Beop Min Kim

Polarization-dependent optical second-harmonic imaging of a rat-tail tendon.

Using scanning confocal microscopy, we measure the backscattered second harmonic signal generated by a 100 fs laser in rat-tail tendon collagen. Damage to the sample is avoided by using a continuous scanning technique, rather than measuring the signal at discrete points. The second harmonic signal varies by about a factor of 2 across a single cross section of the rat-tail tendon fascicle. The signal intensity depends both on the collagen organization and the backscattering efficiency. This implies that we cannot use intensity measurements alone to characterize collagen structure. However, we can infer structural information from the polarization dependence of the second harmonic signal. Axial and transverse scans for different linear polarization angles of the input beam show that second harmonic generation (SHG) in the rat-tail tendon depends strongly on the polarization of the input laser beam. We develop an analytical model for the SHG as a function of the polarization angle in the rat-tail tendon. We apply this model in determining the orientation of collagen fibrils in the fascicle and the ratio gamma between the two independent elements of the second-order nonlinear susceptibility tensor. There is a good fit between our model and the measured data.

Collagen structure and nonlinear susceptibility: Effects of heat, glycation, and enzymatic cleavage on second harmonic signal intensity†

Helical macromolecules such as collagen and DNA are characterized by nonlinear optical properties, including nonlinear susceptibility. Because collagen is the predominant component of most biological tissues, as well as the major source of second harmonic generation (SHG), it is reasonable to assume that changes in harmonic signal can be attributed to structural changes in collagen. The purpose of this study is to determine whether various modifications of collagen structure affect second harmonic intensity.

Nonlinear finite-element analysis of the role of dynamic changes in blood perfusion and optical properties in laser coagulation of tissue

A nonlinear finite-element program was developed to simulate the dynamic evolution of coagulation in tissue considering temperature and damage dependence of both the optical properties and blood perfusion rate. These dynamic parameters were derived based on the Arrhenius rate process formulation of thermal damage and kinetics of vasodilation. Using this nonlinear model, we found that the region of increased blood flow that formed at the periphery of the coagulation region significantly reduces the heat penetration. Moreover, increased scattering in the near-surface region prevents light penetration into the deeper region. Therefore, if the dynamic parameters are ignored, a relatively significant overestimation of the temperature rise occurs in a deeper area resulting in an overestimation in predicted depth of coagulation. Mathematical modeling techniques that simulate laser coagulation may not provide reliable information unless they take into account these dynamic parameters.

Influence of pulse duration on ultrashort laser pulse ablation of biological tissues

Ablation characteristics of ultrashort laser pulses were investigated for pulse durations in the range of 130 fs-10 ps. Tissue samples used in the study were dental hard tissue (dentin) and water. We observed differences in ablation crater morphology for craters generated with pulse durations in the 130 fs-1 ps and the 5 ps-10 ps range. For the water experiment, the surface ablation and subsequent propagation of stress waves were monitored using Mach-Zehnder interferometry. For 130 fs-1 ps, energy is deposited on the surface while for longer pulses the beam penetrates into the sample. Both studies indicate that a transition occurs between 1 and 5 ps.

Simple high-speed confocal line-scanning microscope

Using a line scan camera and an acousto-optic deflector (AOD), we constructed a high-speed confocal laser line-scanning microscope that can generate confocal images (512 x 512 pixels) with up to 191 frames/s without any mechanically moving parts. The line scanner consists of an AOD and a cylindrical lens, which creates a line focus sweeping over the sample. The measured resolutions in z (depth), x (perpendicular to line focus), and y (direction of line focus) directions are 3.3 mum, 0.7 mum and 0.9 mum, respectively, with a 50x objective lens. This confocal microscope may be useful for analyzing fast phenomena during biological and chemical interactions and for fast 3D image reconstruction.

Effects of high repetition rate and beam size on hard tissue damage due to subpicosecond laser pulses

We report the effects of the repetition rate and the beam size on the threshold for ultrashort laser pulse induced damage in dentin. The observed results are explained as cumulative thermal effects. Our model is consistent with the experimental results and explains the dependence of the threshold on repetition rate, beam size, and exposure time.

Physical characterization of ultrashort laser pulse drilling of biological tissue

Abstract Ultrashort laser pulse ablation removes material with low-energy fluence required and minimal collateral damage. The ultimate usefulness of this technology for biomedical application depends, in part, on characterization of the physical conditions attained, and determination of the zone of shockwave and heat-affected material in particular tissues. Detailed numerical modeling of the relevant physics (deposition, plasma formation, shockwave generation and propagation, thermal conduction) are providing this information. A wide range of time scales is involved, ranging from picosecond for energy deposition and peak pressure and temperature, to nanosecond for development of shockwave, to microsecond for macroscopic thermophysical response.

Frequency doubling of ultrashort laser pulses in biological tissues

Theoretical and experimental studies of second-harmonic generation (SHG) in biological tissues was performed by use of ultrashort laser pulses (<1 ps). A simplified one-dimensional model for the generation and the propagation of frequency-doubled light inside tissue was developed. This model was tested in vitro against measurements of pig and chicken tissue and human tooth. The experimental results indicate that the intensity of SHG varies significantly among tissue types and between test sites in individual tissue. Possibilities of using this nonlinear tissue property in imaging and diagnostics are discussed.

External Line-Cavity Wavelength-Swept Source at 850 nm for Optical Coherence Tomography

We constructed a novel and compact wavelength-swept laser source at the 850-nm region using an external line cavity in a modified Littman configuration filter. The angle alignment stability was improved by separating the function of output coupling and the wavelength selection of diffraction grating with gold coating. Our swept source produces laser output at the center wavelength of 849 nm and the average power of 2 mW with the full-width at half-maximum of 48 nm. The system performance is discussed and a two-dimensional image of a biological sample is presented

Quantification of the wound healing using polarization-sensitive optical coherence tomography

We use polarization-sensitive optical coherence tomography (PS-OCT) to monitor the wound healing process in vitro and in vivo, which are affected by various drugs. Five rabbit subjects are used for in vitro studies and another five are used for in vivo studies. The in vitro studies are conducted to compare the PS-OCT images with histopathology. For each subject, three biopsy lesions are created on each ear: one site is not treated (control); the second site is treated with sphingosylphosphorylcholine, which is expected to promote healing; and the last is administered with tetraacetylphytosphingosine, which negatively affects the healing process. Each site is examined with a PS-OCT system at 1, 4, 7, 10, and 14- days after wound generation. The variations of phase retardation values caused by the collagen morphology changes on wound sites are quantified for all cases. Our results suggest that PS-OCT may be a useful tool for visualization of collagen fiber regeneration and for quantification of various drug effects during the wound healing process.