Second harmonic generation in a sputtered LiNbO3 film on MgO

Journal ofCrystal Growth 45 (1978) 355—360 © North-Holland Publishing Company

SECOND HARMONIC GENERATION IN A SPUTTERED LiNbO3 FILM ON MgO K. NUNOMURA, A. ISHITANI, T. MATSUBARA and I. HAYASHI Central Research Laboratories, Nippon Electric Co., Ltd., Kawasaki City, Kanagawa, 213, Japan Phase-matched optical second harmonic generation with a Nd:YAG laser and a GaAs laser was observed in an rf sputtering LiNbO3 4 andfilm the conversion guide deposited efficiency on a with (l11)MgO a Nd:YAG substrate. laser (TM The efficiency of the SIIG with a GaAs laser (TM0 — TM2) was about i0 0 — TM1)was about 10-6.

1. Introduction

2. LiNbO3 epitaxial growth

An optical waveguide consisting of an optical nonlinear material is potentially effective for Second Harmonic Generation (SHG), because of the high concentration of fundamental power in a narrow region. Another advantage is that phase matching can be achieved by utilizing the inherent waveguide mode dispersion [1]. Several experimental observations of SHG in waveguides have been reported previously [2—I1]. The optical waveguide used in the past SHG experiments mostly consisted of a linear guide on a noniinear substrate. A nonlinear guide on a substrate structure is expected to be more efficient. However, fabrication of a nonlinear waveguide with a good crystaffine perfection is very difficult. On the other hand, LiNbO3 optical waveguides have been prepared by various methods, such as CVD [12], LPE [13,14], epitaxial growth by melting [15], ion-diffusion [16,171, and rf sputtering [181. Active devices, such as electrooptical and acoustooptical modulators have been fabricated with an ion-diffused LiNbO3 surface layer waveguide. LiNbO3 is an excellent nonlinear material. Especially, the nonlinear susceptibility component d33 of LiNbO3, through which the harmonic wave can be excited in the waveguide structure, is about 130 times larger than the nonlinear coefficient of quartz. This paper describes heteroepitaxial growth of LiNbO3 on MgO substrates and the results of SHG experiments in this waveguide.

Substrate material selection is very severe, because of lattice parameter matching and restrictions for the refractive index. MgO was found to be a promising substrate material. That is because the MgO(l 11) plane and the LiNbO3 z-plane have the same oxygen ion framework, although the crystal structures differ from each other, i.e. cubic and hexagonal, respectively. The oxygen ion positions on the MgO(l 11)

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I I Fig. 1. Oxygen ion framework on the z-plane LiNbO3 (111) plane MgO (0). 355

(.)

and

356

K. Nunomura et al.

/ Second harmonic generation in sputtered LiNbO3

plane and LiNbO3 z-plane are shown in fig. 1. LiNbO3 oxygen ions appear in a periodically deviated position 0.23 A with respect to MgO. However, the lattice mismatch between the geometrical center of

the=than less LiNbO3 0.2%; layer and MgO=constant oxygen ion position is a0 4.2112 ah)/~.~/~ao] A MgOthe Xlattice 100 0.2 ah

=

—

,

5.1483 A

,

LiNbO3 lattice constant

The MgO refractive index is 1.735, and that of LiNbO3 is 2.289 (ordinary) and 2.201 (extraordinary) at 6328 A wavelength. Rf sputtering is a more appropriate fabrication technique for obtaining thin films with accurate thickness control. LiNbO3 ceramic disks of 15 cm in diameter and 4 mm thick were prepared as targets. MgO(1 11) plane wafers were chemically polished with hot phosphoric acid. A typical growth condition is given in table 1. Electron diffraction patterns of the deposited film, are shown in fig. 2. Figs. 2a and 2b are reflective electron diffraction (RED) patterns of the film with the electron beam along the [2111 direction and the [1101 direction of the MgO substrate, respectively, Fig. 2c shows the RED pattern for film with a smooth surface. A transmission electron diffraction pattern of the film indicates six-fold symmetry as shown in fig. 2d. These electron diffraction patterns show that the deposited film is the z-plane of LiNbO3 and has a micro twin structure. It also shows that the in-plane epitaxial relations are [v-axis] LiNbO3 1/ [1lOIMgO and [x-axis] LiNbO3 /1 [21 l]MgO. The halfwidth of the X-ray rocking curve taken for the (006) reflection was 0.5—1.5°. The lattice constant of the film was about 0.5% larger than that of

U. Fig. 2. Electron diffraction patterns of film.

the

sputtered LiNbO3

bulk LiNbO3, suggesting that the sputtered films contamed Ar. The surface roughness, measured by TARY STEP at various film thicknesses, is shown in fig. 3. On increasing the film thickness, there was a tendency for the surface to become rougher. Film surface smoothness is very important for optical waveguide use. The measured optical propagation loss for the zero order mode was less than 10 dB/cm at 6328 A in a LiNbO3 film with less than 0.6 jim thickness. In order to utilize the ferroelectric LiNbO3 nonlinear optical effect, the sputtered film must be poled. This was confirmed by a mechanical compression piezoelectricvoltage test that the as-sputtered film was in the poled state, and the growth surface was the positive electrode side.

Table 1 A typical sputtering condition Target composition Target—substrate distance Gas content Gas pressure rf power rate Deposition Substrate temperature

film on MgO

Li2 0/Nb2 0~ = 55/45—58/42 5 cm Ar (60%) + 02 (40%) 2.6 X 1022 Torr 0.1 1.7 pm/h W/cm 620—660 °C

K. Nunomura et al.

Film Thickness (p.m) 0.2

Surface

/ Second harmonic generation in sputtered LiNbO3 film on MgO

Roughness

bOA

—.---------_~.~~,~.

Table 2

Nonlinear

polarization in sputtered LiNbO3 waveguide; prop-

agation direction is along the z-axis; the x-axis is perpendicular to the film surface.

T 0.4

0.6

~

TE mode

TM mode

(SH)

(SH)

____________—

~

___________

TEinode (fundamental)

0

P~=daiE~,

TMmode

0

P~=d3iE~+da3E~

(fundamental)

1~ ~ I 0

~

III ~

—i

I op.m~__

Fig. 3. Surface roughness for various thickness films, measured by TARY STEP.

3. SHG in the sputtered LiNbO3 film waveguide The nonlinear susceptibility tensor for the sputtered LiNbO3 films with a micro twin structure was investigated. The relation between the phase matched film thickness and efficient mode coupling was investigated. Because the twin size is sufficiently small compared to the wavelength, the effective susceptibility tensor of the deposited filmnonlinear can be obtained by adding the tensors of both twins. The nonlinear susceptibility tensor for bulk crystalline

~

2d1 sE~E~

=

in the twin structure. The nonlinear polarization, interesting component, d33 (107 d3’~”)[19], remains induced by the electric fields, of the fundamental TE and TM mode, is shown in table 2. The conversion from the fundamental TM mode to the second harmonic TM mode, in which second harmonics are induced mainly through d 33, is most efficient. It must be noted that the SHG in this waveguide is independent from the fundamental wave propagation direction. The calculated dispersion curves for TM modes at the 1 .06 jim fundamental and 0.53 pm harmonic frequency in the LiNbO3 z-plane film on a MgO sub_________________________________________ 53F 2 3

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2.2

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— ———

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deposited

15

Lii

TM 0

0

357

0

0

d15

0

0

TM1

0.5

.

FILM THICKNESS

d31

d31

d33

0

0

0

Fig. 4. Dispersion curves

for

TM2 1.0

(,aml

TM modes in the waveguide,

consisting of LiNbO3 film and MgO substrates, calculated on

Though the d22 component disappears, the most

bulk LiNbO3 optical constants.

K. Nunomura eta!. / Second harmonic generation in sputtered LiNbO

358

3 film on MgO

Table 3 The calculated conversion efficiency and the propagation length for the maximum conversion in the waveguide with small deviations from the complete phase matched thickness (tm) Deviation from tm

TM0—TM5 tm = 3700 A

TM0—TM2 tm = 9400 A

OA 105A A

6,0.4mm 7Xl0~5mm 10 0.2 mm 52Xx i0~,

6X 103,l.3mm 3X10~5mm 2 x 10~,0.7 mm a. 0

strate are shown in fig. 4 using the optical constants of bulk LiNbO 3. The phase matched film thickness of TM0(fundamental)—TM1(second harmonic) interaction and TM0—TM2 interaction are 0.37 and 0.94 pm respectively. The theoretical conversion efficiency was estimated for bulk crystal data, according to the SHG theory reported in ref. [20]. It was assumed for the calculation that the fundamental powder was l0~W with 0.1 mm beam diameter and that the propagation loss for fundamental TM0, second harmonic TM1 and TM2 was 5,15 and 30dB/cm, respectively. The maximum conversion efficiency and propagation length for the maximum conversion in the waveguide with small deviations from the cornplete phase matched thickness are given in table 3. The TMO—TM2 conversion with a large spacial coupling is remarkably efficient.

4. Experiments on SHG with a Nd:YAG and a GaAs laser Experiments on SHG in the sputtered LiNhO3 film were accomplished using a Nd:YAG laser or a GaAs laser as a fundamental source. Sputtered LiNbO3 films with tapered thickness were used in waveguide

order

to

achieve

complete

phase

matching.

The

tapered film was fabricated by moving a mask above the substrate during sputtering. The sputtered film used in the present SHG experiment was 0.33 pm thick at one edge and 0.41 pm thick at the opposite edge, resulting in tapering of 40 A/mm. The propagation loss was 6 dB/cm for the TE0 mode, 7.5 dB/cm for the TM0 mode, and 10.5 dB/cm for the TM1 mode at 6328 A. The ordinary and extraordinary refractive indices of the LiNbO3 film, which were cal-

I

0.52

I

0.53 Wavelength

0.54

I

0.55

(/Lm)

Fig. 5. Spectrum of the second harmonic wave with Nd:YAG aser.

the measured values of prism coupling angles for a He—Ne laser, were 2.28 ±0.01, and 2.21 ±0.01, respectively. A 0.1 mm beam diameter Nd:YAG laser was fed into the film through a Ti02 prism coupler in the TM0 mode. A bright green beam trace was observed on the film surface when the phase matched thickness was selected by sliding the tapered film transversely. The generated light spectrum is shown in fig. 5. The variation in second harmonic power when the film is displaced is shown in fig. 6. The second harmonic power was strongly dependent on the film thickness. The phase matched thickness was 0.37 pm, and the second harmonic wave was in the TM1 mode. The propagation length for maximum conversion was approximately 1 mm. The measured power values of the fundamental and second harmonic wave emitted from the waveguide edge were 6.8 X 1O~and 7.8 X l0~W, respectively. The measured values of the conversion efficiency and phase matched thickness were well in agreement with the theoretical values. Second harmonic generation with a GaAs laser in a LiNbO3 waveguide has not been reported yet. The difficulty is due to the low absolute power of a GaAs laser. In order to realize SHG, a nonlinear culated from

K. Nunomura eta!.

Transverse

Displacement

—0.5

-P.O

/ Second harmonic generation in

+0.5

+1.0

I

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3 film on MgO

359

in spite of much higher power than that from the

1mm)

0

sputtered LiNbO

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YAG laser. The phase matched thickness was 0.6 pm, the second harmonic was in the TM2 mode. The inter-

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-

>‘

0 UI C 4,

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action length was less than 0.5 mm. The maximum pulsed peak power of the second harmonic radiation observed at the waveguide edge was about 10_6 W, while the propagation power of the fundamental wave was 1 0_2 W. This is probably that first observation of SHG with GaAs laser in a LiNbO3 optical waveguide.

-

5. Conclusion

-

A heteroepitaxial film of LiNbO3 was deposited on a (111)MgO substrate by the rf sputtering method.

0 0

2 •

0

o

0

0

0

& 0

QjjXIdXP —30

-20

I

—10

I

0

I

+10

I

+20

+30

(3700A) Variation of Film Thickness

(A)

Fig. 6. SHG relative power with Nd:YAG laser versus transverse displacement, namely, film thickness variation.

waveguide with high efficiency is necessary. The waveguide, consisting of a LiNbO3 film on a MgO substrate prepared in this experiment, is designed for the GaAs laser SHG, that is, the large refractive index difference makes it possible to utilize the large nonlinear susceptibility d33 with phase matching for GaAs laser wavelength. The wavelength for the GaAs laser used as fundamental light source was 0.86 pm. The maximum pulsed power was about 10—i W (duty cycle 0.25%). The laser had a stripe geometry Al~Ga1..~As/GaAsdouble hetero structure, which was designed to deliver higher power than conventional stripe geometry lasers. The GaAs laser beam, focused to 0.1 mm diameter through 40X and IX microscopic objective, was fed into the film waveguide through a Ti02 prism in the TM0 mode. The film waveguide was about 0.6 jim in thickness with 140 A/mm tapered ratio. A faint bule light was observed by the naked eye in a dark room when the proper phase matched thickness was selected. It was faint because of the poor eye sensitivity at the SHG wavelength and the low duty cycle,

The as-sputted film was z-plane LiNbO3 with a micro twin structure and was in the poled state. Visible seeond harmonic waves appeared in a tapered thickness waveguide when the proper phase matched thickness was selected. Conversion efficiencies from a YAG laser and a GaAs laser were about 106 for TM0—TM1 coupling and about iO~ for TMO—TM2 coupling, respectively. This high efficiency was due to utilizing the large nonlinear component d33 of LiNbO3. The demonstration of the highly efficient SHG in the sputtered LiNbO3 waveguide indicates the possibility of realizing a small blue coherent light source.

Acknowledgement The authors wish to thank Mr. Sakai for polishing the MgO substrates, Dr. Shiroki for loaning the Nd:YAG laser, and Dr. Yonezu for supplying GaAs lasers.

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Suematsu, Y. Saaki and K. Shibata, App). Phys. Letters 23 (1973) 137. Tang and John M. Telle, AppI. Phys. [4] Bor-Uei Chen, C.L. Letters 25 (1974) 495. [51W.K. Burns and A.B. Lee, Appi. Phys. Letters 24 (1974)

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/ Second harmonic generation in sputtered LiNbO3 film on MgO

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