Fabrication and characterization of optical waveguides in KTiOPO4John D. Bierlein, August Ferretti, Lothar H. Brixner, and William Y. Hsu

Citation: Applied Physics Letters 50, 1216 (1987); doi: 10.1063/1.97913 View online: http://dx.doi.org/10.1063/1.97913 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/50/18?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Study on photorefractive effect in KTiOPO4 waveguides Appl. Phys. Lett. 65, 2539 (1994); 10.1063/1.112628 Characterization of optical waveguides in KTiOPO4 by second harmonic spectroscopy J. Appl. Phys. 76, 4999 (1994); 10.1063/1.357211 Optical characterization of KTiOPO4 periodically segmented waveguides for secondharmonic generation of bluelight J. Appl. Phys. 74, 4298 (1993); 10.1063/1.354393 Proton and ammoniumexchanged optical waveguides in KTiOPO4 J. Appl. Phys. 73, 3608 (1993); 10.1063/1.352919 Fabrication and characterization of planar ionexchanged KTiOPO4 waveguides for frequency doubling Appl. Phys. Lett. 58, 19 (1991); 10.1063/1.104436

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Fabrication and characterization of optical waveguides in KTiOP04

John D. Bierlein, August Ferretti, lothar H. Brixner, and William Y. Hsu E. L du Pont de Nemours and Company, Central Research and Development Department, a) Experimental Statio/l, Wilmington, Delaware 19898

(Received 14 January 1987; accepted for publication 4 March 1987)

Planar and channel optical waveguides have been fabricated in KTiOP04 (KTP) using ion exchange processes. The exchange kinetics are diffusion limited and depend strongly on composition and surface orientation. Increases in surface refractive index of up to 0.23 and waveguide thicknesses of up to 15 fim have been observed. A simple waveguide modulator has been fablicated which shows electro-optic r coefficients in the waveguide region to be similar to bulk KTP.

KTiOPO~ (KTP) is a relatively new material that has been shown to have superior properties for several nonlinear optical applications. I Its high nonlinear optical d coeffi­cients, high optical damage threshold, and thermally stable phase-matching properties make it useful for second-har­monic generation and, its large linear electro-optic r coeffi­cients and low dielectric constants make it attractive for var­ious electro-optical applications such as modulators and Q switches. 2 KTP also has an electro-optic waveguide modula­tor figure of merit nearly double that for any other inorganic materia12 and hence should be useful for active integrated optical devices provided optical waveguides can be fabri­cated. From the known refractive indices of K TP and RbTiOP04 I waveguiding may be possible if a layer of Rb­rich Rbx K l .. " TiOP04 can be fabricated on the surface of a KTP substrate. Similar effects may occur for Cs and n. This letter summarizes recent experiments aimed at fabricating optical waveguides in KTP, describes their properties, and discusses possible applications.

KTP is chemically stable up to about 1000 "C, is not hygroscopic, and has been shown to have relatively mobile potassium ions.} Solid solutions exist in the MTiOPO" (K TP) structure where Ai can be K, Rb, Tl, Cs (partial), or NH4 or any combination of these ions and solid solutions exist between MTiOP04 and MTiOAsO i . These properties permit a variety of waveguide fabrication techniques to be attempted for fabricating optical waveguides in K TP. One technique which has been particularly successful is ion ex­change where waveguides have been fabricated using molten nitrate salts ofRb, Cs, and TL The KTP surfaces of interest are first polished and then immersed into the molten salt for the required diffusion time, typically less than 4 h. The sam­ple is then examined for waveguiding by the moline tech­nique4 using a 45-45-90 rutile prism and He-Ne laser with A = 0.633 pm.

The ion exchange conditions and waveguiding results are summarized in Table I, where d is the diffusion depth and I:1n the increase in surface refractive index. An error function distribution is assumed for the refractive index pro­files in the ion exchanged regions, a distribution which agrees well with the ion concentration profile as determined

a) Contribution No. 4153.

by electron microprobe measurements and shown for a typi­cal Rb-exchanged sample in Fig. 1. The maximum increase in surface refractive index observed for rubidium (f:.n = 0.02) is close to the value that would result for nearly complete ion exchange forming a RbTiOP04 surface on a KTP substrate. l The increase in surface refractive index for all these ion exchanged guides generally scales with the elec­tronic polarizabilities of the exchanged ions relative to potas­sium.

The ion exchanged waveguides are stable both at room temperature and in accelerated aging tests, and, provided the diffusion temperature remains below about 450 "C, the ex­change process does not introduce any noticeable surface defects. Hence optical attenuation is expected to be low. Near and above 450 °C, slight surface etching occurs in some samples during the exchange and the results given in Table I for the x-cut Rb-exchanged and the z-cut Cs-exchanged samples were obtained after the surfaces were repolished.

The results given in Table I show the ion diffusion in KTP to be highly anisotropic, being much greater along thez axis (c or polar direction) and, when measured, being higher on a negative z surface (positive pyroelectric coefficient) than a positive z surface. The diffusion anisotropy correlates well with the large anisotropy observed in ionic conductiv­ities and dielectric properties.2 The variations in diffusion into the different polar surfaces are observed in other materi-

TABLE L KTP waveguide characteristics.

Ion

Rb

Rb

Rb

Cs

Tl

Surface Temp. type eel

x 450

z( +) 350

z( -) 350

z 450

z 335

Time (h)

3.3

4

4

4

4

No. of Mode modes type

0 TE TM

3 TE 3 TM

3 TE 2 TM

11 TE 8 TM

4 TE 4 TM

d (Itm)

1.3

4.0 4.0

6.5 6.5

l3 13

1.6 1.6

0.02

0.019 0.018

O.GOR 0.008

0.028 0.019

0.23 (l.18

1216 AppL Phys. Lett. 50 (18),4 May 1987 0003-6951/87/181216-03$01.00 © 1987 American Institute of Physics 1216 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 68.107.83.4

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0.8 tRb] -- :: edt (Z/4.4)

~I~ [RIl] a

.0 .0

.~ 0:; a: ................

c 0.6 0 .... g .. -<: <» y 0:: (I

0.4-u 'a 0.\1 N

a E 0 z 0.2

o 2 :5 4 5 6 Depth Z (p-m 1

FIG.!. Depth profile for Rb ion exchange in KTP.

aIs5 and result from differences in surface adsorption and reactivities. Additional variations in diffusion kinetics are observed along the z axis from changes in local ionic proper­ties. Some crystals had regions of varying pyroelectric and dielectric properties depending on crystal defects, incorpo­rated O-H, etc. The diffusion rate generally scales with ionic conductivity, a result which is expected since ionic conduc­tivity and diffusion are closely related.

Experiments aimed at measuring diffusion constants and activation energies indicate that the Rb-K-exchange process does not obey simple diffusion kinetics. The effective waveguide thickness and An were found to be nearly inde­pendent of diffusion time from 0.25 to 4 h at the typically used 350°C diffusion temperature and also nearly indepen­dent of diffusion temperature from 350 to 400 0c. Also, post­annealing a Rb-exchanged guide in air from 300 to 350 °C for 30 min to 2 h did not significantly change d or I1n. These results indicate the effective exchange or diffusion rate for the Rb-K system is initially high and then decreases signifi­cantly after some point in the exchange process. This large change in exchange or diffusion rate can be explained by assuming a very low diffusion constant for Rb and K in Rb­rich Rb~Ktx TiOP04 and a high constant in KTPo Initial measurements of some dielectric properties on single-crystal RbTiOP04 (RTP) show a much lower (-WOX) ionic conductivity than in KTP and hence ionic diffusion is also expected to be much lower. Exchanging K with the larger Rb ion in a KTP surface layer will also tend to block conduc­tion channels which further lowers ionic conductivity. Hence, during ion exchange, as the surface rubidium con-

1217 Appl. Phys. Lett., Vol. 50, No. ,8,4 May ,987

(a)

20 p..

(b)

FIG. 2. Rb-exchanged channel. waveguide in KTP; (a) top view, (b) end VIew.

centration increases, the diffusion constants at the surface decrease which will suppress further ion exchange and result in the equilibrium ion distribu tion shown in Fig. I. Although such an equilibrium distribution is unusual, it is consistent with diffusion theory. This type of behavior is, of course, an advantage for optical waveguide devices since it allows for rapid waveguide fabrication at relatively low temperatures but also permits thermally stable properties.

In addition to fabricating planar guides, channel wave­guides have been fabricated in z-cut surfaces using Rb ion exchange and a metal mask. The channels are defined by initially applying a suitable resist to the crystal surface prior to depositing the meta! (e.g., - 500 A Au or AI) then lifting off the resist. After ion exchange, channels with well-defined edges are obtained that show very little light scattering. Ini­tial measurements show optical attenuation to be less than 0.4 db/em. A typical channel waveguide is sh.own in Fig. 2 using phase contrast microscopy. These channel guides ap­pear to be optically uniform over the width of the channel and show very little if any evidence oflateral ion diffusion. A simple amplitUde waveguide modulator using crossed polar­izers was demonstrated by eiectroding both z surfaces and applying a field across the substrate. The measured electro­optic coefficient r33 - (n 2/n 3 )IY23 in the channel was found to be 23 pm/V which is similar to that for the KTP sub­strate. 2

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Several practical device possibilities are suggested by these results. The large increases in surface refractive indices that occur suggest that devices with short waveguide bend radii can be fabricated which would greatly increase sub­strate utilization over other materials. In addition, since the diffusion process occurs at relative low temperatures and the waveguides are stable at high temperatures, it should be pos­sible to fabricate devices using simple additive processes with large an channels at bends and low t:..n channels where large optical field penetrations are needed (e.g., switches, etc.). Also, the large diffusion anisotropies observed imply that lateral diffusion problems would be nearly eliminated when fabricating channel waveguides, an effect demonstrat­ed above. Finally, the extremely large t:..n observed in Tl ion exchanged guides suggests other nonlinear optical devices could be fabricated which take advantage oflong interaction lengths with high optical fields. A good example is second­harmonic generation in the waveguide using the d33 coeffi­cienL Such a device could be optimized over a much broader range of wavelengths than is presently possible using bulk KTP.

1218 Appl. Phys. Lett.. Vol. 50, No. 18,4 May i 987

In conclusion, we have shown that both planar and channel optical waveguides with a wide range of an can be fabricated in KTP, demonstrated a simple waveguide modu­lator, and discussed several other potential integrated opti­cal and nonlinear optical devices. Work is ongoing to fabri­cate specific devices and these will be reported on in future communications.

The authors acknowledge Airtron, Inc. for supplying the KTP crystals, J. B. Brown for fabricating crystal sub­strates and for electrical and optical measurements, J. H. Kelly for m-line spectrometry, and J. W. Brennan for elec­tron microprobe analyses.

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