David Kupka

Self-referenced interferometry for the characterization of axicon lens quality

A simple interferometer for the characterization of axicon lenses is presented. The phase cone acquired by a wave propagating through an axicon, when interfered with a collinear reference wave, produces a nearly cylindrically symmetric self-referenced interference pattern from which the distortions of the axicon surface may be readily obtained. Comparison with two-dimensional off-axis interferometry is used to validate the self-referenced technique. The measurements are based on retrieval of the accrued spatial phase distribution from interference fringes with on- and off-axis reference beams and are found to be equivalent. We use the ellipticity of the phase maps to qualify axicon lenses, which are expected to exhibit radial symmetry and engage the self-referential capability of the on-axis method to derive deviation maps that characterize the surface quality of the axicons.

Simplified ultrafast pulse shaper for tailored polarization states using a birefringent prism

A new polarization pulse shaping method utilizing a birefringent prism as both the spectrally dispersing and polarization separating element is presented and analyzed. The method of appropriate prism design is first examined, followed by calibration technique and experimental demonstration of the pulse shaper. Using phase-only modulation by means of a spatial light modulator, we obtain near-transform limited pulses. Furthermore, a sinusoidal spectral phase imparted on the pulse is retrieved and qualitatively compares well with the theoretical target field.

Polarization control and tomography for nonlinear microscopy

In optical microscopy, the polarization state of the focal field strongly influences formed images due to its interactions with the sample and the effective focal spot size. We demonstrate experimentally that control over the spatial profile of the focal field polarization state improves spatial resolution in laser-scanning third harmonic generation (THG) microscopy. The focal field is manipulated by imaging a spatial light modulator to the focal plane of a moderate-numerical aperture microscope. The resolution enhancement arises from exploiting the suppression, in isotropic media, of THG for circularlypolarized field polarization. By synthesizing a focal field whose polarization state changes from linear at the beam center to circular beyond radius rs, we quench THG beyond rs. A transverse spatial resolution of up to 2 times is demonstrated. Targeted manipulation necessitates measurement techniques that allow us to determine of the focal field polarization state. We develop two such techniques to characterize the field. We use a nano-particle with known third-order susceptibility to localize THG scattering to a small focal volume. Scanning this nano-probe through the focal volume of the microscope allows for complete reconstruction of the vector point spread function. Under moderate focusing conditions, where the recorded THG signal is dominated by the incident paraxial polarization component, the spatial polarization state is determined non-iteratively via three linear-polarization projection THG measurements. Under tight focusing conditions, polarization scrambling occurs such that the input and focal fields are dissimilar, and we introduce an algorithm for focal field retrieval through the collection of far-field THG images.

Optimization of Third Harmonic Conversion Efficiency in the Presence of a Spatially Localized Plasma

A computational optimization of parameters leading to maximal third harmonic conversion efficiency in the presence of a laser-induced plasma is introduced, building on previous experimental work. The model contains full spatial and temporal dynamics for nonlinear interaction and pulse dispersion. Optimization of a localized plasma for a significant further enhancement of conversion efficiencies is suggested by the modeling.

Coherence modulated third harmonic generation for winterface vibrational spectroscopy

Contributions to coherent vibrational modulation of third harmonic generation across an interface include a bulk coherent second hyper-Raman scattering and an interface-specific Fresnel boundary modulation. Interface scans permit separation of the bulk and interface signals.

Complete vector focal field characterization via nanoprobe induced nonlinear far field signals

An algorithm fully characterizing vector focal plane third order polarization densities and fundamental fields for tightly focused beams through analysis of far field third harmonic intensities is presented.

Advanced vector polarization shaping and applications of ultrafast laser pulses

We demonstrate complete control over the polarization, phase and amplitude state of an ultrafast laser pulse using a single, linear spatial light modulator. Characterization and applications of vector pulse shaping will also be discussed.