OSA/CIPS Brown Bag Seminar Series

Thursday, Nov 12, 2009

12 noon , RLE Haus Conference Room 36-428

Generating optical near-fields from afar via Absorbance Modulation

Hsin-Yu Tsai

Diffraction limits the resolution of far-field optics to approximately half the wavelength of illumination. The high spatial frequency components of an optical field, which carry high-resolution spatial information, decay exponentially and therefore exist only in the optical near-field. Absorbance modulation is a way to generate optical near fields from the far-field by combining photochemistry with appropriate illumination.

Absorbance modulation relies on a thin film of photochromic molecules, referred to as the absorbance-modulation layer (AML). The AML is originally opaque, becomes transparent upon a shorter wavelength, l1, illumination, and returns to opaque upon a longer wavelength, l2, illumination. When the AML is illuminated with a bright spot at l1 and a ring-shaped spot at l2 simultaneously, a sub-wavelength transparent aperture forms in the vicinity of the node of the l2 ring. This aperture can be generated dynamically and the size of the aperture is not limited by diffraction, but by the photochromic parameters of the molecules and the ratio of the intensities at the two wavelengths. By placing the AML atop a substrate, and scanning the substrate relative to the far-field optics, one can employ the optical probe that penetrates through the aperture for nano-patterning or nanoscopy.