The excitation and collection of optical signals using lenses form the basis for many applications in imaging, nephelometry, fluorometry, and spectroscopy. While lenses are needed for imaging systems, their use is not so obvious for volume sensing applications. Here, we study the excitation and collection of fluorescence signals to show that lensless systems generally provide a stronger signal compared to lensed systems for the case of extended Lambertian-like sources, such as LEDs. To elucidate this result, we provide a foundational framework to analyze the signal collection efficiency from an arbitrary detection volume with and without lenses when extended sources and detectors are used. A combination of factors, including the limited numerical aperture, the use of extended sources/detectors, and the requirement of a finite imaging distance between the source/detector, lenses, and the sample, limits the performance of the lensed system compared to the lensless system. Our theoretical and experimental results indicate that conventional wisdom based on the assumption of point-like sources and detectors should not always be followed. We provide a systematic approach for analyzing and simplifying the design of low-cost, lensless fluorometers and nephelometers without sacrificing their performance, reporting a sub-ppb level detection limit for measuring tryptophan-like-fluorescence in drinking water.

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Recent reports of upconversion and white light emission from graphitic particles warrant an explanation of the physics behind the process. We offer a model, wherein the upconversion is facilitated by photo-induced electronic structure modification allowing for multi-photon processes. As per the prediction of the
model, we experimentally show that graphite upconverts infrared light centered around 1.31 μm to broadband white light centered around 0.85 μm. Our results suggest that upconversion from shortwave infrared (∼3 μm) to visible region may be possible. Our experiments show that the population dynamics of the electronic states involved in this upconversion process occur in the timescale of milliseconds.

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We present a compact, CMOS compatible, photonic integrated circuit (PIC) based spectrometer that combines a dispersive array element of SiO2-filled scattering holes within a multimode interferometer (MMI) fabricated on the silicon-on-insulator (SOI) platform. The spectrometer has a bandwidth of 67 nm, a lower bandwidth limit of 1 nm, and a peak-to-peak resolution of 3 nm for wavelengths around 1310 nm.

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Distinguishing chemically similar particles in a complex environment has been a challenging problem in spectroscopy, such as micro-Raman spectroscopy. Here, we show that it is possible to distinguish particles from their spectroscopic signals in a modulated optical trap, where the trapping field also acts as an excitation source. Using the overdamped Langevin equation, we report that spectroscopic signals averaged over a certain signal acquisition time exhibit several discrete minimas at unique modulating frequencies dependent on their drag coefficient, exemplified herein as a function of the particle size. In typical experimental conditions, such minimas can be as low as ∼50% of the signal of an unmodulated trap.

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A real-time, portable and low-cost detection method for the detection of fecal contamination in drinking water has been in demand, especially in developing countries. Studies [1,2] have demonstrated correlation between the intensity of Tryptophan-like Fluorescence (TLF, Excitation/Emission = 280 nm / 350 nm) from environmental water samples to the number of colony forming units (CFUs) of TTCs (thermotolerant coliforms) present in the water sample. In this work, we validate that TLF-fluorometry is applicable for detection of fecal contamination for the range of water samples collected from various important sources in the Kathmandu valley by analyzing their microbiological and UV-fluorescence properties.

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The field of silicon photonics has experienced widespread adoption in the datacoms industry over the past decade, with a plethora of other applications emerging more recently such as light detection and ranging (LIDAR), sensing, quantum photonics, programmable photonics and artificial intelligence. As a result of this, many commercial complementary metal oxide semiconductor (CMOS) foundries have developed open access silicon photonics process lines, enabling the mass production of silicon photonics systems. On the other side of the spectrum, several research labs, typically within universities, have opened up their facilities for small scale prototyping, commonly exploiting e-beam lithography for wafer patterning. Within this ecosystem, there remains a challenge for early stage researchers to progress their novel and innovate designs from the research lab to the commercial foundries because of the lack of compatibility of the...

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We experimentally show that azimuthally apodized circular gratings can allow light to penetrate further into the structure, and create a focus of ≈ 10 μm, allowing for biomedical applications such as OCT.

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Characterizing a monolayer of biological molecules has been a major challenge. We demonstrate nanophotonic waveguide enhanced Raman spectroscopy (NWERS) of monolayers in the near-infrared region, enabling real-time measurements of the hybridization of DNA strands and the density of submonolayers of biotin–streptavidin complex immobilized on top of a photonics chip. NWERS is based on enhanced evanescent excitation and collection of spontaneous Raman scattering near nanophotonic waveguides, which for a 1 cm silicon nitride waveguide delivers a signal that is more than 4 orders of magnitude higher in comparison to a confocal Raman microscope. The reduced acquisition time and specificity of the signal allows for a quantitative and real-time characterization of surface species, hitherto not possible using Raman spectroscopy. NWERS provides a direct analytic tool for monolayer research and also opens a route to compact microscopeless lab-on-a-chip devices with integrated sources, spectrometers, and detectors fabricated using a mass-producible complementary metal oxide semiconductor technology platform.

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