Download Color center production by femtosecond-pulse laser

ISSN 1054-660X, Laser Physics, 2006, Vol. 16, No. 2, pp. 331–335.
NONLINEAR OPTICS
AND SPECTROSCOPY
© MAIK “Nauka /Interperiodica” (Russia), 2006.
Original Text © Astro, Ltd., 2006.
Color Center Production by Femtosecond-Pulse Laser
Irradiation in Fluoride Crystals
L. C. Courrol1, R. E. Samad2, L. Gomes2, I. M. Ranieri2, S. L. Baldochi2,
A. Z. Freitas2, S. P. Morato2, and N. D. Vieira, Jr.2, *
1 Laboratório
de Espectroscopia Óptica, CEETEPS, Fatec-SP, Praça Coronel Fernando Prestes 30, São Paulo, Brazil
de Lasers e Aplicações, CNEN-IPEN-SP, Av. Prof. Lineu Prestes 2242, São Paulo, Brazil
2 Centro
*e-mail: [email protected]
Received July 8, 2005
Abstract—Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in LiF and LiYF4
single crystals by ultrashort high-intensity laser pulses (60 fs, 12.5 GW). An intensity threshold for color center
creation of 1.9 and 2 TW/cm2 was determined in YLF and LiF, respectively, which is slightly smaller than the
continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser
+
beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as F2, F 2 , and
+
F 3 . The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived
+
F 2 centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.
DOI: 10.1134/S1054660X06020216
1. INTRODUCTION
Color centers are lattice vacancy defects trapping
electrons or holes. They are usually created in single
crystals at room temperature by irradiation with ionizing radiation [1].
When LiYF4 crystals are exposed to high intensities
of pumping radiation in the UV/visible spectral regions
or ionizing particles, degradation of the applicationrelated characteristics and performance results. The
study of defects produced, such as color centers, is then
a useful approach, which can be applied to understand
the relevance of such degradation processes and their
microscopic mechanisms. The threshold for color center creation was 1.9 TW/cm2 for LiYF4.
Color center lasers have some important optical
characteristics if created with particular spatial dimensions: they can modulate the material refraction index,
creating waveguides or photonics devices.
Recently, it was shown that it is possible to create
color centers with interaction of ultrashort-pulse lasers
in crystals [2, 3] and glasses [4]. In this case, the defects
may be produced with dimensional control [5] by
focusing high-intensity ultrashort laser pulses in the
material. Due to their absorption, a modulation of the
material’s refraction index occurs, creating waveguides
or photonics devices [6].
In this work, we show that it is possible to produce
stable color centers in YLiF4 (YLF) and in LiF fluoride
crystals with dimensional control. The optical properties of the color centers produced by irradiation with
ultrashort laser pulses and by electron beam were investigated, allowing the determination of the center cre-
ation intensity threshold and therefore the discussion of
the basic formation mechanisms of these centers. The
optical absorption and emission properties of these
color centers were measured.
2. MATERIALS AND METHODS
In this paper, we study the creation of color centers
in pure LiF and YLF irradiated by intense laser light.
The crystals were grown by the Czochralski technique
under Argon atmosphere.
The absorption spectra of all samples were measured at room temperature in the 200- to 900-nm range,
using a Varian Spectrometer Cary 17 D. The emission
spectra were obtained by exciting the samples (1 mm in
thickness) with a 150-W xenon lamp passing thought a
monochromator. The emissions of the samples were
analyzed with a second 0.5-m monochromator (Spex)
and a PMT detector. The signal was analyzed with an
EG&G 7220 lock-in amplifier and processed by a computer. A time-resolved luminescence spectroscopy
technique was employed to measure the luminescence
at 710-nm, induced by resonant laser excitation. The
excitation system consists of a short pulse tunable optical parametric oscillator (OPO from OPOTEK)
pumped by the second harmonic of a Q-switched Quantel Nd-YAG laser. This laser system delivers pulses of
10 mJ with a pulse duration of 4 ns and repetition rate
of 10 Hz. The time-dependence luminescence of the
europium was detected by a S-20 Hammamatsu PMT
and analyzed using a 200-MHz Tektronix TDS 410 digital oscilloscope. The relative errors in the emission
331
332
COURROL et al.
Seeder
Pump
830 nm, 60 fs
78 MHz, nJ pulses
1 kHz@ 527 nm
13 W
Amplifier
830 nm, 750 µJ, 60 fs, 1 kHz
M2 = 1.6
f = 83 mm lens
Fig. 1. Experimental setup.
measurements are estimated to be <5%, while errors in
the lifetime measurements are <10%.
A Ti:Sapphire Chirped Pulse Amplified (CPA) laser
system operating at 830 nm was used, producing a train
of 640-µJ, 60-fs pulses at 1 kHz, in a beam with a M2 =
1.6 and a peak power of 12.5 GW. The beam was
focused by an 83-mm lens to a radius of 12 µm. The
beam waist was focused inside the crystal samples. The
scheme of the experimental setup is shown in Fig. 1.
The irradiations were performed at room temperature.
3. RESULTS
Figure 2a shows a photograph of the effect of the
focused laser beam impinging into a bulk LiF crystal
with the beam waist located inside it. A green emission
and white light (continuum [7–9]) generation along the
Spectral characteristics of color centers in LiF
Absorption
wavelength, nm
Emission
wavelength, nm
F
F3
F2
248
316, 374
444
678
+
F3
448
541
645
910
960
1120
Color center
+
F2
–
F2
beam path can be seen. Figure 2b shows the scheme of
the shapes seen in the photograph, where the green light
appears first and then the continuum, indicating that the
green emission begins at a lower intensity than the continuum. After the laser irradiation, only the green emission geometry was preserved inside the crystal, forming a green track when viewed under white light.
The LiF crystal has a wide optical gap (~11.8 eV),
which makes it a very good medium to study the formation of color centers. The simplest color center, formed
by an electron trapped in an anion vacancy, is the F center. When two, three, or four F centers are aggregated,
F2, F3, and F4 centers are formed, respectively. When
ionized or when an electron is additionally trapped by
these centers, positively or negatively charged color
–
+
centers are formed ( F 2 or F 2 centers in the case of F2
center), respectively. The table shows the known spectral characteristics of such color centers in LiF crystals.
In Fig. 3 the absorption spectra obtained for the LiF
+
crystals are shown. F 3 and F2 centers are responsible
for the blue absorption band, called the M band, due to
+
the 448-nm ( F 3 ) and 444-nm (F2) absorption peaks. In
addition to the main M absorption band, the red absorp+
tion band peaking at the 632-nm band is due to the F 2
center (ionized F2 center).
Correspondent absorption bands can be estimated
by the Mollwo–Ivey [10] relations for absorption bands
in YLF. They can be observed in the spectrum of Fig. 3:
+
F = 296 nm, F2 = 473 nm, F3 = 343 nm, and F 2 =
650 nm, all shifted to the infrared.
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2006
COLOR CENTER PRODUCTION BY FEMTOSECOND-PULSE LASER
333
(‡)
LiF crystal
Laser beam
Green emission
region
(b)
White light
(c)
Fig. 2. (a) Green emission and white light generation along the beam path while under irradiation by the femtosecond pulses (the
pulses came from the left); (b) schematic representation of the light shapes of the preceding photograph: the color centers are created
before the beam waist (focal point) and before the white light. There are no color centers after the beam waist, because crystal break+
down occurs at the waist position, scattering the laser beam; (c) emission of F 3 centers when excited by white light (the laser
entered the sample from the top surface); note that these tracks start near the entrance surface of the sample, grow larger in radius,
and then get smaller near the beam focus position.
α, cm–1
25
YLF
LiF
20
15
10
5
0
200
300
400
500
600
700
800
Wavelength, nm
Fig. 3. Absorption spectra of the tracks created in LiF and YLF crystals by 640 µJ, 60 fs laser pulses.
The emission spectra obtained by exciting the samples at 630 nm are shown in Fig. 4 for both LiF and YLF
crystals. Several differences are observed in the intensity of the bands. The emission band present in YLF at
LASER PHYSICS
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2006
710 nm is very weak in LiF. Fixing the emission at
710 nm, we obtained the excitation spectra and
observed that the position, width, and quantity of the
peaks changed, indicating the different kinds of color
334
COURROL et al.
Signal, arb. units
0.035
λexc = 630 nm
0.030
YLF
LiF
0.025
0.020
0.015
fs
0.010
0.005
0
0.030
YLF
0.025
λem= 710 nm
LiF
0.020
×10
0.015
0.010
0.005
0
350
400
450
500
550
600
650
700
Wavelength, nm
Fig. 4. Emission spectra obtained for LiF and YLF with excitation at 630 nm (upper graphic) and excitation spectra obtained fixing
the emission at 710 nm (lower graphic).
+
centers produced. In LiF:F2 (441 nm), F 3 (458 nm) and
+
F2
(600 nm) excitations are observed in this spectra. In
YLF, only two bands are observed at 430 and 650 nm,
+
F2 and F 2 , respectively.
+
The pursuit for stable F 2 centers in crystals has
been an important issue for building lasers. If both thermal and photo-stable centers are present, they can have
high energy output and good efficiency [11]. Also, the
spectral output is shown to be very wide. For YLF crystals, the calculated emission cross section for F2 was
approximately 2 × 10–17 cm2, considering the measured
lifetime of 22 ns (710 nm). We believe the stable centers produced in YLF can be useful to make ultrafast
color center lasers, as well.
In order to determine the YLF color center creation
intensity threshold, the samples were placed in the
focused laser beam before the waist, in a position where
no color centers were formed, and were moved towards
the waist until color centers were observed by naked
eye to be formed by the laser pulses. The sample position relative to the lens was measured, and the pulse
intensity was calculated at the sample position. Considering the laser spot size w given by the laser beam propagation law [12],
2
w = w0
⎛ M 2 λ ( z – z 0 )⎞
1 + ⎜ ---------------------------⎟ ,
2
⎝
⎠
πw 0
(1)
where λ = 830 nm is the laser wavelength, z0 = 19.8 cm
is the beam waist position relative to the lens, w0 =
25 µm is the beam waist, M2 = 1.6 is the beam quality
factor, and z = 17.5 cm is the sample position where
color centers were observed to be formed by the laser
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COLOR CENTER PRODUCTION BY FEMTOSECOND-PULSE LASER
pulses, resulting in w = 390 µm. In this measure, the
pulse energy was 547 µJ and the pulse width was 60 fs,
resulting in a color center creation threshold of
1.9 TW/cm2 for YLF. By applying a method recently
developed [2, 13] to estimate the threshold for color
center formation in LiF, its value was determined to be
2.04 TW/cm2. In both cases, this is a first-approximation effect, because nonlinear effects were not considered.
To explain the color center production by the
ultrashort pulses, we propose a mechanism based on
the electron avalanche that develops due to a strong initial nonlinear multiphoton absorption [14, 15]. This
avalanche promotes many electrons to the conduction
band, where they acquire kinetic energy from the laser
field and create vacancies by impact with the fluorine
ions. When this anion vacancy traps an electron (neutralizing the vacancy charge), an F center is created.
The second step in the production of defects involves
the migration of primary defects and the formation of
complex defects. These secondary processes depend on
temperature and intensity rate. The difference in using
a focused laser beam is that the density of primary
defects formed is much higher than the usual methods,
and therefore the probability of aggregation is very
much increased. This can be seen by the ratio of the F
to the M bands in YLF (Fig. 3). This ratio is much
higher when the centers are produced by regular ionizing radiation [16]. In the region after the green emission
track in Fig. 2, there is no significant color center formation. In this case, we propose that there is equilibrium between the creation and destruction processes
that occur simultaneously during the ultrashort-pulse
irradiation, since the fundamental and the harmonics
[17] of the pumping laser can be absorbed by the
defects as well as the white light generation. These centers are generally unstable with the combination of temperature and intense light [18].
The production of stable color centers in pure YLF
samples was obtained as a consequence of the interaction of an ultrashort laser pulse with the material. We
think that a controllable refractive index change can be
achieved by adjusting femtosecond laser irradiation
parameters and subsequent annealing conditions. In
this way, one can induce refractive index change for the
crystals to fabricate internal diffraction gratings, optical
waveguides, etc., useful for three-dimensional integrated optics devices.
4. CONCLUSIONS
We created color centers in LiF and YLF crystals
with high-intensity ultrashort laser pulses and mea-
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No. 2
2006
335
sured their absorption and emission spectra. We propose that the mechanism responsible for the creation of
centers is a multiphotonic process depending on the
crystal energy gap as well as the higher color center
density formation that induces their aggregation.
+
Stable F 2 color centers were produced in YLF and
their emission cross section was calculated to be
approximately 2 × 10–17 cm2.
ACKNOWLEDGMENTS
The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo, FAPESP, for support
under the grant 00/15135-9.
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