Label Free Biosensor Based On A Planar Optical Waveguide

Published Date: 02 Nov 2017

Abstract

In this article we will review some common planar optical waveguide based sensors for label-free biosensing applications including conventional dielectric waveguides, reverse symmetry waveguides, anti-resonant reflecting optical waveguides (ARROW) and metal clad leaky waveguide (MCLW), while their configurations, sensing mechanisms and sensitivities will be discussed together, after that some actual sensing platforms will be introduced, with which we will present their development from the beginning and recent progress as well. In particular, we will pay more attention to emphasize the reverse symmetry waveguide and metal clad leaky waveguide based biosensors (MCLW) due to their specific characteristics on deep-probe evanescent wave sensing for detection of micron-scale biological objects.

Introduction

Bio-sensing technology based on an optical system is defined as using a wide range of optical sensing devices called biosensors for their vast applications in biomedical research, health care, pharmaceutical science, environmental monitoring etc.[1-3], by transducing a biochemical interaction on the probe interface to a physical signal with good signal to noise ratio rapidly. Fig 1shows the general configuration of a refractive index based optical label-free biosensor.

Over several decades intensive research for rapid and sensitive optical sensors has been conducted and has resulted in a variety of techniques. In some cases, the reason we use optical bio-sensing systems for detection over many biophysical techniques mainly bases on the feature of label-free, never requiring any fluorescent method (dyes, radiolabels or enzymes) and resulting in much more convenient than conventional method . For its special inner structure and sensing principle, we can get rid of the external effect of electromagnetic interference so that it is great potential to be used as remote sensing devices and can provide multiplexed measurement within only one single equipment. In addition, optical biosensors are relatively easily fabricated, low cost and with much smaller scale so that they can be availably used as the center component of a photonic integrated circuit system (PICs)[4], which can offer real-time data monitoring and requiring for different biochemical interactions.

Fig. 1 – General illustration of a label-free optical sensor. Reprinted with permission from refs.[33]

A great number of label free optical sensors have been fabricated and reported for biochemical detection including dielectric optical waveguide sensors [5- 8], metal-clad leaky optical waveguide sensors (MCLW) [9-13], anti-resonance reflecting optical waveguide (ARROW) based sensors [14-16], resonant mirrors (RM) [17,18],surface plasmon resonance (SPR) sensors [19-23], photonic crystal sensors [24-26] and kinds of optical interferometry based sensors [27-30] etc. For evaluating various optical sensors performance in their respective applications we need to induce a set of requirement listed by Zemel.for describing an ‘ideal’ chemical sensor [31],within which contained 3 most important benchmarks: stability, sensitivity and selectivity.

Each sensor that could be used for biochemical detection is supposed to be stable enough so that the measurement will be reversible corresponding to its attribute of reversibility, if not, it makes no sense. However an ‘ideal sensor’ which means totally stable does not exist, but in some cases we can adjust other factors to improve the stability even if their decreasing the sensitivity.

The term of sensitivity is a quantitative factor referred as magnitude of induced transduction signal change resulted from the level of chemical analyte (for example, antibodies or enzymes) concentration or the lowest increment of concentration change that can be detected in the sensing environment [32], thus the optical sensors with better sensitivity could distinguish a weaker signal from the background.

The selectivity of a biochemical sensor is recognized as the ratio of the ability to detect what is of interest over the sensor’s ability to detect what is not of interest. Similar to the term of sensitivity, the higher of the selectivity, the better.

These sensors displayed just now have got a great progress throughout these years especially for their rapid and sensitive measurement, however detection of objects with a size scale more than 150 nm like microorganisms, DNA, proteins and particles may not be available, in that case we need to some slight adjustment with their structures and delicate fabrication steps, like deposition operations, etching, and various chemical or physical treatment. [27-30]

Over past several decades, the development on optical label-free based bio-sensing method is tremendous, fascinating and a remarkable interest. Until now there exist various label-free structures based on optical principle for investigation and research. Throughout this article some label-free bio-sensing systems based on planar optical waveguides will be reviewed, simultaneously their structures, sensing principle and sensitivity will be shown as well.

These system will be divided into 2 main sections which are consist of different materials and structures for dielectric waveguide sensor and metal clad waveguide sensor, and each section will be divided into subsection as well.

Content

Over the past several decades, there have been intensive research and investigation on the theory [34-36] and applications of optical waveguides, however there was not any institute or scientist discovering its great potential of interest on sensing applications until Tiefenthaler and Lukosz did it [5]. They fabricated a conventional planar optical waveguide and applied it for humidity and gas sensing by detecting the refractive index (RI) change caused by the material contained in the cover medium with help of grating coupler. Since then, the optical waveguide based sensors have achieved great and tremendous advances in some sensing applications like humidity sensing, heavy metal detection, chemical measurement, micro-scale objects detection etc. [38-41]

Elementary structure

Basically, a planar optical waveguide is a three-layer dielectric configuration containing a high index material (functioning as a waveguide film, F) sandwiched between the substrate (S) and the cover medium (C) within which the analyte exists for detection, See Fig.2 for the basic waveguide structure.

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Fig. 2-Basic optical waveguide configuration and illustration of beam guided inside the waveguide film by total internal reflection (TIR) at the interface of two boundaries

The basic dielectric waveguide consists of 3 dielectric layers with different RIs shown in Fig.2. The beam will be reflected when traveling to the interface between different materials with respective RI while inducing different angles to each material. However in some special cases when the beam travels from the material with higher RI to another with lower RI at or larger than a critical angle (θCritical, shown in Fig.2) of incidence, it will be totally reflected back into the higher RI material with almost no energy loss. This phenomenon is named as total internal reflection (TIR), a common physical effect with important practical significance for biochemical sensing. The critical angle shown in Fig.2 can be calculated with an equation as below:

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(1)

Arising of a guide mode will take place only when the phase of the light in the guide film after twice reflections matches with each other while the , called self-consistency described in Fig.3.

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Fig.3 –Guided modes will arise if the wave fronts at a and b are in phase because of the constructive interference.

The beam traveling in the guide film will be given extra phase shift (shown in Fig.3) after twice reflection on the interface of guide film. The incident angle when guide mode arising is called mode angle (θm). Thus the criteria can be written as:

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(2)

Where m = 0, 1, 2, 3... is the order of mode. The phase shift can be described as below [42,43]:C:\Users\A-Bing\AppData\Roaming\Tencent\Users\317197942\QQ\WinTemp\RichOle\N(A~6@]1AT@GKC4`CH$(()7.jpg

(3)

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(4)

(5)

Where ρ = 0, 1 corresponding to TE and TM modes. N stands for the effective refractive index given by nF sinθ, which means in guided mode θm can be transduced to Nm.

Sensing mechanism and sensitivity

The evanescent field which is induced by Tiefenthaler and Lukosz [5] will be produced in the medium with a low RI when light travels in the guide film by TIR.

So this field will exist in both substrate and cover medium. This phenomenon which is described theoretically in [43] can be expressed as: light outside a waveguide film forms an exponentially decayed wave in perpendicular direction of the interface. Typically the evanescent field has a penetration depth around 100 nm into the cover layer depending on different configurations. With this extension depth we can know clearly the basic mechanism of sensing for waveguide from Eq.2 – Eq.5, at a given concentration shift of nC, it will influence the phase shift on the film/cover surface then influence the total equation, for maintaining guided modes we need to change some external parameters such as incident angles which can be detected and recorded by the right devices. Finally we can get a sensorgram describing the relationship between intensity and incident angle θ or effective refractive index N as N=nF sinθ.

(a) C:\Users\A-Bing\AppData\Roaming\Tencent\Users\317197942\QQ\WinTemp\RichOle\)[{7{1%A0_S0E@R9(Q)BEHD.jpg

(b)

Fig.4 – Illustration for explanation of sensing principle. (a) shows process of light coupling into and out of waveguide and recorded by the detector. (b) shows the sensorgram within which exists a comparison for different concentration.

Usually we use evanescent field for sensing two types of ambient shifts: either, a liquid or gaseous sample treated as cover medium altering the refractive index nC, or objects inside the sample continuous falling to the sensor surface forming a thin adlayer while continuous changing refractive index. This sensing principle can be developed as a useful method for monitoring the adsorption and binding process of biological and chemical objects. From Eq.2 we can derive the sensitivity for RI change in the cover medium(Eq.6 for common case and Eq.7 for adlayer case) as below[43]:

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(6)

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(7)

Light coupling methods

For different system configurations and applications, we need to take special attention for coupling methods. Fig.5 shows four commonly available coupling systems including grating coupler, prism coupler and two types of end-face couplers for optical waveguide.

End-face coupling can be completed via either an objective lens or a common optical fiber by focusing the beam onto the end-face of the waveguide, this method ensures the minimum energy loss which is very suitable for remote communication while in some cases may not be appropriate for sensing purpose based on its not demanding technically and disadvantage os stability resulting from vibrations of the layout.

Prism coupling method is a convenient way of coupling beam into waveguide film by a prism with a slightly higher RI than the substrate. At some controllable incident angle there will be TIR taking place at the interface of prism and substrate of waveguide, resulting in a evanescent filed which will pass through the thin substrate into guide film, thus with controllable incident angle we can get different guided modes respectively. The theory describing prism couples was first introduced in the Soviet Union[44], from then on, various studies represent theories and applications for optical sensing as a basic light coupling method because this coupling method never requires any complicated optical components structure and is relatively precise and simple.

Grating coupling system takes advantage of the phenomenon diffraction induced by shining the laser beam with a certain wavelength on a grating system, coupling will take place when the controllable incident angle matches with the coupling condition so that the guided modes can be determined by the angle, requiring and recording data we can get the final sensorgram. In 1980`s, Lukosz et al. discovered and designed first grating coupler layout monitoring phase shift resulting from binding of molecules included in the sample with high sensitivity[45]. This coupling method contains an obvious advantage over the previously displayed methods that it is a good reproducible coupling layout since any extra optical elements are not necessary.

Usually we use 3 types of grating coupling systems including input grating coupler as shown in Fig.5 (a), output grating coupler which will functions as an output system directly connecting with a detector while the light will be coupling into waveguide film via an objective lens [46] or optical fiber, and reflection grating coupler with which we detect the intensity of directly reflected beam so that getting a sensorgram between reflectance and incident angle.

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a b

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c d

Fig.5 - Four commonly available coupling systems for optical waveguide. (a) Grating coupler. (b) End-face coupling by lens. (c) Prism coupler. (d) End-face coupling by fiber.

PART I: Dielectric optical waveguide sensor

Conventional optical waveguide sensor

A conventional or asymmetrical optical waveguide owns a basic three layers configuration shown in Fig.2 while the sensing mechanism has been shown in the previous chapter. Here the term conventional/asymmetrical means that in this structure, the refractive index of substrate nS is always larger than that of the cover medium writer as ns > nC.

This planar waveguide was introduced by Tiefenthaler and Lukosz functioning as a RI sensing switch due to the evanescent filed extending into cover medium with help of grating coupler and achieved the effective index down to 1.4 x 10-3 [5,45 ]. In 1986,M. Seifert, K. Tiefenthaler etc. demonstrated an integrated optical biosensor based on the same grating configuration to detect enzyme activities by measuring the RI change induced by biological interactions taking place on the surface of waveguide film successfully and got the resolution down to nC.=5 × 10−5 [37]. After that, Tiefenthaler and Lukosz showed a theoretical explanation for sensing principle of waveguide and grating couplers [43]. The same year they adjusted the coupling configuration from input grating coupler to output one while using an objective lens for coupling [46]. In 1998 Ursula Bilitewski. etc. described a direct optical affinity sensors based on total three types of grating couplers for real-time monitoring of the binding reaction and thus reported and evaluated the data of kinetic and thermodynamic [47]. The Fig.6 shows different configurations of grating couplers.

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a

C:\Users\A-Bing\AppData\Roaming\Tencent\Users\317197942\QQ\WinTemp\GE\28C759AE-4A16-4253-8FB2-5B5890CCE40E.jpg

b c

Fig.6 shows schematics for different grating coupling systems. (a) for input grating coupler. (b) for output grating coupler. (c) for reflection grating coupler. Reprinted with permission from refs [48].

The previous mentioned methods are all based on searching for relationship between the coupling angle and output intensity guided light at a certain wavelength emitting by a laser, however it is also possible to ensure a constant incident angle while scanning the incident wavelength with another source system which is named as the wavelength interrogated optical sensor (WIOS) [49]. In 2003, K. Cottier and M. WikiAn etc. demonstrated the WIOS configuration and applied it for bulk refractometry and affinity measurement [49] with a detection limit of <10 -6 for RI detection, 0.3 pg/mm for tiny molecules detection. Fig. 7 shows the schematic of WIOS configuration. There exist two types of WIOS configurations for respective sensing purpose: (b) type contains single grating with different periods for in and output pad, (c) type show a uniform grating with different thickness.

b

a c

Fig.7 – Schematic of WIOS system. Reprinted with permission from Ref.[49].

In 2002 and 2004, Cunningham etc. [50, 51] made use of a resonant diffractive grating as a binding platform for real-time monitoring the binding of biochemical events (named as resonant grating waveguide system shown in Fig.8. RGW for short); this system has been commercialized as BIND® by SRU Biosystems. In 2006, Yih, J. etc. performed a set of theoretical analyses on resonant diffractive grating system for optimizing the configuration and fabrication method in order to gain a higher sensitivity [52]. RMG system is good at monitoring the binding process of small molecules to proteins [50, 53, 54], however has an evident advantage of monitoring mass redistribution for live cells after special treatment of test agents [55]. There exists a weakness of measuring cells with the evanescent field that the size of analytes is always limited by penetration depth (around 100nm ) extended into the sample medium.

Fig.8 – Schematic of a RGW system. The detector requires the reflected light with a narrow band of frequency at which guided mode is satisfied, while the light source has a broad band. Reprinted with permission from Ref. [50].

Resonant Mirror sensor

In 1993, Cush.et al. described a unique optical waveguide sensing system consisting of four-layer dielectric materials, named it the resonant mirror (RM) biosensor [56, 57] and demonstrated the operation principle, sensing instrumentation and applications. Actually the RM sensor belongs to the area of evanescent filed sensor, owing a similar configuration (shown in Fig.9) to the previous basic structures which consist of three-layer materials guide film, substrate and cover medium, however RM sensor is different caused by unique constructions as, first the total sensing system is located on top of a prism without spacer between the substrate and prism, and another, thickness for guide film about 100 nm with a sufficiently thin substrate (~1um) of slight lower refractive index than the prism.

Light is incident onto the interface between prism and thin substrate with a controllable angle, at some angle the effect of TIR occurs resulting a evanescent field which will pass through the ultra-thin substrate into the guide film, thus at some certain incident angle, guided modes will arise while the intensity of reflected beam will draw down intensively due to matching all other right parameters such as RI of cover medium, therefore similar to conventional waveguide with help of RM system we can get a sensorgram between intensity and incident angle, however instead of acquiring data for intensity the RM system is usually used for measuring the phase shift of reflected light caused by TIR occurring on the interface [58, 59].

For over a decade the RM has been used for various bio-sensing applications [57-62] such as monitoring of interactions of various molecules in micron-scale and the RM based sensing system has been commercialized by NeoSensors with achieving detection limit down to 0.1 pgmm-2 [63].

Fig.9 – Schematic of a resonant mirror sensing system. Reprinted with permission from Ref.[56]

For these sensor types all mentioned previously have respective sensitivity and detection limit due to their specific configuration, however the common point they own altogether is that the RI of layers keeping a rule as: nF > nS > nC, in which case we take them as conventional or asymmetric optical waveguide sensor. By comparison, next we will introduce another particular waveguide sensor with similar configuration expect for the reversed RI of between substrate and cover medium, which means for this sensing platform we need to always keep nF > nC> nS so that achieving some certain purpose great of interest for applications.

The reverse-asymmetric waveguide sensor

In 2002, R. Horvath et.al [64, 65] presented a theoretical analysis and explanation of a reverse-symmetry optical waveguide, demonstrated the basic principle of fabrication method and showed some potential deep-probe sensing applications especially for micron-scale objects such as bacteria (1–5µm) and even higher cells (5–20µm) [65, 68] due to its higher penetration depth in comparison with conventional case (shown in Fig.10). In their articles, they presented three different implementation for fabricating a substrate with lower RI compared with cover medium, air below freestanding waveguides (RI= 1), mesoporous materials (mean RI≈1.2) and corrugated surfaces with the nanometer scale (mean RI≈1.2), calculated their respective sensitivity and probing depth [64, 66] .Then they applied this sensor for monitoring the phase transition of DMPC (dimyristoylphosphatidylcholine) lipid bilayer around critical temperature [67] and on-line monitoring of bacteria [41] , finally the detection limit has achieved down to 60 cellsmm-2(TM mode) or 78 cellsmm-2 (TE mode), much better and that in RM [41]. In 2004 this group demonstrated this fabrication for monitoring attachment and spreading of living cells as well, proving the potential application in real time monitoring of biological molecules interactions and cell morphology. Fig.10 (b) shows their schematics.

a b

Fig.10 – Schematic of a reverse symmetry waveguide. (a) The comparison of penetration depth between normal symmetry waveguide (black profile) and reverse symmetry structure (gray profile). (b) Three types of substrate assembled with different inner structure. Reprinted with permission from Ref. [64].

It is evident to see from Fig.10 that the penetration depth for reverse symmetry waveguide is much higher than conventional case, through which we can achieve a much better sensitivity, so that it is great potential of applying for measurement of micron-size objects like cells, bacteria etc. shown in this figure clearly.

For its specific configuration of taking air or porous polymer as a support fulfill with air inside, we can treat this waveguide configuration as a free standing waveguide. Fig.11 shows the calculated comparison of sensitivity and power flowing extending into the cover film between conventional waveguide and reverse symmetry case for TE and TM mode with increasing of guide film.

Fig.11 – Schematics of Sensitivities (solid lines) and power flowing fraction in the cover medium (dashed lines) the waveguide film thickness. (a) for TE modes. (b) for TM modes. Reprinted with permission from Ref [65].

In 2002, Qi et al [69] fabricated a simple reverse symmetry multimode waveguide sensor which is comprised of a 100-um-thick quartz plate fixed on a PMMA container working as a flow-cell for a liquid sample, introducing a pair of prism couplers (nPism > nF) contacting with the film surface on each edge so that the air space between two prisms will function as a substrate with ultra-low RI (configuration shown in Fig.12a).

a b

Fig.12 – Schematic of two types of free-standing multimode waveguide sensors. (reverse symmetry configuration based). Reprinted with permission from Ref [69,70].

For a multimode waveguide, the highest order of guided mode will be given the largest phase shift in response to its highest penetration depth into the sample, naturally it is most sensitive to any change taking place in the cover medium [69].

Qi et al made use of a pair of prisms for light input coupling and output coupling,

adjusted the angle of incident light until sure of the highest guided mode excited, then fixed the incident angle, started to record data while introducing samples with slight different RI without changing the maximum mode number. Using this method Qi et al achieved a detection limit down to 3x10-5 RIU, however detection range is limited to 1.5x10-3 RIU.

For overcoming the drawback of rather narrow detection range for this system,

N. Skivesen et al [70] presented an alternative free-standing multimode waveguide sensor which owns a similar configuration except for a thinner guide film and specific grating coupling system (made of polystyrene with a RI slight higher than guide film), the schematic is shown in Fig.12 (b).

By introducing the specific grating system we can easily control the coupling mode angle in the guide film, while placing a photodiode on the end-facet of the glass film for recording data to get a sensorgram between incident angle and output intensity shown in Fig.13. With this sensing system N. Skivesen et al achieved the detection limit down to 5x10-5 RIU which is a little bit lower however improving the detection limit up to 0.52 RIU which is broad enough to be used as a refractometer for various sensing applications including both liquids and gases.

a b

Fig.13 – (a) Schematics of mode with intensity for different samples. (b) Resolution of different RI measurement. Reprinted with permission from Ref.[70].

ARROW sensor

In 1986, M. A. Duguay et al presented a new type of dielectric optical waveguide (anti-resonance reflecting optical waveguide, ARROW for short) structure [71] by inserting an anti- resonant reflector into a conventional waveguide made of special materials and showed its feature of low-loss propagation on a high index substrate. Two years later, T. Baba and Y. Kokubun et al optimized the basic setup to reduce the propagation loss with help of a transparent Ti02/Si02 interference cladding, and presented a stacked 3D-configuration for enhancing its performance [72]. In 1989, T. Baba and Y. Kokubun et al proposed another ARROW type based on evanescent field anti-resonant reflector called ARROW-B type [73]. Following the new ARROW-B type, some scientist developed and demonstrated other types of ARROW within which hollow-ARROW is the most notable configuration. Fig.14 shows the respective configuration for convention ARROW and hollow-ARROW.

a b

Fig.14 – Schematics for two ARROW types (a) conventional structure with the RI distribution. (b) hollow-core structure. Reprinted with permission from Ref. [72].

Usually ARROW consists of a five-layer waveguide structure (shown in Fig.14) where

the beam will confined inside the guide film by TIR at the interface between film and cover medium, while at another surface light will be experience an anti-resonant reflection (reflectivity >99.9%) because of the two coatings deposited on the silicon substrate. In the case of TIR at upper surface, this kind of structure can also be used as evanescent field sensing system. In 1996, D. Jimenez et al [73] demonstrated an interferometric detection configuration first time by inducing an ARROW structure as a sensing component for a theoretical RI measurement. After that, F. Prieto et al [74,75] showed a set of theoretical study for optimizing parameters, fabricated a multilayer ARROW system and used it for applications and got a detection limit of 2×10-5[76].

In 2004, D. Yin et al [77] demonstrated the design and fabrication method for a hollow-core ARROW sensor, and Campopiano, S. et al [78]use a similar configuration introducing liquid into the hollow core, exhibiting a RI change of 9×10-4.

.

Part II: Metal Clad Optical Waveguide sensor

Dip-type Metal clad leaky waveguide sensor (MCLW)

The dip-type metal clad waveguide was first demonstrated by Salamon et al [79] for measuring morphology of biological molecules, in which a metal layer is deposited on the inner surface of substrate(usually a prism ). At that time this configuration was presented as a coupled plasmon-waveguide resonator (CPWR) and showed an improvement of the optical sensitivity and measurement range such as anisotropy in lipid bilayer membranes, membrane-related proteins and molecular interactions within membranes etc [79-85]. However this configuration of CPWR sensor is equivalent to what was treated as a metal-clad leaky waveguide (MCLW) before [86-88].Fig.15 (a) shows the basic structure.

.

a b

Fig 15 – (a) Configuration for a MCLW sensing system. (b) The schematic of sensing principle.

The MCLW sensing system has a similar configuration compared with the conventional dielectric waveguide expect for an extra metallic layer (typically Au or Ag with tens of nanometers thickness) introduced between the guide film and prism functioning as a substrate. However, the sensing principle for two configuration types is slightly different, in conventional structure the light will penetrate into the guide film by an evanescent field while be confine inside by total inner reflection , but for MCLW system the light will be totally internally reflected at the interface of film and cover medium, while at another boundary normal reflectance will take place so that partly of light will transmit through the metal layer again with the intensity measured by a detector.

Based on feather of waveguide, the mode will arise by adjusting the incident angle directly, which will result in an intensive dip of reflectance. In same way, by choosing different cover medium with respective RI, the coupling angle will not be same either.

So, searching for the relationship between coupling angle and reflectance in response to different concentrations is the main sensing mechanism (shown in Fig.15 b) for dip-type MCLW sensing system.

The most notable advantage of introducing MCLW system as a biochemical sensor is duo to the specific of this inserted metallic layer, which can be passed through by the modes even if the RI for film and substrate is reversed. In this way, it is possible to improve the penetration depth of evanescent field into cover medium for further higher interaction.

In 2005, N.Skivesen et al [89] presented an article for optimization of MCLW sensors.

With help of Fresnels’ equations, N.Skivesen et al calculated both angle modulation and resonance width in the sensorgram and list a comparison graph sensitivity with SPR effect in terms of sensivity. Then in 2007, N.Skivesen et al [90] used this dip-type MCLW system for cell detection, showing a detection limit of 8–9 cells/mm2. In 2009, V. Singh and D. Kumar [91] presented a theoretical study about a five-layer (an affinity layer is introduced on guide film surface) WCLG system for detecting Pseudomonas and Pseudomonas-like bacteria and showed a set of design optimization parameters for sensing micro-scale objects. In 2011 D. Kumar and V. Singh [92] presented another theoretical study of modeling a nonlinear asymmetrical metal clad waveguide while enhancing its sensitivity.

Peak-type Metal clad leaky waveguide sensor (MCLW)

The peak-type MCLG system was first demonstrated by Zourob et al in 2003 as a MCLG sensor for biochemical sensing applications. Then the following years Zourob et al use it for detection of RI, protein layers, bacteria and other applications, showing a great potential for measurement of micron-scale objects in the sample.

The peak-type MCLW owns a same configuration as the dip-type (Fig.15 a), however the metallic layer sandwiched between guide film and prism has a much larger imaginary part of dielectric constant than that of dip-type, which means value of ε＇＇peak >>ε＇＇dip corresponding to a strong absorption for light with a right frequency by the metallic layer, and the metal thickness is about several nanometers. Fig.16 shows the basic configuration and sensing principle.

Fig.16 –The sensorgram for a peak-type MCLW.

The peak-type MCLG consists of the same configuration as dip-type except for thickness of metal layer, and the sensing mechanism is a litter different. For both MCLG types, they work at a reflection mode. To the dip-type, the guide mode will arise when the light after twice reflections is in phase, while resulting in a strong power level decreasing in reflected light. However, before the critical angle for total internal reflection at interface of film/cover medium, the light can pass through, which means the power level is supposed to increase slowly due to Fresnel’s Equations for normal reflection. Now we will care about the two critical, one is for total inner reflection and another is for guide mode arising. At the first critical point when TIR taking place, the guide mode may not happen to be excited in most cases, thus most of light will be reflected back into the prism and detected by a detector even if a portion of light absorbed by metal layer with a larger ε＇＇, however at the second critical angle , most of light will be combined and guided inside the film resulting in a dip point shown in Fig.16. So for the area between two critical points, it is explainable that both larger ε＇＇and multi times normal transmittance induced by short guide distance in the film drag the reflectance from the peak to the dip.

In 2003, M. Zourob et al [93] demonstrated this MCLW configuration and used it for detecting micron-scale objects such as soluble proteins and bacteria and achieved a better sensitivity than the conventional method. The same year, M. Zourob et al [94] used it for fluorescence system making a detection limit down to 10-13 M for fluorescein solution. In 2005, they [95,96] used it for detection of paraoxon and achieved detection limit down to 6 nm while presenting a rapid detection and better selectivity. In 2007, N.Skivesen et al [90] used two types MCLW systems for cell detection showing a detection limit of 8–9 cells/mm2, and list some comparisons with another widely used sensing platform SPR. Fig.17 shows that the dip-type metal-clad waveguide sensor performs best all-round compared to the SPR biosensor.

In 2005, M. Zourob et al made use of MCLW

a b

Fig.17 – Schematic of comparisons with different sensing platforms. (a) Sensitivities to RI change in adlayer. (b) Sensitivities to adlay growth.

Outlook

1: left hand material

2:plasma substrate for reverse symmetry waveguide

3:photonic crystal

4: Goos–H¨ anchen (GH) shift:1: Goos–Hänchen shift as a probe in evanescent slab waveguide sensors 2: plasma substrate for reverse symmetry waveguide

5:nanopous for ARROW