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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.

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analyte's refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.

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We have generated pulsed, high power, sodium resonance radiation by sum frequency mixing the 1.06 microm and 1.32 microm outputs of two Nd:YAG lasers with an average power conversion efficiency of 30%. The wavelength of the sum radiation was tuned across the full Doppler width of the sodium-vapor D(2) absorption by tuning the wavelength of either Nd:YAG laser with intracavity etalons. The wavelength of the 1.32 microm Nd:YAG laser was also tuned by injection seeding with a GaInAsP/InP diode laser. We have used this sodium resonance radiation for the lidar observation of the earth's naturally occurring atomic-sodium layer at 90 km altitude.

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