The black-brown pigment eumelanin acts as a vital barrier to the harmful UV radiation. With an ingenious ability to effectively dissipate 99.9% of the incoming UV energy to the heat within few picoseconds, eumelanin serves as a natural photo-protectant. To unravel the nature of the energy dissipation we study the eumelanin pigment and its major building block using ultrafast broadband transient absorption spectroscopy. The excited state decay was found to be fluence independent for all excitation energies, suggesting that electronic excitations are also spatially localized. The short excited state lifetime – on the order of a few picoseconds – leads to a suggestion that the energy is dissipated through excited-state intramolecular proton transfer, which we examined via comparison with pH-dependent TA spectroscopy of the DHICA and related building blocks.
An en face coherence gated camera equipped with adaptive optics (AO) has been constructed for imaging single cells in the living human retina. The high axial resolution of coherence gating combined with the high transverse resolution of AO provides a powerful imaging tool whose image quality can surpass either methodology performing alone. The AO system relies on a 37-actuator Xinetics mirror and a Shack-Hartmann wavefront sensor that executes up to 22 corrections per second. The coherence gate is realized with a free-space Michelson interferometer that employs a scientific-grade 12-bit CCD array for recording 2-D retinal interferograms. Images were collected of microstructures the size of single cells in the in vivo retina. Early results suggest that a coherence gated adaptive optics camera should substantially improve our ability to detect single cells in the retina over the current state-of-the-art AO retina cameras, including conventional flood illuminated and confocal scanning laser ophthalmoscopes. To our knowledge, this is the first effort to combine coherence gating and adaptive optics.
We consider anisotropic stress in substrates coated with vacuum- deposited optically-anisotrpic thin films. Birefringent films with tilted-columnar and normal-columnar nanostructures and thin film helocoidal bianisotropic mediums are considered. In the case of tilted- columnar nanostructures, the sign and relative magnitude of the stresses parallel to the deposition plane and perpendicular to the deposition plane depend on deposition angle. We show that an ion-assisted overcoat, applied with the primary purpose of protecting optical properties, can be designed to provide effective compensation of stress.
The serial bideposition (SBD) technique is used to produce biaxial thin films with large linear birefringence and chiral (handed) media with large circular birefringence. Thin film wave plates and chiral reflectors of SBD silicon that have been fabricated show promise for applications in the wavelength range 800-2200nm. In particular we have deposited quarter-wave plates of metric thickness less than 1micrometers for the 800nm wavelength used in CD players, and half-wave plates of metric thickness about 3micrometers for the 1550nm optical communications wavelength. An observation of the Bragg resonance in a silicon chiral film in the 600-800nm wavelength range where the absorption is high but the linear birefringence is approximately equals 0.35, suggests possible applications even at the 633 HeNe wavelength. Polarizing elements that we have designed and fabricated for use with circularly polarized light in the visible and near infrared spectral regions are described. These include a two-layer Fabry-Perot filter that uses a structural phase discontinuity instead of a physical spacer layer to define the wavelength of a narrow spectral hole at the center of the Bragg dip. Further strategies, also based on phase discontinuities, are explored for adapting other filter designs from isotropic thin film filter theory to the circular case.