1. Expression of optical functions
(1)Development of optical elements employing nanoimprint lithography
Nanoimprinting is a technology whereby a mold in the form of a fine structure is precisely transferred to a target material.With its high productivity and potential for low cost performance, this method has been reported in numerous studies, and in most cases, resin is used as a base material.Our laboratory aims to express new optical functions through formation of a fine structure on the surface of a highly durable substrate glass material.
・Nanomold fabrication technology
This laboratory is developing the know-how for micro/nano fabricating a mold material that excels in thermal durability and mechanical strength.Using electron beam lithography, interference exposure and laser lithography, a fine resist pattern is formed. The pattern is subsequently transferred to the mold surface through a dry etching process.
A thermal nanoimprint apparatus is utilized to make a micro/nano structured glass surface under a vacuum or a nitrogen atmosphere.
As for UV nanoimprint lithography, a quartz mold is pressed to UV curing resin coated on the substrate surface, and then after a certain period of UV irradiation, the mold is detached.
・Optical devices developed to date
By making use of the aforementioned processes in collaboration with private companies, optical devices of anti-reflective lens, hybrid refractive-diffractive lens and diffraction gratings have been developed.
・Electrical nanoimprint with voltage application
Imprinting method in which voltage is applied to a micro/nano-structured mold has enabled formation of micro/nano structure with no need of conventional elements of high temperature or high pressure.
Furthermore, when combined with wet etching, we are able to produce a high-aspect ratio responding to the concentration distribution of alkali in the glass that is formed by application of voltage. Thus, this technology shows promise to be applied to optical devices.
(2)Development of plasmon optical devices
As a method to detect slight refractive-index change and weak fluorescence, application of localized plasmon using metallic nanoparticles of Au and Ag, as well as application of propagating plasmon using a metal-coated prism, has been established
In order to develop a system to detect cells and proteins labelled by fluorescent reagents in a way of multipoint detection over a large area, the propagating plasmon type has advantages, though it requires building a specialized optical system.
In collaboration with the National Institute of Advanced Industrial Science and Technology, our laboratory developed a grating-based plasmon-enhanced chip that allows for the detection of weak fluorescence through a commercial microscope.
The metal used is Ag. Observation through a microscope on fluorescence-labeled cells on this chip proves that its fluorescence imaging is ten times brighter compared to a chip without a diffraction grating.
To date, much research has been conducted on hydrogen sensing using plasmon resonance on palladium (Pd).
Compared to Au or Ag, the plasmon dip (drop in reflectance during coupling of excitation light into a metal) is overly broad or not detected in most cases. For this reason, correlation between sensitivity of hydrogen detection and plasmon resonance was not always clear.
This laboratory has verified that a clear plasmon dip appears in a palladium grating that was prototyped based on grating parameters calculated through rigorous coupled wave analysis (RCWA).
However, a study on the relation between reflective power and hydrogen concentration on the grating surface by radiating incident excitation light (λ=850nm) into the grating revealed that the reflectance change in the s polarization is similar to that in the p polarization whereby the plasmon is excited.
Currently, further study is being done on what causes it to express some characteristics unrelated to the plasmon resonance.