Silicon photonic crystal sensors have become very attractive for various
optical sensing applications. Using silicon as a material platform provides
the ability to fabricate sensors with other photonic devices on a single
chip. In this paper, a new optical sensor based on optical resonance in a
one-dimensional silicon photonic crystal with an air defect is theoretically
studied for refractive index sensing in the infrared wavelength region.
The air defect introduces a cavity into the photonic crystal, making it
suitable for probing the properties of a gas found within the cavity. This
photonic crystal nanocavity is designed to oscillate at a single mode with a
high quality factor, allowing for refractive index sensing of gases with a
high sensitivity. A method is presented to maximize the sensitivity of the
sensor and to obtain a very narrow bandwidth cavity mode for good sensor
resolution. We change the thickness of the air layers linearly in the
photonic crystals on both sides of the nanocavity and show that a sensitivity
of 1200 nm RIU
Optical sensors are used for a variety of applications in science, technology, and the environment. Most of these optical sensors measure the variations of optical properties as a result of a change in the refractive index of a gas or a liquid. Various structures have been investigated as optical sensors such as fiber Bragg gratings (Fang et al., 2010), step-index fibers and photonic crystal fibers (Lee et al., 2011; Udd and Spillman, 2011), fiber-assisted surface plasmon resonances (Ma et al., 2009), and photonic crystals (Liu and Salemink, 2012; Jágerská et al., 2010; Kang et al., 2010; Bougriou et al., 2013). Recently, optical resonance in photonic crystals has received increased attention for refractive index sensing applications.
Photonic crystals are periodic structures that exhibit a band gap for a range
of wavelengths. Photonic crystals with one- and two-dimensional (1-D, 2-D)
band gaps have been studied for a variety of refractive index sensing
applications. Below, we give some of the most recent and important examples.
A new design based on a 2-D photonic crystal slab with a triangular lattice
pattern has been used for optofluidic sensing with a sensitivity of
636 nm RIU
Although these 2-D photonic crystal optical sensors can be used for refractive sensing of liquids and gases, they all have limitations. The fabrication of these sensors is difficult and complicated and the sensitivities that can be achieved by these optical sensors are not high. Because of easy fabrication, 1-D photonic crystals have attracted a lot of attention for fabrication of different components in silicon-based photonic integrated circuits. Techniques for fabrication of 1-D dielectric photonic crystal at IR wavelengths are mature and well developed. It is now possible to fabricate 1-D photonic crystals with layer thicknesses reaching the nanometer scale. 1-D dielectric multilayer structures, or photonic crystals, have been used in many photonic devices as filters (Vangala et al., 2014), microcavities (Chen et al., 2014b) and optical sensors (Sreekantha et al., 2013; Chen et al., 2014a; Frascella et al., 2013; Nunes et al., 2010).
Several 1-D photonic crystal optical sensors have been suggested for
refractive index sensing. A biosensor configuration based on the excitation
of surface electromagnetic waves in a 1-D photonic crystal has been proposed
(Sreekantha et al., 2013). They show that the sensitivity of the proposed
configuration is much higher compared to the sensitivity of conventional
surface plasmon resonance biosensors. A refractive index sensor consisting of
a 1-D photonic crystal embedded in a microfluidic channel has been
investigated, yielding a sensitivity of 836 nm RIU
Optical resonance sensors based on photonic crystals have several advantages. Photonic crystal cavities typically have high quality factors and very small dimensions, which makes them a promising building block for integrated lab-on-a-chip systems. In a typical photonic crystal cavity, the optical field is mostly located in the high index material, making it difficult to probe the properties of the analyte found within the hollow part of the cavity. To overcome this drawback, we use an air nanocavity in the defect region of a 1-D photonic crystal.
In this paper, a 1-D photonic crystal nanocavity is investigated for
refractive index sensing of gases at an IR wavelength of 1.55
1-D photonic crystal nanocavity.
One of the methods used for analysis of photonic crystal structures is the
transfer matrix method. We use it to investigate the transmission of light in
a 1-D photonic crystal nanocavity as shown in Fig. 1. The electric and
magnetic fields inside the layers are shown by
The 1-D photonic crystal nanocavity, shown in Fig. 1, consists of an air
nanocavity (layer C) that is introduced as a defect in a 1-D photonic crystal
consisting of silicon (A) and air (B) layers. Silicon and air layers are used
in order to achieve high contrast of the refractive index between the high
index and the low index adjacent layers. To study this structure for gas
sensing applications, one would place the structure in a gas cell that has an
inlet for injection of gas into the cell and an outlet for gas evacuation.
Then, an optical signal from a laser diode (for example, in the wavelength
range of 1.4–1.7
First, the gap map method is used to calculate the photonic crystal band
gap. The band gaps as a function of the thickness ratio of air and silicon
layers,
The 1-D photonic crystal air nanocavity is simulated using the transfer
matrix method. The method was explained in the last section in detail, is
used to calculate the reflection and transmission properties of the cavity,
and gives the amplitude of waves reflected by and transmitted through the
cavity. When we choose the thickness of the Si layer
Band gap map of 1-D photonic crystal with silicon and air layers.
Transmission of the 1-D photonic crystal air nanocavity for
The cavity wavelength and quality factor have been plotted in Fig. 4, as a
function of the cavity length. The wavelength of the defect mode shifts to
higher wavelengths almost linearly, when the cavity length is increased. The
quality factor decreases as the wavelength shifts to the edges of the band
gap, which results in a broadened bandwidth, and peaks at a wavelength near
the middle of the band gap. For sensing applications, it is desirable to have
a mode with a narrow bandwidth and, thus, we must choose the parameters so
that the quality factor peaks at the operation wavelength. One can see that
the cavity has a high quality factor on the order of
10
The cavity wavelength and quality factor as a function of cavity
length for
To study the photonic crystal air nanocavity for refractive index sensing of
gases, we consider gases of different refractive indices, such as air (
The sensitivity of the sensor is defined by
Transmission and spectral position of the cavity mode for different gases with air as the reference medium. The mode has a very narrow bandwidth of 4.3 pm.
Sensitivity for different values of
To study the effect of the air layer on the sensitivity of the optical
sensor, we selected
Sensitivity as a function of the air layer length for
The wavelength shift as a function of the refractive index.
The results are for cavities corresponding to band gaps
The shift in the cavity mode wavelength
Transmission and spectral position of the cavity mode for
different gases, with
1-D photonic crystal nanocavity with the thickness of the air layers
changing linearly. The thickness of the air layer is increased linearly from
0.22 to 0.55
In this section, we present a method for optimizing the sensor sensitivity.
In sensing applications, a sensor is required to have high sensitivity and
good resolution. We showed above that the sensitivity is increased as the air
layer is made thicker. However, in optical resonance sensors, the FWHM
bandwidth of the cavity mode is very important and affects the sensor
resolution. If the bandwidth is broad, detecting a small change in the gas
refractive index becomes difficult. In order to investigate this problem, we
consider the sensor with
Transmission and spectral position of the cavity mode for the 1-D photonic crystal nanocavity with linearly changing air layer thickness as shown in Fig. 10. The mode has a very narrow bandwidth, yielding a sensor with good resolution.
Now, we propose a method by which the thickness of the air layers in the 1-D
photonic crystals are changed linearly. We change the air layer thickness in
each photonic crystal linearly by a constant value. A 1-D photonic crystal
nanocavity with linearly changing air layer thicknesses is shown in Fig. 10.
The thickness of the air layer in the photonic crystal on the left side of
the cavity is increased as
The thickness of the air layer in the photonic crystal on the right side of
the cavity is decreased as
Sensitivity of the optimized sensor that has the structure shown in Fig. 10.
A 1-D photonic crystal nanocavity was investigated for refractive index
sensing of gases at a wavelength of 1.55