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Sensor to angle of incidence or wavelength of light or to refractive index of surrounding media on the basis of Surface Plasmon-Polariton Resonance

№39 Department of Optoelectronics polariton
N.L. Dmitruk, О.І. Mayeva, S.V. Mamykin, M.V. Sosnova

	a)	b)

a) AFM image of the surface of the sensors which are shown on the inset as matrix of sensitive elements fabricated on GaAs wafer with diffraction grating;
b)  Application of the sensor in the device for the measurement of liquid refractive index. Red line shows the light path of the laser. 1 is the laser λ =633 nm; 2 is the container for liquid of triangle prism shape; 3 is the cylindrical lens; 4 is the sensitive element with holder and preamplifier; 5 is the system for signal processing and indication.
  Dimensions – 305mm*75mm*95mm, weight - 0.4kg.

1. Application  For industry, medicine, environment protection.                             
2.  Description
The sensitive element of the sensor (Fig.1) is the photodetector based on the Schottky barrier (Au/GaAs) with periodically corrugated interface. The surface plasmon resonance (SPR) can be excited here by the light of p-polarization with certain wavelength and angle of incidence. The polaritonic wave is localized on the boundary between air and metal. It is very sensitive to the surface conditions and optical constants of surrounding medias. The surface plasmon electromagnetic wave penetrates through the metal film to the space charge region of the Schottky barrier where it generates electrons and holes. They are being separated by build in electric field of space charge region and generate the photocurrent (Ip,s).
The measurement procedure is to find the conditions where maximum of the photocurrent is observed. Then angle, wavelength or polarization of the light can be found using calibrated curves. In case if all these parameters known the refractive index of surrounding media can be obtained.
This procedure can be simplified by choosing the working point (angle of light incidence) of device at the middle of the slope of SPR so the small changes in the angle of incidence (light wavelength, polarization or refractive index of surrounding media) lead to big changes in the photocurrent. The larger slope of SPR the higher sensitivity can be obtained. The quality of SPR can be characterized by parameter of polarization sensitivity (Ip/Is). This is relation between photocurrent excited by p-polarized light where SPR exist to photocurrent excited by s-polarized light where SPR can not be excited.  
In comparison with other SPR based sensors the proposed one has important advantages:1) using the same structure to excite and to record the SPR in consequence of which the mechanical scanning system is simplified (if it is required); 2) compactness and low specific consumption of materials (gold for example); 3) the possibility of big scale production by means of microelectronic industry.
 
3. Main technical parameters

        
Parameter                                                                                     Value
Structure                                                                                    Au/GaAs
Active Plasmon Carrying Metal                                                          Au
Metal thickness, nm                                                                        45
Polarization sensitivity, Ip/Is                                                            >3:1
Angle halfwidth of photocurrent maximum , ΔΘ                                 1.77°
The limit of sensitivity to refractive index of tested liquid media, Δn     0.00016