Hartmann-Shack wavefront sensing for nonlinear materials characterization.

We present two new techniques exploiting a Hartmann-Shack wavefront sensor to characterize the optical self-focusing effect of nonlinear materials. We demonstrate that the defocus Zernike coefficient (C5) can be used to quantify nonlinear optical properties of materials. In the first technique proposed, the wavefront of a collimated laser beam transmitted through a nonlinear sample is analyzed with different irradiance values. In the second technique,instead of conventional detectors, a Hartmann- Shack sensor is used in a Z-scan setup. The methods are demonstrated by measuring the nonlinear refractive indices of CS2 and Quartz, using femtosecond Ti:sapphire lasers at 76 MHz and 1 KHz repetition rate.

Second-harmonic microscopy of ex vivo porcine corneas.

The ex vivo cornea of porcine eyes has been studied with second-harmonic microscopy with a laboratory-built system to examine the structure of collagen fibrils at different length scales, as well as the image dependence on polarization and wavelength of the illumination source. We found that collagen fibrils can effectively be visualized with second-harmonic microscopy, in agreement with previous findings, at different wavelengths of the illumination. The same laser source used for imaging may also be used to induce changes to the corneal tissues that are observable both in the linear and second-harmonic imaging channels. Such studies are essential first steps towards a future high-resolution optical characterization technique for simultaneous corneal surgery and wound healing of the human eye.

Optical characterization of probes for photon scanning tunnelling microscopy.

The photon scanning tunnelling microscope is a well-established member of the family of scanning near-field optical microscopes used for optical imaging at the subwavelength scale. The quality of the probes, typically pointed uncoated optical fibres, used is however, difficult to evaluate in a direct manner and has most often been inferred from the apparent quality of recorded optical images. Complicated near-field optical imaging characteristics, together with the possibility of topographically induced artefacts, however, has increased demands for a more reliable probe characterization technique. Here we present experimental results obtained for optical characterization of two different probes by imaging of a well-specified near-field intensity distribution at various spatial frequencies. In particular, we observe that a sharply pointed dielectric probe can be highly suitable for imaging when using p-polarized light for the illumination. We conclude that the proposed scheme can be used directly for probe characterization and, subsequently, for determination of an optical transfer function. which would allow one to deduce from an experimentally obtained image of a weakly scattering sample the field distribution existing near the sample surface in the absence of the probe.

Near- and far-field second-harmonic imaging of quasi-phase-matching crystals.

Second-harmonic scanning near- and far-field optical microscopy of an electric-field poled KTiOPO4 quasi-phase-matching crystal has been accomplished. This has been done in order to reveal the walls that form the intersections between inverted and non-inverted crystal domains. The domain walls are seen clearly only in images recorded by means of second-harmonic generation because of a large nonlinear contrast, and they appear as bright stripes when studied in a reflection geometry but they are dark when studied in transmission. The images show that the duty cycle of the quasi-phase-matching crystal differs from the ideal and that the walls are not completely smooth. These effects, in combination with the observed scattering from the domain walls, are expected to lower the output of the crystal when used for frequency doubling. We conclude that the wall thickness is no more than approximately 100 nm, which makes it a suitable test object for the resolution capabilities of scanning near-field optical microscopes that are used for nonlinear imaging.

Characterization of near-field optical probes.

Radiation and collection characteristics of four different near-field optical-fiber probes, namely, three uncoated probes and an aluminum-coated small-aperture probe, are investigated and compared. Their radiation properties are characterized by observation of light-induced topography changes in a photosensitive film illuminated with the probes, and it is confirmed that the radiated optical field is unambiguously confined only for the coated probe. Near-field optical imaging of a standing evanescent-wave pattern is used to compare the detection characteristics of the probes, and it is concluded that, for the imaging of optical-field intensity distributions containing predominantly evanescent-wave components, a sharp uncoated tip is the probe of choice. Complementary results obtained with optical phase-conjugation experiments with the uncoated probes are discussed in relation to the probe characterization.

Coupling of surface-plasmon polaritons to directional far-field radiation by an individual surface protrusion.

An experimental study of surface-plasmon polariton scattering by an individual surface protrusion on a silver film has been performed. Both in- and out-of-plane scattering have been investigated by imaging with a photon-scanning tunneling microscope at various probe-to-sample distances. For imaging in close proximity we observed a bright spot with rims on the shadow side, but, for imaging at larger distances, the location of the bright spot shifts away from the direction of the incoming surface-plasmon polariton. We estimate a pronounced peak in the far-field scattering pattern at an angle of ~20 degrees from the sample plane, which is in good agreement with recent theoretical studies and indicates that the same protrusion could possibly be used for local launching of surface-plasmon polaritons in a reversed illumination configuration.