Kimmo Saastamoinen

Interferometric interpretation for the degree of polarization of classical optical beams

We introduce an interferometric interpretation for the degree of polarization as a quantity characterizing the ability of a light beam to generate polarization modulation when it interferes with itself. The result is confirmed experimentally in Youngʼs interferometer with beams of controlled degree of polarization and by comparing to a standard polarimetric measurement. The new interpretation is a consequence of the electromagnetic interference law that we formulate for stationary, quasi-monochromatic, partially polarized light beams in time domain. Our work provides fundamental insight into the role of polarization in electromagnetic coherence and interference.

Spatial coherence measurement of polychromatic light with modified Young’s interferometer

Partial spatial coherence is a fundamental concept in optical systems. Theoretically, the normalized mutual coherence function gives a quantitative measure for partial spatial coherence regardless of the spectral nature of the radiation. For narrowband light the degree of spatial coherence can be measured in terms of the fringe modulation in the classic Youngs two-pinhole interferometer. Though not commonly appreciated, with polychromatic radiation this is not the case owing to the wavelength dependence of diffraction. In this work we show that with a modified two-beam interferometer containing an achromatic Fresnel transformer the degree of spatial coherence is again related to the visibility of intensity fringes in Youngs experiment for any polychromatic light. This result, which is demonstrated both theoretically and experimentally, thus restores the usefulness of the two-pinhole interferometer in the measurement of the spatial coherence of light beams of arbitrary spectral widths.

Detection of electromagnetic degree of coherence with nanoscatterers: comparison with Young's interferometer.

We show theoretically that the (spectral) electromagnetic degree of spatial coherence of a random, stationary light beam can be measured by using two dipolar nanoscatterers instead of aperture diffraction as in traditional Youngs interferometer. The method is based on considering individually the correlation functions associated with the six polarization states that make up the coherence (two-point) Stokes parameters and observing separately the visibilities and the locations of the intensity fringes created by the interfering dipole fields, leading to a complete characterization of the beams second-order spatial coherence. The novel technique, although introduced in this work for beams, paves the way toward the detection of spatial coherence in nonparaxial optical near-fields for which the use of nanoscatterers is necessary.

Detection of partial polarization of light beams with dipolar nanocubes.

We confirm experimentally that the degree and state of polarization of a random, partially polarized electromagnetic beam can be obtained by probing the field with a nanoscatterer. We use a gold nanocube on silicon substrate as a local scatterer and detect the polarization characteristics of the scattered far field, which enables us to deduce the state of partial polarization of the field at the nanoprobe site. In contrast to previous beam characterization methods where spatial resolution is limited by the pixel size of the detector, the accuracy of the current technique is specified by the particle size. Our work is the first step towards polarization-state detection of random optical near fields for which the use of nanoprobes is required.

Surface Plasmons Carry the Pancharatnam-Berry Geometric Phase

Surface plasmon polaritons (SPPs) are electromagnetic surface waves that travel along the boundary of a metal and a dielectric medium. They can be generated when freely propagating light is scattered by structural metallic features such as gratings or slits. In plasmonics, SPPs are manipulated, amplified, or routed before being converted back into light by a second scattering event. In this process, the light acquires a dynamic phase and perhaps an additional geometric phase associated with polarization changes. We examine the possibility that SPPs mediate the Pancharatnam-Berry phase, which follows from a closed path of successive in-phase polarization-state transformations on the Poincaré sphere and demonstrate that this is indeed the case. The geometric phase is shown to survive the light→SPP→light process and, moreover, its magnitude agrees with Pancharatnams rule. Our findings are fundamental in nature and highly relevant for photonics applications.

Dynamic control of optical transmission through a nano-slit using surface plasmons

We demonstrate how the optical transmission by a directly illuminated, sub-wavelength slit in a metal film can be dynamically controlled by varying the incident beams phase relative to that of a stream of surface plasmon polaritions which are generated at a nearby grating. The transmission can be smoothly altered from its maximum value to practically zero. The results from a simple model and from rigorous numerical simulations are in excellent agreement with our experimental results. Our method may be applied in all-optical switching.

Broadband spatiotemporal axicon fields

We study the properties of broadband optical fields produced by two classes of axicons: reflective axicons creating fields with a frequency-independent cone angle, and diffractive axicons that generate fields with frequency-independent transverse scale. We also consider two different types of illumination: spectrally completely coherent pulses and spectrally incoherent (stationary) light assuming that the spectra are the same in both situations. In the former case we evaluate the spatiotemporal shape of the output field, and in the latter case its spatiotemporal coherence properties. Physical reasons for the substantially different fields produced by the two types of axicons are identified. Our results are useful for optical applications in which joint spatial and temporal field localization is desired.

Measurement of the degree of temporal coherence of unpolarized light beams

We measure the electromagnetic degree of temporal coherence and the associated coherence time for quasi-monochromatic unpolarized light beams emitted by an LED, a filtered halogen lamp, and a multimode He–Ne laser. The method is based on observing at the output of a Michelson interferometer the visibilities (contrasts) of the intensity and polarization-state modulations expressed in terms of the Stokes parameters. The results are in good agreement with those deduced directly from the source spectra. The measurements are repeated after passing the beams through a linear polarizer so as to elucidate the role of polarization in electromagnetic coherence. While the polarizer varies the equal-time degree of coherence consistently with the theoretical predictions and alters the inner structure of the coherence matrix, the coherence time remains almost unchanged when the light varies from unpolarized to polarized. The results are important in the areas of applications dealing with physical optics and electromagnetic interference.