Pulsed infrared (IR) light in the 1.8 - 2 μm region can modulate neural activities with high spatial and temporal precision. However, the mechanisms underlying these photothermal interactions are not fully understood. Here we investigate the IR modulation of axon and muscle activities using the crayfish (Procambarus clarkii) opener neuromuscular preparation. A modulated fiber coupled laser diode (λ = 2 μm) is used to deliver pulsed light with durations between 10 – 500 ms. Twoelectrode current clamp (TECC) is performed to stimulate and monitor the neural activities. Laser-induced temperature changes are measured by an open patch pipette simultaneously with TECC. We find that IR pulses can reversibly inhibit or block axon (Na<sup>+</sup> ) and muscle (Ca<sup>2+</sup>) spikes. In axons, single IR pulses can suppress the action potential (AP) amplitude and duration and increase the interspike interval. In addition, the rates of AP depolarization and repolarization are also modulated by IR pulses. Individual IR pulses can also block muscle fiber Ca<sup>2+</sup> spikes. The IR-induced decrease in the input resistance (8.4%) can be a contributing factor for the inhibition phenomena reported here.
Mid-infrared spectroscopy and imaging has attracted significant interest due to the presence of characteristic vibrational resonances of numerous molecular compounds in the fingerprint region. However, the imaging resolution in the midinfrared can be significantly weakened by the relatively large spot sizes of the beam used to target the absorption resonances of the sample. Photothermal spectroscopy and imaging as a label-free and nondestructive technique has shown great potential for pushing the imaging resolution to values below the mid-infrared diffraction limit while offering enhanced chemical sensitivity and specificity. Mid-infrared photothermal spectroscopy relies on a pump-probe setup, in which a shorter wavelength probe (removed from the targeted resonances) is co-aligned with the resonant pump beam. The modulated photothermal signal is then detected by a lock-in amplifier, whose output can provide information on the signal amplitude as well as phase. However, most commonly, amplitude images have been evaluated, without considering the recorded photothermal phase. We will provide a detailed analysis of the mid-infrared photothermal amplitude and phase information from a melamine bead sample embedded in a 2.5 μm-thick 4-Octyl-4’-Cyanobiphenyl (8CB) liquid crystal environment. Studies related to thermal transport phenomena as well as increases in imaging sensitivity will be presented for sub-diffraction limited resolution, label-free imaging. The concept of wavelength optimization to minimize thermal blurring effects at the zerocrossing for the out-of-phase contribution is introduced as a novel approach to enhance spatial resolution and sensitivity in mid-infrared label-free photothermal imaging.
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.
A Tm/Ho co-doped mode-locked soliton fiber laser design is presented with stable and low noise single-pulsing operation at a repetition rate of 135.2 MHz and a transform limited pulse duration of 375 fs. The fiber laser is directly core pumped at a wavelength of 790 nm. In single-pulsing operation, the fiber laser is centered at a wavelength of 1983 nm and can be continuously tuned over an 8 nm bandwidth. The fiber laser consists of a linear cavity which allows scaling of the repetition rate further by reducing the cavity length and utilizing the high pump absorption at 790 nm and efficient absorption/emission dynamics without photodarkening. In addition, co-doping with Tm/Ho increases the efficiency of the lasing with enhanced cross-relaxation rates. Stable mode-locked operation with reduced ripples in the optical spectrum and high signal-to-background ratios in the RF spectrum is observed. A low relative intensity noise with an rms fluctuation level of 0.13 % (frequency interval of 10 Hz to 1 MHz) and a low phase noise with a timing jitter of 20 fs (frequency range of 100 Hz to 1 MHz) characterizes the mode-locked laser.
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.