The laser-induced damage threshold (LIDT) is a commonly used method for testing optical thin films, where the sample is exposed to laser radiation of a defined intensity and observed for laser-induced damage. Sensitive evaluation of the damage is essential for the experiment and different approaches are used for this purpose. This work introduces a new approach to LIDT evaluation based on the second harmonic generation (SHG) principle for the detection of defects in thin films. The process of SHG is very sensitive to changes in the symmetry of the crystal lattice of the material, which can be very well exploited for the observation of defects and various changes in thin films. We developed a setup able to track the polarization-dependent SHG from the samples with micrometer resolution for a broad range of incident angles in both reflection and transmission regimes. We use this setup to study the polarization and angular dependence of SHG in pristine and irradiation-affected areas on various thin films (Si3N4, TiO2,...). Our measurements show that SHG makes it possible to reliably detect spots with subtle laser-induced changes in the material that are hardly detectable or even undetectable by scanning electron microscopy or other commonly used methods.
In this study, we introduce a technique for creating 3D speckle patterns, which can be utilized in photoluminescence tomography based the single-pixel camera technique. Initially, we showcase our capability to generate independent speckle patterns at different planes near the focal plane, facilitating z-axis resolution imaging. Using simulated data, we explored the effects of various factors and, informed by these analyses, developed an experimental setup capable of producing 3D speckle patterns in reality. Our results indicate that by precisely adjusting the phase distribution of the input field, we can reproduce the 3D patterns with the level of simulation-experiment correlation exceeding 0.9. We further demonstrate the application of actual 3D speckle patterns in 3D photoluminescence tomography using a single-pixel camera and discuss its promise for investigating defects in optical materials.
Various modes of spatial modulations in compressive imaging can vastly affect the measurement quality and reduce the number of measurements needed. While there have been experiments focusing on different modulations, usually only a handful have been tested and in varying conditions. We propose a scheme unifying the testing parameters while exploiting our dual hyperspectral compressive microscope to create a range of conditions in one measurement. In it, the influence of light collection from the DMD, including its diffraction, can be considered. With this, we proceeded to test a total of 20 modulations and their modifications. Simulations using the same parameters were conducted with and without added noise. In the experimental setup, all modulations have been tested across three samples and three different resolutions to provide a comprehensive comparison. Individual effects and complications have also been identified, providing tips for modulation testing and deployment.
Absorption spectroscopy and photoluminescence (PL) decay measurement count among the most used spectroscopic methods to characterize optical materials. Therefore, an extension of these techniques into the imaging mode – hyperspectral imaging (HSI) and fluorescence lifetime imaging (FLIM) – is immensely useful and commonly used.
The measurement of transmittance HSI and FLIM is seldom combined in a single system due to the necessity to use very different working conditions. For HSI, high light intensity needs to illuminate a sample as its image is typically scanned across a slit. On the contrary, the most common FLIM technique, time-correlated single-photon counting (TCSPC), uses low-light conditions and laser beam scanning.
We present a versatile multimodal microscopic system that combined transmittance HSI with FLIM using a single setup based on compressed sensing, namely on the so-called single-pixel camera (SPC). This method makes it possible to reconstruct an image by using only a fraction of the measurements necessary compared to the number of pixels. The use of the SPC concept allowed us to create multidimensional HS and FLIM maps, which are inherently matched. Moreover, their spectral and temporal resolution follows the resolution of the used spectrometer and TCSPC setup.
Our system is also highly versatile with respect to image magnification and field of view. Owing to the use of a digital micromirror device, we can switch without any modification of the setup, from the large field of view (3.5 × 2.6 mm) to the imaging on the micrometer scale limited only by the microscope lens.
We demonstrate the functionality of the setup in both HS and FLIM mode on the testing samples of fluorescent dyes, luminophore (LuAG:Ce), and the cells of Convallaria.
Digital micromirror device (DMD) serves in a major part of computational optical setups as a means of encoding an image by a desired pattern. The most prominent is its usage in the so-called single-pixel camera experiments, where light reflected from a DMD is collected onto a single-pixel detector. This often requires efficient and homogenous collection of light from a relatively large chip on a small area of an optical fiber or spectrometer slit. This effort is moreover complicated by the fact that the DMD acts as a diffractive element – this becomes especially prominent in the infrared (IR) spectral region. The light diffraction causes serious spectral inhomogeneities in the light collection. We studied the effect of light diffraction via whiskbroom hyperspectral camera. Based on the knowledge, we designed a variety of different approaches to light collection, which use a combination lenses, off-axis parabolic mirrors, diffuser, light concentrator, and integrating spheres. By using an identical optical setup we mapped the efficiency and spectral homogeneity of each of the approaches. The selected benchmark was the ability to collect the light into fiber spectrometers working in the visible and IR range (up to 2500 nm). As expected, we have found the integrating spheres to provide homogeneous light collection, which however suffers from a low efficiency. The best compromise between the performance parameters was provided by a combination of an engineered diffuser with an off-axis parabolic mirror. We used this configuration to create a computational microscope able to carry out hyperspectral imaging of a sample in a broad spectral range (400-2500 nm) and to map photoluminescence (PL) decay via time-correlated single photon counting technique. This allowed us to create one-to-one maps of absorption and PL inhomogeneities in samples. We see such setup as an ideal tool to study properties of luminophores and the effect of inhomogeneities on the PL properties.
Spectrum of light which is emitted or reflected by an object carries immense amount of information about the object. A simple piece of evidence is the importance of color sensing for human vision. Combining an image acquisition with efficient measurement of light spectra for each detected pixel is therefore one of the important issues in imaging, referred as hyperspectral imaging. We demonstrate a construction of a compact and robust hyperspectral camera for the visible and near-IR spectral region. The camera was designed vastly based on off-shelf optics, yet an extensive optimization and addition of three customized parts enabled construction of the camera featuring a low f-number (F/3.9) and fully concentric optics. We employ a novel approach of compressed sensing (namely coded aperture snapshot spectral imaging, abbrev. CASSI). The compressed sensing enables to computationally extract an encoded hyperspectral information from a single camera exposition. Owing to the technique the camera lacks any moving or scanning part, while it can record the full image and spectral information in a single snapshot. Moreover, unlike the commonly used compressed sensing table-top apparatuses, the camera represents a portable device able to work outside a lab. We demonstrate the spectro-temporal reconstruction of recorded scenes based on 90×90 random matrix encoding. Finally, we discuss potential of the compressed sensing in hyperspectral camera.
Pb(Zr,Ti)O3 (PZT) is a ferroelectric material interesting for its high dielectric constant and piezoelectric response. PZT thin films can be prepared by various methods, e.g. pulsed laser deposition, chemical vapor deposition, sol-gel and, most frequently, sputtering. Though the magnetron sputtering is used more frequently, PZT thin films can be prepared also by ion-beam sputtering (IBS). In this paper we study the deposition process of PZT thin films in our IBS system with a possibility of ion-beam assisted deposition (IBAD), which has the advantage that more energy can be added to the growing layer. We show how in our system the resulting layers, mainly their quality, the Pb content, which is important for the creation of the perovskite crystal structure, and the resulting crystal structure are influenced by the oxygen flux during the deposition for the samples grown on the silicon substrate with and without an intermediate Ti seeding layer.
Compressed sensing (CS) is a branch of computational optics able to reconstruct an image (or any other information) from a reduced number of measurements – thus significantly saving measurement time. It relies on encoding the detected information by a random pattern and consequent mathematical reconstruction. CS can be the enabling step to carry out imaging in many time-consuming measurements. The critical step in CS experiments is the method to invoke encoding by a random mask. Complex devices and relay optics are commonly used for the purpose. We present a new approach of creating the random mask by using laser speckles from coherent laser light passing through a diffusor. This concept is especially powerful in laser spectroscopy, where it does not require any complicated modification of the current techniques. The main advantage consist in the unmatched simplicity of the random pattern generation and a versatility of the pattern resolution. Unlike in the case of commonly used random masks, here the pattern fineness can be adjusted by changing the laser spot size being diffused. We demonstrate the pattern tuning together with the connected changes in the pattern statistics. In particular, the issue of patterns orthogonality, which is important for the CS applications, is discussed. Finally, we demonstrate on a set of 200 acquired speckle patterns that the concept can be successfully employed for single-pixel camera imaging. We discuss requirements on detector noise for the image reconstruction.
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