Our innovations in fabrication technique pave the way for further miniaturization of carbon fiber ultra-microelectrode arrays. We believe these advances to be key steps to enable a shift from labor intensive, manual assembly to a more automated manufacturing process.
We report on the mid-infrared nonlinear photothermal spectrum of the neat liquid crystal 4-octyl-4′-cyanobiphenyl (8CB) using a tunable Quantum Cascade Laser (QCL). The nonequilibrium steady state characterized by the nonlinear photothermal infrared response undergoes a supercritical bifurcation. The bifurcation, observed in heterodyne two-color pump–probe detection, leads to ultrasharp nonlinear infrared spectra similar to those reported in the visible region. A systematic study of the peak splitting as a function of absorbed infrared power shows the bifurcation has a critical exponent of 0.5. The observation of an apparently universal critical exponent in a nonequilibrium state is explained using an analytical model analogous of mean field theory. Apart from the intrinsic interest for nonequilibrium studies, nonlinear photothermal methods lead to a dramatic narrowing of spectral lines, giving rise to a potential new contrast mechanism for the rapidly emerging new field of mid-infrared microspectroscopy using QCLs.
We report a technique to measure the mid-infrared photothermal response induced by a tunable quantum cascade laser in the neat liquid crystal 4-octyl-4 0 -cyanobiphenyl (8CB), without any intercalated dye. Heterodyne detection using a Ti:sapphire laser of the response in the solid, smectic, nematic and isotropic liquid crystal phases allows direct detection of a weak mid-infrared normal mode absorption using an inexpensive photodetector. At high pump power in the nematic phase, we observe an interesting peak splitting in the photothermal response. Tunable lasers that can access still stronger modes will facilitate photothermal heterodyne mid-infrared vibrational spectroscopy. Photothermal spectroscopy has rapidly emerged as the most sensitive label-free optical spectroscopic method, rivaling even fluorescence spectroscopy. The method has been shown to be remarkably sensitive in the visible region of the spectrum with reports of yoctomole sensitivity, eventually culminating in the observation of single molecule response 1,2,28 at room temperature. This unexpected sensitivity has led to rapid development of photothermal methods in the visible region, both for spectroscopy 3-5 and for imaging nanoparticles and organelles with high signal-to-noise ratio. 6,7 Extension of the photothermal technique to the midinfrared region is particularly attractive because the presence of a large number of characteristic normal modes of molecules in the so-called "fingerprint" region of the electromagnetic spectrum allows for spectroscopy and imaging without requiring a perturbing label. The standard instrument of choice for vibrational infrared spectroscopy remains Fourier transform infrared spectroscopy (FTIR) using cryogenically cooled detectors along with a $1200 K Globar blackbody source. But the lack of table-top stable high brightness sources and a fundamental quantum limit on the detectivity of broadband cryogenic mid-infrared detectors has translated to a lack of progress: the state-of-the-art 8 has not advanced significantly in several decades. Detection of the absorption of infrared radiation is still performed using narrow band-gap cryogenically cooled detectors made of indium antimonide (InSb) or mercury-cadmium-telluride (MCT), 8 which both are intrinsically less sensitive than the best available visible photodetectors. With the advent of tunable quantum cascade lasers (QCLs) as table-top high brightness sources, there is now hope of a rapid transformation in the field of midinfrared spectroscopy.9,10 The spectral brightness of these table-top QCL sources actually can exceed that of synchrotrons and other large relativistic electron-accelerator-based sources.
Our recent work has showed that diffractively coupled nanoplasmonic arrays for Fourier transform infrared (FTIR) microspectroscopy can enhance the Amide I protein vibrational stretch by up to 10(5) times as compared to plain substrates. In this work we consider computationally the impact of a microscope objective illumination cone on array performance. We derive an approach for computing angular- and spatially-averaged reflectance for various numerical aperture (NA) objectives. We then use this approach to show that arrays that are perfectly optimized for normal incidence undergo significant response degradation even at modest NAs, whereas arrays that are slightly detuned from the perfect grating condition at normal incidence irradiation exhibit only a slight drop in performance when analyzed with a microscope objective. Our simulation results are in good agreement with microscope measurements of experimentally optimized periodic nanoplasmonic arrays.
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