Full text of DOI:10.1111/j.1151-2916.2002.tb00285.x

J. Am. Ceram. Soc., 85 [6] 1387–90 (2002)
journal
New Materials and Processes for Integrated Optics
James S. Wilkinson
Optoelectronics Research Centre, University of Southampton, Southampton, Hampshire SO17 1BJ, United Kingdom
Materials and processes for integrated optics are faced with
increasing demands from telecommunications and instrumen-
tation applications. Progress in three aspects of novel materials
and processes are described: titanium-diffused sapphire
waveguides and lasers, interferometric excimer laser ablation
of waveguide gratings, and surface studies by waveguide
surface plasmon resonance.
generation of short-wavelength radiation, and silicon for realiza-
tion of electronic circuits and detectors. We have previously shown
that titanium may be diffused into sapphire to give spectroscopic
behavior similar to that of bulk-doped crystals,5 demonstrated
waveguide fabrication by titanium diffusion in sapphire,6 and
realized the first waveguide laser in Ti-diffused sapphire.7 Re-
cently lasers with lower pump power thresholds have been
produced by optimizing the device geometry.
Waveguide lasers were fabricated by diffusing 270 Ϯ 7 nm
thick, nominally 3 ␮m wide, stripes of titanium oxide, deposited
by thermal evaporation from Ti2O3 powder, into 10 mm ϫ 10 mm
c-cut sapphire wafers. Diffusion took place in an argon atmosphere
at 1700 Ϯ 60°C for a duration of 60 min plus ϳ10 min each for
heating and cooling. The sample was cut and polished to yield a
device 4 mm long, and mirrors were attached to the endfaces. The
mirrors had a transmission of 90% at 500 nm and reflectivities of
Ͼ98% at wavelengths between 760 and 810 nm. Transverse
magnetic (TM) polarized pump radiation from an argon ion laser
operating at 514.5 nm was chopped and end-fire coupled into a
waveguide through the input mirror. The chopper operated to
repeatedly pass pump radiation for 1 ms and block it for 19 ms, to
reduce heating of the sample. Guided output radiation was col-
lected with a microscope objective lens and unabsorbed pump
radiation was separated from laser signals using a cold mirror with
an edge at 700 nm. Using appropriate filters, the pump and signal
radiation were measured using silicon photodetectors, and the wave-
guide mode profiles were measured using a silicon CCD camera.
The waveguide laser signal output power is shown as a function
of launched pump power in Fig. 1 for two different channels. The
launched pump power is estimated from the pump power at the
output and the signal output power is averaged over 100 pump
pulses for each pump power. The threshold pump power is 210 Ϯ
10 mW for both lasers and the slope efficiencies are ϳ0.06% for
both lasers. Assuming that the quantum efficiency is 80% and
taking the mirror transmission to be 2%, the round-trip loss is
estimated as 12 dB for both lasers. Deviations in linearity in the
power characteristics are believed to be due to thermal effects.
Laser spectra were measured, for laser 2 in Fig. 1 using an optical
spectrum analyzer, with pump power 85 mW above threshold, and
are found to consist of two main peaks at 788 and 791 nm. The
lasers operated between 780 and 885 nm for different pump
powers and chopper duty cycles.
I. Introduction
NTEGRATED optics is experiencing a period of unprecedented
growth, primarily fueled by the need for components for
I
high-capacity optical telecommunications systems, but also stim-
ulated by requirements for screening of chemical species for
medical, environmental, and biotechnological applications. Well-
established integrated optical technologies such as silica-on-
silicon,1 titanium diffusion in lithium niobate,2 and ion exchange
in glass3 provide ready solutions for many of these applications.
However, the need for greater functionality in integrated optical
circuits, for optical gain at new wavelengths in compact devices,
and for low-cost devices, is driving research into new materials
and processes for integrated optics.
In this paper, progress in research into three example systems is
presented. First, the realization of waveguides and waveguide
lasers by diffusion of titanium into sapphire and the prospects for
a low-threshold broadly tunable Ti:sapphire laser are discussed.
Second, the writing of relief gratings in ion-exchanged glass
waveguides and in thin waveguide overlayers by interferometric
laser ablation is described and results are given for waveguide
transmission and reflection spectra. Finally, waveguide surface
plasmon resonance studies of the electrochemical underpotential
deposition of a copper monolayer at the surface of a waveguide
overlaid with a thin gold film are described.
II. Titanium-Diffused Waveguides and Lasers in Sapphire
The titanium sapphire laser is a versatile laboratory tool for
spectroscopic applications, with a tuning range from 650 to 1050
nm,4 making it well suited to tunable short-pulse operation.
Realization of a Ti:sapphire waveguide laser on a chip promises
low-threshold operation for ready pumping by solid-state sources,
rendering it compatible with incorporation into compact spectro-
scopic instruments. Further, sapphire is a versatile substrate for the
growth of ZnO for the generation of acoustic waves, GaN for
The mode intensity profiles for the laser mode and the pump
mode for laser 1 were measured during lasing, and are shown in
Figs. 2(a) and (b), respectively.
The intensity profiles in Fig. 2 are TM polarized and show
fundamental mode operation with the signal mode having an
approximate width of 10 ␮m and depth of 5 ␮m at full width 1/e
peak intensity. The pump mode is shown on the same scale and is
substantially narrower; at wavelengths below 500 nm the
waveguide supported more than one mode. White-light waveguide
absorption measurements, which quantified the Ti3ϩ absorption,
showed that the waveguides had an average titanium concentration
equivalent to 0.13 Ϯ 0.02 wt% Ti2O3 in Al2O3 by comparison with
published results for bulk crystals.4 These measurements yielded
no evidence of significant absorption due to Ti4ϩ, which causes
absorption in the gain band and lowers the material figure of merit.
These results show a considerable reduction in pump power
threshold compared with the 1.2 W reported previously for
J. Ballato—contributing editor
Manuscript No. 188061. Received December 18, 2000; approved December 4,
2001.
Presented at the 102nd Annual Meeting of the American Ceramic Society,
Photonic Materials, Components and Applications Symposium, May 1–3, 2000, St.
Louis, MO.
The Optoelectronics Research Centre is an interdisciplinary center supported by
the U.K. Engineering and Physical Sciences Research Council.
*Member, American Ceramic Society.
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Journal of the American Ceramic Society—Wilkinson
Vol. 85, No. 6
Fig. 1. Ti:sapphire waveguide laser characteristics.
Ti-diffused sapphire lasers.7 However, the low slope efficiency
indicates either that the quantum efficiency is low or that the
waveguide propagation losses or mirror coupling losses are high.
Future work will investigate these parameters and is expected to
lead to devices with substantially lower thresholds and higher slope
efficiencies. Work is also in progress to realize waveguides with
smaller spot sizes, to integrate wavelength selection devices, and to
operate the lasers continuously (CW) with appropriate cooling.
have been written by photolithography and etching in titanium-
diffused waveguides in lithium niobate overlaid with a thin silicon
film to enhance modal interaction with the grating.9 We have
studied the use of thin indium tin oxide film overlayers on glass
waveguides for enhancement of evanescent chemical sensing and
electrochemical control of sensing reactions. Recently we have
realized submicrometer relief gratings in thin InOx films overlaid
on Kϩ-ion-exchanged waveguides using interferometric laser ab-
lation at 248 nm.10
Waveguides were fabricated by ion exchange of BK7 glass in
molten potassium nitrate through aluminum mask openings rang-
ing from 3 to 8 ␮m at 400°C for 11 h. A 135 nm thick film of InOx
was deposited by DC magnetron sputtering in 100% O2 to cover a
length of 25 mm of the 40 mm long waveguides. The overlaid
waveguides were exposed to a high-contrast UV fringe pattern
with period 514 nm using the three-mirror interferometer de-
scribed previously.8 The spatial distribution of the laser output was
homogenized during recording using a rotating plate. Gratings of
length 16 mm were produced on each set of waveguides with an
average pulse energy density of 60 mJ/cm2 using between 5 and
100 pulses. An AFM micrograph of part of a grating ablated using
5 pulses is shown in Fig. 3, where the peak-to-peak grating depth
is 35 nm, and scales are in nanometers.
III. Excimer Laser Ablation of Surface Waveguide Gratings
The application of excimer lasers to the fine machining of
optical materials has recently been intensively investigated. Inter-
ferometric laser ablation has the potential for realizing strong
reflection gratings in compact waveguide devices, with the advan-
tages of flexibility of reflection wavelength selection and minimi-
zation of the number of process steps. We have recently demon-
strated such reflection gratings near 1300 nm, written in thallium-
ion-exchanged glass waveguides by interferometric laser ablation
at 193 nm.8 However, the exposure caused substantial damage to
the waveguides, resulting in high broadband losses. Bragg gratings
The polarized transmission spectra of the waveguides were
measured by coupling the ASE spectrum of an erbium-doped fiber
amplifier into each waveguide with a monomode fiber and
coupling the output signal into an optical spectrum analyzer using
Fig. 2. (a) Signal mode intensity profile. (b) Pump mode intensity profile.
Fig. 3. AFM micrograph of an ablated grating in a 135 nm InOx film.

June 2002
New Materials and Processes for Integrated Optics
1389
Fig. 5. Integrated optical surface plasmon resonance (IOSPR) sensor.
confinement of the optical fields to the surface under investiga-
tion,15 while the waveguide configuration allows ready tailoring of
the interaction strength and integration of dense arrays of such
sensors through photolithographic replication.
Fig. 4. Waveguide transmission and reflection spectra.
Pyrex glass wafers 50 mm square were coated with an alumi-
num film and channels 3 ␮m wide were opened in this film, in the
form of Y-junctions as shown in Fig. 5, giving rise to a diffusion
mask. The coated wafer was then immersed in molten KNO3 at
389°C for 7.4 h, to form potassium-ion-exchanged waveguides,
and the aluminum mask was removed. The surface of the glass was
treated to promote gold adhesion by refluxing in a solution of
(3-mercaptopropyl)trimethoxysilane in water and 2-propanol in
the ratio 1:1:46 for 15 min.
The wafer was patterned with gold pads 30 Ϯ 5 nm thick and 3
mm long on one arm of each of the waveguide Y-junctions, as
shown in Fig. 5. The gold film serves both to guide the surface
plasma wave (SPW) and as a working electrode in electrochemical
studies. The second waveguide is used to provide a reference
output allowing compensation for the effects of input power
fluctuations; the device transmittance being defined as T ϭ
a multimode fiber. These spectra were then normalized to that
obtained with the monomode fiber butted directly to the multi-
mode fiber. Reflection spectra were obtained using a fiber coupler
at the input. An example of the TE-polarized transmission and
reflection spectra for an 8-␮m wide waveguide overlaid with the
grating shown in Fig. 3 are given in Fig. 4.
The transmission spectrum shows a broadband loss of about 4
dB, due principally to fiber-waveguide coupling loss but also
including transition losses (due to modal mismatch between the
coated and uncoated regions of the waveguide) and propagation
losses due to scattering and absorption in the overlayer. The
grating transmission showed a clear notch at 1546.7 nm with a
depth of ϳ4.5 dB and a bandwidth at full width at half-maximum
power of 0.08 nm. The reflection spectrum shows that the reflected
power is coupled into the backward-traveling waveguide mode,
with a peak reflection coefficient of ϳ40%. No detectable grating
response was observed for the TM polarization. The grating
reflection wavelength showed a near-linear dependence on nomi-
nal waveguide width of ϳ0.25 nm/␮m over the range from 3 to 8
␮m. Recently, strong gratings have also been written by interfero-
metric laser ablation in thin Ta2O5 films on glass waveguides.11
High-quality relief gratings have been written directly in high-
index overlayer films on glass waveguides. The films act both to
provide a low ablation threshold material to prevent damage to the
underlying waveguides and to increase the field strength at the
surface, thereby increasing the overlap with the corrugation. These
devices exhibit narrowband reflection of readily adjustable wave-
length and strength and high polarization selectivity, which may
find application, for example, as reflectors in waveguide lasers, or
in the construction of photonic crystal waveguides.
P
Reference/PSensing. The electrodes were connected by a track to
pads at the edges of the substrate. Finally, a 700 nm thick layer of
Teflon AF 1600 (n Ϸ 1.31) was deposited on the surface of the
substrate and patterned to form windows exposing only the gold
electrodes on each Y-junction device. This device design matches
the velocity of the surface plasma wave at the gold surface and the
waveguide mode, for visible wavelengths at analyte indexes near
water, resulting in resonant coupling observed as a sharp reduction
in the transmittance.12
An open cylindrical silica cell, 20 mm in diameter and 25 mm
high, was clamped to the waveguide surface using a silicone
rubber O-ring seal, and filled with 10Ϫ3M Cu2ϩ in 0.1M perchloric
acid (n ϭ 1.334). A three-electrode cyclic voltammetry circuit was
established by introducing a platinum wire counter electrode and a
saturated calomel reference electrode (SCE) into the cell, with the
gold film pads acting as the working electrodes. Permanent contact
was made to the gold electrodes outside the cell at the edge of the
substrate. All three electrodes were connected to a conventional
potentiostat, allowing the potential of the working electrode, E,
with respect to the reference electrode, to be controlled and the
current, I, through the working electrode to be measured.
Figure 6 shows the optical transmission and the electrochemical
current against the applied potential after several potential cycles.
The first part of the potential scan, from 0.5 to 1.6 V, causes the
anodic oxidation of the gold surface, as is shown by the current
peak at about 1.1 V and the corresponding drop in transmission.15
The cathodic scan shows the corresponding reduction peak in the
current and increase in the optical transmission. The reduction is
completed at about 0.7 V, where the optical transmittance has
returned to its preoxidation value. As the potential decreases from
0.7 to 0.4 V, the transmittance increases because of the alteration
of the ionic distribution of the double layer at the metal–electrolyte
interface. This ionic redistribution involves the chemisorption of
anions at the gold, and results in a small displacement current. At
a potential of 0.24 V a peak in the current is observed, which is due
IV. Surface Studies by Waveguide Surface
Plasmon Resonance
The potential of integrated optical surface plasmon resonance
sensors has previously been demonstrated in immunosensing for
waterborne pesticide analysis.12 The combination of electrochem-
ical techniques with waveguide sensors, through use of conducting
overlayers of doped indium oxide, silver, or gold, allows electro-
chemical control and reversal of sensing reactions and may yield
complementary information on the nature of electrochemical
processes at surfaces.13 Underpotential electrochemical deposition
(UPD) of monolayers of metal adsorbates on noble metal surfaces
occurs below the Nernst potential required for electroplating, when
the adsorbate atoms are bonded to the noble metal atoms more
strongly than to a surface of the adsorbate metal.14 The electroad-
sorption process may be expressed as Cu0(ads) 7 Cu2ϩ(sol) ϩ
2eϪ, where ads and sol refer to adsorbed copper and copper ions
in solution, respectively. Surface plasmon resonance studies of
such surface reactions have great sensitivity due to the close

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Journal of the American Ceramic Society—Wilkinson
Vol. 85, No. 6
Fig. 6. Optical and electrochemical response of IOSPR sensor.
to the deposition of a monolayer of Cu onto the gold film surface.
The deposition of this monolayer of ϳ0.3 nm thickness gives rise
to a 10% drop in the optical transmittance. Integration of the peak
current over time during deposition gives a charge of 0.41
mC/cm2, which is close to the theoretical value of 0.44 mC/cm2 for
a monolayer of Cu2ϩ ions transferring two electrons per ion to an
ideal Au(111) surface. The direction of the potential scan is
reversed at 0.1 V and a peak in current is observed at 0.29 V,
corresponding to stripping of the Cu monolayer, and the transmit-
tance increases correspondingly.
The processes involved in the electrochemical oxidation and
reduction of a gold surface and the underpotential deposition of
Cu2ϩ on gold may be continuously monitored in situ with high
sensitivity using a compact integrated optical surface plasmon
resonance sensor. The waveguide approach is ideally suited for
multisensor integration and the results compare favorably with
those obtained using ellipsometry and reflectance measure-
ments.16,17 Sensor arrays for interrogation of surface films are
expected to find wide application in chemical sensing applications,
and combined monitoring of electrochemical and optical parame-
ters provides a powerful combination for multiparameter surface
sensing.18
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V. Conclusion
Growth in telecommunications applications of integrated optics
has stimulated the exploitation of planar optical waveguides in a
wide range of fields. Many new applications require materials with
different properties to conventional waveguide materials, new
fabrication processes, or new techniques to characterize surface
processes. An example of progress in research in each of these
three areas has been described. Work on sapphire waveguides has
been primarily concerned with the realization of lasers, which
show potential as compact, low-cost, low-power devices; however,
other applications in high-temperature or hostile environments are
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production of strong high-quality reflection gratings for wave-
length selection in integrated devices. Finally, interfacial studies
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been described, where the surface sensitivity of planar waveguides
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Acknowledgments
We thank the many colleagues who have contributed to the work of the Integrated
Optics and Microstructures Group at Southampton University.
Council for Research, Florence, Italy, 2000.
Ⅺ