Zirconia and zirconia-organically modified silicate distributed feedback waveguide lasers tunable in the visible

Article abstract of DOI:10.1063/1.1512949

Zirconia and zirconia-organically modified silicate waveguides of refractive index from 1.56 to 1.64 and thickness from 0.6 to 1.4 μm were prepared by the sol-gel method. Narrow linewidth (<0.5nm) lasing was observed in dye-doped zirconia and zirconia-organically modified silicates waveguides. Tuning of the output wavelength was achieved by varying the period of the gain modulation generated by a nanosecond Nd:yttritium-aluminum-garnet laser at 532 nm. Tuning ranges were 586-618 nm and 629-657 nm for rhodamine 6G and rhodamine B, respectively. The threshold pump energy was about 50 μJ for rhodamine 6G-doped zirconia film on glass substrates. 2002 American Institute of Physics.

Full text of DOI:10.1063/1.1512949

Zirconia and zirconia-organically modified silicate distributed feedback waveguide
lasers tunable in the visible
Dennis Lo, Lei Shi, Jun Wang, Guo-Xuan Zhang, and Xiao-lei Zhu
Citation: Applied Physics Letters 81, 2707 (2002); doi: 10.1063/1.1512949
View online: http://dx.doi.org/10.1063/1.1512949
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 15
7 OCTOBER 2002
Zirconia and zirconia-organically modified silicate distributed feedback
waveguide lasers tunable in the visible
Dennis Lo,a) Lei Shi, Jun Wang, and Guo-Xuan Zhang
Physics Department, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong
Xiao-lei Zhu
Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, Shanghai, China
͑Received 5 July 2002; accepted 19 August 2002͒
Zirconia and zirconia-organically modified silicate waveguides of refractive index from 1.56 to 1.64
and thickness from 0.6 to 1.4 ␮m were prepared by the sol-gel method. Narrow linewidth
(Ͻ0.5 nm) lasing was observed in dye-doped zirconia and zirconia-organically modified silicates
waveguides. Tuning of the output wavelength was achieved by varying the period of the gain
modulation generated by a nanosecond Nd:yttritium–aluminum–garnet laser at 532 nm. Tuning
ranges were 586–618 nm and 629–657 nm for rhodamine 6G and rhodamine B, respectively. The
threshold pump energy was about 50 ␮J for rhodamine 6G-doped zirconia film on glass
substrates. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1512949͔
Zirconia (ZrO2) is a very useful optical material because
of the wide optical transparency and the high mechanical
strength and resistance to chemical reaction. Combined with
its high refractive index, zirconia films on glass or quartz
substrates hold good promise for a wide range of applica-
tions in integrated optics. Recently the sol-gel method has
been successfully developed for the preparation of zirconia,
titania and silica thin films.1 In particular Sorek et al. pre-
pared and characterized zirconia and zirconia-organically
modified silicates ͑ORMOSIL͒ waveguides.2 Zevin et al.
demonstrated zirconia waveguide amplifiers doped with a
number of laser dyes.3 The optical properties of infrared dye
in zirconia films were reported by Casalboni et al.4 Thick
zirconia films up to 30 ␮m were developed by Del Monte
et al.5
The active device such as laser is a critical enabling
technology to integrated optics. Thin film waveguide lasers
are desired for their efficient coupling with planar lightwave
circuit. The sol-gel method is particularly relevant for the
fabrication of active device since large number of functional
components ͑e.g., rare-earth elements, semiconductors, or-
ganic dyes͒ can be introduced into the glass matrix.1,6 Laser
action from thin film structure can be induced using the dis-
tributed feedback ͑DFB͒ configuration.7 We recently demon-
strated a distributed feedback waveguide laser based on
titania–silica thin film prepared by the sol-gel method.8
Multiple-mode lasing and optical confinement were ob-
served. Zirconia thin films appear to be superior to titania
films in view of the optical transmission of zirconia deep into
the UV and the absence of catalytic photodegradation of or-
ganic dopants.2 Hence, zirconia seems ideal as a host matrix
for functional organic dopants.
were prepared in low temperature on glass or fused quartz
substrates. A periodic gain modulation was generated by a
frequency-doubled Nd:yttritium–aluminum–garnet ͑YAG͒
laser in the film. Lasing commenced ͑evidenced by spectral
collapse͒ when the pump energy was 50 ␮J for R6G-doped
zirconia films. Single mode laser action was observed in both
zirconia and zirconia–ORMOSIL films. The preparation of
good optical quality zirconia and zirconia–ORMOSIL wave-
guide was key to the demonstration of tunable DFB laser
action. The main difficulty in preparing zirconia films from
zirconia alkoxides laid in the rapid hydrolysis and subse-
quent precipitation of colloidal zirconia upon water addition
to the zirconia precursors.2 Ganguli and Kundu were the first
ones to prepare transparent zirconia films by dissolving zir-
conia propoxide in dried solvents.9 Alternatively the zirconia
precursor can be stabilized by complexing agents such as
acetic acid or acetylacetonate.10 Hybrid organic/inorganic
zirconia–ORMOSIL films can also be prepared by the incor-
poration of an organic modifier ␥-glycidyloxypropylti-
methoxysilane ͑GLYMO͒. The presence of GLYMO in the
films served to reduce the porosity and to improve the me-
chanical strength. The addition of R6G and RB dyes made
zirconia and zirconia–ORMOSIL film laser active.
We largely followed the preparation procedures of Sorek
et al. to fabricate dye-doped zirconia and zirconia–
ORMOSIL films on glass or fused quartz substrates.2 The
refractive indices of the glass and fused quartz substrates
were 1.51 and 1.46, respectively. The starting solutions con-
sisted of zirconium n-propoxide and acetic acid. After the
solutions were magnetically stirred for an hour, a few drops
of 2-propanol were added to adjust the viscosity that in com-
bination of the speed of spin coating determined the thick-
ness of the films. The water needed for hydrolysis was mixed
with acetic acid ͑1:3 by volume͒ and introduced drop by
drop to the solutions. The molar ratio of zirconium
n-propoxide to acetic acid was about 1:4 in the final solu-
tions. Finally, laser dyes were added until the desired con-
centration was reached. For zirconia–ORMOSIL films, the
organic modifier GLYMO was introduced in the initial solu-
We report in this letter the observation of tunable DFB
laser action from dye-doped zirconia and zirconia–
ORMOSIL thin films. Rhodamine 6G ͑R6G͒ and rhodamine
B ͑RB͒ doped zirconia and zirconia–ORMOSIL thin films
a͒
Electronic mail: dlo@phy.cuhk.edu.hk
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0003-6951/2002/81(15)/2707/3/$19.00 2707 © 2002 American Institute of Physics
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2708
Appl. Phys. Lett., Vol. 81, No. 15, 7 October 2002
Lo et al.
tions. Dye-doped zirconia and zirconia–ORMOSIL films
were obtained by spin-coating glass or fused quartz sub-
strates with the final solutions. Cladded on one side by the
low index substrate and the other by air, the film on substrate
structure behaved as an asymmetric waveguide. Depending
on the viscosity of the solution and the spinning speed, films
of thickness varying from 0.6 to 1.4 ␮m were obtained. Pre-
pared at room temperature, the refractive index for both zir-
conia and zirconia–ORMOSIL films was 1.56 to 1.57. Re-
fractive index up to 1.64 was obtained when the films were
kept at 60 °C for 2 days. Atomic force microscopy revealed
that the surface of the films appeared to be amorphous with a
random distribution of zirconia gel particles.
The waveguiding properties of doped and undoped zir-
conia and zirconia–ORMOSIL films were characterized us-
ing a commercial prism coupler ͑Metricon model 2010͒ at
633 nm. Multiple waveguide modes ͑for both TE and TM͒
were observed for films of thickness greater than 1 ␮m. Only
one mode was allowed for films of thickness of about 0.6
␮m. Refractive index and the thickness of the film were si-
multaneously determined from the propagation angles of the
waveguide modes. Comparing the scanning prism coupler
results for zirconia and for zirconia–ORMOSIL films, it was
revealed that zirconia–ORMOSIL films had better
waveguiding properties as evidenced by the sharp and better
resolved guiding modes. Indeed we were to make waveguid-
ing zirconia–ORMOSIL films of thickness up to 2.8 ␮m,
whilst zirconia films of similar thickness often had poor
waveguiding properties ͑absence of waveguide modes͒ prob-
ably resulting from poor surface smoothness. For compari-
son purpose, the well-studied titania–silica films had a ten-
dency to phase separate and the surface integrity of films of
thickness Ͼ2 ␮m was difficult to maintain. There were no
measurable differences in refractive index for dye-doped and
undoped waveguides for dye concentrations up to 2
ϫ10Ϫ3 M/l. A Hitachi spectrophotometer was used to study
the absorption/transmission properties of the waveguides.
The transmission trace of a typical undoped zirconia wave-
guide of 0.6 ␮m thickness on fused quartz showed excellent
transmission from 210 nm to the near infrared. For zirconia–
ORMOSIL films, increased absorption in the deep UV ͑210–
280 nm͒ was observed. Figure 1 showed the absorption spec-
tra of RB-doped ͑a͒ and R6G-doped ͑b͒ zirconia waveguides.
The R6G-doped waveguide had an absorption peak near 530
nm, thus allowing efficient pumping at 532 nm by
frequency-doubled Nd:YAG laser. On the contrary the ab-
sorption peak of RB was at 580 nm. High RB concentration
of 5ϫ10Ϫ2M/l was achieved in the zirconia films which
partly made up for the weaker absorption of the pump laser,
whilst the highest concentration for R6G was 7ϫ10Ϫ3 M/l.
Narrow linewidth laser action was achieved in both dye-
doped zirconia and zirconia-ORMOSIL waveguides. The op-
tical arrangement was identical to that used in our previous
experiments in obtaining DFB laser action in dye-doped sol-
gel silica slabs and in titania–silica waveguides.6,11 Briefly a
holographic grating of 1800 lines/mm served as a beamsplit-
ter for the frequency-doubled output from a 6 ns Nd:YAG
laser of relatively low spatial coherence. Incidenton the grat-
FIG. 1. Absorption spectra for RB-doped zirconia film and R6G-doped
zirconia film.
into Ϯ1 orders of approximately equal intensities. The dif-
fracted beams were redirected to combine on the zirconia or
zirconia–ORMOSIL films at an intersection angle of 2␪,
forming a fringe pattern. A periodical modulation of gain
was thus created in the dye-doped glass film that served as
the gain medium. The output wavelength of the dye-doped
FIG. 2. Laser emission spectra for the DFB R6G-doped zirconia waveguide
laser on glass substrate ͑top͒. Data of angle tuning vs theoretical fit ͑bot-
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ing, the pump beam from the Nd:YAG laser was diffracted
tom͒.
93.180.53.211 On: Mon, 10 Feb 2014 08:50:56

Appl. Phys. Lett., Vol. 81, No. 15, 7 October 2002
Lo et al.
2709
FIG. 4. Data of angle tuning vs theoretical fit for RB-doped zirconia–
ORMOSIL waveguide laser.
FIG. 3. Laser emission spectrum as the DFB RB-doped waveguide laser on
glass substrate.
from 629 to 657 nm. The threshold pump energy for RB was
higher by a factor of 2 probably a result of the weaker ab-
sorption of the pump.
zirconia waveguide laser followed the Bragg condition at
␭Lϭ2␩⌳/M, where ␩ was the refractive index of the gain
medium at ␭L , ⌳ was the period of the gain modulation ͑the
fringe pattern͒, and M was the Bragg reflection order. In the
present experiments, second order Bragg condition was sat-
isfied (Mϭ2). When pumped by another laser at wavelength
␭P , the modulation period ⌳ is given as ⌳ϭ␭p/2 sin ␪. Tun-
Sol-gel glass films of thickness Ͼ10 ␮m were required
for volume holography, optical storage, and information
processing.12 Films of such thickness can be fabricated by
the introduction of an organic modifier into the inorganic
network to reduce stress during evaporation. As mentioned
earlier, GLYMO was used as the organic modifier in the
fabrication of the zirconia–ORMOSIL waveguides. We pre-
pared doped and undoped hybrid zirconia–ORMOSIL films
on glass substrates by spin coating. The thickness of the
hybrid films was up to 2 ␮m and the refractive index up to
1.64. We demonstrated DFB laser action in RB-doped
zirconia–ORMOSIL waveguide of thickness of 1.34 ␮m and
a refractive index of 1.57. Wavelength tuning around 650 nm
was achieved with a range of 27 nm. Figure 4 showed the
angle tuning data versus the theoretical fit. Good agreement
was observed. The zirconia–ORMOSIL film thickness can
be increased if dip-coating technique is applied.
ing of ␭ can be achieved by varying the intersection angle
L
and thus the gain period. A 0.3 m spectrograph/array detector
system was employed for spectral measurement of the laser
output. The time wave forms were measured using a PIN
photodiode and a fast digital oscilloscope.
DFB laser action was observed in R6G-doped zirconia
film on glass substrate. The film had a thickness of 0.62 ␮m
and a refractive index of 1.56. The dye concentration was
4ϫ10Ϫ3 M/l. The threshold pump laser energy was about 50
␮J. Accounting for reflection and transmission losses of the
pump laser, the actual energy delivered to the film was about
8 ␮J. Tuning of the output wavelength was achieved by vary-
ing the intersection angle ͑␪͒ of the crossing beams. Single
mode lasing was observed throughout the tuning range from
586 to 618 nm ͑top of Fig. 2͒. The laser output spectra had a
structure of a single prominent peak near the center of the
gain profile ͑600 nm͒. As the laser was tuned way from the
gain center, significant amplified spontaneous emission ap-
peared in the background. The linewidth of the laser output
was narrow at about 0.5 nm which was in fact the resolution
limit of our detection system. The bottom of Fig. 2 showed
the experimental data of the angle tuning versus the theoret-
ical fit of Bragg condition ͑solid line͒, in which the refractive
index 1.56 determined independently by the prism coupler
was used. Good agreement was seen. As the pump energy
increased from 50 to 350 ␮J, the DFB laser output energy
increased linearly. We also observed DFB laser action in RB-
doped zirconia wavguides. The initial dye concentration was
2ϫ10Ϫ3 M/l. Narrow linewdith lasing was demonstrated
͑Fig. 3͒. Angle tuning of the output wavelength was obtained
This work is supported in part by RGC Earmarked Re-
search Grant of the Hong Kong SAR Government No 4366/
99E.
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