Optics & Laser Technology 170 (2024) 110183 Available online 17 October 2023 0030-3992/© 2023 Elsevier Ltd. All rights reserved. Full length article Photonic spin Hall effect-based ultra-sensitive refractive index sensor for haemoglobin sensing applications Lokesh Ahlawat a , Kamal Kishor a , Ravindra Kumar Sinha a, b, * a TIFAC-Centre for Relevance and Excellence in Fiber Optics and Optical Communication, Department of Applied Physics, Delhi Technological University, Delhi, India b Gautam Buddha University, Greater Noida, Gautam Budh Nagar, Uttar Pradesh 201312, India A R T I C L E INFO Keywords: Photonic spin Hall effect Refractive index sensor Surface Plasmon Resonance Haemoglobin Spin-orbit coupling ABSTRACT A highly sensitive Photonic Spin Hall Effect (PSHE)-based layered sensor is proposed which detects changes in refractive index of biological sample. A simple and elegant design measures haemoglobin concentrations using Surface Plasmon Resonance (SPR) effect. It consists of a BK7 prism, a thin gold layer, an analyte layer of hae- moglobin and BK7 glass as the substrate. The average angle sensitivity (S R ) has been calculated and found to be 191 ◦ /RIU, 208 ◦ /RIU and 171 ◦ /RIU respectively for low, normal and high haemoglobin concentrations. The Figure of Merit (FOM) for the proposed structure is found to be 896RIU 1 , 812RIU 1 and 3166RIU 1 respectively for low, normal and high concentrations of oxygenated haemoglobin. 1. Introduction Researchers have always shown a keen interest in spin-dependent splitting (SDS) ever since the discovery of the space-quantization of electrons through the Stern-Gerlach experiment [1]. The PSHE having close resemblance to Spin Hall Effect (SHE) in condensed matter physics [2], manifests itself as the splitting of Right Hand Circularly Polarized (RHCP) and Left Hand Circularly Polarized (LHCP) light perpendicular to the plane of incidence. This phenomenon occurs when a linearly polarized (TE or TM) Gaussian beam interacts with an optical interface. The SDS arises in both refected and transmitted light waves. It arises due to the accumulation of phase in RHCP and LHCP components of refected/transmitted light wave. The phase acquired by the compo- nents of refected/transmitted light is an outcome of light-matter interaction. This phase is referred to as Berry Phase [3]. It has the same magnitude for RHCP and LHCP but opposite in nature (∓δ + r ). Since, phase is responsible for controlling the magnitude and direction in which the light wave propagates, as a result, RHCP and LHCP which have opposite phases, splits up on the optical interface [4]. The gap created between RHCP and LHCP can be enhanced using Wave Guiding Surface Plasmon Resonance (WG-SPR) [4] and Long Range Surface Plasmon Resonance (LRSPR) [5] and this enhanced gap is referred to as Giant Photonic spin Hall effect (G-PSHE). The PSHE can be manipulated by controlling the incident beam polarization [6], beam waist [5], refractive index of interface [7], doping [8], angle of incidence and phase [9]. The spin–orbit coupling of light plays a pivotal role in the accumulation of opposite phases in RHCP and LHCP. Evidence of spin–orbit coupling of light in PSHE is seen in the transversality condi- tion of light, which couples the direction of propagation ( k → ) and spin polarisation of light ( σ → ) [3]. Another evidence is encountered in the case of Berry phase (ϕ B ), which is a manifestation of Coriolis effect in a non-inertial frame of reference attached to the beam. It couples up the total angular momentum ( j → ) of the individual wavevector and angular velocity ( Ω → ) with which frame of reference is rotated in laboratory frame. The laboratory frame is rotated in order to incorporate non- collinear wavevectors present in the gaussian beam [10]. Further, an interesting manifestation of PSHE based spin–orbit coupling is also seen in the coupling between spin (σ) and orbital (l) angular momentum of light, which are interconvertible due to the conservation of the total angular momentum of light [11] (see Table 1). The frst theoretical phenomenon similar to PSHE has been discussed by Fedorov in 1955 [12], and this was further experimentally realized by Imbert in 1972 [13]. After the discovery of Berry phase in 1984 [14] and light’s orbital angle momentum by L. Allen in 1992 [15], the fundamental concept of spin–orbit interaction (SOI) of light behind the PSHE was established in 2004 by Onada et al. [16] and in 2006 by Bliokh and Bliokh [17]. By adopting the quantum weak measurement * Corresponding author at: TIFAC-Centre for Relevance and Excellence in Fiber Optics and Optical Communication, Department of Applied Physics, Delhi Technological University, Delhi, India. E-mail address: dr_rk_sinha@yahoo.com (R.K. Sinha). Contents lists available at ScienceDirect Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec https://doi.org/10.1016/j.optlastec.2023.110183 Received 16 June 2023; Received in revised form 25 September 2023; Accepted 5 October 2023