Optical coating devices
We actively do research to develop various precision multilayer optical coating devices such as wideband high reflection mirror, broadband antireflection coating, beam combiner, beam splitter, UV narrow bandpass filter, etc., which are required for different departmental projects.
Above mentioned optical coating devices are used in many applications involving the use of high power lasers. Therefore, it is necessary to know the laser induced damage threshold (LIDT) value of a coating before its use. For that purpose, LIDT measurement facility has been set-up, which is being upgraded now to measure non-linear optical properties of thin film coatings.
Developing computational codes
The design and development of optical coating mirrors or filters require the exact values of optical constants such as refractive index and extinction-coefficient as well as thickness of the thin film layers. In that context, we have made computational programs using Python, MATLAB, and SCILAB to determine the optical constants and thickness of thin film layers from the measured transmission/ reflection spectrum.
We have also made program to calculate spectral as well as angular dependent reflection, transmission and absorption in multilayer coatings. It can also estimate the effect of surface or interface roughness on the reflectivity of the multilayer coatings. We can also retrieve thickness and refractive index of individual layers in a multilayer structure by reverse fitting the measured optical spectrum (Reflection/Transmission) of the multilayer.
Tuning thin film properties
In many cases, one needs refractive index values which do not exist in conventional coating materials, but can be obtained by varying the (i) deposition process parameters, (ii) mixing two materials of contrasting refractive indices, and (iii) changing the deposition configurations from normal to oblique angle deposition. We have performed all the possible methods to achieve the desired refractive index values.
The mixed oxide HfO2-SiO2 thin films with varying silica fraction are prepared by vapor phase mixing of HfO2 and SiO2 materials, which gives a wide range of refractive index values from 1.46 to 1.96. The LIDT of the mixed oxide films is improved from 18 J/cm2 to 45 J/cm2 with increasing silica fraction. It is found that the refractive index values of HfO2 thin films can be tuned from 1.32 to 1.96 by varying angle of deposition known as oblique angle deposition technique.
Surface and interface roughness in multilayer coatings significantly affects the transmission/reflection of the coating devices, which ultimately affects their performances. Therefore, we have played with various process parameters to minimize surface roughness of thin film coatings. We have obtained a surface roughness of 1.1 nm for the EB evaporated HfO2 thin film.
Mechanical stability of multilayer coating devices depends on residual stress and elastic modulus of individual layers. We have utilized AFAM and substrate curvature method to measure elastic modulus/Indentation modulus and residual stress of thin films, respectively in order to optimize these properties by optimizing oxygen partial pressure, substrate temperature, angle of deposition etc. during deposition.
Multilayer coating mirrors and filters
Significant research is being carried out to improve the LIDT as well as spectral stability of multilayer mirrors. After optimizing deposition process parameters for high LIDT multilayer coating, the LIDT of the coating is further improved by post-deposition annealing for 2 hours in ambient condition at different temperature. It is observed that HfO2/SiO2 multilayer high reflection mirror shows higher LIDT value of 77 J/cm2 at annealing temperature of 500oC.
Omnidirectional mirror reflects both polarizations of light at all incidence angles over at least some spectral band width. Effort has been made to design and develop omnidirectional multilayer mirror in the visible region with wider bandwidth. To the best of our knowledge, the observed omnidirectional band width (Δλ= 76 nm) of the fabricated TiO2/SiO2 multilayer device is found much wider than that reported in literature for periodic multilayer.
Tunable photonic crystals
Tunable photonic crystals (PCs) in the THz region have received considerable attention in recent years, in which the PBG and the defect mode of the PCs are tuned by the external parameters such as electric field, magnetic field, temperature, and pressure in order to explore novel THz photonic devices. We have recently proposed one dimensional photonic crystal (1DPC) systems based on polymers, and semiconductors which can be tuned by pressure, and temperature, respectively.
We have utilized the elasto-optic behaviour of the polymer materials for tuning properties of the 1DPCs. The PMMA/SiO2 1DPC can be used as a switchable mirror, while the PMMA/PS 1DPC filter exhibits higher pressure sensitivity making it suitable for pressure sensor application. We have also demonstrated the feasibility of using moderately doped silicon (m-Si) semiconductor for developing 1DPCs that can have thermally tunable omnidirectional PBG and defect mode in the THz region in the low temperate range from 40-100K, which can be used to develop future THz mirrors and filters, and thermal sensors.