We have characterized the refractive index sensing properties of a compact refractometric sensor based on a grated silicon photonic wire. A resolution of $10^{-5}$ in refractive index has been measured

Hoekstra, H.J.W.M.

Kauppinen, L.J.

Ridder, R.M. de

Hoekstra, Hugo

de Ridder, R.M.

University of Twente Research Information

Refractometric Sensor Based on Silicon Photonic Wires  L.J. Kauppinen, H.J.W.M. Hoekstra, and R.M. de Ridder University of Twente, MESA+ Institute for Nanotechnology,  Integrated Optical MicroSystems Group,  7500 AE Enschede, The Netherlands Email: l.j.kauppinen@ewi.utwente.nl  Summary We have characterized the refractive index sensing properties of a compact refracto-metric sensor based on a grated silicon photonic wire. A resolution of 10-5 in refractive index has been measured.  Introduction  Integrated optical (IO) refractometric sensors have particularly interesting applications in label-free biosensing and in measuring chemical compositions. A current trend in the research of optical refractive-index sensors focuses on making the sensors compact and highly sensitive [1-3]. We have investigated the refractometric sensing properties of grated silicon photonic wires (GSPW), fabricated using a 248-nm deep UV lithography process [4]. Sharp spectral phenomena of a grated waveguide exhibit strong sensitivity to changes of ambient refractive index while the compact size of the GSPW allows dense integration of several IO sensors on a chip and the use of very small sample volumes, down to the picolitre level.  A small cladding-index change Δnclad causes the first-order Bragg wavelength λB of a grating to shift according to:  ΛΔ≈Δ cladB nηλ 2 ,  (1) in which Λ is the period of the grating and η is the fraction of energy density in the cladding. Hence by detecting a wavelength shift, a change of the cladding index can be observed. To fully exploit the sharp spectral features of such a waveguide grating, we monitor the change in transmitted power at a stopband edge. As the cladding index changes, the spectral position of this edge shifts according to (1). A change in transmitted power ΔT depends on the slope of the stopband edge and on the cladding-index change according to: cladnTT Δ∂∂Λ≈Δλη2 ,  (2) Therefore, a steep stopband edge and a large energy density fraction in the cladding (η) will yield high sensitivity to cladding-index changes.  Discussion  The refractive-index sensing element of our sensor is a 400 periods long waveguide grating, see the inset of Fig. 1. With a silicon waveguide width of less than 350 nm and a grating period of 450 nm, a very compact sensor is obtained. As a demonstration of monitoring on-chip chemical reactions, we measured the refractive-index change of a solution during evaporation of isopropanol from water. Isopropanol has a slightly higher refractive index (nIPA=1.365) than water (nw=1.315) and it also evaporates much faster than water. Therefore, the refractive index of an isopropanol-water solution exposed to air will slowly decrease due to evaporation. To study this phenomenon we applied a droplet of isopropanol solution to our sensor chip and monitored the evaporation process [5]. The wavelength of the light source was fixed at the stopband edge and the transmission was recorded as a function of time, as shown in Fig 1. It can be clearly seen that the stopband edge slowly drifts through the monitoring point, corresponding to a total index change of approximately 1.5*10-3. The detection limit for sensing changes in bulk refractive index with our setup was evaluated by using water with different sugar concentrations. To eliminate evaporation, a flow cuvette system was used. From the measurements it was found that a sugar concentration down to 0.2 mg/ml can be detected. The sensor’s response on this sugar concentration is shown in Fig. 2. The concentration of 0.2 mg/ml corresponds to a refractive-index change Δnclad of 2.8*10-5. The achieved resolution of the sensor is limited by the thermal stability of our setup (0.1 oC). With an improved design, involving a reference sensor, sensitivity of the order of 10-6 is expected.  Conclusions  A compact refractometric sensor, based on silicon photonic wires, shows an index resolution of the order of 10-5. Due to the compact size of this sensor, the achieved sensitivity predicts good results for the envisaged label-free bio-sensing application using immobilized antibodies.  0 100 200 300 400 500 60000.10.20.30.40.50.60.7Time [s]Transmission [a.u.]Δn=1.5*10-3Transmission [a.u.] 0 50 100 150 200 250 300 3500.40.50.60.70.80.911.10.2mg/ml 0.2mg/mlwaterwater waterTime [s]Transmission [100 nW]Transmission [100 nW] Fig 1. Sensor response to the refractive index change due to evaporation of isopropanol from a solution in water. Inset: SEM image of the grating. Fig 2. Sensor response to alternating 0.2 mg/ml sucrose solution and pure water.  References  [1] Katrien De Vos, et al., Biosensors and Bioelectronics 24, 2528–2533 (2009) [2] Nina Skivesen, et al., Optics Express 15, 3169-3176 (2007) [3] A. Densmore, et al., Optics Letters, 33, 596-598 (2008) [4] Silicon Photonics Platform, http://www.epixfab.eu/ [5] L.J.Kauppinen, et al., Sensors & Actuators: B. Chemical 139, 194-198 (2009) 

English

Refractometric sensor based on silicon photonic wires

We have characterized the refractive index sensing properties of a compact refractometric sensor based on a grated silicon photonic wire. A resolution of 10^-5 in refractive index has been measured

NARCIS 

http://doc.utwente.nl/69507/1/Refractometric_Sensor_Based_on_Silicon_Photonic_Wires.pdf

Refractometric sensor based on silicon photonic wires

Abstract

Similar works

Full text

Available Versions

University of Twente Research Information

University of Twente Research Information

NARCIS