g., give constant diffraction efficiencies over the wavelength band while the field propagation after the elements remains wavelength-dependent. Furthermore, elements producing symmetric signals are shown to work with arbitrarily polarized or partially polarized incident planar broadband fields. The performance of the elements is illustrated by numerical examples and some practical issues related to their fabrication are discussed.

High-efficiency broadband diffractive elements based on polarization gratings

A method is introduced for designing paraxial-domain diffractive elements working over a broad frequency range. The method is based on space-variant manipulation of the state of polarization by form-birefringent binary diffractive structures. It is shown that any scalar phase transmission function can be realized by use of such polarization-modulating structures and that at least in some cases it is even possible to exceed the scalar paraxial-domain upper bounds of diffraction efficiency over a broad frequency band.

Increase of spatial coherence by subwavelength metallic gratings.

We study the coherence changes in partially coherent beams transmitted through binary metallic gratings. The interaction of Gaussian Schell-model beams with grating structures supporting surface plasmons is rigorously modeled using the Fourier modal method and the coherent mode representation of partially coherent fields. Our numerical results show that, by choosing suitable parameters for the grating, the degree of coherence of the beam can be significantly increased. The studied approach offers new possibilities to alter the coherence properties of fields using nanophotonic components.

Micropatterned luminescent optical epoxies.

We study the fabrication and optical properties of micropatterned luminescent optical epoxy samples. Five different photoluminescent materials were added to epoxy resin to form luminescent epoxies of different colors and micropatterned gratings were imprinted on the surface of the samples. The absorbance spectra of the unpatterned epoxy samples were measured with spectrometer and the luminescence intensities of all samples were measured using custom made bispectrometer. The methods used in this work offer an efficient and straightforward way to produce micro- or nanopatterned luminescent optical epoxies for various applications, such as LED coatings and solar concentrators.

Imaging with partially coherent light: elementary-field approach.

Numerical modeling of bright-field and dark-field imaging with spatially partially coherent light is considered. The illuminating field is expressed as a superposition of transversely shifted fully coherent elementary fields of identical form. Examples of imaging under variable coherence conditions demonstrate the computational feasibility of the model even when the coherence area of the illumination is in the wavelength scale.