Development of the Biopolymeric Optical Planar Waveguide with Nanopattern

doi:10.4236/jsemat.2011.12009

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Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 56-61

doi:10.4236/jsemat.2011.12009 Published Online July 2011 (http://www.SciRP.org/journal/jsemat)

Development of the Biopolymeric Optical Planar

Waveguide with Nanopattern

Seung H. Yoon1, Won T. Jeong1, Kyung C. Kim1, Kyung J. Kim2, Min C. Oh2, Sang M. Lee3

1Mechan ical Engineerin g, Pusan National University, Pusan, Korea; 2Electrical Engin eering, Pusan National University, Pusan, Ko-

rea; 3Pusan National University, Pusan, Korea.

Email: [email protected]

Received May 18th, 2011; revised June 15th, 2011; accepted Jun e 25th, 2011.

ABSTRACT

This paper demonstrates for fabricating the biopolymric optical planar waveguide. Gelatin and chitosan were mixed

with ratio of 9 to 1 and sti rred at 70˚C with 1300 rpm. The blended biopolymer was spincoated on silicon substrate with

500 rpm and then dried in the oven at 50˚C. The refractive indices of the prepared biopolymer clad and core layers of

the waveguide were measured by the ellipsometry. The measured refractive indices of the two layers were obtained to

be 1.516 and 1.52, respectively. The nanograting was successfully imprinted on surface of the biopolymeric waveguide.

Keywords: Biopolymeric Optical Waveguide, Nanograting

1. Introduction

This work aims to develop highly sensitive all-biopoly-

meric planar Bragg grating biosensor. Biopolymer [1-4]

is biocompatible material which is developed by blend-

ing the chitosan and gelatin [5]. The optical planar wa-

veguide is composed of a clad and core layers. The re-

fract ive i nd ex o f t he c o re layer is sl ig htl y hi g he r t ha n tha t

of the clad layer so that light can be guided in the wave-

guide. The Bragg grating is printed on the developed

optical planar waveguide by using the nanoimprint tech-

nique.

2. Theory

This section provides the basic mathematical foundation

that will be used to design and perform data analysis of

the proposed planar waveguide grating sensor. This sec-

tion starts by providing the expressions needed to calcu-

late the propagation constants for the slab waveguide in

terms of the waveguide geometric and optical properties.

Then the coupled mode equations used to determine

the grating properties are presented. The waveguide

geometry of interest in this paper is illustrated in Figure

1, and consists of a biopolymer core bounded by air

above and a biopolymer clad below. This waveguide is

fabricated on a silicon substrate. The refractive indices of

air, clad and core layers are denoted by n1, n2, and n3

respec-

Figure 1. Schematic of the slab waveguide with surface

corr ugati on gr a ting.

tively and the core thickness is given by tg. Also seen in

Figure 1 are the corrugations of pitch and depth a. The

most important parameter in the design of Bragg grating

in slab waveguide is the effective index. This effective

index is found in the usual way [6] by solving the wave

equation and applying the continuity boundary condi-

tions at the respective core/cladding interfaces of the

wave guide s hown i n Figure 1. T he guid ed mod es have a

propagation constant βs such that k0n3 < βs < k0n2, where

n1 < n3. This solution process lead s to the following tran-

scende nt al equation that yields the propagation constant:

tanh

tpq

=

−





(1)

*Korean Government (NRF-2009).

Development of the Biopolymeric Optical Planar Waveguide with Nanopatter n

where

( )

22 2

0ys

h nk

= −

( )

2 22

10s

q nk

= −

( )

2 22

10s

p nk

= −

and k0 ≈ ω/c = 2π/λ. Given a set of

refractive indices n1, n2, and n3 and the waveguide thick-

ness, tg, of the planar waveguide, and the source wave-

length,

, Equation (1) in general yields a number of so-

lutions for the propagation constant, βs. However, the

source wavelength and the waveguide thickness are re-

stricted in the present study such that only one propaga-

tion mode is supported, and therefore Equation (1) has

only one solution of interest. As a result, the effective

index of the planar waveguide is given by neff = βsλ/2π.

The corrugated structure into the waveguide leads to a

corresponding periodic perturbation of the refractive in-

dex distribution. Each groove of the grating acts like a

weak mirror, and the cumulative effect of all of the weak

reflectors results in a very strong combined reflection

centered on what i s kno wn as the B ragg wave le ngth. The

Bragg wavelength is related to the effective index calcu-

lated above and the grating period, Λ, by [6]

b eff

nΛ

(2)

which when expressed in terms of the propagation con-

stant is given by

β λΛ

(3)

where λb is the Bragg wavelength and λ is the central

wavel ength of t he optical sour ce.

3. Experiment

3.1. The Production of Biopolymer through

Blending of Chitosan and Gelatin

The biopolyme r was de veloped in this stud y by ble nding

chitosan and gelatin. The buffer solution which was

composed of sodium acetate and acetic acid dissolved

chitosan and gelati n in liquid state. In ord er to co ntrol the

refractive index of the material, the ratios of chitosan to

gelatin were 1:9, 2:8, 5:5, 8:2 and 9:1. Figure 2 shows

procedures for blending the biopolymer.

There are issues addressed in the process for dissolv-

ing the biopolymers. As the first issue, viscosity of chi-

tosan in liq uid state increase s when the amount of chito-

san i n the buf fer incre ases. W hen ble nding c hitosa n with

gelatin in the liquid state, conglomeration of chitosan

may occur and accordingly it causes surface quality.

Another issue is that chitosan in acetic acid is not com-

pletely insoluble and remained to b e par tic le state .

To measure the refractive index of the sample, surface

of the sample must be kept clean and flat. In order to

mitigate the problems mentioned above, dissolving the

amount of chitosan was controlled to prevent increase in

viscosity of chitosan solution, and chitosan particles exi-

Figure 2. Schematic of blending process of a biopo lymer.

sting in the solution have b een removed using the centri-

fuge .

3.2. Fabrication of Biopolymeric Optical

Waveguide

The ratio of chitosan and gela tin was varied to ob tain the

slightly different refractive indices of the biopolymers

which consisted of the biopolymeric waveguide. The

biopolymer to be used in core layer was mixed with the

ratio of chitosan of 0.05 g and ge lati n of 1 .03 g. The clad

layer was mixed with the ratio of chitosan of 0.03 g and

gelatin of 1.05 g. The biopolymer was spincoted on the

silicon wafer with 550 rpm for the 30 s and then baked

for 5 hours at 50˚C. The ellipsometer was used to meas-

ure the refractive index of the spincoted film of the bio-

polymer.

3.3. Optical Butt Coupling

The optical planar waveguide was butt-coupled to inves -

tigate the light p ropagation into the waveguide. T he light

is incident into the optical fiber which was butt coupled

to the waveguide, and then the light transmitted through

the waveguide was monitored by the CCD camera, as

sho wn in Figure 3.

The Bragg grating which will be imprinted on the

biopolymeric planar waveguide obtained in this study is

schematic of the optical waveguide, indicating that the

grating pitch was determined by the effective refractive

index of and thick ness of waveguide.

3.4. Imprint of Bragg Grating on the

Biopolymric Waveguide

Holographic grating, as shown in Figure 5, is imprinted

on the photoresist spincoated on the silicon substrate

with grating period obtained by calculating the optical

waveguide. PDMS grating was molded on the PDMS by

using the photoresist grating on the silicon substrate.

PDMS grating mold production process and a grating

impr i ntin g tec hni que are shown in F igures 6 and 7.

Development of the Biopolymeric Optical Planar Waveguide with Nanopatter n

Figure 3. Schematic of optical butt coupling setup.

Figure 4. Sche matic of the opti cal waveg ui de .

Figure 5. Sche matic of holographic lithography of the grat-

ing.

Figure 6. Sche matic of PDM S grating mol d.

Figure 7. Schematic of grating imprinting process.

4. Results and Discussion

The ratio of blending of chitosan and gelatin makes the

refractive index of the biopolymeric material changed.

The blended biopolymer was coated on the substrate with

the proper velocity of spincoating and time. The condi-

tion for spincoating is shown in Table 1. The thickness

of the spincoated layer of the biopolymer was measured

to be 300 nm - 800 nm.

However, the biopolymer layer was spincoated more

than once to obtain the thick layer of biopolymer as thick

as 8 µm - 10 µm. The following method which may be

able to increase the thickness of biopolymer layer was

employed. The biopolymer was spincoated on the silicon

wafer and then the buffer solution was evaporated in or-

der to increase the viscosity of the spincoated biopoly-

meric layer. The clad layer with the more than 8µm

thickness o f the sa mple was ob tained in or der to meet the

requirement of optical coupling and then the core layer in

the 4 µm thickness was spincoated on the clad layer to

complete fabrication of the optical waveguide. Figure 8

shows surface quality of the coated biopolymer. A cer-

tain amount of conglomerated chitosan particles was ob-

served. Figure 9 shows magnification of the photo

sho wn in Figure 8.

Table 1. Claddi ng layer thickness.

Spin-Coating

Velocity (rpm)

Spin-Coating

Time (s) Th ickness

(µm) Dry

Temperature (˚C)

Dry Time

(hr)

500 30 0.640 50 5

500 30 4.8 50 5

Figure 8. Photo of t he surface o f biopolymer.

Figure 9. Photo of the sample is pro duced.

Development of the Biopolymeric Optical Planar Waveguide with Nanopatter n

The ellipsometer was used to measure the refractive

indices of biopolymer layers. As a result, the measured

refractive indices are n = 1.516 for the clad layer and n =

1.520 for the core layer, respectively, as shown in Figure

10. Chan ge in t he ra tio of chi t osan a nd ge lati n ena ble s us

adjust the refractive index.

The clad thickness of the sample waveguide, which

was coupled with the light, was more than 8.5 μm, and

the core thickness of the sample was more than 4.5 μm.

The transmitted light through the waveguide was meas-

ured by using the CCD camera, as sho wn in Figure 11.

5. Conclusions

The biopolymric optical planar waveguide and Bragg

grating were developed in this study. Gelatin and chito-

san were blended with the proper ratio to develop the

biopolymers with the different refractive index. The re-

fractive indices of the spincoated biopolymer clad and

core layers of the waveguide were obtained to be used in

the planar waveguide. Then, the Bragg grating was suc-

cessfully imprinted on the biopolymeric waveguide. The

biopolymeric planar waveguide Bragg grating, which is

biocompatible, implantable, and biodegradable, will have

Figure 10. Meausred ref ractive index of biopolymer by using the ellipsometer.

Figure 11. CCD i mage for light coupl ing measurements.

Development of the Biopolymeric Optical Planar Waveguide with Nanopatter n

Figure 12. AFM images of the imprinted grating on t he biopolymeric waveguide.

a great potential in application for biomedical diagnosis

and monitoring as well as militar y and environmental

6. Acknowledgements

This work was supported by the National Research

Foundation Korea Grant funded by the Korean Govern-

ment (MEST) (NRF-2009-0076655).

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