Mid-infrared (IR) fibers have been extensively investigated due to their applicability in chemical sensing and remote laser delivery, among others. Materials such as chalcogenides and fluoride glasses transmit mid-IR wavelengths with low practical losses. However, their low glass transition temperatures make them chemically unstable, even at room temperatures, resulting in performance degradation over time. Semiconductors, such as germanium, have a wide transmission window in the mid-IR region, and offer significantly improved chemical stability. In this research, germanium-core, borosilicate-cladded fibers were drawn by a ‘rod in tube’ method using a mini draw tower assembled in-house at 1000°C, which is significantly lower than the drawing temperatures of 2000-2200°C for conventional silica fibers. Typical drawn fibers had a 40 μm core diameter and 177 μm cladding diameter. Transmission electron microscopy (TEM) studies showed that diffusion of oxygen and silicon from the cladding to the core during the drawing process was minimal, with diffusion distances of the order of 10s of nm. This is encouraging for mid-IR transmission, since the presence of oxygen in the fiber core is known to increase transmission losses in the mid-IR spectrum. This low diffusivity is presumably due to the relatively low drawing temperature. Transmission losses through the fibers were measured with a quantum cascade laser (QCL) and the losses were found to be in the 3-9 dB/cm range in the spectral range of 5.75-6.3 μm.
Mid-infrared photothermal spectroscopy is a pump-probe technique for label-free and non-destructive sample
characterization by targeting intrinsic vibrational modes. In this method, the mid-infrared pump beam excites a
temperature-induced change in the refractive index of the sample. This laser-induced change in the refractive index is
measured by a near-infrared probe laser using lock-in detection. At increased pump powers, emerging nonlinear
phenomena not previously demonstrated in other mid-infrared techniques are observed.
Nonlinear study of a 6 μm-thick 4-Octyl-4’-Cyanobiphenyl (8CB) liquid crystal sample is conducted by targeting the
C=C stretching band at 1606 cm<sup>-1</sup>. At high pump powers, nonlinear signal enhancement and multiple pitchfork
bifurcations of the spectral features are observed. An explanation of the nonlinear peak splitting is provided by the
formation of bubbles in the sample at high pump powers. The discontinuous refractive index across the bubble interface
results in a decrease in the forward scatter of the probe beam. This effect can be recorded as a bifurcation of the
absorption peak in the photothermal spectrum. These nonlinear effects are not present in direct measurements of the
Evolution of the nonlinear photothermal spectrum of 8CB liquid crystal with increasing pump power shows
enhancement of the absorption peak at 1606 cm<sup>-1</sup>. Multiple pitchfork bifurcations and spectral narrowing of the
photothermal spectrum are demonstrated. This novel nonlinear regime presents potential for improved spectral
resolution as well as a new regime for sample characterization in mid-infrared photothermal spectroscopy.
The use of reliable tissue-simulating phantoms spans multiple applications in spectroscopic imaging including device calibration and testing of new imaging procedures. Three-dimensional (3D) printing allows for the possibility of optical phantoms with arbitrary geometries and spatially varying optical properties. We recently demonstrated the ability to 3D print tissue-simulating phantoms with customized absorption (μ<sub>a</sub>) and reduced scattering (μ<sub>s</sub>`) by incorporating nigrosin, an absorbing dye, and titanium dioxide (TiO<sub>2</sub>), a scattering agent, to acrylonitrile butadiene styrene (ABS) during filament extrusion. A physiologically relevant range of μ<sub>a</sub> and μ<sub>s</sub>` was demonstrated with high repeatability. We expand our prior work here by evaluating the effect of two important 3D-printing parameters, percent infill and layer height, on both μ<sub>a</sub> and μ<sub>s</sub>`. 2 cm<sup>3</sup> cubes were printed with percent infill ranging from 10% to 100% and layer height ranging from 0.15 to 0.40 mm. The range in μ<sub>a</sub> and μ<sub>s</sub>` was 27.3% and 19.5% respectively for different percent infills at 471 nm. For varying layer height, the range in μ<sub>a</sub> and μ<sub>s</sub>` was 27.8% and 15.4% respectively at 471 nm. These results indicate that percent infill and layer height substantially alter optical properties and should be carefully controlled during phantom fabrication. Through the use of inexpensive hobby-level printers, the fabrication of optical phantoms may advance the complexity and availability of fully customizable phantoms over multiple spatial scales. This technique exhibits a wider range of adaptability than other common methods of fabricating optical phantoms and may lead to improved instrument characterization and calibration.
Photothermal imaging in the mid-infrared enables highly sensitive, label-free microscopy by relying on bond-specific characterization of functional groups within the samples. In a pump-probe configuration, the mid-infrared (mid-IR) pump laser is tuned to characteristic vibrational modes and through localized absorption thermal changes in the refractive index are induced. The shorter wavelength probe scatter can be detected with lock-in technology, utilizing highly sensitive detectors at telecommunication wavelengths. This mitigates the need of complex detector technology as required for traditional infrared spectroscopy/Fourier Transform Infrared Spectroscopy.<p> </p> The presented photothermal system integrates a high brightness quantum cascade laser that can be tuned continuously over a spectral range of interest with a fiber probe laser. <p> </p>Fiber laser technology features a compact footprint and offers robust performance metrics and reduced sensitivity to environmental perturbations compared to free-space laser configurations. In systematic spectroscopy studies where the probe laser parameters were modified, we demonstrate that the signal-to-noise ratio can be significantly enhanced by utilizing a mode-locked laser compared to a continuous-wave laser. With a raster-scanning approach, photothermal spectroscopy can be extended to hyperspectral label-free mid-infrared imaging to combine spectral content with localized sample details. By tuning the pump laser to the amide-I absorption band around 1650 cm<sup>-1</sup> in biological tissue samples, the spectral characteristics can provide insight into the secondary structure of proteins (e.g. amyloid plaques; alpha-helix, beta-sheet). We present the versatility of our mid-IR photothermal system by analyzing histopathological tissue sections of cancerous tissue in a non-contact, non-destructive approach with good sensitivity.
A mid-IR photothermal imaging system is presented that features an integrated ultrafast erbium-doped fiber probe laser
for the first time. With a mid-IR tunable quantum cascade laser (QCL) as the pump laser, vibrational molecular modes
are excited and the thermally-induced changes in the refractive index are measured with a probe laser. The custom-built,
all-fiber ultrafast probe laser at telecommunication wavelengths is compact, robust and thus an attractive source
compared to bulky and alignment sensitive Ti:sapphire probe lasers. We present photothermal spectra and images with
good contrast for a liquid crystal sample, demonstrating highly sensitive, label-free photothermal microscopy with a
mode-locked fiber probe laser.
Infrared absorption spectroscopy offers direct access to the vibrational signatures of molecular structure. Although
absorption cross sections are nearly 10 orders of magnitude larger than the Raman cross sections, they are small in
comparison with those of fluorescent labels. Sensitivity improvements are therefore required in order for the method to
be applicable to single molecule/monolayer studies. In this work, we demonstrate a plasmon enhanced vibrational
spectroscopy technique which allow for the measurement of molecule specific signatures at the monolayer level.
Specifically, we show 4-5 order of magnitude enhancement of the amide-I and II backbone signature of protein
monolayers, the signal resulting from only zeptomole quantities of molecules.
Extraordinary optical transmission through nanoholes has recently taken much interest for its promise to a wide range of
applications. Enhancement of non-linear optical phenomena and the development of sensitive biosensors are among the
leading ones. As a result of recent studies on the subject, it is now widely accepted that either the non-trivial interaction
of the localized and extended surface plasmons or only the localized surface plasmons (for direct transmissions) are
responsible from the extra-ordinary light transmission effect. On the other hand, there is little conceptual understanding
for controlling the localized surface plasmonic behavior of the individual apertures and their coupling to the extended
surface plasmons. In this letter, an intuitive and straightforward picture of the extra-ordinary light transmission
phenomena is developed using basic antenna principles for the elementary plasmonic excitations and hybridization of
these plasmonic excitations in complex nano-apertures. As an example, the model is successfully applied to explain the
experimentally observed plasmonic response of the complex rectangular coaxial apertures. Experimentally measured
red-shifting of the plasmonic resonances of the rectangular coaxial-apertures with respect to those of the simple
rectangular aperture arrays are successfully described and the asymmetric nature of the plasmonic resonances are
explained in relation to strong shape anisotropies. Further enhancement of the extra-ordinary light transmission is also
predicted by the model and experimentally demonstrated by using rectangular coaxial aperture arrays as a result of
significantly larger net dipole moment in the apertures. Model is also verified by rigorous 3D-FDTD calculations.
It has been theoretically predicted and experimentally shown that circular coaxial aperture arrays have higher
transmissivities with respect to simple circular ones. This observation is mainly attributed to the propagating waveguide
modes supported by the circular coaxial unit cell. In this letter, we investigate extraordinary light transmission in simple
rectangular and coaxial rectangular aperture arrays through decaying TE waveguide modes at mid-infrared wavelengths.
We demonstrate enhanced transmissions for the rectangular coaxial aperture arrays with respect to simple ones
indicating that the enhancement of extraordinary light tranmission in coaxial structures can not be simply explained by
the presence of propagating waveguide modes. Using 3-D FDTD simulations and experimental analysis of the localized
plasmons at the aperture rims of the individual apertures, the nature and the enhancement of extraordinary light
transmission for the coaxial apertures are shown. Shape anisotropy of the apertures is utilized for polarization control of
the transmitted light through the total suppression of the desired polarizations. Depolarization ratios larger than the
commercially available holographic wire grid polarizers are obtained. The reported results indicate the underlying
physics of enhanced extraordinary transmission in coaxial aperture arrays is intricate and merits further scientific
attention while practical applications are possible through the controlling of the aperture shapes.
The recent development of Scanning Near-Field IR Microscopy (SNIM) has resulted in the first ever high-resolution IR images of single living cells. We discuss extensions of this method using table-top tunable OPO-based ultrafast lasers and other tunable lasers as sources. Vibrational spectral provide an intrinsic mechanism of contrast in biological systems, without the need for any radioactive or fluorescent labels. Using the capability of SNIM for obtaining sub- wavelength resolution images and spectral allows for breaking of the femtogram barrier in biological systems. This provides a new technique for imaging sub-cellular features, and characterization of a single bacterium.
Infrared absorption microspectroscopy is a useful technique to analyze biological tissues, as it can rapidly and non- destructively provide quantitative information about the molecular composition of tissue on a small spatial scale. At the Stanford Picosecond Free Electron Laser Center, a Scanning Near-field Infrared Microscope (SNIM) with the Free Electron Laser (FEL) as its illumination source has been used for in situ microspectroscopic characterization of constituents in human atherosclerotic tissue. The system consists of a Near-field Scanning Optical Microscope utilizing a tapered chalcogenide fiber as the scanning probe. The Stanford mid-infrared FEL provides high power infrared radiation that can be easily coupled into the chalcogenide fiber and whose wavelength is continuously tunable from 3 to 15 micrometers. With the FEL, the SNIM can acquire an image at a single wavelength of a 200 micrometer square region with 2 micrometer spatial resolution in under 30 minutes. It can also obtain infrared spectra at sub- wavelength resolution. The SNIM was used to examine unstained, frozen microtone sections of human atherosclerotic lesions. Spectra from localized regions in the sample were taken and analyzed to determine the distribution of various protein, lipid, and mineral constituents among the tissue microstructures. These findings were compared with results obtained by polarization microscopy and traditional histological staining techniques. The molecular information obtained in these studies can potentially lead to a greater understanding of atherosclerosis. Moreover, they demonstrate the usefulness of SNIM towards micrometer-scale vibrational microspectroscopy.
We have developed a scanning near-field infrared microscope (SNIM) that utilizes the Stanford picosecond free electron laser as its illumination source. Infrared spectroscopy is a sensitive technique for characterizing materials. However, the spatial resolution of conventional infrared microscopy is limited to a few micrometers due to diffraction. The SNIM overcomes this limitation by using infrared near-field optics to obtain sub-wavelength resolution. The system is built around a near-field scanning optical microscope (NSOM) head, in which a tapered infrared transmitting fiber is mounted as the scanning probe. The Stanford picosecond free electron laser, which provides high power infrared radiation with a wavelength that is continuously tunable from 3 to 15 micrometers, is then coupled to the fiber. In combination with the FEL, the SNIM can obtain infrared spectra of localized regions smaller than one micrometer and acquire images at a chosen wavelength with sub-micrometer resolution. The most promising aspect of SNIM is in the development of 'vibrational nanospectroscopy.' Images have been obtained of biological tissue such as kidney sections using the intrinsic amide absorption in the tissue proteins to provide contrast, instead of relying on an externally introduce stain or marker. Images of lithographically patterned semiconductor samples have also been obtained, revealing subsurface features in gallium arsenide.
Topological transitions in membrane liquid crystals formed by biological lipid-water systems have been the subject of much recent interest. We have developed an x-ray diffraction system capable of initiating pressure jumps of up to 3 kbar in about 5 ms. Time-resolved x-ray diffraction patterns were obtained (approximately 9 ms each) at the National Synchrotron Light Source using two state-of-the-art CCD based detectors developed at Princeton. Numerous Bragg diffraction patterns were obtained in studying the effect of pressure on the simplest topological transitions in membranes, the lamellar to hexagonal phase transition. The patterns from one of the detectors were recorded with a signal-to-noise sufficient to measure peak positions, peak widths, and integrated areas to an accuracy adequate to test models and mechanisms of phase transition kinetics. Additional longer time-scale studies were performed using optical turbidity measurements and were found to be consistent with x-ray studies. Transition rates were found to vary by nearly 5 orders of magnitude as the difference between the final pressure and the equilibrium transition pressure was varied. As the magnitude of the pressure jump in these lyotropic systems is increased, the transition mechanism is determined not only by the rate at which water and lipid molecules transform from one phase to the new emerging phase, but also by the need for water transport. Finally, it was found that the lamellar phase acts as an intermediate phase in transitions between the gel phase and the hexagonal phase, induced by very large pressure jumps (> 2 kbar).