1. Design of Alternately Stacked ZnS/MgF2 and CdS/MgF2 Ultra-Thin Multilayer Optical Filters
Vemuri SRS Praveen Kumara,b, Parinam Sunitaa,b, Mukesh Kumara,b, Parinam Krishna Raoa, Neelam Kumaria,b , Vinod Karara,b and Amit L. Sharmaa,b
a CSIR-Central Scientific Instruments Organisation, Chandigarh, India,
b Academy of Scientific & Innovative Research (AcSIR), CSIR-CSIO, Chandigarh
ABSTRACT
RESULTS AND DISCUSSION
Thin film optical filters selectively transmit light in a particular range of wavelengths and block other region. In the present work, multilayer thin film interference filters are designed using two different material combinations ZnS (RI 2.35) with MgF2 (RI 1.38) and CdS (RI 2.529) with MgF2 for the wavelength of 550 nm to check whether one combination can be used as an alternate to other without compromising the transmission or reflection performance.
A total of three interference filters are designed – one 13 layers micro cavity filter, 7 layers and 13 layers interference filters using alternate stack of high and low refractive index materials.
Quarter wave optical thickness (λ/4) of individual layers were used for all the designs.
All the simulations are done taking substrate as BK7 with refractive index (1.52).
FILMSTARTM software is used to simulate the transmittance and reflectance performance.
The average reflectance for 7 and 13 layers for both combinations is ~87 % and ~97% (over the wavelength range nm) respectively and average transmittance for 13 layers micro-cavity filter is ~83% in both cases.
Index (n)
Index (n)
Layer Number
Layer Number
Index (n)
Layer Number
Figure 1 Refractive index profile for ZnS/ MgF2 (a) 13 layers micro cavity (b) 7 layers, and (c) 13 layers with quarter wave stack of alternating high and low indices
%Transmittance
%Reflectance
Wavelength (λ nm)
Wavelength (λ nm)
INTRODUCTION
%Reflectance
Interference phenomenon is being used extensively in ultra-thin multilayer optical filters for their extensive applications in the area of optics, avionics, helmet mounted displays, sensors, fibre optics and space applications.
Conventional discrete layer thin film filters consist of alternating quarter wave stacks having a constant refractive index contrast. Due to this, the filter performance outside the stop band suffers with high number of ripples or higher order harmonics.
The typical interference filters for designing narrow band filters are designed by the principle of Fabry-Perot cavity filter. Such filters usually consist of two parallel distributed Bragg reflectors separated by a spacer layer, whose refractive index will be same or different from the one used for the Bragg reflectors.
The spacer layer usually has optical thickness of half wavelength and other layers are designed with quarter wave optical thickness. Such filters transmit light at the wavelength for which the spacer layer thickness will be half wave. Other wavelengths are reflected thereby producing the narrow band filter.
Wavelength (λ nm)
Figure 2 (a) Transmittance spectra of 13 layers micro cavity filter (b) Reflectance spectra of 7 layers and (c) Reflectance spectra of 13 layers for normal angle of incidence
Index (n)
Index (n)
Layer Number
Layer Number
Index (n)
Layer Number
Figure 3 Refractive index profile for CdS/ MgF2 (a) 13 layers micro cavity (b) 7 layers, and (c) 13 layers with quarter wave stacks of alternating high and low indices
DESIGN METHODOLOGY
%Transmittance
The design specification and methodology of the alternately stacked multi layer optical filters for mirror and notch filters are shown below:
 Design of multilayer interference filter:
Glass / (HL)3 H / air  7 Layer design
Glass / (HL)6 H / air  13 Layer design
Design of narrow band filter:
Glass / (HL)3 2L (LH)3 / air  13 Layer micro-cavity design
where H represents high refractive index material and L represents low refractive index material
%Reflectance
Wavelength (λ nm)
Wavelength (λ nm)
%Reflectance
Wavelength (λ nm)
Figure 4 (a) Transmittance spectra of 13 layer micro cavity filter (b) Reflectance spectra of 7 layer and (c) 13 layer for normal angle of incidence
Table 1: Filter design and performance parameters
Material Combination
ZnS/MgF2
CdS/MgF2
Parameters
No. of Layers
Type of Filter
%R/%T
FWHM
(nm)
S.No. 1 7
Reflective
95.52 (%R)
~345
97.52 (%R)
~362 2 13
99 (%R)
~229
~256 3
Micro-cavity
95.72 (%T) ~8
(%T) ~5
CONCLUSION
The peak transmittance of 13 layer micro-cavity filter design for ZnS/MgF2 and CdS/MgF2 are % and respectively which are almost same but FWHM of CdS/MgF2 (~5) is better than the ZnS/MgF2 (~8).
7 and 13 layers reflective filter designs of alternately stacked multilayers of ZnS/MgF2 and CdS/MgF2 achieved an average reflectance of 97.52% and 99% respectively at central wavelength of 550 nm within the visible spectrum.
These filters can be used for applications requiring high reflection mirrors and polarized interference filters and it is suggested that CdS can be used as an alternate to ZnS for the same.
ACKNOWLEDGEMENTS
Funding from CSIR through 12th FYP project grant OMEGA/PSC0202 is gratefully acknowledged. Authors are thankful to The Director, CSIO Chandigarh for his constant support and cooperation
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