Diffraction-limited hyperspectral mid-infrared micro-ellipsometry.

The recent introduction of quantum cascade lasers (QCL) in infrared spectroscopic ellipsometry led to decisive improvements in measurement times and signal-to-noise ratios of this powerful analytical method. In this contribution, we present another significant enhancement leading to the first, to the best of our knowledge, diffraction-limited micro-ellisometry setup in the mid-infrared spectral range with a spatial resolution better than 13.3 µm. The fast spectral tunability of the QCL combined with phase-modulated polarization enabled simultaneous acquisition of broadband (900 cm-1-1204 cm-1) high-resolution (1 cm-1) hyperspectral Ψ, Δ-cubes in a scanning approach in reasonable time scales. The spatial resolution of the QCL micro-ellipsometer was experimentally characterized by the knife-edge method and measurements of a resolution test target. Furthermore, the hyperspectral ellipsometric investigation of a polymer multilayer cross section and the portrait window of a 200-euro bank note demonstrate the capabilities of diffraction-limited QCL micro-ellipsometry.

Diffraction-limited hyperspectral mid-infrared single-pixel microscopy.

In this contribution, we demonstrate a wide-field hyperspectral mid-infrared (MIR) microscope based on multidimensional single-pixel imaging (SPI). The microscope employs a high brightness MIR supercontinuum source for broadband (1.55 [Formula: see text]-4.5 [Formula: see text]) sample illumination. Hyperspectral imaging capability is achieved by a single micro-opto-electro-mechanical digital micromirror device (DMD), which provides both spatial and spectral differentiation. For that purpose the operational spectral bandwidth of the DMD was significantly extended into the MIR spectral region. In the presented design, the DMD fulfills two essential tasks. On the one hand, as standard for the SPI approach, the DMD sequentially masks captured scenes enabling diffraction-limited imaging in the tens of millisecond time-regime. On the other hand, the diffraction at the micromirrors leads to dispersion of the projected field and thus allows for wavelength selection without the application of additional dispersive optical elements, such as gratings or prisms. In the experimental part, first of all, the imaging and spectral capabilities of the hyperspectral microscope are characterized. The spatial and spectral resolution is assessed by means of test targets and linear variable filters, respectively. At a wavelength of 4.15 [Formula: see text] a spatial resolution of 4.92 [Formula: see text] is achieved with a native spectral resolution better than 118.1 nm. Further, a post-processing method for drastic enhancement of the spectral resolution is proposed and discussed. The performance of the MIR hyperspectral microsopce is demonstrated for label-free chemical imaging and examination of polymer compounds and red blood cells. The acquisition and reconstruction of Hadamard sampled 64 [Formula: see text] 64 images is achieved in 450 ms and 162 ms, respectively. Thus, combined with an unprecedented intrinsic flexibiliy gained by a tunable field of view and adjustable spatial resolution, the demonstrated design drastically improves the sample throughput in MIR chemical and biomedical imaging.

Advances in mid-infrared spectroscopy enabled by supercontinuum laser sources.

Supercontinuum sources are all-fiber pulsed laser-driven systems that provide high power spectral densities within ultra-broadband spectral ranges. The tailored process of generating broadband, bright, and spectrally flat supercontinua-through a complex interplay of linear and non-linear processes-has been recently pushed further towards longer wavelengths and has evolved enough to enter the field of mid-infrared (mid-IR) spectroscopy. In this work, we review the current state and perspectives of this technology that offers laser-like emission properties and instantaneous broadband spectral coverage comparable to thermal emitters. We aim to go beyond a literature review. Thus, we first discuss the basic principles of supercontinuum sources and then provide an experimental part focusing on the quantification and analysis of intrinsic emission properties such as typical power spectral densities, brightness levels, spectral stability, and beam quality (to the best of the authors' knowledge, the M2 factor for a mid-IR supercontinuum source is characterized for the first time). On this basis, we identify key competitive advantages of these alternative emitters for mid-IR spectroscopy over state-of-the-art technologies such as thermal sources or quantum cascade lasers. The specific features of supercontinuum radiation open up prospects of improving well-established techniques in mid-IR spectroscopy and trigger developments of novel analytical methods and instrumentation. The review concludes with a structured summary of recent advances and applications in various routine mid-IR spectroscopy scenarios that have benefited from the use of supercontinuum sources.

Mid-infrared DMD-based spectral-coding spectroscopy with a supercontinuum laser source.

We present a mid-infrared spectroscopic system based on a spectral-coding approach enabled by a modified digital micromirror device (DMD). A supercontinuum source offering a confined mid-infrared laser beam is employed to perform gas measurements with this system. The performance, flexibility, and programmability enabled by the DMD is experimentally demonstrated by gas-cell measurements (CO2, CH4, N2O, NO2 and CO). Full spectra are acquired in 14 ms at 10 nm spectral resolution and in 3.5 ms at 40 nm spectral resolution. Further, we employ the system for stand-off open-path spatially resolved CO2 measurements that fully exploit the laser emission properties - the bright and highly-collimated supercontinuum beam is scanned by a galvo mirror over a retroreflector array at a scalable remote distance. The measurement concept models a passing gas emitter under lab conditions; time and spatially resolved CO2 absorbance gas-plume images in the mid-infrared range are obtained.

Towards Real-Time In-Situ Mid-Infrared Spectroscopic Ellipsometry in Polymer Processing.

Recent developments in mid-infrared (MIR) spectroscopic ellipsometry enabled by quantum cascade lasers (QCLs) have resulted in a drastic improvement in signal-to-noise ratio compared to conventional thermal emitter based instrumentation. Thus, it was possible to reduce the acquisition time for high-resolution broadband ellipsometric spectra from multiple hours to less than 1 s. This opens up new possibilities for real-time in-situ ellipsometry in polymer processing. To highlight these evolving capabilities, we demonstrate the benefits of a QCL based MIR ellipsometer by investigating single and multilayered polymer films. The molecular structure and reorientation of a 2.5 µm thin biaxially oriented polyethylene terephthalate film is monitored during a stretching process lasting 24.5 s to illustrate the perspective of ellipsometric measurements in dynamic processes. In addition, a polyethylene/ethylene vinyl alcohol/polyethylene multilayer film is investigated at a continuously varying angle of incidence (0∘- 50∘) in 17.2 s, highlighting an unprecedented sample throughput for the technique of varying angle spectroscopic ellipsometry in the MIR spectral range. The obtained results underline the superior spectral and temporal resolution of QCL ellipsometry and qualify this technique as a suitable method for advanced in-situ monitoring in polymer processing.

Sub-second quantum cascade laser based infrared spectroscopic ellipsometry.

Laser-based infrared spectroscopic ellipsometry (SE) is demonstrated for the first time, to the best of our knowledge, by applying a tunable quantum cascade laser (QCL) as a mid-infrared light source. The fast tunability of the employed QCL, combined with phase-modulated polarization, enabled the acquisition of broadband (900-1204 cm-1), high-resolution (1 cm-1) ellipsometry spectra in less than 1 second. A comparison to a conventional Fourier-transform spectrometer-based IR ellipsometer resulted in an improved signal-to-noise ratio (SNR) by a factor of at least 290. The ellipsometry setup was finally applied for the real-time monitoring of molecular reorientation during the stretching process of an anisotropic polypropylene film, thereby illustrating the advantage of sub-second time resolution. The developed method exceeds existing instrumentation by its fast acquisition and high SNR, which could open up a set of new applications of SE such as ellipsometric inline process monitoring and quality control.