Optics

The photoelectric effect in a Ge-on-Si single-photon avalanche detector (SPAD) at an ultralow energy in incident pulsed laser radiation is considered in the frame of the classical theory of the electrodynamics of continuous media. It is shown that the energy of incident laser radiation which is shared among a huge number of electrons in a Ge matrix can concentrate on only one of these through the effect of the constructive interference of the fields re-emitted by surrounding electrons. Conservation of energy in this case is upheld because of a substantial narrowing of the effective bandgap in heavily doped p-Ge, which is used in the design of the SPAD considered. Full article

The present study is aimed at designing anti-reflective (AR) engraving on the input–output surfaces of a rectangular light-guide. We estimate AR efficiency, by the transmittance level in the angular range, determined by the light-guide. Using nano-engraving, we achieve a uniform high transmission over a wide range of wavelengths. In the past, we used smoothed conical pins or indentations on the faces of light-guide crystal as the engraved structure. Here, we widen the class of pins under consideration, following the physical model developed in the previous paper. We analyze the smoothed pyramidal pins with different base shapes. The possible effect of randomization of the pins parameters is also examined. The results obtained demonstrate optimized engraved structure with parameters depending on the required spectral range and facet format. The predicted level of transmittance is close to 99%, and its flatness (estimated by the standard deviation) in the required wavelengths range is 0.2%. The theoretical analysis and numerical calculations indicate that the obtained results demonstrate the best transmission (reflection) we can expect for a facet with the given shape and size for the required spectral band. The approach is equally useful for any other form and of the facet. We also discuss a simple way of comparing experimental and theoretical results for a light-guide with the designed input and output features. In this study, as well as in our previous work, we restrict ourselves to rectangular facets. We also consider the limitations on maximal transmission produced by the size and shape of the light-guide facets. The theoretical analysis is performed for an infinite structure and serves as an upper bound on the transmittance for smaller-size apertures. Full article

Photoacoustic imaging (PAI) is a powerful imaging modality that relies on the PA effect. PAI works on the principle of electromagnetic energy absorption by the exogenous contrast agents and/or endogenous molecules present in the biological tissue, consequently generating ultrasound waves. PAI combines a high optical contrast with a high acoustic spatiotemporal resolution, allowing the non-invasive visualization of absorbers in deep structures. However, due to the optical diffusion and ultrasound attenuation in heterogeneous turbid biological tissue, the quality of the PA images deteriorates. Therefore, signal and image-processing techniques are imperative in PAI to provide high-quality images with detailed structural and functional information in deep tissues. Here, we review various signal and image processing techniques that have been developed/implemented in PAI. Our goal is to highlight the importance of image computing in photoacoustic imaging. Full article

Visible Light Communication (VLC) has gained popularity in research and business in the last decade. This technology aims to combine lighting and communication into a single device. For now, this technology has been thoroughly studied for an indoor environment, but it is sufficiently mature nowadays to consider its outdoor-environment potentials. The key outdoor challenges are the weather variabilities and smoke particles in cities due to pollution or fires. The aim of this is the study and quantification of the weather and smoke particles’ impact on a short-range optical communication thanks to a simulator. This article’s novelty is the inclusion of the effects of smoke in a short-range outdoor VLC system channel model. This smoke model, which comes from the fire engineering field, states that smoke attenuation is independent of the wavelength, starting from high visibility to 5 m. The visibility represents the distance up to which an object can be distinguished against the background. The effects of fog and smoke are studied in the case of two outdoor VLC scenarios. Smoke and fog models have analogous equations to express the optical attenuation they induce, using the visibility concept. Taking into account the actual light-emitting diode (LED) lamp radiation pattern, the simulator computes the power at the receiver side and the channel attenuation coefficients for a given fog or/and smoke outdoor setting. The main result drawn in this paper is that the channel attenuation levels due to fog and smoke are both in the same order of magnitude, starting from the visibility of about 1 km. The attenuation induced by fog is higher under this threshold of 1 km. Full article

Photons in interferometers manifest the functional ability to simultaneously navigate both paths through the device, but eventually appear at only one outlet. How this relates to the physical behaviour of the particle is still ambiguous, even though mathematical representation of the problem is adequate. This paper applies a non-local hidden-variable (NLHV) solution, in the form of the Cordus theory, to explain photon path dilemmas in the Mach–Zehnder (MZ) interferometer. The findings suggest that the partial mirrors direct the two reactive ends of the Cordus photon structures to different legs of the apparatus, depending on the energisation state of the photon. Explanations are provided for a single photon in the interferometer in the default, open-path, and sample modes. The apparent intelligence in the system is not because the photon knows which path to take, but rather because the MZ interferometer is a finely-tuned photon-sorting device that auto-corrects for randomness in the frequency phase to direct the photon to a specific detector. The principles also explain other tunnelling phenomena involving barriers. Thus, navigation dilemmas in the MZ interferometer may be explained in terms of physical realism after all. Full article

Optical fibre micro/nano tips (OFTs), defined here as tapered fibres with a waist diameter ranging from a few microns to tens of nanometres and different tip angles (i.e., from tens of degrees to fractions of degrees), represent extremely versatile tools that have attracted growing interest during these last decades in many areas of photonics. The field of applications can range from physical and chemical/biochemical sensing—also at the intracellular levels—to the development of near-field probes for microscope imaging (i.e., scanning near-field optical microscopy (SNOM)) and optical interrogation systems, up to optical devices for trapping and manipulating microparticles (i.e., optical tweezers). All these applications rely on the ability to fabricate OFTs, tailoring some of their features according to the requirements determined by the specific application. In this review, starting from a short overview of the main fabrication methods used for the realisation of these optical micro/nano structures, the focus will be concentrated on some of their intriguing applications such as the development of label-based chemical/biochemical sensors and the implementation of SNOM probes for interrogating optical devices, including whispering gallery mode microcavities. Full article

Operando and precisely probing aqueous pH is fundamentally demanded, both in chemical and biological areas. Conventional pH probes, subjected to the larger size, are probably unfit for application in some extreme scenarios, such as a trace amount of samples. In this paper, we have further developed the pH sensor that leverages the microfiber Bragg grating with an ultra-compact size down to an order of magnitude of 10⁻¹⁴ m³. Using the electrostatic self-assembly layer-by-layer technique, the functional film consisting of sodium alginate, which harnesses a pH-dependent hygroscopicity, is immobilized on the fiber surface. Consequently, the alteration of aqueous pH could be quantitatively indicated by the wavelength shift of the grating resonance via the refractive index variation of the sensing film due to the water absorption or expulsion. The grating reflections involving fundamental mode and higher order mode exhibit the sensitivities of −72 pm/pH and −265 pm/pH, respectively. In addition, temperature compensation can be facilitated by the recording of the two reflections simultaneously. Furthermore, the modeling and simulation results predict the pivotal parameters of the configuration in sensitivity enhancement. The proposed proof-of-concept enriches the toolbox of pH sensor for catering to the need of detection in some extremely small spaces—for example, the living cells or the bio-tissues. Full article

To improve pulse contrast in chirped pulse amplification petawatt laser systems, the regenerative amplifier is substituted with a multipass amplifier at the Shanghai Superintense Ultrafast Laser Facility (SULF). To reduce the consequent angular dispersion of the broadband spectrum, a double-grating stretcher is established in the SULF front end. A grating compressor is set up for the 10-PW front end to obtain 20-TW output. An accurate adjustment method of grating attitude (angular position) is presented, which references the direction of gravity, improving dispersion management and focusing ability of the beam. After a pulse passes the front end compressor, its duration and phase in the frequency domain are measured, and the duration can be continuously compressed to <24 fs. Full article

In this work, we present a feasibility study of integrated optofluidic chips to measure the ionic content in water using differential absorption spectroscopy. The second overtone of the OH-stretch vibration of water is used as indicator for both the type and concentration of the dissolved ions. The optofluidic chips are based on silicon nitride (TripleX) containing Mach–Zehnder interferometers (MZI) with two 5 cm sensing paths for the sample and reference arms, respectively. Simulations show that, theoretically, the determination of both the type and concentration of a mixture of four electrolytes is possible with the techniques presented. However, the performance of the chips deviated from the expected results due to the insufficient reproducibility and precision in the fabrication process. Therefore, at this early stage, the chips presented here could only determine the ion concentration, but not differentiate between the different ion types. Still, this work represents the first steps towards the realization of an online and real-time sensor of ionic content in water. Full article

Thermal energy storages (TES) are increasingly important for storing energy from renewable energy sources. TES that work with liquid storage materials are used in their most efficient way by stratifying the storage fluid by its thermal density gradient. Mixing of the stratification layers during stand-by periods decreases the thermal efficiency of the TES. Tank sidewalls, unlike the often poorly heat-conducting storage fluids, promote a heat flux from the hot to the cold layer and lead to thermal convection. In this experimental study planar particle image velocimetry (PIV) measurements and background-oriented schlieren (BOS) temperature measurements are performed in a model experiment of a TES to characterise the influence of the thermal convection on the stratification and thus the storage efficiency. The PIV results show two vertical, counter-directed wall jets that approach in the thermocline between the stratification layers. The wall jet in the hot part of the thermal stratification shows compared to the wall jet in the cold region strong fluctuations in the vertical velocity, that promote mixing of the two layers. The BOS measurements have proven that the technique is capable of measuring temperature fields in thermally stratified storage tanks. The density gradient field as an intermediate result during the evaluation of the temperature field can be used to indicate convective structures that are in good agreement to the measured velocity fields. Full article

The novel 3D imaging and reconstruction capabilities of plenoptic cameras are extended for use with continuous scalar fields relevant to reacting flows. This work leverages the abundance of perspective views in a plenoptic camera with the insertion of multiple filters at the aperture plane. The aperture is divided into seven regions using off-the-shelf components, enabling the simultaneous capture of up to seven different user-selected spectra with minimal detriment to reconstruction quality. Since the accuracy of reconstructed features is known to scale with the available angular information, several filter configurations are proposed to maintain the maximum parallax. Three phantoms inspired by jet plumes are simulated onto an array of plenoptic cameras and reconstructed using ASART+TV with a variety of filter configurations. Some systematic challenges related to the non-uniform distribution of views are observed and discussed. Increasing the number of simultaneously acquired spectra is shown to incur a small detriment to the accuracy of reconstruction, but the overall loss in quality is significantly less than the gain in spectral information. Full article

The calibration of a multi-camera system for volumetric measurements is a basic requirement of reliable 3D measurements and object tracking. In order to refine the precision of the mapping functions, a new, tomographic reconstruction-based approach is presented. The method is suitable for Volumetric Particle Image Velocimetry (PIV), where small particles, drops or bubbles are illuminated and precise 3D position tracking or velocimetry is applied. The technique is based on the 2D cross-correlation of original images of particles with regions from a back projection of a tomographic reconstruction of the particles. The off-set of the peaks in the correlation maps represent disparities, which are used to correct the mapping functions for each sensor plane in an iterative procedure. For validation and practical applicability of the method, a sensitivity analysis has been performed using a synthetic data set followed by the application of the technique on Tomo-PIV measurements of a jet-flow. The results show that initial large disparities could be corrected to an average of below 0.1 pixels during the refinement steps, which drastically improves reconstruction quality and improves measurement accuracy and reliability. Full article

We present a method to generate any genuine electromagnetic partially coherent source (PCS) from correlated, stochastic complex screens. The method described here can be directly implemented on existing spatial-light-modulator-based vector beam generators and can be used in any application which utilizes electromagnetic PCSs. Our method is based on the genuine cross-spectral density matrix criterion. Applying that criterion, we show that stochastic vector field realizations (corresponding to a desired electromagnetic PCS) can be generated by passing correlated Gaussian random numbers through “filters” with space-variant transfer functions. We include step-by-step instructions on how to generate the electromagnetic PCS field realizations. As an example, we simulate the synthesis of a new electromagnetic PCS. Using Monte Carlo analysis, we compute statistical moments from independent optical field realizations and compare those to the corresponding theory. We find that our method produces the desired source—the correct shape, polarization, and coherence properties—within 600 field realizations. Full article

In this paper, we propose the use of a microfluidic channel with flow focusing technique to fabricate solid state polymeric microlasers to precisely control sizes for mass production. Microlasers are made from a solution of UV curable polymer, namely polyethylene glycol diacrylate (PEGDA) with a molecular weight of 700 and rhodamine 6G laser dye at two different volumetric ratios (polymer to dye) of 4:1 and 2:1, respectively, which are used as the dispersed phase. A reservoir filled with liquid polydimethylsiloxane (PDMS) was used to cure the microlasers via UV lamp. A microchannel made of (PDMS) and size of 200 µm was used in this paper; mineral oil was selected as the continuous phase. Two experiments are conducted by fixing the pressure flow for the dispersed phase to 188 mbar and 479.9 mbar, respectively. In both experiments, the pressure of the continuous phase (mineral oil) was varied between 1666.9 mbar and 1996.9 mbar. The measurement of the fabricated microlasers’ size was performed with the aid of the MATLAB Image Processing Toolbox by using photographs taken with a CMOS camera. The tunability of the highest size, ranging from 109 µm to 72 µm, was found for the PEGDA to dye ratio of 2:1 (188 mbar) and average standard deviation of 1.49 µm, while no tunability was found for the 4:1 ratio (188 mbar). The tunability of the microlaser’s size, ranging from 139 µm to 130 µm and an average standard deviation value of 1.47 µm, was found for the 4:1 ratio (479.9 mbar). The fabricated microlasers presented a quality factor Q of the order 10⁴, which is suitable for sensing applications. This technique can be used to control the size of the fabrication of a high number of solid state microlaser based UV polymers mixed with laser dyes. Full article

Flow measurement in porous media is a challenging subject, especially when it comes to performing a three-dimensional (3D) velocimetry at the micro scale. Volumetric flow measurement techniques such as defocusing and tomographic imaging generally involve rigorous procedures, complex experimental setups, and multi-part data processing procedures. However, detailed knowledge of the flow pattern at the pore and subpore scales is important in interpreting the phenomena that occur inside the porous media and understanding the macro-scale behaviors. In this work, the flow of an oil inside a porous medium is measured at the pore and subpore scales using refractive index matching (RIM) and shadowgraph imaging techniques. At the macro scale, flow is measured using the particle image velocimetry (PIV) method in two dimensions (2D) to confirm the volumetric nature of the flow and obtain the overall flow pattern in the vicinity of the flow entrance and at the far field. At the micro scale, the three-dimensional (3D) flow within an arbitrary volume of the porous medium was quantified using 2D particle-tracking velocimetry (PTV) utilizing the law of conservation of mass. Using the shadowgraphy method and a single camera makes the flow measurement much less complex than the approaches using laser light sheets or multiple cameras with multiple viewing angles. Full article