Leaf-inspired luminescent solar concentrator based on two-stage photoconversion.

Geometrical gain of a luminescent solar concentrator is drastically increased by laying out a luminescent fiber in a luminescent plate with air gap around it and attaching a photovoltaic (PV) cell to the tip of the fiber. The plate converts an incident photon to a first photoluminescence (PL) photon, and the fiber converts it to a second PL photon. Thus, the fiber carries the optical power as a leaf vein transports water and nutrients. The probability of the first PL photon resulting in the second PL photon reaching the PV cell can be measured by exciting a single spot on the plate with a laser beam. In experiment, 2 mm-thick, 50 mm-square and 50 mm-diameter circular devices were assembled with off-the-shelf components. For each case, geometrical gain exceeded 1000 and this probability averaged over the incident area was of the order of 0.01. Connecting multiple small-area devices to a single PV cell with a clear fiber would increase geometrical gain further and alleviate the absorption and scattering of PL photons during waveguiding.

Position detection of a beta particle emitter by utilizing self-absorption in a scintillation fiber.

Self-absorption in a plastic scintillation fiber can be utilized to determine the incident position of single beta particles. A dichroic mirror directs scintillation photons with shorter wavelengths to one Si photomultiplier and those with longer wavelengths to another. An index calculated from the two signals is a monotonic function of the distance between the tip of the fiber and the incident point. Once this relation is known, one can determine the distance from the two measurables. In an experiment, such a calibration curve was acquired to detect the position of a 90Sr source up to a distance of 240 cm. The average total number of photoelectrons for a single beta particle was about 15-17. Depending on the propagation distance in the scintillation fiber, they were unevenly divided by the two photodetectors.

Luminance, color gamut, and energy-harvesting characteristics of luminescent layers placed above a solar cell.

One can display images and harvest energy by utilizing ambient light with a stack of an optical shutter array, a luminescent layer and a solar cell. In our experiment, a luminescent layer and a corresponding color filter were attached to a polycrystalline Si solar cell with 13% power conversion efficiency. For each configuration using BBOT, Coumarin 6 and Lumogen F Red 305, the power conversion efficiency was measured to be 6.7%, 8.0% and 8.9%, respectively. The luminance of these configurations was proportional to the illuminance in all cases. Its color gamut was comparable to the National Television System Committee standard.

Spectral study on utilizing ambient light with luminescent materials for display applications.

A luminous reflective display can be constructed by placing an electro-optic shutter on the stack of a luminescent layer, a color filter, and a reflector in this order. The luminescent materials convert a part of the incident light to photoluminescence photons. The reflector redirects the downward photon flux toward an observer. The color filters prevent the photons with unwanted wavelengths from being reflected. The upward spectral flux from this multi-layer structure is formulated. Experiments with off-the-shelf components revealed more than three-fold increase in spectral flux and up to 55% color gamut extension, compared with a control device without luminescent materials.

Concentric re-emission pattern from a planar waveguide with a thin uniform luminescent layer.

When a beam of light excites a single spot on a thin luminescent layer embedded in a planar waveguide, a concentric re-emission pattern is observed. An analytical expression is formulated by following the series of events in the waveguide: generation of angle-dependent photoluminescence spectra, reflection at the waveguide-air boundary, absorption by the luminescent layer, and generation of next-generation photoluminescence. The formula reproduces the peak radii observed in the experiments with some organic dyes. It provides insights for the re-emission events in a luminescent solar concentrator and the cross talk in an energy-harvesting display based on photoluminescence.

Cross talk and optical efficiency of an energy-harvesting color projector utilizing ceramic phosphors.

A color projector screen was fabricated by filling three kinds of ceramic phosphor powders in the periodic hollow columns formed in a ${50}\;{{\rm mm}}\; \times \;{50}\;{{\rm mm}}\; \times \;{10}\;{{\rm mm}}$50mm×50mm×10mm acrylic waveguide. When a blue laser beam excited a single spot on the screen, a disk-shaped cross-talk pattern appeared. Its intensity was 5 orders of magnitude lower than that of the excited spot. The solar cells attached to the waveguide edge harvested less than 0.8% of the incident optical power. The photons scattered by the phosphors are responsible for these characteristics, and the use of non-scattering luminescent materials is desired for improving them.

Illumination uniformity of an edge-lit backlight with emission angle control.

Viewing range of a liquid crystal display can be controlled by a liquid crystal device inserted between a light source and a light-guide of an edge-lit backlight unit. Here, we propose an output coupler with a vertical optical window through which light is extracted from a light-guide. Rays with large propagation angles and those with small angles have a comparable probability of hitting this window. As a result, a single set of these output couplers can provide uniform light extraction for both settings of a wide viewing range and a narrow one. Ray tracing simulations confirm these findings.

Dual-LED imaging for finger liveliness detection and its evaluation with replicas.

Unattended fingerprint identification systems need to reject input attempts by a replica. Previously, we proposed detecting the color changes of a finger during an input action as a signature of liveliness. To improve its reliability, a dual-LED imaging system is investigated. It employs two LEDs with peak emissions at 530 and 630 nm to cover the spectral ranges where live fingers show characteristic changes. Using nine types of replicas and the live fingers of 42 participants, we have recorded and analyzed 153 input trials. The two groups of data are successfully separated by some defined criteria.

Recognition of living fingers with a sensor based on scattered-light detection.

To prevent fraud by use of replicas of human fingers for identity verification, we propose to check variations in color and in a series of previously acquired fingerprint images. Movement of a finger on a human hand induces blood movement, and light scattered inside the finger carries this life-related information. In experiments, we found characteristic changes in color and luminance extracted from the central parts of fingerprint images. Some criteria for recognition of living tissue were tested with input provided by more than 30 participants. Inasmuch as the response of a human body to an external stimulus can be checked by our method with no additional components, this technology can raise the security level of a fingerprint identification system.

Active optical interconnect based on liquid-crystal grating.

We propose a transparent plate of a liquid-crystal grating to be used as a light guide for optical interconnection. To this end, we are empowering the connection itself with active functions, such as switching, wavelength division, power adjustment, etc. In experiments, we built a grating based on in-plane switching. It contains vertically-aligned nematic liquid-crystal molecules between a glass plate with a high refractive index (light guide) and another glass plate with a pair of interdigitated electrodes. Entering a TM wave from an edge of the light guide, we have demonstrated that the activation of diffraction and intensity adjustment for the guided light are both possible. Because a TE wave is barely diffracted, the device also exhibits polarization division capability.

Theoretical considerations for arrayed waveguide displays.

An arrayed waveguide display (AWD) is proposed in which a linear emitter array is coupled to a waveguide array upon which liquid-crystal (LC) switches are formed. Images are displayed by line-by-line control of light, including emission by the emitter array, injection into the waveguide array, propagation inside, and extraction by use of the LC switches. With mostly polymeric materials, an AWD can be made thin and tough. Its light-use efficiency can be made larger than that of a conventional transmissive LC display. Because of these potential advantages, an AWD is well suited for mobile applications.