Aloka Sinha’s research while affiliated with Indian Institute of Technology Ropar and other places

The development of Liquid crystals (LCs) based technology is happening at a quick pace to design various switchable optical devices due to the exceptional electro-optical properties of LCs. The purity of an LC is the primary concern for these applications. Here, we propose a straightforward and effective optical method to detect the purity of an LC using surface plasmon resonance phenomena. The Kretschmann configuration is used in the proposed technique, and an LC cavity is formed over the metal layer using a glass substrate. Various impurities are added in the pure LC, which disturbs the molecular arrangement of the LC molecules, and hence, the refractive index of LC changes. We have numerically calculated and experimentally observed the shift in the resonance angle for the impure LC as compared to the pure one. The impurity in the LC is evident from the significant shift in the resonance angle. The experimentally measured sensitivity of the proposed technique is around 150⁰/RIU, which is comparable to the other Kretschmann configuration-based sensors. This sensitivity is suitable for LC material, especially for their uses in optics and photonics applications. In comparison to the existing LC purity detection method, the key advantages of the proposed method are its lightweight, compact design, label-free detection, and real-time monitoring capabilities.

In this Letter, a phase-shifting angular shearing interferometer has been proposed for the application in optical surface metrology (SM) by using a combination of a wedge-shaped liquid crystal (LC) cell and a polarization phase shifter. The demonstration of this angular shearing interferometer for step height measurement is accomplished with the help of a phase-shifting technique. Four phase-shifted interferograms produced by a geometrical phase shifter are subjected to a simplified Wiener deconvolution method, which resembles a simple analysis technique for shearing interferograms in comparison to alternative approaches. A simulation study has been conducted to validate the proposed technique. The experimental results show an accuracy of $5.56{\% }$ 5.56 % for determining the step height, which also agrees with the results obtained by atomic force microscopy. Owing to the tunability of birefringence, the proposed LC-based angular shearing interferometry technique will be useful to control spatial resolution in optical metrology.

A polarized low-coherence interferometer (PLCI) based on a liquid crystal (LC) wedge is designed and an associated demodulation method encompassing the tunability feature is proposed for tunable, standalone optical sensing. The application of an electric field to the LC material effectively decreases the birefringence value and the related dispersion relation, which in turn enhances the resolution of detection. The effect of the electric field on the demodulation of the cavity length is addressed by the successive determination of the centroid positions of the PLCI interferograms. Through a comprehensive study of numerical simulation, the effectiveness of the proposed approach is explored relative to the conventional envelope detection methodology. In order to verify this method, an experiment with Fabry-Perot based fiber optic displacement sensor is investigated using 5CB LC wedge based PLCI setup in presence of electric field. The measurement accuracy of the cavity length is found to be 0.74% of full scale, rendering it to be more precise and robust than the conventional envelope detection method.

A novel nanocomposite system has been prepared by dispersing multiferroic bismuth ferrite nanoparticles (BiFeO$_3$) in a bent-core nematic liquid crystal (8-F-OH) that exhibits cybotactic clusters. Transition temperature, optical textures, order parameter (\emph{S$_{m}$}), and dielectric spectroscopy experiments are performed in the doped system, and the results are compared with the pure one. The main experimental outcome is that the doped system has increased orientational order parameters, even though the cybotactic cluster size is reduced due to the incorporation of multiferroic BiFeO$_3$ nanoparticles. The transition temperature, as observed under polarising optical microscopy, clearly indicates a reduction of $1 - 2~ ^\circ{\rm C}$ in the doped system compared to the pure one, and we conjecture this is due to the disordering of the cybotactic cluster in the doped system. Based on the experimental findings, a Landau-de Gennes-type free energy model is developed. The model qualitatively explains the increased mean order parameter and the disordering of cybotactic clusters with increasing polarization value of nanoparticles. This is corroborated by experimental findings.

Polarization handling using an external source is highly desirable in applied optics and photonics to increase the degree of freedom of an optical system. Here we report an electrically controlled polarization beam splitter (PBS) by sandwiching the nematic liquid crystal (LC) between two equilateral prisms. The presented LC-PBS is operated in two different modes: non-splitting mode and polarization splitting mode. The externally applied voltage can switch the mode of the PBS, which makes the device active and flexible. The proposed electrically controlled PBS exhibits features such as bistability with highly stable modes, large splitting angle, wider operating range, and ease of fabrication with lower cost.

We demonstrate a Liquid Crystal (LC) core channel waveguide-based low threshold electro-optic switch by employing in-plane switching configuration and show its application for polarization selection device. The waveguide is fabricated on an indium tin oxide (ITO) coated glass substrate using the laser direct writing lithography technique. We have experimentally shown that the Freedericksz transition voltage of LC molecules is ∼ 1 V, and it relatively requires a low threshold voltage of ∼ 2 V to switch on the waveguide. The measured extinction ratio of the device is greater than 18 dB, along with a low insertion loss of ∼ 3.27 dB as compared to the conventional LC waveguides. The fabricated device is further demonstrated as a voltage-controlled polarization selector that passes either TM or TE-polarized light with a polarization extinction ratio of 28 dB and 25 dB, respectively. The proposed device exhibits the features such as low voltage control, low optical and insertion loss, high extinction ratio, and an externally controlled polarization selector, which are the key requirements in integrated photonics.