Multilayer dielectric stack Notch filter for 450-700 nm wavelength spectrum

Abstract

In this work, a multilayer dielectric optical notch filters design is proposed based on TiO2 and SiO2 alternating layers. Titanium dioxide (TiO2) is selected for its high refractive index value (2.5) and Silicon dioxide (SiO2) as a low refractive index layer (1.45). These filters are conventionally envisioned for overpowering of powerful laser beams in research experiments, to obtain good signal-to-noise ratios in Raman laser spectroscopy. It is precarious that light from the pump laser should be blocked. This is attained by inserting a notch filter in the detection channel of the setup. In addition to spectroscopy, notch filters are also useful in laser-based florescence instrumentation and biomedical laser systems. The designed filter shows a high quality with an average transmission of more than 90% in 450-535 and 587-700 nm bandwidths. And a stop band region between 536-586 nm shows a transmission of 3% only with an optical density of greater than 3, which makes it a promising element to be used as a notch filter.

Keywords

• Notch filter
• Optical density
• Distributed Bragg Reflector (DBR)
• visible spectrum

1. Introduction

In this work, the design of a Notch filter based on TiO2/SiO2 is proposed at a central wavelength of 561 nm with an FWHM of 50 nm. Titanium dioxide (TiO2) is selected for its high refractive index value (2.5)[ 5 ] and Silicon dioxide (SiO2) as a low refractive index layer (1.45)[ 5 ]. TiO2 is a vital dielectric material with a wide band-gap energy and high refractive index that can make it useful in the fabrication of multilayer thin films due to its high optical properties. For instance, its high transmittance and high refractive index in the visible region (380-760 nm) make it valuable to be employed in the production of the optical filter and window glazing [ 6, 7 ].

In the designing of optical filters, the behaviour of the entire multilayer system is anticipated on the basis of the properties of the individual layers in the stack [ 8 ]. Hence to attain the optimum performance, it is important to optically characterize and accurately determine the thickness of the individual layers. We designed this filter with a less possible number of layers with high transmission in pass band region and high reflection is obtained in the stop band. Open-source software, Open Filters, is

Computer Optics and Nanophotonics / M.A. Butt, S.A. Fomchenkov, S.N. Khonina between multiple reflected waves [ 10, 11 ]. Optical density (OD) is used to see the blocking specification of a filter and is associated with the amount of energy transmitted through it. It uses a logarithmic scale to describe the transmission of light through a highly blocked optical filter, particularly useful when the transmission is extremely small. A high optical density value indicates very low transmission of light and low optical density indicates high transmission. For instance, OD=1 relates to a transmittance value of 0.1, and OD =8 corresponds to a transmittance value of 10-8. It can be expressed as [ 12 ]: (

) = 10− 100 100 = − log

… … … … … … … … … … … … … . absorption of the filter.

3. Filter design and discussion

For the filters having OD ≥ 3 the effects of multiple reflections are insignificant because of the low reflectance and strong Multilayer thin films have an extensive wavelength tunability which gives an optical response that is desired for a specific application. Distributed Bragg Reflectors (DBRs)[ 13,14 ] consisting of alternating high and low refractive index material pairs are the most commonly used mirrors in FP filters, due to their high reflectivity. However, DBRs have high reflectivity for a selected range of wavelengths known as the stop band of the DBR. Its reflectance usually depends on the constructive or destructive interference of light reflected at consecutive boundaries of different layers of the stack. The performance of the multilayer devices highly depends on the interface formed between the alternating layers. Therefore an appropriate sequencing of the layers of suitable dielectric materials and their thicknesses is critical for achieving the desired spectral response and application. Therefore, it is important to optimize the coating conditions in the designing process [ 15, 16 ]. In our previous work, we proposed multilayer dielectric filter based on TiO2 and SiO2 materials because of their excellent optical properties [ 17 ]. Therefore, TiO2 and SiO2 are chosen as high and low refractive index materials, respectively. The choice of materials is made on the basis of low absorption and high index contrast in the wavelengths of interest. The notch filter is designed for visible spectrum ranges from 450-700 nm with FWHM of 50 nm. The optimized thickness of the layers is shown in table 1. The total

The designed filter has maximum transmission of 3% in the stop band. The OD of the filter is calculated by using an eq. (1) which provides a value greater than 3.5 (Transmission is 0.0003%). It shows a promising result for the notch filter. The optical density of the notch filter is plotted in figure 3.

100 90 80 ) 0 450 5 ty4 i s n e d l a ic3 t p O

Wavelength (nm)

Computer Optics and Nanophotonics / M.A. Butt, S.A. Fomchenkov, S.N. Khonina 4. Effect of the angle of incidence of light on the central wavelength and FWHM

In all dielectric stack filters, the transmission depends on the angle of incidence. The central wavelength of the blocking region shifts to shorter wavelengths and FWHM increases as the angle of incidence is increased. It can be seen from figure 2, when the angle of incidence of light increases, a noticeable increase in the FWHM of the bandwidth of stop band is seen which shifts towards smaller wavelength. And an increase in the OD is also noticed which is around 3.9 with a slight decrease in the transmission of the band-pass region. Table 2 summarizes the effect of the incidence angle of light on the filters FWHM and central wavelength.

Angle of Incidence (Degrees) 0 30

Central wavelength (nm) 561 542

5. Conclusion

Acknowledgements References

In this work, a multilayer dielectric optical notch filter design is presented which is based on TiO2/SiO2 alternating layers. These filters provide an average transmission of more than 90% in region 450-535nm and 587-700 nm. The transmission of the stop band 536-586 nm is around 3%. The OD of this filter is greater than 3.5 which shows the high blocking specification of a filter and is associated with the amount of energy transmitted through it. With an increase in the incident angle of light, the central wavelength of the notch filter shifts toward smaller wavelength.

This work was supported by the Ministry of Education and Science of the Russian Federation and the Russian Foundation for Basic Research (grant No. 16-29-11698-ofi_m, 16-29-11744-ofi_m).

References

[1] Macloed HA . Thin film optical filters . McGraw-Hill , 1989 .

[2] Kazanskiy NL , Serafimovich PG , Popov SB , Khonina SN . Using guided-mode resonance to design nano-optical spectral transmission filters . Computer Optics 2010 ; 34 ( 2 ): 162 - 168 .

[3] Kazanskiy NL , Kharitonov SI , Khonina SN , Volotovskiy SG , Strelkov YuS . Simulation of hyperspectrometer on spectral linear variable filters . Computer Optics 2014 ; 38 ( 2 ): 256 - 270 .

[4] Kazanskiy NL , Kharitonov SI , Khonina SN , Volotovskiy SG . Simulation of spectral filters used in hyperspectrometer by decomposition on vector Bessel modes , Proc. of SPIE 2015 ; 9533 : 95330L - 7 pp.

[5] Weber MJ . Handbook of Optical Materials . CRC Press: Boca, Raton, London, New York, Washington, 2003 .

[6] Hasan MM , Malek ABM , Haseeb ASMA , Masjuki HH . Investigations on TiO2 and Ag based single and multilayer films for window glazings . ARPN Journals of Engineering and Applied Sciences 2010 ; 5 : 22 - 28 .

[7] Butt MA , Fomchenkov SA . Thermal effect on the optical and morphological properties of TiO2 thin films obtained by annealing a Ti metal layer . J. Korean Phys. Soc . 2017 ; 70 ( 2 ): 169 - 172 .

[8] Hinczeweski DS , Hinczeweski M , Tepehen FZ , Tepehen GG . Optical filters from SiO2 and TiO2 multi-layers using sol-gel spin coating method . Solar Energy Materials and Solar Cells 2005 ; 87 ( 1-4 ): 181 - 196 .

[9] Larouche S , Martinu L. Optical filters: Open-source software for the design, optimization, and synthesis of optical filter . Appl. Opt . 2008 ; 47 ( 13 ): C219 - C230 .

[10] Eckerle KL , Hsia JJ , Mienlenz KD , Weidner VR . Regular spectral transmittance . NBS special Publication 1987 ; 250 ( 6 ): 1 - 59 .

[11] Zhang ZM . Optical properties of layered structures for partially coherent radiation . Heat Transfer.Proceedings of the Tenth International Heat Transfer Conference. G.F. Hewitt 1994 ; 2 : 177 - 182 .

[12] Zhang ZM , Gentile TR , Migdall AL , Datla RU . Transmittance measurements for filters of optical density between one and ten . Appl. Opt . 1997 ; 36 ( 34 ): 8889 - 8895 .

[13] Butt MA , Fomchenkov SA , Ullah A , Verma P , Khonina SN . Biomedical bandpass filter for fluorescence microscopy imaging based on TiO2/SiO2 and TiO2/MgF2 dielectric multilayers . J. Physics. Conf. Series 2016 ; 741 : 012136 .

[14] Ullah A , Butt MA , Fomchenkov SA , Khonina SN . Indium phosphide all air-gap Fabry-Perot filters for near Infrared spectroscopic applications . J. Physics. Conf. Series 2016 ; 741 : 012135 .

[15] Kheraj VA , Panchal CJ , Desai MS , Potbhare V . Simulation of reflectivity spectrum for non-absorbing multilayer optical thin films . Pramana-Journal of Physics 2009 ; 72 ( 6 ): 1011 - 1022 .

[16] Richter F , Kupfer H , Schlott P , Gessner T , Kaufmann C. Optical properties and mechanical stress in SiO2/Nb2O5 multilayers . Thin Solid Films 2001 ; 389 ( 1- 2 ): 278 - 283 .

[17] Butt MA , Fomchenkov SA , Ullah A , Habib M , Ali RZ . Modelling of multilayer dielectric filters based on TiO2/SiO2 and TiO2/MgF2 for fluorescence microscopy imaging . Computer Optics 2016 ; 40 ( 5 ): 674 - 678 . DOI: 10 .18287/ 2412 -6179-2016-40-5- 674 -678.