Filamentation and damage in fused silica induced by tightly focused femtosecond laser pulses
A. Couairon,
1
L. Sudrie,
2
M. Franco,
2
B. Prade,
2
and A. Mysyrowicz
2
1
Centre de Physique Théorique, École Polytechnique, CNRS UMR 7644, F-91128, Palaiseau Cedex, France
2
Laboratoire d’Optique Appliquée, École Nationale Supérieure des Techniques Avancées–École Polytechnique, CNRS UMR 7639,
F-91761 Palaiseau Cedex, France
Received 8 April 2004; revised manuscript received 24 June 2004; published 31 March 2005
We investigate experimentally and numerically the damage tracks induced by tightly focused NA=0.5
infrared femtosecond laser pulses in the bulk of a fused silica sample. Two types of irreversible damage are
observed. The first damage corresponds to a permanent change of refractive index without structural modifi-
cations type I. It appears for input pulse energies beyond 0.1 J. It takes the form of a narrow track extending
over more than 100 m at higher input powers. It is attributed to a change of the polarizability of the medium,
following a filamentary propagation which generates an electron-hole plasma through optical field ionization.
A second type of damage occurs for input pulse energies beyond 0.3 J type II. It takes the form of a
pear-shaped structural damage associated with an electron-ion plasma triggered by avalanche. The temporal
evolution of plasma absorption is studied by pump-probe experiments. For type I damage, a fast electron-hole
recombination is observed. Type II damage is linked with a longer absorption.
DOI: 10.1103/PhysRevB.71.125435 PACS numbers: 79.20.Ds, 61.80.Ba, 72.20.Jv, 42.65.Jx
I. INTRODUCTION
The controlled deposition of laser energy in transparent
solids is of crucial importance for several applications such
as micromachining of optical materials,
1–4
biomedical
technologies,
5
or three-dimensional optical data storage.
6,7
Laser damage often constitutes a limiting factor for the de-
velopment of these applications because it can prevent the
transmission of energy in the bulk of transparent media. The
understanding of the mechanisms responsible for damage in
dielectrics is therefore the subject of intense investigation,
while the structural modifications induced by focusing fem-
tosecond laser pulses are currently applied to the fabrication
of optical devices such as waveguides in glasses,
2,8
gratings,
9,10
fiber gratings,
11
couplers,
3
or photonic crystals.
12
In this paper, the propagation of femtosecond laser pulses
in fused silica is investigated experimentally and numeri-
cally, under tight focusing conditions. In this case, unlike in
surface damage experiments,
13
the laser pulses propagate in
the bulk of the sample and cause permanent damage to the
material without ablating the surface. Fundamental aspects
of laser-dielectric interaction are studied, in particular the
effect of basic processes occurring in the presence of a high
laser field such as photoionization, free-carrier absorption,
carrier-carrier interaction, as well as self-induced effects
such as self-phase modulation or self-focusing that can dra-
matically affect the propagation. Inspection of the damage
tracks provides useful diagnostics on the pulse propagation
in the medium. These experimental observations are com-
pared with numerical results from a code that describes the
propagation of the laser pulse coupled with the evolution of
the electron plasma generated by avalanche and photo-
ionization. Both experimental and numerical results show
that the laser pulse propagates in the form of a filament. The
local intensity and the density of the plasma generated during
the propagation of the pulse can be obtained accurately by
means of comparisons of the damage tracks with isodensity
contours of the electron density computed numerically.
The outline of this paper is the following: First, we study
the shift toward the laser of the damage caused along the
propagation axis when the energy of the incident laser pulse
varies. From this observation, the self-focusing of the laser
beam by the optical Kerr effect is quantitatively estimated.
Measurements of the transmission of the pulses as a function
of the incident energy are then presented and compared with
the results of numerical simulations. Thereafter, we analyze
the tracks of permanent damage. Comparisons of these tracks
with the numerical results allows us to infer the electron
density corresponding to the threshold of permanent damage.
This paper ends with pump-probe measurements which make
it possible to follow the temporal evolution of plasma ab-
sorption. A maximum value for the density of the plasma
generated in the medium is given.
II. EXPERIMENTAL SETUP
We have used two types of laser sources. The first source
is a cw-pumped regenerative Ti: S oscillator-amplifier laser
system Coherent RegA delivering a train of pulses of 2 J
energy at 800 nm with a duration of 160 fs FWHM and a
repetition rate of 200 kHz. The second laser source is an
experimental chain developed in our laboratory. It is a CPA
Ti:sapphire laser consisting of an oscillator, a regenerative
amplifier, and two power amplifiers. This chain is able to
deliver pulses with a central wavelength of 800 nm, an en-
ergy of 7 mJ, and a duration of 50 fs at a repetition rate of
1 kHz.
The laser beam is focused in the bulk of a transparent
solid medium. The diagram of Fig. 1 gives a simplified over-
all picture of the experimental setup. The sample and the
focusing objective are mounted on computer-controlled
translation stages with micrometric precision. The entrance
face of the sample is perpendicular to the propagation axis z.
The sample can be moved along three axes x, y, z by three
stepping motors. A camera placed after the sample enables us
PHYSICAL REVIEW B 71, 125435 2005
1098-0121/2005/7112/12543511/$23.00 ©2005 The American Physical Society 125435-1