Michel Piché

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.

Beam reshaping and self-mode-locking in nonlinear laser resonators

Abstract The modal structures of near-concentric laser resonators with a saturable gaussian gain and a Kerr nonlinearity are investigated numerically. As a consequence of self-focussing, the output power from unstable resonators tends to increase as a function of on-axis nonlinear phase-shift, and their beams are more concentrated at the center of the gain medium; on the other hand, stable resonators lead to larger beams and, in most cases, lower output power. Self-mode-locking should take place in unstable resonators and in some stable resonators with small beam waists, as they favor the guild-up of intense signals. Bistable behaviour is predicted when the pumping rate is modulated about the threshold value for laser operation.

Photonic bandgaps in periodic dielectric structures

Abstract Photonic bandgaps are defined as frequency intervals for which propagation of electromagnetic waves is forbidden in all 4π steradians within a dielectric structure with a periodic index of refraction. Such structures consist, for example, of dielectric spheres in suspension or air holes in a dielectric material, with a spatial period comparable to the electromagnetic wavelength. The principal feature of periodic structures is their ability to perturb the density of electromagnetic states within the structures. Since photonic bandgap materials can essentially suppress all states, the radiative dynamics within the materials can be strongly modified. By changing the atom-field radiative coupling, photonic bandgap materials could lead to the inhibition of spontaneous emission; if a local defect is introduced within the structure, it will behave like a high-Q microcavity. The existence of bandgaps can be predicted from a classical treatment of the vector wave equation. The use of the plane-wave expansion method can lead to accurate results but introduces two problems related to the dielectric discontinuities and the plane-wave cutoff. Experimental investigations at microwave frequencies have demonstrated many of the properties of photonic bandgap structures.

Two-photon excitation fluorescence microscopy with a high depth of field using an axicon

In conventional two-photon excitation fluorescence microscopy, the numerical aperture of the objective determines the lateral resolution and the depth of field. In some situations, as with functional imaging of dynamic events distributed in live biological tissue, an improved temporal resolution is needed; as a consequence, it is imperative to use optics with a high depth of field to simultaneously image objects at different axial positions. With a conventional microscope objective, increasing the depth of field is achieved at the expense of lateral resolution. To overcome this limitation, we have incorporated an axicon in a two-photon excitation fluorescence microscopy system; measurements have shown that an axicon provides a depth of field in excess of a millimeter, while the lateral resolution is maintained at the micrometer scale. Thus axicon-based two-photon microscopy has been shown to yield a high-resolution projection image of a sample with a single 2D scan of the laser beam while maintaining the improved tissue penetration typical of two-photon microscopy.

Self-mode locking of solid-state lasers without apertures.

Numerical simulations that take into account propagation, self-focusing, and gain saturation in solid-state lasers reveal that self-mode locking can take place in such lasers even in the absence of apertures. The combination of self-focusing and gain saturation produces a differential gain that favors the growth of short pulses and eliminates cw oscillations. Nonsymmetrical cavities can provide a substantial differential gain when they are operated near a stability limit.

Needles of longitudinally polarized light: guidelines for minimum spot size and tunable axial extent.

Optical beams exhibiting a long depth of focus and a minimum spot size can be obtained with the tight focusing of a narrow annulus of radially polarized light, leading to a needle of longitudinally polarized light. Such beams are of increasing interest for their applications, for example in optical data storage, particle acceleration, and biomedical imaging. Hence one needs to characterize the needles of longitudinally polarized light obtained with different focusing optics and incident beams. In this paper, we present analytical expressions for the electric field of such a nearly nondiffracting, subwavelength beam obtained with a parabolic mirror or an aplanatic lens. Based on these results, we give expressions of the transverse and longitudinal full widths at half maximum of the focal lines as a function of the width of the incident annular beam and we compare the performances of the two focusing systems. Then, we propose a practical solution to produce a needle of longitudinally polarized light with a tunable axial extent and a transverse width reaching the theoretical limit of 0.36λ.

Pulse shaping and passive mode-locking with a nonlinear Michelson interferometer

Abstract A Michelson interferometer, where one branch contains a Kerr medium with a fast response time, can shorten the duration of laser pulses and act as a passive mode-locking element. The device is analyed and experimental results, obtained with a TEA-CO 2 laser using germanium as the Kerr medium, are shown and discussed.

Single transverse mode oscillation from an unstable resonator Nd:YAG laser using a variable reflectivity mirror

Abstract Single transverse mode pulses were obtained from a Nd:YAG laser using an output mirror with a parabolic reflectivity profile in an unstable Cassegrain resonator. The experimental results show a smooth near-field beam intensity distribution and a narrow, single lobe far-field pattern. Output pulses of up to 150 mJ have been obtained with a 4 mm rod.

Generation of a beam of fast electrons by tightly focusing a radially polarized ultrashort laser pulse

The generation of an electron beam through longitudinal field acceleration from a tightly focused radially polarized (TM01) laser mode is reported. The longitudinal field is generated by focusing a TM01 few-cycle laser pulse (1.8 μm, 550 μJ, 15 fs) with a high numerical aperture parabola. The created longitudinal field in the focal region is intense enough to ionize atoms and accelerate electrons to 23 keV of energy from a low density oxygen gas. The characteristics of the electron beam are presented.

Direct-field electron acceleration with ultrafast radially polarized laser beams: scaling laws and optimization

In the past few years, there has been a growing interest for direct-field electron acceleration with ultra-intense and ultrafast radially polarized laser beams. This particular acceleration scheme offers the possibility of producing highly collimated mono-energetic relativistic attosecond electron pulses from an initial cloud of free electrons that could be produced by ionizing a nanoparticle. In this paper, we describe how electron energy scales with laser power and we explain how the beam waist size and the pulse duration can be optimized for maximal acceleration. The main conclusion of our work is that an electron can effectively reach the high-intensity optical cycles of this particular beam and be optimally accelerated without the necessity of being released by photoionization near the pulse peak.