<p>We have proposed and demonstrated position-sensitive detectors based on the spectral changes in fluorescent waveguides. The first prototype is a transparent heat-shrink tubing containing an organic luminescent dye at its core. With a laser beam incident on this linear fluorescent tubing, the redshift in the photoluminescence (PL) spectrum observed at its edge increases with the distance from the incident point. The range for position sensing is 2 cm. It is extended to 280 cm by adopting a scintillating fiber in our second experiment. Two-stage conversion enables two-dimensional position detection. We have attached two linear fluorescent tubing to a planar 50 mm × 50 mm × 8 mm fluorescent waveguide. When a laser beam excites the first luminescent material at a single spot in the planer waveguide, PL photons propagate to its edges and excite the second luminescent material in the two linear waveguides. Photon division between these linear waveguides gives the first coordinate. The second coordinate is given by the redshift in the linear waveguides. We have observed that the maximum error in position estimation is 1.5 mm. Unlike the conventional semiconductor technologies, no electronic components are required for the sensor head. This robust technology might be suited for deployment in large-scale harsh environments.</p>
We propose to utilize spectral information of photoluminescence (PL) photons for position sensing. Suppose that a beam of radiation is incident at a certain point on a rectangular plate in which luminescent materials are uniformly dispersed. With an optical fiber attached to its edge surface, the PL photons are guided to a spectrometer. The spectrum of the PL photons is red-shifted due to self-absorption in the plate. The magnitude of the red-shift is enhanced as the PL photons propagate longer distance inside the plate. Hence, we can determine the distance by quantifying this spectral change. In experiment, we let a laser beam (wavelength 450nm) normally incident on an acrylic plate containing luminescent materials (40 mm × 40 mm × 2.9 mm). The incident position on the plate was varied and from each spectrum recorded we calculated chromaticity coordinates in the CIE1931-XYZ color space. With one sample plate, the coordinate <i>x</i> increased from 0.23 to 0.29 monotonically when we increased the horizontal distance on the plate from 2mm to 20mm. In another sample, the chromaticity coordinates behaved differently but the monotonic relation remained valid. We now have calibration curves for the position. This sensing technique might be suited for long-range position detection, usage in harsh environments and for insertion to narrow places.
We have demonstrated one-dimensional position sensing based on red-shift of photoluminescence (PL) spectra. This technology can be extended to two-dimension by the two-stage PL conversion technique described here. A planar waveguide contains a first luminophore to convert an incoming radiation to PL photons. A linear waveguide contains a second luminophore and two of them sandwich the planar waveguide such that the second luminophore absorbs the PL photons emitted by the first luminophore. Spectral analysis of the PL photons exiting the two linear waveguides gives the coordinate of the incident position along the direction of the linear waveguide. The coordinate perpendicular to this direction is determined by comparing the PL intensities propagating in the two linear waveguides. This is analogous to the charge division principle utilized in a position-sensitive proportional counter as well as a tetra-lateral semiconductor detector. In the current case, we are dividing the PL photons emitted by the first luminophore to the two light-sensitive regions facing to each other. The use of optical fibers allows one to build optical sensors without electric components at the sensing sites. Based on this technology, a robust large-scale radiation monitoring system might be constructed. In a proof-of-concept experiment, we fabricated a 50 × 50 × 8 mm sensor head using coumarin6 as a green emitter and Lumogen F Red 305 as a red emitter. The maximum error for estimating the incident spots in the upper-left 20 × 20 mm region of the sensor head was 1.5mm.
One can convert a luminescent solar concentrator to a display by projecting intensity-modulated light on it. We fabricated a 95 mm×95 mm×10 mm screen by sandwiching a thin coumarin 6 layer with two acrylic plates. We removed the light source in a commercial projector and fed a blue laser beam into its optics. It displayed monochrome images on the screen clearly. A photodiode covered a 10 mm×10 mm region on the edge surface of the screen. As we pulsed the laser, the photodiode output varied synchronously. Its output indicates that a fully covered version would harvest up to 71% of the incoming laser power. However, a ghost image was noticeable when we displayed a high-contrast still image. We address two aspects in design considerations. First, tiling small modules will reduce the thickness of a large-area projection system and alleviate its self-absorption loss. For seamless tiling, we can attach output couplers to the surface of the transparent plate and extract photoluminescence (PL) photons in each module. Second, the origin of the ghost image is the PL photons reflected at the plate–air interface inside the screen. Thinning the transparent plate facing the projector will eliminate such an optical cross talk.
One can convert a Luminescent Solar Concentrator (LSC) to an energy-harvesting display by scanning a laser beam on it. By incorporating a guest-host system of liquid crystal (LC) and dye materials in an LSC, the power of photoluminescence (PL) utilized for either display or energy-harvesting can be adjusted to the changes in ambient lighting conditions. We have measured basic characteristics of an LC/dye cell with twisted-nematic (TN) alignment. These are absorption of the laser light, PL radiation pattern, contrast of luminance, spreading of the PL generated by a narrow laser beam, and their dependencies on the bias. The results are similar to those of the LC/dye cell with antiparallel (AP) alignment with the following exceptions. First, absorption by the TN cell depends on the bias for both polarization components of the excitation light, while the AP cell exhibits a bias dependency only for the component polarized along the alignment direction. Second, the PL from the TN cell is mostly polarized along the alignment direction on the exit side of the cell while the PL from the AP cell is mostly polarized along its alignment direction. These observations can be attributed to the fact that the polarization plane of a linearly polarized light rotates as it propagated the TN-LC layer. For both AP and TN cells, low-intensity PL is observed from the whole cell surfaces. This can degrade the contrast of a displayed image. Bias application to the cell suppresses this effect.
One can convert a luminescent solar concentrator to a display by scanning a laser beam on it. When a guest–host system of liquid crystal (LC) and dye materials are incorporated, absorption of excitation light and the radiation pattern of photoluminescence (PL) can be adjusted to changes in lighting condition. The resolution of a displayed image can be degraded by PL spreading in the LC/dye layer. Its contrast can be limited by the PL induced by ambient light. In the experiment, we fabricated a 22×25 mm2 cell that contained 0.5 wt. % coumarin 6 in a nematic LC host. The alignment was antiparallel and the gap was 6 μm. Using a blue laser beam of 0.04 mm FWHM, the PL intensity distribution was measured to be 0.20 mm FWHM at zero bias. It became slightly wider at 10 V. For contrast evaluation, we measured PL spectra under two conditions. First, the center of the cell was irradiated by a 1.7-mW blue laser beam. Second, the whole cell was uniformly exposed to light from a fluorescent lamp at illuminance of 800lx. The contrast of luminance was calculated to be 1.4×105. The optical power reaching its edge surfaces was measured and roughly agreed with the prediction by a simple model.
Elongated dye molecules orient themselves with surrounding liquid crystal molecules. We propose to incorporate such a guest-host cell in a screen of a projection display. This configuration might be applied for digital signage to be installed on building walls. Dual-mode operation is realized by the bias applied to the cell. In display-mode, the dye molecules are oriented in parallel to the substrate of the cell. When excited by ultra-violet light, photoluminescence (PL) is generated. Because it is mostly perpendicular to the long axis of the molecule, it exits the cell efficiently. In powerharvesting mode, they are oriented vertically. The PL generated by ambient light is directed to edge surfaces where solar cells are mounted. In experiment, we fabricated a cell with commonly-available materials (coumarin 6 and a nematic liquid crystal). Anti-parallel alignment condition was adopted. We recorded PL spectra from the cell for the two excitation conditions. First, the center of the cell was irradiated by a 1.69mW blue laser beam. Second, the whole cell was uniformly exposed to the light from a fluorescent lamp at illuminance of 800lx. From the measured spectra for these cases, the contrast of luminance is calculated to be 3.2 ×10<sup>5</sup> . This factor is improved to 5 7.5×10<sup>5</sup> by attaching a polarizer sheet on the cell surface. The optical power reaching its edge surfaces is measured and it roughly agrees with the prediction by a simple model neglecting self-absorption. Development of phosphor materials with a large Stokes shift is desired to boost performance of the proposed system.