Optical channel waveguides with trapezoidal-shaped cross sections

in KTiOPO4 crystal fabricated by ion implantation

Feng Chen a,*, Yang Tan a, Lei Wang a, Dong-Chao Hou a, Qing-Ming Lu b

a School of Physics and Microelectronics, Shandong University, Ji’nan 250100, Shandong, Chinab School of Chemistry and Chemical Engineering, Shandong University, Ji’nan 250100, Shandong, China

Received 30 March 2007; accepted 21 July 2007

Available online 27 July 2007

Abstract

Optical channel waveguides were fabricated in KTiOPO4 crystal by He+-ion implantation using photoresist masks with wedged-shaped cross

sections. Semi-closed barrier walls with reduced refractive indices inside the crystal constructed the enclosed regions to be channel waveguides

with trapezoidal-shaped cross sections. The m-line as well as end-fire coupling arrangements were performed to characterize the waveguides with

light at wavelength of 632.8 nm. The propagation loss of the channel waveguides was determined to be as low as �2 dB/cm after simple post-

irradiation thermal annealing treatment in air.

# 2007 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/apsusc

Applied Surface Science 254 (2008) 1822–1824

PACS : 42.82.Et; 85.40.Ry; 42.70.Mp

Keywords: Channel waveguides; Ion implantation; KTiOPO4 crystal

1. Introduction

Waveguide structures confine light within sizes of a few

microns, where high optical intensities could be acquired over a

relatively long distance even in low powers. As a result,

capabilities of frequency doubling of many non-linear materials

could be improved considerately via geometries of waveguides

[1–4]. Potassium titanyl phosphate (KTiOPO4) is well-known for

its large non-linear optical coefficients and high optical damage

threshold, from which high-quality commercial lasers in visible

spectral regimes have been realized via frequency doubling of

infrared lasers [5]. Waveguides in KTiOPO4 were produced by

proton exchange [6], pulsed laser deposition (PLD) [7], and ion

implantation [3,4,8–12]. As one of the most successful

techniques for material-property modification, ion implantation

has shown its unique capability for fabricating waveguide

structures in many optical materials (for a recent review, see Ref.

[4]). By choosing ions with suitable energies, species, or doses,

the waveguide parameters could be conveniently determined,

which allows the fabrication of reliable photonic devices.

Particularly, irradiation of some light ions, e.g. He or H, often

* Corresponding author. Tel.: +86 531 88364655; fax: +86 531 88565167.

E-mail address: drfchen@sdu.edu.cn (F. Chen).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.07.160

creates an optical barrier with reduced refractive index at the end

of the ion track inside the substrate material mostly via nuclear

energy deposition, constructing a waveguide layer together with

the cladding air [3]. Planar waveguides in KTiOPO4 has been

produced by implantations of a few light ions at suitable doses,

e.g. H, He, Li, or B [3,4,8–11]. By using stripe masks with

rectangular-shaped cross sections, buried channel waveguides

were formed by successive multi-energy implants into a

KTiOPO4 planar waveguide substrate, which often requires a

two-step implantation processing to create an optical barrier

layer and sidewalls, respectively [12]. Special designed masks

with suitable shapes may simplify the fabrication process of

channel waveguides, which had been tried for potassium niobate

(KNbO3) [13] and potassium sodium strontium barium niobate

(KNSBN) [14]. In this letter, we reported on, to our knowledge

the first time, the formation of channel waveguides with

trapezoidal-shaped cross sections by performing one-step

multiple-energy He+-ion bombardment with masking of a series

of photoresist stripes with wedged-shapes.

2. Experiments and results

The KTiOPO4 samples we used were with sizes of

7 � 5 � 1.5 mm3 along x-, y-, and z-axes, respectively. A

F. Chen et al. / Applied Surface Science 254 (2008) 1822–1824 1823

thick-film positive photoresist was spin-coated onto one

optically polished x–y facet (7 � 5 mm2) of the samples at

5000 rpm for 50 s, forming a photoresist-mask with thickness

of �5 mm. After coating, the sample was pre-baked at 110 8Cfor 4 min. A mask consisting of open stripes with widths of

�5 mm and a spacing of 45 mm between the adjacent

channels was used for subsequent UV exposure. After this the

photoresist was developed and the structured resist film was

post-baked at 125 8C for 30 min. With this processing, a

series of strongly wedged smooth stripes (pointing at y-axis of

the crystal) with a period of 50 mm were deposited on the

surface of the sample wafer. In addition, for comparison some

areas of the sample were not covered by the photoresist for

formation of planar waveguides. The implantation was

performed directly using the photoresist stripe masks at a

1.7 MV tandem accelerator at Peking University with He+

ions with energies of (1.9 + 2.0 + 2.1) MeV and at fluences of

(2.7 + 2.7 + 4.5) � 1015 cm�2, respectively [Fig. 1(a)]. After

removing the photoresist mask, we polished the end faces of

the sample to reach optical quality for end-face coupling

of light. The cross sections of waveguides were imaged

by a microscope with a reflected polarized light (Olympus

Fig. 1. Schematic of the waveguide fabrication processes in KTiOPO4 (a),

microscopic photographs of the cross sections of planar (b), and channel

waveguides with trapezoidal shapes (c). PM, photoresist mask; BW, barrier

walls; PW, planar waveguide; CW, channel waveguide. The sample crystal axes

were marked.

BX51M, Japan). To reduce the scattering loss and additional

absorption induced by the ion irradiation in the waveguides,

the sample was annealed at 200 8C for 30 min in an open oven

with air ambience.

Fig. 1(b) and (c) shows the microscopic photographs (�500)

of the cross sections of planar and channel waveguide in

KTiOPO4 samples, respectively. As one can see, in the planar

case, together with air, the waveguide was constructed by an

optical barrier layer with reduced refractive index, which was

located at depth of �5.2 mm inside the substrate. With the

blocking effect of the photoresist stripes at the wedged edges,

the incident ions experienced different penetration depth inside

the crystal, creating semi-closed barrier walls with trapezoidal-

shaped sections. These special barrier walls constructed a low-

index well, which confines the enclosed region to be channel

waveguides [see Fig. 1(c)].

For investigation of the guided properties of the sample, we

performed an m-line arrangement to measure the dark-mode

spectroscopy of the planar waveguide in the photoresist-

uncovered region at the wavelength of 632.8 nm with a prism

coupler (Metricon 2010). There were four transverse electric

(TE) and six transverse magnetic (TM) dark-line modes

observed, respectively. Based on these results, the refractive

index profiles of the waveguide were reconstructed by the

reflectivity calculation method (RCM) [15]. Moreover, by

forming a damage or amorphous layer (barriers) mostly at the

end of ion track, the ion-induced nuclear damage was often

considered to be the main reason for the waveguide formation.

Fig. 2 shows the comparison of refractive index variations (Dnx

and Dnz) (reconstructed by RCM) of the KTiOPO4 planar

waveguides (for both as-implanted and annealed) and nuclear

damage distribution (based on Stopping and ranges of ions in

matter or SRIM simulation, code 2006) [16] of the crystal

induced by the He-ion implantation. As it is indicated, the He

irradiation induced a damage layer at the depth of �5.2 mm

inside the crystal, creating a maximum atomic displacement of

Fig. 2. Refractive index variations (Dnx and Dnz; before and after annealing at

200 8C for 30 min in air) and damage distribution (solid line) vs. the waveguide

thickness of KTiOPO4 sample in the photoresist-free surface region induced by

the He-ion implantation.

Fig. 3. Measured near-field intensity distribution of (a) the quasi-TE00 mode of the as-implanted channel waveguides and (b) the quasi-TM00 mode of the sample after

thermal annealing at 200 8C for 30 min from the output facet obtained by CCD camera.

F. Chen et al. / Applied Surface Science 254 (2008) 1822–18241824

�11%; as a result, the decreases of refractive indices

(Dnx � �0.03; Dnz � �0.05) happened at almost the same

location, which confined waveguide structures for both TE and

TM modes. The optical barrier was broadened by using these

triple-energy implants, which may reduce the leaky effect of the

modal field in the waveguide. We also found that the refractive

index profiles of the sample had very slight changes after

annealing treatment at 200 8C for 30 min, which exhibited

somehow good thermal stability.

We implemented the end-fire coupling arrangement to

investigate the channel waveguide properties. A 40�microscope objective lens was used to couple the incident

polarized light into the waveguides, and another 40� lens

collected it from the rear facet of the crystal. Finally, the

crystal’s output facet was imaged onto a CCD camera. For

the as-implanted sample, the scattered light from the output

face of the sample was very strong, which was usually caused

by the ion implantation induced defects or color centers (via

electronic damage). Nevertheless, we obtained the near-field

intensity distribution of the quasi-TE00 mode from the output

facet of the as-implanted channel waveguides [see Fig. 3(a)],

whilst no light in TM modes was coupled out from the

waveguide. This means the light in TM modes experienced

higher attenuation than TE-polarized light. For the

sample after annealing at 200 8C for 30 min in air, the

waveguide quality was considerably improved, and conse-

quently, quasi-TM modes of the channels were observed.

Fig. 3(b) shows the image of the near-field intensity profile of

the quasi-TM00 modes for the annealed channel waveguide.

In addition, the light scattering was significantly reduced

after the annealing, which is in agreement with some

previous work [10]. We estimated the propagation losses of

the channel waveguides by directly measuring the power of

the coupled light into and out of the end faces of the sample.

Considering the imperfect aperture matching, crystal edge

polishing (�70% launching efficiency) and Fresnel reflec-

tions at the air–crystal interfaces (�15% correction), the

values of annealed channel waveguide attenuation were

determined to be �2 and �5 dB/cm for the TE- and TM-

polarized light at wavelength of 632.8 nm, respectively.

Lower losses were expected for further optimization of the

waveguide fabrication process and the post-implantation

annealing treatment.

3. Summary

We reported on the fabrication of channel waveguides with

trapezoidal-shaped cross sections in KTiOPO4 crystals by

triple-energy He+-ion implantation assisted with special

photoresist masking. The waveguides remain relatively stable

and exhibited acceptable low propagation losses after simple

post-irradiation thermal treatment in air. In addition, it should

be pointed out that the barrier wall-confined channel waveguide

allowed propagation of both TE- and TM-polarized light

simultaneously, consequently offering the feasibility of SHG in

the KTiOPO4 waveguides.

Acknowledgements

This work was supported by the National Natural Science

Foundation of China (grant No. 10505013), Natural Science

Foundation of Shandong Province (grant No. Y2005A01), SRF

for ROCS, SEM.

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