DOI: 10.1007/s00339-006-3853-0
Appl. Phys. A 87, 87–90 (2007)
Materials Science & Processing
Applied Physics A
a. r
´
odenas
1
d. jaque
1,✉
c. molpeceres
2
s. lauzurica
2
j.l. oca
˜
na
2
g.a. torchia
3
f. agull
´
o-rueda
4
Ultraviolet nanosecond laser-assisted
micro-modifications in lithium niobate
monitored by Nd
3+
luminescence
1
Grupo de Espectroscopia L
´
aser GIEL, Departamento de F
´
ısica de los Materiales, Facultad de Ciencias,
Universidad Aut
´
onoma de Madrid, 28049 Madrid, Spain
2
Centro L
´
aser U.P.M., Edificio Tecnol
´
ogico “La Arboleda”, Campus Sur U.P.M.,
Carretera de Valencia km. 7,300, 28031 Madrid, Spain
3
Grupo de
´
Optica, Departamento de F
´
ısica Aplicada, Facultad de Ciencias F
´
ısicas, Universidad de Salamanca,
Plaza de la Merced s/n, 37008 Salamanca, Spain
4
Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, Spain
Received: 8 November 2006/Accepted: 11 December 2006
Published online: 1 February 2007 • © Springer-Verlag 2007
ABSTRACT
This work reports on the microstructural mod-
ifications produced by nanosecond ultraviolet ablation in
neodymium doped lithium niobate crystals. The neodymium
ions have been used as optical probes to determine the exten-
sion and nature of the modified bulk material. From micro-
luminescence experiments we have been able to determine the
spatial distribution of the UV ablation induced material densi-
fication, local disorder and defect creation. Results have been
compared to those previously obtained from femtosecond irra-
diated lithium niobate crystals.
PACS
42.55.Rz; 42.62.Fi; 42.70.-a
1 Introduction
Magnesium oxide-doped lithium niobate (MgO:
LiNbO
3
) is presently accepted as one of the most versatile ma-
terials for applications in integrated optics [1]. When doped
with
Nd
3+
ions, MgO:
LiNbO
3
becomes a versatile solid state
laser material capable of continuous wave laser light gener-
ation in different spectral ranges [2]. Micro and nano surface
relief techniques are regarded as powerful tools for the im-
provement of the potential applications of
LiNbO
3
crystals by,
for example, the development of micro-diffraction elements
and non-linear photonic crystals of high index contrast [3–6].
In particular, nanosecond UV-induced surface relief has been
already used in lithium niobate crystals for the fabrication of
micrometric and sub-micrometric gratings and brag reflec-
tors [7, 8]. The main advantages of this technique are the low
sample contamination, fast processing speed and outstanding
spatial resolution. Nevertheless, the possible existence and
nature of permanent microstructural modifications induced by
nanosecond UV irradiation in
LiNbO
3
crystals is, nowadays,
still an open question. From an applied point of view, the ex-
istence of modified lithium niobate in the surroundings of the
irradiated zones could determine the functionality of the ob-
tained photonic structures. From a fundamental point of view,
✉ Fax: +34-914978579, E-mail: daniel.jaque@uam.es
the knowledge about the extension and nature of nanosecond
ultraviolet-assisted micro-modifications would provide infor-
mation about the light-matter interactions, especially when
compared with those produced in different time scales (fem-
tosecond laser pulses) or wavelengths.
In this work we report on nanosecond UV laser ablation
of neodymium doped MgO:
LiNbO
3
crystals. The character-
ization of the ablated depths obtained under different irradi-
ation doses has been used to determine the ablation threshold.
We have used the
Nd
3+
-ions as optical probes to elucidate
how the nanosecond UV surface relief process has modified
the properties of the underneath bulk material. The infor-
mation extracted from the micro-luminescence properties of
Nd
3+
-ions has been used to localize induced permanent stress,
local disorder and defects. The comparison with previous data
concerning permanent structural modifications caused by in-
frared (IR) femtosecond ablation has revealed the importance
of both pulse wavelength and duration.
2 Experimental
The single domain
Nd
3+
-doped MgO:
LiNbO
3
crystal used in this work was grown from the melt by the
Czochralski method with neodymium and magnesium oxide
concentrations of 0.5 and
4at. %
, respectively. The sample
was a
10 × 2 × 5mm
3
prism, with its two (010) faces pol-
ished to optical grade. For nanosecond UV laser ablation
we have used a diode-pumped frequency tripled Nd:
YVO
4
(Spectra Physics HIPPO) emitting
12 ns
pulses at
355 nm
with
a repetition rate of
15 KHz
. The laser beam was focused to
a
10 µm
spot by using a long distance (
f = 89 mm
) objective
(
N.A. = 0.1
). The sample was mounted on a motorized stage
with a spatial resolution better than
1 µm
. Ablation was done
at normal conditions of temperature and pressure. After the
writing process, the lateral faces of the crystal were polished
in such a way that the cross sections of the ablation grooves
were characterized by performing lateral scanning electron
microscopy (SEM).
In order to study the microstructural modifications in-
duced by laser ablation, we have performed micro-lumine-
scence and micro-Raman experiments. For this purpose we
used a Renishaw Ramascope 2000 micro-spectrometer and