Feruz Ganikhanov

High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy.

We demonstrate a new approach to coherent anti-Stokes Raman scattering (CARS) microscopy that significantly increases the detection sensitivity. CARS signals are generated by collinearly overlapped, tightly focused, and raster scanned pump and Stokes laser beams, whose difference frequency is rapidly modulated. The resulting amplitude modulation of the CARS signal is detected through a lock-in amplifier. This scheme efficiently suppresses the nonresonant background and allows for the detection of far fewer vibrational oscillators than possible through existing CARS microscopy methods.

Broadly tunable dual-wavelength light source for coherent anti-Stokes Raman scattering microscopy

The signal and idler beams from a picosecond, synchronously pumped optical parametric oscillator (OPO) provide the two colors necessary for coherent anti-Stokes Raman scattering (CARS) microscopy. The OPO provides a continuously tunable frequency difference between the two beams over a broad range of Raman shifts (100-3700 cm(-1)) by varying the temperature of a single nonlinear crystal. The near-infrared output (900-1300 nm) allows for deep penetration into thick samples and reduced nonlinear photodamage. Applications of this light source to in vivo cell and ex vivo tissue imaging are demonstrated.

Towards CARS Endoscopy.

We provide a proof-of-principle demonstration of CARS endoscopy. The design utilizes a single mode optical fiber with a focusing unit attached to the distal end. Picosecond pump and Stokes pulse trains in the near infrared are delivered through the fiber with nearly unaltered spectral and temporal characteristics at intensities needed for endoscopy. CARS endoscopic images are recorded by collecting the epi-CARS signal generated at the sample and raster scanning the sample with respect to the fiber. This CARS endoscope prototype represents an important step towards in situ chemically selective imaging for biomedical applications.

Multimodal nonlinear optical imaging of collagen arrays

We report multimodal nonlinear optical imaging of fascia, a rich collagen type I sheath around internal organs and muscle. We show that second harmonic generation (SHG), third harmonic generation (THG) and Coherent anti-Stokes Raman scattering (CARS) microscopy techniques provide complementary information about the sub-micron architecture of collagen arrays. Forward direction SHG microscopy reveals the fibrillar arrangement of collagen type I structures as the main matrix component of fascia. SHG images detected in the backward direction as well as images of forward direction CARS microscopy show that the longitudinal collagen fiber bundles are further arranged in sheet-like bands. Forward-THG microscopy reveals the optically homogeneous content of the collagen sheet on a spatial scale of the optical wavelength. This is supported by the fact that the third harmonic signal is observed only at the boundaries between the sheets as well as by the CARS data obtained in both directions. The observations made with THG and CARS microscopy are explained using atomic force microscopy images.

Imaging skeletal muscle using second harmonic generation and coherent anti-Stokes Raman scattering microscopy.

We describe experimental results on label free imaging of striated skeletal muscle using second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy. The complementarity of the SHG and CARS data makes it possible to clearly identify the main sarcomere sub-structures such as actin, myosin, acto-myosin, and the intact T-tubular system as it emanates from the sarcolemma. Owing to sub-micron spatial resolution and the high sensitivity of the CARS microscopy technique we were able to resolve individual myofibrils. In addition, key organelles such as mitochondria, cell nuclei and their structural constituents were observed revealing the entire structure of the muscle functional units. There is a noticeable difference in the CARS response of the muscle structure within actin, myosin and t-tubule areas with respect to laser polarization. We attribute this to a preferential alignment of the probed molecular bonds along certain directions. The combined CARS and SHG microscopy approach yields more extensive and complementary information and has a potential to become an indispensable method for live skeletal muscle characterization.

Femtosecond optical parametric oscillator based on periodically poled stoichiometric LiTaO3 crystal.

Efficient optical parametric oscillation is demonstrated in periodically poled stoichiometric lithium tantalate crystal pumped by a mode-locked Ti:sapphire laser. The optical parametric oscillator (OPO) delivers a maximum average power of more than 345 mW in signal and 180 mW in idler beams. The OPO is continuously tunable across the 940-1350 nm wavelength range in its signal branch, delivering nearly transform-limited 160-180 fs pulses. Despite the onset of high absorption loss in the crystal in the mid-IR, more than 40 mW of power is obtained for the idler beam tunable within the 4.3-4.6 microm wavelength range. The high parametric gain in the crystal allows tunable OPO operation at a repetition rate as high as 760 MHz with 50 mW of output power.

CARS imaging with a new 532-nm synchronously pumped picosecond OPO

A new, synchronously pumped picosecond OPO for CARS microscopy is presented. It is based on non-critically phasematched interaction in LBO pumped by a frequency-doubled modelocked Nd:Vanadat laser at 532 nm. Within the parametric process a tuneable pair of two different wavelengths in the NIR range is generated (Signal <680 ...990 nm, Idler 1150...>2450 nm). In this system they are extracted from the cavity at the same mirror and therefore propagating collinear at the same beam path. Due to the mechanism of their generation there is no jitter between Signal and Idler. Though the wavelengths are different the GVD is negligible for this picosecond pulse duration. As a result the two pulse trains are spatially and temporally perfectly matched. The pulses generated are close to transform limit with about 5-6 ps pulse duration, excellent beam quality (M2 < 1,1) and high pointing stability. The output power for Signal and Idler is about 1 W each @ 4 W pump power. The tuning mechanism is split into two parts - temperature tuning for rough variations and fast angular BRF tuning for the fine adjustment of the output wavelength. The perfect spatial and temporal overlap make the described OPO an ideal and nearly hands-free laser source for CARS microscopy with a tuneable energy difference 1,400 ... >10,000 cm-1. The absolute wavelength range is resulting in high penetration depth and low photo damage of the analyzed samples. Finally some CARS-images are presented and the latest results and methods for further sensitivity enhancements are shown.

Ultrafast decay of high frequency optical phonon mode in KTiOPO4

Decay of the strongest optical phonon mode in KTiOPO4 was directly traced using femtosecond coherent anti-Stokes Raman scattering spectroscopy. Dephasing of the Raman active mode at ∼700 cm−1 proceeds with the nonlinear polarization dephasing time of 495 ± 10 fs. The dephasing is solely due to the phonon energy decay with corresponding homogeneous linewidth of 21.4 ± 0.5 cm−1. Low temperature linewidth of 14.7 cm−1 is estimated from our data assuming that down-conversion phonon relaxation process is dominant. Our results can help to understand stimulated Raman generation and oscillation of Stokes wave in laser systems where Raman gain is critically dependent on dephasing time.

Efficient picosecond optical parametric oscillator based on periodically poled lithium tantalate

Generation of transform limited picosecond pulses continuously tunable across wavelengths in the near-infrared (995–1340 nm) and midinfrared (2.1–3.6 μm) range is demonstrated in an optical parametric oscillator (OPO) based on a periodically poled stoichiometric lithium tantalate crystal. The OPO, which is synchronously pumped by a mode-locked Ti:sapphire laser, delivers about 1 W in combined average power at a repetition rate of 74 MHz, which represents close to 45% extraction efficiency in output power. With wavelength and bandwidth controlled via an intracavity diffraction grating, about 2.7 ps pulses with 3.8–10.9 cm−1 bandwidth (full width at half maximum) were generated across the tuning range in the near-infrared thus making the OPO an optimal source for use in various nonlinear spectroscopy and microscopy applications that require high spectral resolution.

Dispersion of the resonant nonlinear optical susceptibility obtained with femtosecond time-domain coherent anti-Stokes Raman scattering

We propose and experimentally demonstrate a method that is capable of resolving both real and imaginary parts of third-order nonlinearity (χ(3)) in the vicinity of Raman resonances. Dispersion of χ(3) can be obtained from a medium probed within microscopic volumes with a spectral resolution of better than 0.10 cm(-1).