Ivan V. Tomov

Observations on the measurement of two-photon absorption cross-section

We have measured two-photon absorption (TPA) cross-sections of four organic molecules in solution. The data show that the nonlinear transmission method without consideration of other nonlinear effects results, under certain conditions, in erroneous values for the TPA cross-sections. We also find that the cross-sections measured by excited-state methods, namely two-photon induced fluorescence and a new excited-state method, which is based on transient spectroscopy following two-photon excitation, are in good agreement with the published data. Therefore, caution is warranted when using the transmission method.

Second-harmonic optical coherence tomography

Second harmonic optical coherence tomography, which uses coherence gating of second-order nonlinear optical response of biological tissues for imaging, is described and demonstrated.

Miniaturized probe based on a microelectromechanical system mirror for multiphoton microscopy

A factor that limits the use of multiphoton microscopy (MPM) in clinical and preclinical studies is the lack of a compact and flexible probe. We report on a miniaturized MPM probe employing a microelectromechanical system (MEMS) scanning mirror and a double-clad photonic crystal fiber (DCPCF). The use of a MEMS mirror and a DCPCF provides many advantages, such as size reduction, rapid and precise scanning, efficient delivery of short pulses, and high collection efficiency of fluorescent signals. The completed probe was 1 cm in outer diameter and 14 cm in length. The developed probe was integrated into an MPM system and used to image fluorescent beads, paper, and biological specimens.

Time resolved heat propagation in a gold crystal by means of picosecond x‐ray diffraction

Characteristic Cu Kα x‐ray pulses of less than 10 ps duration at 300 Hz repetition rate are generated in a x‐ray diode driven by 1.8 ps, 193 nm pulses. Both light and x‐ray pulses are utilized in pump–probe experiment to study the heat propagation in a Au(111) crystal lattice. Changes in the x‐ray diffraction pattern in the first 100 ps are clearly observed with about 10 ps resolution. The system allows for detection of temperature changes of 20 °C in the 8–20 ps time frame.

Effect of thermal self‐defocusing of degenerate four wave mixing in absorbing media

The distortion of a phase conjugate wave produced by degenerate four wave mixing as a consequence of thermal self‐defocusing is reported. The thermally induced refractive index changes cause modulation in the phase of the phase conjugate wave. The thermal self‐defocusing effect may impose a limiting factor in high repetition rate nonlinear devices.

A high repetition rate, picosecond hard x‐ray system, and its application to time‐resolved x‐ray diffraction

A new technique for generation of hard x‐ray picosecond pulses is presented. Excitation of a x‐ray diode with 10 ps ultraviolet light pulses produces characteristic x‐ray pulses with duration in the range of 10–100 ps at a repetition rate of 300 Hz. The x‐ray pulses are synchronized with picosecond accuracy to the optical pulses of the laser system as well to other ultrafast phenomena. Results of picosecond time‐resolved x‐ray diffraction are reported.

Temperature dependence of degenerate four wave mixing in azo dye doped polymer films

Experimental results of the temperature dependence of the nonlinear optical response of disperse red 1 and methyl red doped in polystyrene film in the range 90–290 K are reported. It is found that the phase conjugate signal intensity resulting from degenerate four wave mixing is decreased by an order of magnitude at low temperature. The decay time of the recorded grating at fixed pump intensity also decreases with decreasing temperature.

Low-noise broadband light generation from optical fibers for use in high-resolution optical coherence tomography

Broadband light generation from a single-mode optical fiber was developed for high-resolution optical coherence tomography (OCT). No noise amplification was observed for light broadened by self-phase modulation. The investigation showed that the intensity noise of light broadened by self-phase modulation in a single-mode optical fiber was much lower than that of continuum light from a microstructure fiber (MSF). The spectral width of a femtosecond input laser pulse was successfully broadened by a factor of 11, and a coherence length of 3.7 microm was achieved with this source. The application of light broadened by a single-mode optical fiber and MSF was compared for use in OCT imaging. The results showed that a single-mode fiber with a small core diameter is a useful way to generate low-noise, broadband light for high-resolution OCT imaging.

High-resolution second-harmonic optical coherence tomography of collagen in rat-tail tendon

A high-resolution second-harmonic optical coherence tomography (SH-OCT) system is demonstrated using a spectrum broadened femtosecond Ti:sapphire laser. An axial resolution of 4.2μm at the second-harmonic wave center wavelength of 400 nm has been achieved. Because the SH-OCT system uses the second-harmonic generation signals that strongly depend on the orientation, polarization, and local symmetry properties of chiral molecules, this technique provides unique contrast enhancement to conventional optical coherence tomography. The system is applied to image biological tissues of the rat-tail tendon. Highly organized collagen fibrils in the rat-tail tendon can be visualized in recorded images.

Rotational multiphoton endoscopy with a 1 μm fiber laser system

We present multiphoton microendoscopy with a rotational probe and a 1 μm fiber-based femtosecond laser. The rotational probe is based on a double-clad photonic crystal fiber, a gradient index lens, a microprism, and a rotational microelectronicmechanical system (MEMS) motor. The MEMS motor has a diameter of 2.2 mm and can provide 360° full-view rotation. The fiber laser provides ultrashort pulses with a central wavelength at 1.034 μm and a repetition rate of 50 MHz. Second-harmonic-generation images of rat-tail tendon and fish scale are demonstrated with the rotational probe-based multiphoton system.