Mid-infrared photothermal heterodyne spectroscopy in a liquid crystal using a quantum cascade laser

Abstract

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.

Optical Photothermal Infrared Microspectroscopy with Simultaneous Raman – A New Non-Contact Failure Analysis Technique for Identification of <10 μm Organic Contamination in the Hard Drive and other Electronics Industries

“…Optical Photothermal Infrared Microspectroscopy with Simultaneous Raman -A New Non-Contact Failure Analysis Technique for Identification of <10 μm Organic Contamination in the Hard Drive and other Electronics Industries Mustafa Kansiz, 1 * Craig Prater, 1 Eoghan Dillon, 1 Michael Lo, 1 Jay Anderson, 1 Curtis Marcott, 2 Abel Demissie, 3 Yanling Chen, 3 and Gary Kunkel 3…”

All-digital histopathology by infrared-optical hybrid microscopy

“…While progress in classical IR microscopy is hindered by fundamental physical limitations (e.g., diffraction limit), our results reveal the potential of IR-OH for superresolution IR imaging, largely improved coverage, and suppression of IR scattering that can be employed for unforeseen, transformative applications in histopathologic imaging. Further, the interferometric detection in IR-OH provides a sensitivity advantage over previous photothermal techniques that relied on beam deflection from IR absorption-induced refractive index change (41,42,44,45,48,53). While our method measures the physical response of expansion directly, deflection methods measure the refractive index change that occurs upon deflection and need higher probe intensity than we have employed.…”

“…The concept of dynamic photothermal changes in morphology (34,35), force (36,37), or near-field coupling (38)(39)(40), using an atomic force microscope cantilever as local probe, has been reported for point-by-point IR measurements. Noncontact optical photothermal microscopy is more recent (41)(42)(43)(44)(45)(46)(47)(48)(49)(50) and typically utilizes a local IR illumination coincident with a highly focused visible probe beam to measure local refractive index change by beam scattered out of the angular acceptance of the objective lens. This method has been applied to chemical imaging of tissue and live cells (44,46,47), bacteria (48), and pharmaceutical tablets (49).…”

Vibrational mid-infrared photothermal spectroscopy using a fiber laser probe: asymptotic limit in signal-to-baseline contrast

“…Sensitive photothermal spectroscopy with high contrast has been demonstrated at visible wavelengths for single molecule detection [4] and imaging of nanomaterials [5][6][7] and biological samples [8] such as heme proteins [9] and mitochondria in live cells [10,11]. With the development of tunable quantum cascade lasers (QCLs) [12,13], photothermal spectroscopy has been recently extended into the mid-infrared (mid-IR) [14][15][16][17].…”

“…As the lenses and the photodetector (PD) after the sample are designed for near-IR wavelengths, the probe beam is automatically filtered from the mid-IR pump beam. The transmitted probe beam is detected in an optical heterodyne configuration [16,25] in an amplified InGaAs PD (ET3000A, EOTech). A voltage-preamplifier (SR560, Stanford Research Systems, 50-times amplification between 10 kHz and 1 MHz in a low noise gain mode) together with a lock-in amplifier (HF2LI, Zurich Instruments) that is synchronized to the pump repetition rate selects the modulated photothermal signal of interest.…”