Milan Poudel

Propagation of ultrashort laser pulses in water: linear absorption and onset of nonlinear spectral transformation

We study propagation of short laser pulses through water and use a spectral hole filling technique to essentially perform a sensitive balanced comparison of absorption coefficients for pulses of different duration. This study is motivated by an alleged violation of the Bouguer-Lambert-Beer law at low light intensities, where the pulse propagation is expected to be linear, and by a possible observation of femtosecond optical precursors in water. We find that at low intensities, absorption of laser light is determined solely by its spectrum and does not directly depend on the pulse duration, in agreement with our earlier work and in contradiction to some work of others. However, as the laser fluence is increased, interaction of light with water becomes nonlinear, causing energy exchange among the pulses spectral components and resulting in peak-intensity dependent (and therefore pulse-duration dependent) transmission. For 30 fs pulses at 800 nm center wavelength, we determine the onset of nonlinear propagation effects to occur at a peak value of about 0.12 mJ/cm(2) of input laser energy fluence.

Study of self-focusing effect induced by femtosecond photodisruption on model substances

We have studied the possible nonlinear effects on tissue and model substance with emphasis on self-focusing effect. The nature of this effect and its biological consequences are discussed. We have studied how the self-focusing action changes with applied laser power and its effect on various depths of the biological materials. Critical power for biological material is calculated, and the peak power required for surgical purposes is found to be greater than the critical power. Furthermore, we discuss other possible effects that self-focusing might have on refractive surgery.

Lifetime and anisotropy decay of excited Coumarin 30 measured by a femtosecond pump–probe technique

We measured the lifetime of Coumarin 30 dye molecules excited with 400 nm in ethanol solution with the pump–probe stimulated emission fluorescence technique using femtosecond laser pulses that provided high temporal resolution of about 50 fs. Both fluorescence and stimulated emission signals were observed. The stimulated emission was induced at 550 nm. The lifetime of the excited state was determined to be (1.9± 0.3) ns and the rotational depolarization time was measured to be (120± 20) ps. Our results are in agreement with data obtained previously by a different technique.

Single-snapshot and intensity-resolved two-photon fluorescence measurements

We present a single-snapshot (SSS) method for obtaining intensity-resolved two-photon fluorescence (TPF). This simple method uses a digital camera to image the TPF spot on a liquid dye jet. By making a comparison between the local laser and TPF intensities, TPF probabilities are reconstructed. We compare our intensity-resolved TPF results with those obtained by the more common intensity scanning (IS) and z-scan methods. The dependence of the TPF probability on intensity obtained by the SSS method for coumarin-30 exhibits a clear maximum around I approximately 4 x 10(12) W/cm(2) and a postsaturation decrease, while no such effects were found in the data obtained by the other methods. Additionally, theoretical models are presented to extract the overall probability from within the volume integral. To our knowledge, we present the first reported measurements of such intensity-resolved TPF.

Two-photon fluorescence of Coumarin 30 excited by optimally shaped pulses

We optimized the two-photon fluorescence (TPF) of a Coumarin 30 dye by using a feedback-controlled femtosecond pulse shaping technique. For optimization we implemented an evolutionary algorithm with a liquid crystal phase-only pulse shaper in a folded 4f setup. The optimization procedure applied to the second harmonic generation, and TPF noticeably improved the output signals and demonstrated good convergence. In addition, signal ratios involving TPF and second harmonic generation (SHG) were successfully optimized. The correlation between TPF and SHG was studied, and it was found to decrease when the pulse shape was close to the optimum. These experimental results are of interest for potential applications of coherent control to complex molecular systems as well as in biomedical imaging.

Nonlinear optical effects during femtosecond photodisruption

Several nonlinear effects (i.e., continuum generation, self-focusing, and material damage) were studied during femtosecond photodisruption. Numerical aperture dependence of white-light continuum generation and material damage were determined and a relation between the two effects was shown. We showed the possibility of reducing nonlinear side effects and at the same time ensuring precise cut by using lenses of a suitable numerical aperture for refractive surgery, cell surgery, and tissue dissection. Other side effects associated with optical breakdown in model substance were also discussed.