Electro-optic phase modulation in ridge waveguides of epitaxial K_0.95Na_0.05Ta_0.71Nb_0.29O_3 thin...

Electro-optic phase modulation in ridgewaveguides of epitaxial

K0.95Na0.05Ta0.71Nb0.29O3 thin films

Christian Herzog, Steffen Reidt, Gorazd Poberaj, and PeterGunterNonlinear Optics Laboratory, ETH Zurich, 8093 Zurich, Switzerland

[email protected]

Abstract: Electro-optic modulation atλ = 1.5 µm has been demonstratedfor the first time to the best of our knowledge in a ridge waveguide phasemodulator produced in cubic potassium sodium tantalate niobate thinfilms epitaxially grown on potassium tantalate substrates exploiting thelarge quadratic electro-optic Kerr coefficient ofR11 = 8.2×10−17 m2/V2.The relative permittivity, Kerr coefficient, and refractive index have beenevaluated for the thin film crystal and are compared to the values meas-ured in bulk crystals. The half-wave voltage times length figure of merit ofthe modulator has been measured to beVπ l = 38 Vcm at room temperature.

© 2007 Optical Society of America

OCIS codes:(160.2100) Electro-optical materials; (160.4760) Opticalproperties; (230.2090)Electro-optical devices; (230.7380) Waveguides, channeled; (310.2790) Waveguides, chan-neled; (310.6860) Thin films, optical properties

References and links1. F. S. Chen, J. E. Geusic, S. K. Kurtz, J. G. Skinner, and S. H.Wemple, “Light Modulation and Beam Deflection

with Potassium Tantalate-Niobate Crystals,” J. Appl. Phys. 37, 388 (1966).2. S. Triebwasser, “Study of Ferroelectric Transitions of Solid-Solution Single Crystals of KNbO3-KTaO3,” Phys.

Rev.114, 63–70 (1959).3. R. Gutmann, J. Hulliger, and H. Wuest, “Growth of Paraelectric and Ferroelectric Epitaxial Layers of

KTa1−xNbxO3 by Liquid-Phase Epitaxy,” Ferroelectrics134, 291–296 (1992).4. R. Gutmann, J. Hulliger, and E. Reusser, “Liquid-Phase Epitaxy of Lattice-Matched KTa1−xNbxO3 Grown on

KTaO3 Substrate,” J. Cryst. Growth126, 578–588 (1993).5. H. Pierhofer, Z. Sitar, F. Gitmans, H. Wuest, and P. Gunter,“New semiconducting substrate for heteroepitaxial

growth of K1−yNayTa1−xNbxO3,” Ferroelectrics201, 269–275 (1997).6. L. A. Knauss, K. S. Harshavardhan, H. M. Christen, H. Y. Zhang, X. H. He, Y. H. Shih, K. S. Grabowski, and

D. L. Knies, “Growth of nonlinear optical thin films of KTa1−xNbxO3 on GaAs by pulsed laser deposition forintegrated optics,” Appl. Phys. Lett.73, 3806–3808 (1998).

7. S. Toyoda, K. Fujiura, M. Sasaura, K. Enbutsu, A. Tate, M. Shimokozono, H. Fushimi, T. Imai, K. Manabe,T. Matsuura, and T. Kurihara, “Low-driving-voltage electro-optic modulator with novel KTa1−xNbxO3 crystalwaveguides,” Jpn. J. Appl. Phys.43, 5862–5866 (2004).

8. K. Nakamura, J. Miyazu, M. Sasaura, and K. Fujiura, “Wide-angle, low-voltage electro-optic beam deflectionbased on space-charge-controlled mode of electrical conduction in KTa1−xNbxO3,” Appl. Phys. Lett.89, 131115-1–131115-3 (2006).

9. H. Y. Zhang, X. H. He, Y. H. Shih, K. S. Harshavardhan, and L.A. Knauss, “Optical and nonlinear optical studyof KTa0.52Nb0.48O3 epitaxial film,” Optics Letters22, 1745–1747 (1997).

10. C. Herzog, S. Aravazhi, A. Guarino, A. Schneider, G. Poberaj, and P. Gunter, “Epitaxial K1−xNaxTa0.66Nb0.34O3thin films for optical waveguiding applications,” J. Opt. Soc. Am. B 24, 829–832 (2007).

11. R. Gutmann, “Liquid phase epitaxy of para- and ferroelectric KTa1−xNbxO3,” Ph.D. dissertation (ETH ZurichNo. 10095, 1993).

12. C. Herzog, G. Poberaj, and P. Gunter, “Electro-optic behavior of lithium niobate at cryogenic temperatures,”submitted to Opt. Commun. (2007).

#82385 - $15.00 USD Received 24 Apr 2007; revised 23 May 2007; accepted 23 May 2007; published 6 Jun 2007

(C) 2007 OSA 11 June 2007 / Vol. 15, No. 12 / OPTICS EXPRESS 7642

13. J. E. Geusic, S. K. Kurtz, L. G. Vanuitert, and S. H. Wemple,“Electro-Optic Properties of Some ABO3 Per-ovskites in Paraelectric Phase,” Appl. Phys. Lett.4, 141 (1964).

14. K. Buse, F. Havermeyer, L. Glabasnia, K. Schlomp, and E. Kratzig, “Quadratic polarization-optic coefficients ofcubic KTa1−xNbxO3 crystals,” Opt. Commun.131, 339–342 (1996).

15. http://www.c2v.nl/frindex.shtml?/products/software/olympios-software.shtml16. K. Fujiura and K. Nakamura, “KTN optical waveguide devices with an extremely large electro-optic effect,” in

Passive Components and Fiber-Based Devices, Pts 1 and 2, Y. Sun, S. S. Jian, S. B. Lee, and K. Okamoto, eds.,vol. 5623 ofProceedings of the Society of Photo-Optical Instrumentation Engineers (Spie), pp. 518–532 (2005).Conference on Passive Components and Fiber-based Devices NOV 09-11, 2004 Beijing, PEOPLES R CHINA.

17. T. Itoh, M. Sasaura, S. Toyoda, K. Manabe, K. Nakamura, andK. Fujiura, “High-frequency response of electro-optic single crystal KTaxNb1−xO3 in paraelectric phase,” Conference on Lasers and Electro-Optics 2005, paperJTuC36, Baltimore, USA (May 2005).

1. Introduction

In recent years, electro-optic crystals of the potassium tantalate niobate (KTN) family haveattracted increased attention for integrated optics due toits large linear and quadratic electro-optic coefficients[1] and the possibility of tuning the phase transition temperature by varyingthe Ta:Nb ratio[2][3][4][5]. The latest achievements in this field include the growth of nonlin-ear optical thin films of KTN on GaAs by pulsed laser deposition[6], electro-optic devices inburied KTN waveguides[7], and a wide-angle KTN beam deflector[8]. For guided optics ap-plications, epitaxially grown thin films promise to combinelow optical losses with bulk-likedielectric and electro-optic properties as long as two requirements are met: good lattice match-ing at the substrate-film interface and an optically flat thinfilm surface. While other groupshave reported KTN thin films on GaAs[6] or MgO[9] substrates,as well as KTN waveguidesembedded in KTN of different composition[7], we have recently demonstrated a technique thatwe think combines the advantages of a good substrate-film interface and an optically flat thinfilm surface[10]. The liquid-phase epitaxial growth of thinfilms of potassium sodium niobatetantalate (KNTN) on KT substrates with an as-grown optically flat surface, slab waveguidingtherein and the measurement of the optical and dielectric properties of these films have beendescribed in this publication. By substituting a small percentage of the potassium with sodiumand performing the growth at a slow growth rate, a very small relative lattice mismatch of lessthan 1×10−4 and an rms surface roughness of approximately 20 nm were achieved.To the best of our knowledge, we present for the first time ridge waveguiding and electro-opticmodulation in KNTN thin films on KT substrates at infrared wavelengths in this paper, exploit-ing the huge quadratic electro-optic coefficient in this material. In the first part of this work,the material composition and dielectric behavior around the phase transistion temperature areassured by Rutherford back scattering (RBS) and interdigital electrode impedance measure-ments, respectively. In a bulk KNTN crystal, the quadratic electro-optic coefficient was meas-ured in a cryostat interferometer setup and related to the dielectric behavior of the specimen.In µm-thick epitaxial films of KNTN grown on KT, ridge waveguideswere produced by pho-tolithography and argon-ion etching. The waveguides were characterized in terms of refractiveindices and propagation losses, using a mode profile reconstruction method and a scattered lightintensity technique, respectively. Using photolithography, electron-beam metal deposition andlift-off techniques, side electrodes were deposited parallel to the waveguides in order to createan electro-optic phase modulator. The performance of this modulator was tested in terms ofhalf-wave voltageVπ and modulation frequency.

2. Materials, growth and electrical properties

Potassium tantalate niobate (KTa1−xNbxO3, abbreviated KTN) is a solid solution of the twomaterials, potassium tantalate (KT) and potassium niobate(KN). Depending on the ratiox



between tantalum and niobium, the Curie temperatureT0 of the phase transition between theparaelectric cubic phase and the ferroelectric tetragonalphase can be gradually shifted from0 K for pure KT (x = 0) to 710 K for pure KN (x = 1.0)[11]. KTN was studied extensively andhas been epitaxially grown on a variety of substrates[9][6][7], but the quality of the substrate-film interface is largely determined by the lattice mismatchbetween the substrate and thin filmmaterials and even the use of several buffer layers producedrelative mismatch values in thelow percent range[6]. In a recent publication[10] we haven shown that by substituting a smallpercentagey of the potassium with sodium, an excellent lattice matching(relative mismatch< 1×10−4) between the KT substrate and the K1−yNayTa1−xNbxO3 thin film can be achievedthat results in a good interface and a thin film surface that does not require polishing to besuitable for integrated optical applications.

Since our last publication[10], the KNTN material composition has been modified fromK0.98Na0.02Ta0.66Nb0.34O3 (called KNTN1 for future reference) to K0.95Na0.05Ta0.71Nb0.29O3

(KNTN2) to shift the phase transition closer to room temperature which significantly increasesthe relative dielectric permittivity and the quadratic electro-optic coefficientR, since the relativedielectric permittivity theoretically diverges atT0 according to the Curie-Weiss law

ε =C

T −T0, (1)

whereC is the Curie constant andT > T0, and the quadratic electro-optic Kerr coefficientR isrelated toε according to

R = ε20(ε −1)2g, (2)

whereε0 ≈ 8.854×10−12F m−1, ε is the relative permittivity of the material and the quadraticpolarization-optic coefficientg is a material parameter that is mainly determined by the oxygenoctahedron in perovskites and therefore is almost constantas a function of temperature andvaries only by a factor of 2-3 in different perovskites.

–190 –150 –110 –70 –30

–120 –80 –40 0 40

1000

2000

3000

4000

Temperature [°C]

Rel

ativ

e pe

rmitt

ivity f = 100 kHz

f = 10 MHzf = 10 MHz

KNTN1

KNTN2

Temperature [°C]T0 = -10 °C

Fig. 1. Relative permittivity of potassium sodium niobate tantalate thin film epitaxiallygrown on potassium tantalate substrate as a function of temperature and frequency. Blackcurve shows improved material quality of present film compared to the one from our pre-vious study[10] (gray curve).

There are two main reasons why we didn’t use KNTN with a Curie temperature above roomtemperature which would allow us to exploit the larger linear electro-optic Pockels coefficientr: first of all, perovskites exhibit domains in the ferroelectric phase which require poling and



might give rise to scattering losses. Secondly, the waveguide phase modulator presented in thispaper relies on the simultaneous propagation of both TE and TM modes in the waveguide withsimilar properties, which is only possible in an optically isotropic material like cubic KNTN.For future applications, work has however been started to grow ferroelectric KNTN thin filmson KT substrates doped with barium, which significantly increases the electric conductivity ofthe KT and allows it to be used as a bottom electrode for polingand to apply the electric fieldto induce the electro-optic effect.

Using the growth process already described previously[10], paraelectric thin films of KNTNof a few micrometers were grown in liquid-phase epitaxy (LPE) on cubic KT substrates oftypically 1 cm× 1 cm× 0.2 cm dimensions. The material quality of the thin film could besignificantly enhanced through optimization of the growth parameters, as substantiated in Fig.1. The graph shows the relative dielectric permittivity of the thin films, measured with an HP4192 impedance analyzer connected to interdigital electrodes deposited on the thin film surface.It can be seen that the present films (in black) show a phase transition temperature ofTc ≈

−10◦C and exhibit a much higher dielectric permittivity than theprevious samples (in gray).The relative dielectric permittivity reachesε = 4000 at the phase transition temperature andε = 2100 at room temperature. As described later in this paper, the refractive index of thethin film material could also be noticeably increased. Theseimprovements resolve the majorlimitation we addressed in the last article: the distinct difference in material properties betweenbulk and thin film KNTN.

Fig. 2. Cryostatic interferometer setup to measure the quadratic electro-optic coefficientR11 of potassium sodium niobate tantalate bulk crystals. Low-frequency phase oscillationsare compensated by an active phase control.

The quadratic electro-optic coefficientR11 was measured in a bulk crystal of compositionKNTN1 in a cryostatic interferometer setup (depicted in Fig. 2 and described in detail in [12])in order to relate it to the values deduced from the modulatorperformance. Problems wereencountered from low-frequency phase variations (probably emanating from local structuralchanges in the material) at cryogenic temperatures that impeded a stable fixation of the work-



ing point, but this could be compensated by an active phase control (APC). The temperaturedependence ofR11 measured in the interferometer setup can be used to deduce the quadraticpolarization-optic coefficientg11 and the Curie temperatureT0 according to

R11 = ε20

C2

(T −T0)2 g11. (3)

Figure 3 displays the measuredR11 values, the least-squares error approximation of Eq. (3) withthe fit valuesT0 = −81.1 ◦C andg11 = 0.17 m4 C−2 and theR11 values calculated from di-electric permittivity measurements of our last publication, showing good agreement. The Curieconstant ofC = 5.8×104 K was also taken from the permittivity data of KNTN1. The widerdivergence of the curve derived from the dielectric measurements can probably be explainedby the following: a bigger part of the volume of the crystal contributes to the coefficient de-duced from the dielectric permittivity when compared to themore local probing of the laserbeam in the interferometer, leading to a somewhat diffused phase transition. A similar effecthas been observed in the past when comparing phase transition curves measured with plateversus interdigital electrodes[10]. The value ofg11 = 0.17 m4 C−2 lies between the values re-ported by Geusic[13] (g11 = 0.136 m4 C−2) and Buse[14] (g11 = 0.211 m4 C−2). Note that thebulk crystal was grown with the older composition and a lowerphase transition temperature,but the composition dependence ofg11 given by Buse[14] allows us to adapt the results to thenew composition and therefore the lengthy process of growing good quality bulk crystals ofKNTN2 composition was not performed.To summarize, we can say that quadratic electro-optic coefficients ofR11≈ 2.5×10−15m2 V−2

at the phase transition temperature and up toR11≈ 2×10−16m2 V−2 atT = T0+30 ◦C, whichcorresponds to room temperature for our present films, are the upper limit set by the bulk valuesand are by any means high enough to justify the production of an electro-optic device.

–100 –80 –60 –400

1.10–15

2.10–15

3.10–15

Temperature [°C]

R11

[m2

V-2

]

0 20 40

R calculatedR measuredCurie-Weiss fit

Fig. 3. Quadratic electro-optic Kerr coefficientR11 of potassium sodium niobate tantalatebulk crystal as a function of temperature. Black points represent values measured by theinterferometer setup, solid black curve indicates Curie-Weiss law and solidgray curve rep-resentsR11 calculated from dielectric permittivity data.

3. Waveguide production and characterization

With KNTN thin films of of composition KNTN2 and good optical quality and high Kerr coef-ficients at hand, the next step was the production of ridge waveguides and their characterizationat room temperature. To this end, waveguides of nominal widths of 3,5,7,9 and 11µm werestructured into the commercial photoresist AZ6632 and thentransferred to the KNTN thin filmby argon ion etching. Scanning electron microscopy (SEM), atomic force microscopy (AFM)



and optical microscopy pictures confirmed an etch depth ofh = 3.4±0.1 µm. After polishingof the crystal facets, infrared light from a Santec TSL-210 laser atλ = 1579 nm was coupledinto the waveguides using a microscope objective. The output of the waveguide was imagedwith a 100 x objective onto a Sensors Unlimited SU320MX-1.7RT infrared InGaAs CCD cam-era. Figure 4 shows an overlay of the waveguide cross sectionof a 7 µm wide ridge, uniformlyilluminated from the back, and the TE mode guided in the waveguide. The inset on the leftdepicts the simulation of the propagating mode and will be discussed later.

Fig. 4. Waveguide profile and TE mode intensity distribution ofλ = 1579 nm laser lightpropagating in a potassium sodium niobate tantalate thin film ridge waveguide. Left insetshows simulated mode profile with excellent agreement to measurement.

Starting from a vertical cut through the intensity distributions of the guided TE and TMmodes, we employed the profile reconstruction method described previously[10] to derive thethicknessd of the film and the refractive index contrast∆n between KNTN film and KT sub-strate. The method works by solving the Helmholtz equation for guided modes as a function ofthe film thicknessd and the refractive index distributionn(y), and minimizing error between themeasured intensity distribution and the theoretical function using a least-square error algorithm.Since the widthw = 7 µm of the waveguide is much larger than the wavelengthλ = 1579 nm,the intensity distribution iny direction can be approximated by a guided slab mode. The resultfor a TE mode is shown in Fig. 5 and shows an excellent agreement. The parameters weredetermined to bed = 3.4±0.1 µm and∆n = (11.0±1.5)×10−3.

Fig. 5. Measured light intensity (gray circles) along a vertical (y) cross section of potassiumsodium niobate tantalate thin film ridge waveguide on potassium tantalate substrate. Solidcurve represents the best-fitting theoretical solution of the Helmholtz equation with thinfilm thicknessd = 3.4±0.1 µm and refractive index contrast∆n = (11.0±1.5)×10−3.



Both this refractive index contrast of∆n = (11.0±1.5)×10−3 and the result of the dielectricpermittivity measurement confirm that the present thin film has dielectric and optical propertiesalmost on par with the bulk material. Figure 6 shows the dispersion curves of bulk KT andKNTN, and the thin film values of the last (at 633 nm) and the present (at 1579 nm) thin films,indicating that bulk values could almost be reached in present films.

Using the geometry and refractive index values as reported here, we performed a simulationof the propagating TE waveguide mode with the commercial OlympIOs software package[15].The resulting mode intensity distribution is shown on the left side of Fig. 4, demonstratingexcellent consistence between measurement and simulation.

Fig. 6. Refractive index dispersion of bulk potassium tantalate (light graysolid curve), bulkpotassium sodium niobate tantalate (dark gray solid curve), and epitaxial potassium sodiumniobate tantalate thin film (solitary points). The thin film refractive index atλ = 1579 nmis almost equal to the bulk value. All values measured at room temperature.

The propagation losses in the waveguide were estimated by imaging the scattered light alongpropagation of a guided TE mode atλ = 633 nm onto an Allied Dolphin F-145B 12 bit b/wCCD camera. Since the scattered intensity is proportional to the local power in the waveguideand the linearity of the optical system including the software was assured, the measured in-tensity as a function of propagation distance follows an exponential decay law with the losscoefficientα in the exponent. The result of this measurement is shown in Fig. 7 and yieldspropagation losses of 7.8±0.5 dB/cm for λ = 633 nm. Since scattering losses decrease withincreasing wavelength, the propagation loss value for infrared wavelengths is believed to besmaller. The relatively high propagation losses can be attributed to the roughness of the etchedsidewalls (rms roughness approximately 55 nm) and the high refractive index contrast at theseinterfaces. Possible measures to reduce the losses will be discussed in section 5. In terms ofboth propagation constants and losses, TE and TM modes are equal within the experimentalerror and no mode mixing was observed, which allows for the fabrication of a waveguide phasemodulator.

4. Electro-optic phase modulator

Using the polarization-maintaining ridge waveguides described above, an integrated electro-optical phase modulator was created by depositing parallelelectrodes on either side of a 7µm



Fig. 7. Measured scattered light intensity atλ = 633 nm (gray curve) along propagationin potassium sodium niobate tantalate thin film ridge waveguide on potassium tantalatesubstrate. Exponential decay fit yields propagation losses of 7.8±0.5 dB/cm (black curve).

wide waveguide. If light is coupled at an angle of 45◦ to the horizontal into the waveguide,both the TE and TM modes are excited inside the waveguide and propagate independently. Byapplying a voltage to the electrodes, the resulting electric field in the waveguide core shifts thephase of one polarization with respect to the other due to theelectro-optic effect. This phasedifference is converted into an amplitude change by an analyzer at the output. An additionalcompensator allows to set the working point of the modulator. A schematic of the setup is shownin Fig. 8. Microscope objectives were used to couple the light into and out of the waveguide.Electrodes consisting of 50 nm gold on top of 10 nm chromium (acting as adhesion promoter)were deposited on the thin film surface by standard photolithography processing, electron beammetal deposition, and a standard lift-off process. The distance between the electrodes iss =16 µm, with the waveguide in the middle.

Fig. 8. Schematic of the setup used to characterize the potassium sodium niobate tantalatethin film ridge waveguide phase modulator working atλ = 1550 nm.

The half-wave voltageVπ , defined as the voltage needed to switch the modulator from min-imum to full intensity or vice versa, is the key figure of meritfor a modulator and allows us to



assert the electro-optic coefficientR values discussed earlier.Vπ was measured by adjusting thecompensator to full darkness at the modulator output and then recording the transmitted lightintensity as a function of the DC voltage applied to the electrodes. The resulting graph is shownin Fig. 9 and yieldsVπ = 99± 2 V at room temperature. Possible methods for reducing thisvalue are discussed in section 5.

Fig. 9. Transmitted light intensity (y-axis) atλ = 1550 nm in potassium sodium niobatetantalate thin film ridge waveguide phase modulator as a function of DC voltageapplied toparallel side electrodes (x-axis). Graph shows a half wave voltage ofVπ ≈ 100 V.

To calculate the electro-optic coefficientR, the strength of the electric field atVπ = 100 Vinside the waveguide core is needed. A Comsol Multiphysics model of the modulator cross-section was created (see Fig. 11) with the geometry and dielectric permittivity values obtainedpreviously. The simulation shows that (1) the electric fieldis rather uniform in the waveguidecore area with a value ofEx ≈ 0.7× 106 V/m and (2) the horizontal componentEx of theelectric field is about 40 times stronger than the verticalEy. At Vπ , electric field strength andKerr coefficients are related by

R11E2x −R12E2

y =λ

n3 l, (4)

whereλ = 1550 nm is the wavelength,l = 3.8 mm is the length of the electrodes andn = 2.18 is the effective mode index. In our caseEy ≪ Ex and thus, Eq. 4 is reduced to

R11 =λ

n3 E2x l

. (5)

UsingEx ≈ 0.7×106 V/m in the equation above yieldsR11 ≈ 8.2×10−17m2/V2. This isabout 40% of the expected bulk value and very high compared toother materials. Utilizing thefollowing expression,

ε =

(

R11

ε20 g11

)1/2

, (6)

and theg11 value obtained in section 2, which has been modified from the measured valueof g11 = 0.17 m4 C−2 for the previous composition tog11 = 0.257 m4 C−2 for the present one



according to the composition dependence given by Buse et al.[14], a relative dielectric permit-tivity of ε ≈ 2000 can be calculated and agrees well with theε = 2100 measured directly. Interms of modulation speed, we were limited by the availability of amplifiers capable of gener-ating the modulation voltage in the 100 V range. We demonstrate electro-optic modulation atf = 10 MHz as shown in Fig. 10, but since the material family of potassium tantalate niobatehas proven its applicability in the GHz range[16, 17] in similar configurations, we have no rea-son to expect any fundamental limitation up to several GHz once the modulation voltage hasbeen reduced by the methods described in the next section.

Fig. 10. Electro-optic modulation atf = 10 MHz in potassium sodium niobate tantalatethin film ridge waveguide phase modulator. Upper curve shows voltage applied to the sideelectrodes in units of half-wave voltage, lower curve shows detected lightintensity.

5. Discussion

The fundamental suitability of epitaxial KNTN thin films on KT substrates for electro-opticalintegrated waveguide devices has been demonstrated and shows promising properties like highlateral index contrasts, allowing small devices, while maintaining excellent material interfacesand good optical surfaces. The relative dielectric permittivity and Kerr coefficients rank amongthe highest of all materials reported so far and all the limitations described in our previouspublication[10] have successfully been addressed. However, to pave the way for real deviceuse, two main restrictions have to be overcome: the half-wave voltageVπ has to be diminishedand the propagation losses have to be reduced. We will first discuss one approach to addressboth issues simultaneously and then discuss about additional measures individually. As statedin sections 3 and 4, the propagation losses mainly stem from the etched side walls of the waveg-uide, while the top surface is of good optical quality, and the half-wave voltage is on the orderof 100 V due to the huge difference in the relative dielectricpermittivity of the KNTN thin film(ε ≈ 2100) and the KT substrate (ε ≈ 240). Accordingly, we suggest a simple modification tothe waveguide production process that will greatly improveboth the loss andVπ values: insteadof etching ridges the full height of the thin film, the etch depth will be reduced toh = 1.4 µm,and 2µm of the KNTN thin film will remain as a pedestal beneath the ridge. Simulations per-formed in OlympIOs ensure the continued monomodality of theridge waveguide. The effectswill be twofold: first of all, the propagating mode has much less interaction with the etchedwaveguide side walls, which reduces propagation losses andsecondly, the electrodes now alsoreside on the thin film material which increases the electricfield strength inside the waveguidecore area. To illustrate this effect, both geometries were modelled in Comsol Multiphysics. Theresult is shown in Fig. 11 and indicates that in case (b) with shallower ridge waveguides, theelectric field componentEx is 7 times stronger than in the present case (a) for the same voltage



applied to the electrodes. Conversely, to achieve the same phase shift, the half wave voltageVπ drops from 100 V to about 14 V. When also taking into account theelectrode lengthl, thefigure of meritVπ l of this suggested geometry is among the lowest values reported so far[16].

Fig. 11. Existing (a) and suggested (b) waveguide geometries to significantly reduce half-wave voltage in potassium sodium niobate tantalate thin film ridge waveguide phase modu-lator. Placing the electrodes on the thin film instead of on the substrate increases the electricfield in the waveguide core by a factor of seven.

Other possible methods to reduceVπ : (1) increasing the length of the waveguide and the elec-trodes, which may potentially increase device size, (2) biasing the modulator with a DC voltage,sinceVπ decreases with a DC bias due to the quadratic electo-optic effect as demonstrated inFig. 9, or (3) changing the material composition to shift thephase transition temperature closerto the operation temperature. In regards to reducing the losses, the etching process itself canbe further optimized, or the waveguide could be covered witha thin layer, SiO2 or Si3N4 forexample, to reduce the surface roughness, though it is difficult to estimate the amount of lossreduction that would be achievable by these means.

6. Conclusion

Electro-optic modulation in paraelectric epitaxially grown KNTN thin films on KT substratesat λ = 1550 nm has been reported here for the first time to the best of our knowledge, exploit-ing a thin film Kerr coefficient of(8.2±1.1)×10−17 m2/V2. While rather simple measureswill allow for a decrease in both propagation losses and the half-wave voltage of the demon-strated ridge waveguide phase modulator, this achievementalready constitutes an importantstep towards more sophisticated integrated optics devices, such as Mach-Zehnder modulatorsor cross-switches, that utilize the outstanding dielectric and electro-optic properties of the KTNmaterial family.

Acknowledgments

We thank Jaroslav Hajfler for the expert polishing of the waveguide facets, Max Dobeli forthe RBS measurements and Shanmugam Aravazhi for the bulk crystals. This work was in partsupported by the TH project ”Electro-optical Microring Resonators for Photonic Applications”,TH13/05-2.



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