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.
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