Enhancement of Goos-Hänchen Shifts in Dielectric Gratings on a Metal Surface

Venkata Jayasurya Yallapragada Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India Arvind Nagarajan Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India Ajith Padyana Ravishankar Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India Gajendra Mulay Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India Venu Gopal Achanta Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai, India

The Goos-Hänchen (GH) shift is a displacement of the centroid of a beam of finite spatial extent, when reflected at the surface of an absorbing medium, or during total internal reflection. This displacement lies in the plane of incidence, and is known to be amplified by excitation of surface modes at the reflecting interface, for example, at Surface Plasmon Resonance (SPR) [1,2]. The sensitivity of this enhanced GH shift, makes it a good candidate for sensor applications, as has been demonstrated using the Kretschmann - Raether configuration [2].

Plasmonic gratings, offer improved portability and scope for miniaturization of such sensors. Dielectric gratings on metal [3], have SPR conditions, and therefore reflection spectra which can be conveniently tailored. Here, we present a method to design such gratings and calculate the GH shift, and proceed to determine the shift experimentally.

We have simulated the reflected field amplitudes using Rigourous Coupled Wave Analysis, from which GH shift is computed using a calculation scheme outlined previously [4]. The grating resonances, can be tailored by optimizing grating parameters, for example, to obtain the resonance at a certain angle of incidence, depending on the requirement for a particular application.

We have designed a grating consisting of stripes of 120 nm thick Poly Methyl Methacrylate (PMMA) on an opticaly thick Gold film on Silicon, which exhibits SPR at an angle of incidence of about 11.5 degrees, using p-polarized light of wavelength 785 nm. Close to this point, a large GH shift is expected. Experiments have been performed using light from a 785 nm diode laser, and a quadrant photodiode for position measurements. These measurements show the presence of a large relative GH shift difference between s and p polarization of 45 micrometres, as is expected from calculations. Such gratings can be used for sensors based on the GH shift.

[1] X. Yin, and L. Hesselink, Applied Physics Letters 89, 261108 (2006).

[2] C. Bonnet et al., Optics Letters 26, 666 (2001).

[3] J. Yoon, et al., Journal of Applied Physics 94, 123 (2003).

[4] VJ Yallapragada, AV Gopal, and GS Agarwal, Optics Express 21, 10878 (2013).

jayasurya@tifr.res.in