Nanofabricated refractive index sensor based on photon tunneling in nano¯uidic channel Jun Kameoka * , H.G. Craighead School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA Received 20 October 2000; received in revised form 15 February 2001; accepted 20 February 2001 Abstract We have fabricated and tested a refractive index sensor based on photon tunneling in a nano¯uidic system. The device comprises an extremely thin ¯uid chamber formed between two optically transparent layers. It can be used to detect changes in refractive index due to chemical composition changes of a ¯uid in the small test volume. Because the physical property measured is a refractive index change, no stainingorlabelingisrequired.Wetestedthedevicewith®vesamples,waterandwaterwith1%ethanol,2%ethanol,5%ethanoland10% ethanol. The sensing was done by measuring the intensity of a re¯ected laser beam incident on the sensing element at around the critical angle. The measured response agrees well with the calculated re¯ectance. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Photon tunneling; Nanofabrication; Nano¯uidics; Refractive index sensor 1. Introduction There are several sensors that are sensitive to refractive index changes, such as surface plasmon resonance systems [1]. In this paper, we describe a device based on photon tunnelinginnanofabricatedchannelthatrequiresonlysmall amount of analyte and no gold layer. The photon-tunneling device con®guration is shown in Fig.1.Therearetwotransparentglasslayersseparatedbya smallgap,withthegapthicknesscomparabletoorlessthan thewavelengthoflaserlight.Thegapis®lledwiththeliquid tobeanalyzed.Collimatedlaserlightilluminatesthedevice from one side. The re¯ection and transmission of light through the system is determined by the refractive indices oftheglassandanalyte,thegapthickness,thewavelengthof light and the angle of incidence of the light. If the incident angleoflaserislargerthanthecriticalanglefortotalinternal re¯ection, calculated for bulk ¯uid, transmission of light throughthesystemcanonlytakeplacebyphotontunneling. This effect is also known as frustrated total internal re¯ec- tion. The photon tunneling probability, in the case of angle of incidence beyond the critical angle, depends strongly on the refractive index of material in the nano¯uidic channel. Therefore, by monitoring the re¯ection or transmission of lightthroughthesystem,therefractiveindexofthematerial in the nano¯uidic channel can be determined. Photon tun- neling technology has been used for several sensing pur- poses, such as measuring pressure [2], measuring liquid level [3], and for photon tunneling microscopy [4,5]. Thecalculationfortheinternalre¯ectionintensitypro®le as the function of incident angle is straightforward from standard optical formula. According to Airry's formula [6], the expression for total re¯ection coef®cients of three-layer system in Fig. 1 can be written as r  r 12  r 23 e 2if 1  r 12 r 23 e 2if 1) In this equation, f is the phase shift due to the internal re¯ection. It can be written as f  2p l d  n 2 1 sin 2 y 1  n 2 2 q 2) Inthisequation, d isthethicknessofnano¯uidicchannel, l the wavelength of light in vacuum. In Eq. 1), r 12 is the re¯ection coef®cient between the ®rst glass layer and the nano¯uidic channel layer and r 23 the re¯ection coef®cient between the second glass layer and the nano¯uidic channel layer. According to Fresnel's formula [7], the re¯ective coef®cients can be written for plane-polarized p-polarized) light wave as r 12  n 2 cos y 1  n 1 cos y 2 n 2 cos y 1  n 1 cos y 2 3) Sensors and Actuators B 77 2001) 632±637 * Corresponding author. E-mail address: jk84@cornell.edu J. Kameoka). 0925-4005/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0925-400501)00769-9