30 Semiconductor Based Optical Sensor

Dr. Ayushi Paliwal and Dr. Monika Tomar

1. Introduction

Optics plays a pivotal role in the field of data transmission, acquisition, information processing and various other applications such as microscopy, spectroscopy, metrology, velocimetry, particle manipulation etc. Nowadays, optics has received much attention in terms of wide range of instruments and tools which are available commercially. Hence, rapid advancement in the development of optical devices and systems leads to fast-moving field of optical sensing and measurement. The optical devices are advantageous as compared to electronic devices due to lack of direct contact, high spatial resolution, resistance to electromagnetic (EM) radiation and relatively easy detection. But they also have few disadvantages i.e. the need for the optical transducer to be transparent at the wavelength used for sensing, the use of labels, interferences etc. Due to enormous advantages of optical devices, their demand and requirement has increased leading to the development of various new optical devices having unprecedented high quality and rich functionalities. The last two decades have witnessed remarkable research and development activity aimed at the realization of optical sensors for detection of gases, chemicals and biological species. First optical chemical sensor was based on the measurement of change in absorption spectrum towards the detection of CO2 and O2 gases. Numerous optical methods are available for realization of optical sensors such as ellipsometry, spectroscopy (luminescence, phosphorescence, fluorescence, Raman), interferometry (white light interferometry, modal interferometry in optical waveguide structures), spectroscopy of guided modes in optical waveguides (grating coupler, resonant mirror), and surface plasmon resonance (SPR). In all these sensors, the detection relies on measuring the change in refractive index, absorbance and fluorescence properties of sensing material molecules in response to the analyte. Among the different methods, SPR is gaining a lot of attention and is increasingly being used for sensing purposes. The potential application of SPR technique for characterization of thin films and monitoring processes at metal-dielectric interfaces was recognized in the late seventies. There are huge number of research papers published on the application of SPR for the examination of biomolecular interaction.

2. Theory of Surface Plasmon Resonance

2.1 Plasmons

According to Drude model, the free electrons oscillate 180 out of phase relative to the driving electric field due to which majority of the metals possess a negative dielectric constant at optical frequencies which is a consequence of their fairly high reflectivity. Furthermore, at optical frequencies metals possess high electron density (1023 −3) which is comparable to that of plasma. Since the mass of positive ions in metal is very large as compared to that of the electrons, we can assume these positive ions to be fixed and the electrons as free particles; however, lattice as a whole remains electrically neutral. When a weak electric field is applied across the metal, electrons are displaced slightly with respect to the ions. The Coulomb force between the electrons and ions pull the electrons back, and induce a restoring force. The resultant of these two forces sets up the longitudinal oscillations amongst the electrons. The coherent and collective oscillations of these electrons are known as plasma oscillations. Just like the quanta of light waves is considered to be a photon; the quanta of plasma oscillations is known as Plasmons.

2.2Surface Plasmons (SPs)

Conductor (metal)-dielectric interfaces as shown in figure 1 are known to support non radiative propagating waves commonly referred to as Zenneck waves, Sommerfeld waves or Fano modes , and in modern Optics they are called Surface Plasmon Polaritons (SPPs). SPs can be considered as quasi-particles originated by the coupling of surface plasma charge oscillations and the EM fields. Thus, SP are charge density oscillations of the free electrons of a metal, which propagate along the interface of a metal and a dielectric. One can conceive SPs as light waves restricted to the surface of a conductor. This field is an evanescent field, i.e. it extends in both media while decaying exponentially as shown in figure 1 (a). Due to the resonant response of the oscillating electrons, the field features a strong enhancement close to the surface. SPs are p-polarized waves and hence can be excited by a transverse magnetically (TM) polarized wave .The schematic of the metal/dielectric interface supporting a surface plasmon mode is shown in figure 1(b). Changes to the dielectric environment in contact with the metal surface strongly alter the resonance condition.

2.3 Excitation of SPs

For the excitation of SPs, the resonance condition i.e. the matching of wave vectors of the incident light and the SPW, should be satisfied. Figure 2 shows the dispersion curves of the direct light and SPs at metal/air interface. In dispersion curve, only the real part of the wave vector is shown in figure 5 and the decaying nature of surface wave along the x-direction has been neglected. When < , the excitation of SPs is termed as non-radiative surface waves, and have been shown by red line in figure 5. When > , metal is transparent (region corresponds to radiative plasmonic decay), and the SPs are called radiative SPs as shown by blue line in figure 2. The green line is corresponding to dispersion relation for light travelling in air in figure 2. For a given ω, the wave vector of SP (kSP) is always higher than the wave vector (ko) of light in vacuum (figure 2). Hence, the wave vector of the light incident through a dielectric medium is always less than that of the kSP at the metal/dielectric interface. This means that direct incident light cannot excite the SP on metal/dielectric interface at any frequency unless special techniques are employed to achieve phase matching. Thus, to excite SPs the wave vector of the incident light should be increased over its free-space value. Various approaches are available to provide this additional wave vector component to excite SPs i.e. high index prism, grating and optical waveguide.

2.6 Prism coupling

The method used in prism couplers to enhance the momentum of an optical wave to allow coupling to the SPW is the attenuated total reflection (ATR) method. In this method the optical wave is totally reflected at the interface between a prism and a thin metal layer. The optical wave evanescently penetrates through the metal layer and excites a SPW at its outer boundary (figure 3).

As observed from figure 4, the wave vector of SP (kSP) for the metal/dielectric (air) interface (green curve) is always is greater than that of the incident light (ko) in air (purple line) as discussed earlier. When light is incident through the prism of refractive index ng, the wave vector of the incident light is increased by a multiple of ng and os given as,

This implies that the fulfilment of the resonance condition is dependent on the frequency (ω) and the angle of incidence (θ) of the light beam. It is interesting to note from figure 1.9 that the kSP (P/M) for the prism/metal interface (red curve), does not even intersect with the maximum value of the kev (brown line) at any frequency (figure 4). This indicates that SPW cannot be excited at prism/metal interface for any arbitrary value of θ or ω.

The kSP (M/D) is highly dependent on the refractive indices of the metal and dielectric media and any changes in their refractive indices results in the shifting of the resonance condition. This property of SPs is widely used in sensing applications.

2.5 Surface Plasmon Resonance (SPR)

SPR phenomena can be understood by the resonant transfer of energy from the incident optical wave to the SPW at the metal/dielectric interface that leads to the excitation of SPs. In order to excite SP in a resonant manner, the incoming electron or light beam (visible and infrared are typical) has to match its impulse with that of the plasmon. In the case of p-polarized light (polarization parallel to the plane of incidence), the resonance is possible at a given λ and θ (incident angle) by passing the light through a block of glass to increase the wavenumber (and impulse). When the condition of resonance is met in the system, the energy of the incident light wave is absorbed and the intensity of the reflected light is reduced. Thus, if the frequency of the incident light is kept fixed, the reflectance shows a sharp dip at resonance due to the transfer of energy to the SPs, and its monitoring could be used for sensing process. SPR can be studied by varying (1) angle of incidence of light (angular interrogation), (2) wavelength of incident light (wavelength interrogation) (3) phase difference between s and p-polarised light (phase interrogation) and (4) intensity of reflected light (intensity interrogation).

SPR based optical sensors

The optical sensors based on SPR have a great growth due to various advantages having wide range of operative parameters and high stability. Basic principles of SPR sensing are explained in the following sections, along with defining the main parameters to be considered to characterize their performances.

3.1 SPR sensing strategy

A SPR optical sensor comprises of an optical system, a transducing medium which interrelates the optical and (bio) chemical domains, and an electronic system (supporting optoelectronic components of sensor) for data processing. The optical system detects the change in refractive index of the system corresponding to any change in quantity of interest. The transducing medium transforms changes in the quantity of interest into changes in the refractive index which may be determined this by optically interrogating the SPR. The optical part contains a source of optical radiation and an optical structure having sensitive functional layer, in which SPW is excited and interrogated. The optical system and the transducing medium affects the sensitivity, stability and resolution of the developed sensors to a great extent. The primary parameters reflecting the performance of the sensor includes its selectivity and response. Since, SPs are highly sensitive to changes of the refractive index in the vicinity of the surface, they are widely recognized as suitable sensing probe. The SPR sensors exploit the properties of SP for the detection of analyte by optical means. Among optical sensors, SPR technology is appreciated for the possibilities of extreme miniaturization and integration and for being a label-free technology. The principle can be exploited for different applications, both for physical sensors and chemical/bio sensors.

3.2 SPR sensors performance parameters

The performance of a sensor is evaluated in terms of four parameters namely sensitivity, detection accuracy, figure of merit and response time. For the best performance of at typical SPR sensor the values of these parameters should be as optimal as possible.

Sensitivity

In the case of angular interrogation, the resonance angle(θres) is determined corresponding to the refractive index of the sensing medium ns, or the corresponding variation in the content of stimulus. If the resonance angle is changed by θres /Δ λres for a change of Δns in the refractive index of the sensing medium (due to ∆s change in stimulus content), then the sensitivity of the sensor is

Larger the shift, higher the sensitivity of the SPR sensor. For surface-functionalized SPR bio and chemical sensors, the sensitivity depends on effective refractive index change and surface refractive index change caused by surface interaction /binding.

Detection accuracy

Detection accuracy (DA) is defined as how accurately and precisely a system can detect the changes in a sample. Smaller is the width of the SPR curve, larger is the detection accuracy. The detection accuracy of the SPR sensor can be defined as the reciprocal of the width (FWHM) of the SPR curve corresponding to 50% reflectivity or normalised output power. If ΔθSPR be the FWHM of the SPR reflectance curve then the detection accuracy of the SPR sensor can be written as

Figure of merit

Figure of merit (FOM) of the SPR is defined as the ratio of the sensitivity (S) and the width (FWHM) of the SPR curve

Response time

The time taken by a sensor to approach its true output when it is subjected to its input is termed as response time. If the response time is short, then the sensor is better. In the case of SPR sensor how quick the SPR dip changes its position to a stable point with the change in sensing samples in the vicinity of sensing surface defines the response time.

Apart from these, other sensor parameters which are important for the commercialization point of view are the shelf life, reusability, miniaturized size, cost etc.

4. Semiconductor metal oxide thin films for sensing applications

In the past few decades, semiconductor metal oxides have emerged successfully as a new class of materials that can justify and satisfy the three basic requirements “sensitivity, stability and selectivity” for a good sensor. Metal oxides with improved kinetics of electron transfer, chemical stability, good biocompatibility, and high adsorption capability pave the way to make desirable surroundings for the immobilization of biomolecule and better bio-sensing features . Although polymers and other nano-materials do show promising results but they lack in terms of stability and selectivity. It is easier to tune the electronic properties of semiconducting oxides and the sensitivity can be enhanced. Also their novel properties like high surface to volume ratio, low toxicity, better solubility make them a potential candidate for different types of sensors (gas/bio/optical). Since then a lot of work has been reported on the detection of various gases with better stability , sensitivity and selectivity using various metal oxides including SnO2, WO3, TiO2, CeO2, SiO2, etc. Out of these metal oxides, SnO2 and WO3 are extensively used for gas sensing applications and show excellent sensitivity and stability. The most popular gas sensors are based on the conductometric detection techniques where material engineering by adding other metal/metal oxides to the hot sensing oxide material is necessary to obtain the desired high sensitivity, selectivity and operating temperatures. Thus, SPR gas sensors using WO3 and SnO2 sensing layer may be attractive for room temperature operation with enhanced sensitivity and selectivity towards desired target gas. For biosensing applications, different metal oxides including CuO, NiO, CeO2, ZnO, SnO2, ZrO2, TiO2 etc. have been employed as matrices.

5. SPR sensitivity enhancement

The sensitivity of the SPR based sensors is an important parameter. To achieve high sensitivity of SPR sensors, the optimization of optical structure is essentially required and suitable design parameters needed. The resonance condition depends on the refractive index of the prism, the dielectric constant and thickness of the metal and sensing layer, and the wavelength of the incident light. In addition to the above factors, FWHM of the SPR reflectance curve representing the absorption losses should be minimum for realization of highly sensitive SPR sensors. However, long range surface plasmons (LRSPs) have strong coupling and enhance the SPR sensor resolution significantly. Therefore, attempts have also been made to explore the LRSPR sensor.

Summary

Theory about the surface plasmon resonance including the concept of plasmons, surface plasmons, excitation of surface plasmons, surface plasmon resonance SPR based optical sensors including their sensing strategy Semicondctor metal oxide thin film based sensors