CSEM Scientific and Technical Report 2008

Enhanced Visual Chemical Sensor Based on Subwavelength Grating Structures
L. Davoine, M. Schnieper
A new type of colorimetric sensor that provides a direct visual indication of chemical contamination has been developed. The detection is based on
the color change of the reflected light after exposure to a gas or liquid. The sensor is a combination of a chemically sensitive dye layer and a
subwavelength grating structure in order to enhance the color change. Low cost fabrication and compatibility to dangerous environments where use
of electricity is not allowed make this device very attractive for gas and liquid sensing in hospitals, industries, traffic, etc.
Visual sensors are widely useful to warn people of chemical or
biological contaminations. Different chemical systems are on
the market, which change their color if exposed to external
chemicals either through air or if immersed in liquids. For
example, the well known lackmus paper changes color
irreversibly depending on the pH of the liquid it has been
immersed in. Current time-temperature indicators, which are
used on packages of sensitive goods such as food or
chemicals, to determine if the package content is still safe to
use or has exceeded its lifetime, utilize the dynamics of a
tailored chemical reaction to achieve a visual color change.
Flexible temperature sensors use liquid crystals to achieve
reversible, visible color changes at very specific temperatures.
A major drawback of these purely chemical approaches is that
they require chemical engineering even for small changes in
their optical appearance or sensitivity. Combining physical and
chemical interactions can minimize the development and
tailoring effort for such customizations. At CSEM gas and
liquid sensors which couple chemical sensing layers with
photonic resonant structures to achieve tailorable visible color
changes are being developed.
The sensor presented here combines a chemically sensitive
dye layer deposited on top of a resonant subwavelength
grating (SWG) structure. The latter can be seen as a resonant
photonic structure that acts as a tailorable reflection filter.
Such structures are fabricated by replicating a subwavelength
grating into a suitable material followed by a subsequent high
index thin film deposition, providing a waveguide layer which
transmits or reflects light depending on the incident angle and
on the color. As the propagation of the light inside the
waveguide strongly depends on the optical properties in the
close vicinity of the waveguide, the evanescent field of the
light in the waveguide senses the chemical dye layer
deposited close by as is shown in Figure 1.
Figure 1: Schematic of the sensitive resonant structure
After exposition to a particular gas or liquid, the optical
properties of the dye thin film are changed and consequently
the resonance properties of the waveguide are changed too.
38
White light
θ
Visible colored reflection
Active dye layer
Thin film layer
Subwavelength
grating
Glass substrate
To show the feasibility of such sensors, Bromocresol Purple
(BCP), a pH indicator, was used as the chemical dye to detect
ammonia gas. A solution of BCP and Polymethylmethacrylate
(PMMA) was deposited on top of a tailored resonant
waveguide. A sol-gel ring cured on a glass plate and glued on
top of the optical structure enabled the sealing of the centre
area while exposing the rest of the structure to ammonia, as
shown in Figure 2.
Gas exposure
Glass
A B C
Glass
Sealing ring
BCP film
Resonant
waveguide
Figure 2: Schematic cross section of the final gas sensor. A: area
exposed to ammonia, B: sealing ring, C: unexposed reference area
The results after ammonia exposure are shown for two
different resonant gratings (see Figure 3). The color shift
between the reference area (C) and the rest of the sensor (A)
due to ammonia exposure is clearly visible. The photonic
structure of both devices was optimized to obtain a strong
visible color change.
A
B
C
Figure 3: Pictures of two visual gas sensors based on BCP film with
two different photonic structures. A: area exposed to ammonia,
B: sealing ring, C: unexposed reference area
A novel class of visual chemical sensors based on
subwavelength grating structures has been demonstated. The
technology proposed here is generic and can be tuned to a
A
range of different chemical reactions. Its main advantage,
however, lies in its simple adjustability to spectral
requirements. By changing the geometry of the resonant
structures, the same dye and chemistry can be used to obtain
different color shifts.
A
B
C