J. Squier

Self-channeling of high-peak-power femtosecond laser pulses in air.

The self-channeling of ultrashort laser pulses through 20 m of air was demonstrated. The channeled pulse was measured to have 0.75 mJ of energy, a diameter of 80 microm FWHM, and a modulated spectrum. All these values were measured to be fairly constant during the propagation of the pulse. A preliminary model is shown to explain these results.

LASER-INDUCED BREAKDOWN BY IMPACT IONIZATION IN SIO2 WITH PULSE WIDTHS FROM 7 NS TO 150 FS

Results of laser‐induced breakdown experiments in fused silica (SiO2) employing 150 fs–7 ns, 780 nm laser pulses are reported. The avalanche ionization mechanism is found to dominate over the entire pulse‐width range. Fluence breakdown threshold does not follow the scaling of Fth∼ √τp, when pulses are shorter than 10 ps. The impact ionization coefficient of SiO2 is measured up to ∼3×108 V/cm. The relative role of photoionization in breakdown for ultrashort pulses is discussed.

Machining of sub-micron holes using a femtosecond laser at 800 nm

Abstract The ability to machine very small features in materials has a number of technological applications. We have ablated holes, by laser ablation, into a metal film. Using 200 fs, 800 nm pulses from a Ti:sapphire laser, focused to a spot size of 3000 nm, we have produced holes with a diameter of 300 nm and a depth of 52 nm. The production of these small features is possible because the effects of thermal diffusion are minimized with the short pulses.

Third harmonic generation microscopy.

Third harmonic generation microscopy is used to make dynamical images of living systems for the first time. A 100 fs excitation pulse at 1.2 aem results in a 400 nm signal which is generated directly within the specimen. Chara plant rhizoids have been imaged, showing dynamic plant activity, and non-fading image characteristics even with continuous viewing, indicating prolonged viability under these THG-imaging conditions.

Dispersion pre-compensation of 15 femtosecond optical pulses for high-numerical-aperture objectives

The excitation efficiency in two‐photon absorption (TPA) microscopy depends strongly — owing to the square dependence of the TPA fluorescence on the excitation intensity — on the temporal width of the excitation pulse. Because of their inherently large frequency bandwidth, ultrashort optical pulses tend to broaden substantially because of dispersion from propagation through the dispersive elements in the microscope. In this paper, the dispersion characteristics of a wide range of microscope objectives are investigated. It is shown that the induced dispersion can be pre‐compensated in all cases for pulses as short as 15 fs. Because of the excellent agreement between the results from theoretical modelling and the experimental data, predictions of the possibility of dispersion control for microscope objectives in general, as well as for even shorter pulses, can be inferred. Since for TPA imaging the background due to single photon absorption processes and scattering is independent of the pulse width, proper dispersion pre‐compensation — which minimizes the pulse duration at the focal point and hence maximizes the excitation efficiency — provides optimal image contrast in TPA microscopy.

Two-photon image correlation spectroscopy and image cross-correlation spectroscopy

We introduce two‐photon image correlation spectroscopy (ICS) using a video rate capable multiphoton microscope. We demonstrate how video rate two‐photon microscopic imaging and image correlation analysis may be combined to measure molecular transport properties over ranges typical of biomolecules in membrane environments. Using two‐photon ICS, we measured diffusion coefficients as large as 10−8 cm2 s−1 that matched theoretical predictions for samples of fluorescent microspheres suspended in aqueous sucrose solutions. We also show the sensitivity of the method for measuring microscopic flow using analogous test samples. We demonstrate explicitly the advantages of the image correlation approach for measurement of correlation functions with high signal‐to‐noise in relatively short time periods and discuss situations when these methods represent improvements over non‐imaging fluorescence correlation spectroscopy. We present the first demonstration of two‐photon image cross‐correlation spectroscopy where we simultaneously excite (via two‐photon absorption) non‐identical fluorophores with a single pulsed laser. We also demonstrate cellular application of two‐photon ICS for measurements of slow diffusion of green fluorescent protein/adhesion receptor constructs within the basal membrane of live CHO fibroblast cells.

REAL TIME TWO-PHOTON ABSORPTION MICROSCOPY USING MULTI POINT EXCITATION

In this communication we present the development of a real time two‐photon absorption microscope, based on parallel excitation with many foci. This pattern of foci is created by a two‐dimensional microlens array. The fluorescence is detected by direct, non descanned detection on a CCD camera. Due to the parallel nature of both excitation and detection it is possible to speed up image acquisition significantly. This makes the instrument especially suitable for studying living specimens and/or real time processes. The optical design of the instrument is discussed and an imaging example is given. We specifically address the relation between the axial sectioning capability and the distance between the illumination foci at the sample.

REGENERATIVE PULSE SHAPING AND AMPLIFICATION OF ULTRABROADBAND OPTICAL PULSES

Regenerative pulse shaping is used to alleviate gain narrowing during ultrashort-pulse amplification. Amplification bandwidths of ~ 100 nm, or nearly three times wider than the traditional gain-narrowing limit, are produced with a modified Ti:sapphire regenerative amplifier. This novel regenerative amplifier has been used to amplify pulses to the 5-mJ level with a bandwidth sufficient to support ~ 10-fs pulses.

Mode locking of Ti:Al 2 O 3 lasers and self-focusing: a Gaussian approximation

We present an ABCD matrix model showing that self-focusing in the laser rod leads to modifications of the Gaussian beam parameters in cw-pumped Ti:Al(2)O(3) lasers. Stabilization of self-mode-locking should result from these beam perturbations. Experimental measurements of beam modifications supporting this model are presented. The role of gain guiding is studied, and the limitations of the model are discussed.

Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping and chirped-pulse amplification

Transform-limited, 18-fs pulses of 4.4-TW peak power are produced in a Ti:sapphire-based chirped-pulsed amplification system at a repetition rate of 50 Hz. Regenerative pulse shaping is used to control gain narrowing during amplification, and an optimized, quintic-phase-limited dispersion compensation scheme is used to control higher-order phase distortions over a bandwidth of ~100 nm. Seed pulses are temporally stretched >100,000 times before amplification.