Low Light Collection Research Articles

Directional Laser from Solution‐Grown Grating‐Patterned Perovskite Single‐Crystal Microdisks

AbstractSolution‐processed, high‐gain, and wavelength‐tunable perovskite single‐crystal microdisk (PVKsc‐MD) lasers have emerged as prospective coherent light sources in advanced nanophotonic designs. However, the inevitable multi‐directional emission from a highly symmetrical cavity leads to low light collection efficiency which greatly hinders its application in integrated optical circuits. Here, we report on surface‐patterned MAPbBr3 PVKsc‐MDs synthesized by a cost‐efficient bottom‐up solution process employing spin‐coating and confined‐growth nanoimprinting. The patterned microdisks have high crystal quality with regular shape and sharp edges and nano‐grating structure on the upper surface. This straightforward process yields surface‐patterned PVKsc‐MD lasers with a low lasing threshold and high quality (Q) factor. In addition, the grating structure patterned on the PVKsc‐MDs reduces the original symmetry of the laser cavity, which improves the emission directionality four times.

A staggered 3-layer DOI PET detector using BaSO4 reflector for enhanced crystal identification and inter-crystal scattering event discrimination capability

The spatial resolution of small animal positron emission tomography (PET) scanners can be improved by the use of crystals with fine pitch and rejection of inter-crystal scattering (ICS) events, which leads to a better quantification of radiopharmaceuticals. On the other hand, depth-of-interaction (DOI) information is essential to preserve the spatial resolution at the PET field-of-view (FOV) periphery while keeping the sensitivity. In this study we proposed a novel staggered 3-layer DOI detector using BaSO4 reflector material for an enhanced crystal identification performance as well as ICS event rejection capability over those of ESR reflector based DOI detectors. The proposed staggered 3-layer DOI detector had 3-layer staggered LYSO crystal arrays (crystal pitch = 1 mm), an acrylic light guide, and a 4 × 4 SiPM array. The 16 SiPM anode signals were read out by using a resistive network to encode the crystal position and energy information while the timing signal was extracted from the common cathode. The crystal map quality was substantially enhanced by using the BaSO4 reflector material as compared to that of the ESR reflector due to the low optical crosstalk between the LYSO crystals. The ICS events can be rejected with BaSO4 by using simple pulse height discrimination thanks to the light collection efficiency difference that depends on the crystal layers. As a result, the total number of events was decreased around 26% with BaSO4 as compared to that of ESR. The overall energy resolution and coincidence timing resolution with BaSO4 were 19.7 ± 5.6% and 591 ± 160 ps, respectively which were significantly worse than 10.9 ± 2.2% and 308 ± 23 ps values of ESR because of the relatively low light collection efficiency with BaSO4 (1057 ± 308 ADC) compared to that of ESR (1808 ± 118 ADC). In conclusion, we found the proposed staggered 3-layer DOI detector using the BaSO4 reflector material with ICS event rejection capability can be a cost-effective solution for realizing high resolution and highly sensitive small animal PET scanners while minimizing the complexity of the SiPM readout circuit.

Morphometric comparisons and novel observations of diurnal and low-light-foraging bees

Low-light adapted bees are substantially understudied components of the bee fauna, particularly in Australia. Whilst several species in Australia are thought to be adapted to low-light conditions, explicit records of these taxa actually foraging at twilight or night are absent from the scientific literature. We present the first observations of Australian bees foraging in low-light conditions as well as the first evidence of low-light foraging behaviour in the colletid bee subfamily, Hylaeinae. Using morphometrics of Australian and more broadly-distributed diurnal, facultative low-light and obligate low-light adapted bees, we explore the use of morphological traits to objectively assess possible low-light behaviour and corroborate low-light collection events. Our results show that it is possible to morphologically distinguish between diurnal and low-light adapted bees, and that there is a spectrum of characters that are associated with low light conditions. We use GIS to show that low-light adapted species occur mostly in the tropics, but that some species have subtropical, arid and even temperate distributions. As low-light foraging behaviour in bees is infrequently reported, it appears that low-light foraging behaviour is more common than currently appreciated, highlighting the need for extended bee-sampling periods and more consistent collection data to increase the understanding of this little-understood aspect of bee behaviour.

Fast hyperspectral phase and amplitude imaging in scattering tissue.

Hyperspectral imaging in scattering tissue generally suffers from low light collection efficiency. In this Letter, we propose a microscope based on Fourier transform spectroscopy and oblique back-illumination microscopy that provides hyperspectral phase and amplitude images of thick, scattering samples with high throughput. Images can be acquired at >0.1 Hz rates with spectral resolution better than 200 cm-1, over a wide spectral range of 450-1700nm. Proof-of-principle demonstrations are presented with chorioallantoic membrane of a chick embryo, illustrating the possibility of high-resolution hemodynamics imaging in thick tissue, based on transmission contrast.

Pulsed dipole radiation in a transformation-optics wedge waveguide designed by azimuthal space compression

A transformation-optics wedge waveguide designed for the simultaneous collection and directional collimation of pulsed dipole radiation is described and tested with numerical simulation. Azimuthal compression of free space toward a narrow fan-shaped waveguide sector allows dipole pulse radiation in free space to be transformed into a directional non-dispersive pulse propagating within that sector. The collection and collimation ability of the proposed structure is compared with classical approaches using metallic wedge mirrors and parabolic mirrors, which inherently allow multiple internal reflections and thus generate significant pulse distortion and low light-collection efficiency. It is shown that the optical pulse generated by the dipole and propagated through the proposed transformation-optics waveguide maintains its original shape within the structure, and demonstrates enhanced optical power.

Enhancing collection efficiency in large field of view multiphoton microscopy

Many multiphoton imaging applications would benefit from a larger field of view; however, large field of views (>mm) require low magnification objectives which have low light collection efficiencies. We demonstrate a light collection system mounted on a low magnification objective that increases fluorescence collection by as much as 20-fold in scattering tissues. This peripheral detector results in an effective numerical aperture of collection >0.8 with a 3-4 mm field of view.

Analysis of the kinestatic charge detection system as a high detective quantum efficiency electronic portal imaging device

Megavoltage x-ray imaging suffers from reduced image quality due to low differential x-ray attenuation and large Compton scatter compared with kilovoltage imaging. Notwithstanding this, electronic portal imaging devices (EPIDs) are now widely used in portal verification in radiotherapy as they offer significant advantages over film, including immediate digital imaging and superior contrast range. However video-camera-based EPIDs (VEPIDs) are limited by problems of low light collection efficiency and significant light scatter, leading to reduced contrast and spatial resolution. Indirect and direct detection-based flat-panel EPIDs have been developed to overcome these limitations. While flat-panel image quality has been reported to exceed that achieved with portal film, these systems have detective quantum efficiency (DQE) limited by the thin detection medium and are sensitive to radiation damage to peripheral read-out electronics. An alternative technology for high-quality portal imaging is presented here: kinesatic charge detection (KCD). The KCD is a scanning tri-electrode ion-chamber containing high-pressure noble gas (xenon at 100 atm) used in conjunction with a strip-collimated photon beam. The chamber is scanned across the patient, and an external electric field is used to regulate the cation drift velocity. By matching the scanning velocity with that of the cation (i.e., ion) drift velocity, the cations remain static in the object frame of reference, allowing temporal integration of the signal. The KCD offers several advantages as a portal imaging system. It has a thick detector geometry with an active detection depth of 6.1 cm, compared to the sub-millimeter thickness of the phosphor layer in conventional phosphor screens, leading to an order of magnitude advantage in quantum efficiency (>0.3). The unique principle of and the use of the scanning strip-collimated x-ray beam provide further integration of charges in time, reduced scatter, and a significantly reduced imaging dose, enhancing the imaging signal-to-noise ratio (SNR) and leading to high DQE. While thick detectors usually suffer from reduced spatial resolution, the KCD provides good spatial resolution due to high gas pressure that limits the spread of scattered electrons, and a strip-collimated beam that significantly reduces the inclusion of scatter in the imaging signal. A 10 cm wide small-field-of-view (SFOV) prototype of the KCD is presented with a complete analysis of its imaging performance. Measurements of modulation transfer function (MTF), noise power spectrum (NPS), and DQE were in good agreement with Monte Carlo simulations. Imaging signal loss from recombination within the KCD chamber was measured at different gas pressures, ion drift velocities, and strip-collimation widths. Image quality for the prototype KCD was also observed with anthropomorphic phantom imaging in comparison with various commercial and research portal imaging systems, including VEPID, flat-panel imager, and conventional and high contrast film systems. KCD-based imaging provided very good contrast and good spatial resolution at very low imaging dose (0.1 cGy per image). For the prototype KCD, measurements yielded DQE(0)=0.19 and DQE(1 cy/mm)=0.004.

Development of a lens-coupled CMOS detector for an X-ray inspection system

A digital X-ray imaging detector based on a complementary metal-oxide-semiconductor (CMOS) image sensor has been developed for X-ray non-destructive inspection applications. This is a cost-effective solution because of the availability of cheap commercial standard CMOS image sensors. The detector configuration adopts an indirect X-ray detection method by using scintillation material and lens assembly. As a feasibility test of the developed lens-coupled CMOS detector as an X-ray inspection system, we have acquired X-ray projection images under a variety of imaging conditions. The results show that the projected image is reasonably acceptable in typical non-destructive testing (NDT). However, the developed detector may not be appropriate for laminography due to a low light-collection efficiency of lens assembly. In this paper, construction of the lens-coupled CMOS detector and its specifications are described, and the experimental results are presented. Using the analysis of quantum accounting diagram, inefficiency of the lens-coupling method is discussed.

A superfiber for position sensitive detectors

Scintillating fibers are being used more and more in science and medicine to provide precise coordinate measurement systems for elementary particles and X-ray detection. However, low light collection efficiency (3–4% per direction typically) is still the most limiting factor. In this paper we present results of Monte Carlo calculations to illustrate some peculiarities of light collection in round fibers. According to our analyses, by certain modifications light collection in round scintillating and waveshifting fibers could be increased to about 20% per direction.