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MODELLING OF MULTILAYER DIELECTRIC FILTERS BASED ON TIO2 / SIO2 AND TIO2 / MGF2
FOR FLUORESCENCE MICROSCOPY IMAGING
M.A. Butt1, S.A. Fomchenkov 12, A. Ullah 3, M. Habib 3, R.Z. Ali 3 1 Samara National Research University, Samara, Russia
2 Image Processing Systems Institute of the RAS - Branch of the FSRC "Crystallography and Photonics " RAS, Samara, Russia,
3 Balochistan University of Information Technology, Engineering and Management Sciences (BUITEMS), Quetta, Pakistan
Abstract
We report a design for creating multilayer dielectric optical filters based on TiO2 and SiO2/MgF2 alternating layers. We have selected Titanium dioxide (TiO2) for high refractive index (2.5), Silicon dioxide (SiO2) and Magnesium fluoride (MgF2) as a low refractive index layer (1.45 and 1.37) respectively. Miniaturized visible spectrometers are useful for quick and mobile characterization of biological samples. Such devices can be fabricated by using Fabry-Perot (FP) filters consisting of two highly reflecting mirrors with a central cavity in between. Distributed Bragg Reflectors (DBRs) 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 stopband of the DBR. This range is usually much smaller than the sensitivity range of the spectrometer. Therefore, bandpass filters are required to restrict the wavelength outside the stopband of the FP DBRs. The proposed filter shows high quality with an average transmission of 97 % within the passbands and the transmission outside the passband is around 3 %. Special attention has been given to keep the thickness of the filters within the economic limits. It can be suggested that these filters are exceptionally promising for florescence imaging and narrow-band imaging endoscopy.
Keywords: Fabry-Perot filter, fluorescence microscopy, dielectric multilayers.
Citation: 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.
Acknowledgments: This work was supported by the Ministry of Education and Science of the Russian Federation and the Russian Foundation for Basic Research (grant No. 14-07-00177). The authors would also like to thank the colleagues at BUITEMS, Quetta, Pakistan for the fruitful discussions and encouragement.
Introduction
Thin film optics is well developed technology. Therefore, many devices such as passband filters, stopband filters, polarizers and reflectors are realized with the help of multilayer dielectric thin films [1], [2], [3]. These devices consist of alternating layers of high and low refractive index materials with particular thicknesses with good knowledge of their refractive index and absorption, Fig. 1. These devices work on the principle of multiple reflections between high and low index materials interface. Distributed Bragg Reflectors (DBRs) are quarter wave thick of the centre wavelength [4]. The high reflection region of a DBR is known as the DBR stopband, and can be obtained by the refractive index contrast between the constituent layers. A broad stopband can be realized by using high index contrast thin films. By inserting a half wave cavity between two highly reflecting mirrors governs an output at the desired wavelength [1]. This kind of device is known as Fabry-Perot (FP) filter. DBRs are the most commonly used mirrors in FP filters, due to their high reflectivity for specific range of wavelengths. FP has numerous applications in laser line filtering, astronomical observations, fluorescence microscope imaging and spectro-scopic instrumentation.
In two photon fluorescence microscopy imaging, a fluorophore absorbs light at its excitation wavelength, and typically emits light at a longer wavelength. The emission spectra of the biological samples has high emission peak at 511 nm which is easy to detect [5] [6]
[7].Therefore, there is a need to develop such filter which can selectively transmit the emitted wavelength and block the excitation wavelength, thus improving the contrast for both sensing and imaging these fluorophores. DBRs consist of alternating layers of high and low refractive index materials. DRBs have high reflectivity for wavelengths around a central wavelength, which is governed by the optical thickness (refractive index x physical thickness) of the constituent layers, and in four times their optical thickness. DBR layers are therefore a quarter-wave thick of the central wavelength.
__Fabry-Perot filter
I I Low index __ material High index material
Distributed Bragg Reflectors (DBRs)
a)
b)
ZDBRlZ
Central cavity
DBR2_
Fig. 1. Schematic diagram of (a) Distributed Bragg Reflectors, (b) Fabry-Perot filter
Filter design and discussion
In this work, the designs of FP filters based on TiO2 / SiO2 and TiO2 / MgF2 are proposed at a central wavelength of 511 nm for biomedical bandpass filter for fluorescence microscopy imaging, Fig. 2. We tried to de-
sign the filters with minimum number of layers and high transmission peak at central wavelength with high transmission and narrow width (FWHM). Open-source software, Open Filters, is used in this work to design and optimize the required filter, it uses transfer matrix method to analyse transmission and reflection of light from layers based on thicknesses and type of materials [8]. Design are optimized to reduce FWHM and maximum transmission for 511 by nm using needle synthesis method (It adds extra layers (called needle) to the design and each time it add a needle, transmission spectrum of filter is calculated. The optimal position of needle to be added between the layers of a filter is based on derivative of Merit Function with respect to thickness of thin layer. The position where derivative is negative needle is added. Mostly single needle is added and transmission spectrum is calculated. Addition of needle stops at the point where there is no improvement in the target transmission spectrum) [8].
A. Material Selection
Based on refractive indices and absorption coefficient of the materials, TiO2 is selected for its high refractive index at 511 nm, while SiO2 and MgF2 are selected material for its low refractive indices at 511 nm. All three materials have low absorption for 511 nm, Fig. 2.
Refractive index_
2.21.8-
1 d ------■-"■-■"--- --
400 450 500 550 600 650 700 Wavelength, nm
Extinction Coefficient, k
0.0015
0.0012-
0.0009
0.0006 -MgF2
__________ ----Si02
0.0003- ..............no2
0
400 450 500 550 600 650 700 Wavelength, nm
Fig. 2. Refractive index(a) and extinction coefficient(b) of SiO2, MgF2 and TiO2
B. Filter 1: FP _ filter design based on TiO2/MgF2
TiO2 and MgF2 are chosen as high and low refractive index materials, respectively. As mentioned in section A, the choice of materials is made on the basis of low absorption and high index contrast in the wavelengths of interest. The design was optimized to 97 % transmission at 511 nm and narrow FWHM of 2 nm. The design comprised of 19 layers with thickness of 1457 nm, Fig. 3.
The optimized thickness of each layer is shown in Table 1. The filter design comprises of two DBRs having 9
layers each and a spacer (layer no. 10) is sandwiched between them. The layers thickness is automatically optimized by using needle synthesis method.
Transmission, % 100
80
60
40
20
0
350 400 450 500 550 600 650 700
Wavelength, nm
Fig. 3. FP filter with two DBRs of TiO2 / MgF2 with 19 layers, 97 % transmission at 511 nm and 2 nm FWHM
Table 1. Layer thickness of TiO2/ MgF2 based FP filter with a passband of 511 nm
Layer no. Materials Thickness (nm)
1 TiO2 99
2 MgF2 85
3 TiO2 53
4 MgF2 92
5 TiO2 53
6 MgF2 94
7 TiO2 53
8 MgF2 96
9 TiO2 53
10 MgF2 98
11 TiO2 118
12 MgF2 61
13 TiO2 43
14 MgF2 110
15 TiO2 53
16 MgF2 98
17 TiO2 52
18 MgF2 94
19 TiO2 53
C. Filter 2:FP _ filter design based on TiO2/SiO2
TiÜ2 and SiÜ2 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, Fig. 2. The design was optimized to 96 % transmission at 511 nm and narrow FWHM of 2 nm, total layers increased to 21 and thickness of 1351 nm, Fig. 4.
Transmission, %
Fig. 4. FP filter with two DBRs of TiO2 / SiO2 with 21 layers, 96 % transmission at 511 nm and 2 nm FWHM
..............MgF2
----Si02
-Ti02
The optimized thickness of each layer is shown in Table 2. In this filter design, FP comprises of two DBRs having 11 and 9 layers each and a spacer (layer no. 12) is sandwiched between them. We obtained same results with two DBRs of 11 layers each but in order to reduce the number of layers. We have selected asymmetric DBRs; this can help in reducing the cost of fabrication. The layers thickness is automatically optimized by using needle synthesis method.
Table 2. Layer thickness of TiO2/SiO2 based FP filter with a passband of 511 nm.
Layer no. Materials Thickness (nm)
1 TiO2 54
2 SiO2 86
3 TiO2 54
4 SiO2 86
5 TiO2 54
6 SiO2 86
7 TiO2 54
8 SiO2 88
9 TiO2 57
10 SiO2 44
11 TiO2 42
12 SiO2 46
13 TiO2 39
14 SiO2 84
15 TiO2 55
16 SiO2 86
17 TiO2 54
18 SiO2 86
19 TiO2 54
20 SiO2 86
21 TiO2 54
D. Filter 3: Bandpass _filter design based on TiO2/SiO2
In order to obtain the desired specific wavelength in full visible region, a bandpass filter was designed based on DBRs. The design approach was based on (2HL)n layer configuration with central wavelength at 511 nm [9], [10], [11]. TiO2 and SiO2 are chosen as high and low refractive index materials, respectively. The design was optimized using needle synthesis methods; total thickness of bandpass after optimization is 2361 nm with 29 layers. Banspass filter has an average transmission of 95 %, minimum and maximum transmission of 94 % and 96 % within passband, respectively.
The combined effect of filter 1 (FP filter TiO2 / MgF2) with filter 3 (Bandpass filter TiO2 / SiO2) and combined effect of filter 2 (FP Filter TiO2 / SiO2) with filter 3 fulfill the requirement of fluorescence microscopic imaging and can transmit the excitation wavelength while suppressing all the wavelengths outside the stopband of the FP DBRs. The optimized layer thicknesses of bandpass filter is shown in table 3.
The transmission spectrum of the resulting filters (filter 1 + filter 3) is shown in fig. 5. It is well noted that the overall transmission spectra in the visible range of the
spectrum shows a characteristic peak at 511 nm with a maximum of 96 % and a small peak at 433 nm. FP filter and pass band filter can be fabricated on same substrate in the form of single optical element with high characteristic peak at 511 nm.
Table 3. Layer thickness of bandpass filter based on TiÖ2 / SiÜ2
Transmission, % 100] 80604020-0~-
Layer no. Materials Thickness (nm)
1 TiO2 126
2 SiO2 54
3 TiO2 46
4 SiO2 59
5 TiO2 45
6 SiO2 55
7 TiO2 36
8 SiO2 71
9 TiO2 44
10 SiO2 78
11 TiO2 129
12 SiO2 47
13 TiO2 85
14 SiO2 37
15 TiO2 141
16 SiO2 18
17 TiO2 180
18 SiO2 103
19 TiO2 85
20 SiO2 111
21 TiO2 74
22 SiO2 95
23 TiO2 77
24 SiO2 112
25 TiO2 76
26 SiO2 107
27 TiO2 74
28 SiO2 100
29 TiO2 97
Resulting spectrum of FP fitter 1 and bandpass filter
1
,-r ■, •■ , , ■ , . V^-350 400 450 500 550 600 650 700
Wavelength, nm
Fig. 5. Resulting spectrum of filter 1 and filter 3 with 3 % transmission outside passband
The transmission spectrum of the combined filters (filter 2 + filter 3) is shown in fig. 6. It can be seen that the overall transmission spectra in the visible range of the spectrum shows a characteristic peak at 511 nm with a maximum of 96 %. FP filter and pass band filter can be fabricated on same substrate in the form of single optical element with high characteristic peak at 511 nm.
Conclusions
In this work, we presented two FP filter designs based on TiO2 / SiO2 and TiO2 / MgF2 combined with two bandpass filters based on TiO2 / SiO2. These filters provide a peak transmission of 96 % and 97 % at 511 nm. By combining two DBRs with FP filter, the transmission outside the passband reduced to 3 %. The combined effect of filters fulfils the requirement of fluorescence microscopic imaging and can transmit the excitation wavelength while suppressing all the wavelengths outside the stopband of the FP DBRs.
Transmission, %_
Resulting spectrum of FP filter 2
^ and bandpass filter
...1 , 1 _
300 350 400 450 500 550 600 650 700 750
Wavelength, nm
Fig. 6. Resulting spectrum of filter 2 and filter 3 with 3 % transmission outside passband
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Authors' information
Muhammad Ali Butt (b. 1985) received a Bachelor's degree in Electrical (Telecommunication) Engineering from Comsats Institute of Information and Technology, Pakistan in year 2008. Then he left Pakistan for acquiring Master's degree from Germany. He attained Master's degree in Electrical Communication Engineering from University of Kassel (2010). He accomplished his PhD degree with Cum Laude in Material Sciences from Universitat Rovira i Virgili, Spain in year 2015. In 2013, he made a research stay at Optoelectronic research Centre (ORC), University of Southampton, England. Currently he works as a Senior Scientist at Samara National Research University, Russia. Research interests are optical waveguides, diffractive optics, and optical filters. E-mail: [email protected] .
The information about author Sergey Alexandrovich Fomchenkov you can find on page 673 of this issue.
Anayat Ullah (b. 1982) is an assistant professor at the Electronic Engineering Department of Balochistan University of Information Technology, Engineering and Management Sciences (BUITEMS), Quetta, Pakistan, where he heads the Electronic Devices Research group (RED). He holds a PhD in Engineering (magna cum laude) from the University of Kassel, Germany (2014). He attained his Master's degree in Electrical Communication Engineering from the University of Kassel, Germany (2010). Prior to this, he received his degree in Electronic Engineering from NED University of Engineering and Technology, Pakistan (2005). His research centers on optical filters and sensors, spectroscopy, nanotechnology and MEMS devices. He has extensively worked with MEMS and NEMS device fabrication and Nano Imprint Lithography. He also has vast experience in the Telecom field and has been associated with Pakistan Telecom Company Limited (2006 - 2008). His current research involves, optical sensors for food processing and bio-medical applications, interference bandpass filters and MEMS device design. E-mail: anayat. ullah@buitms. edu.pk.
Mohsin Habib (b. 1991) received a Bachelor's degree in Electronic Engineering from Baluchistan University of Information Technology, Engineering and Management Sciences (BUITEMS), Pakistan in year 2013. Then he was appointed as
Resident Engineer by Interactive Group of Companies. From Jan 2015, he is working as Lecturer of Electronic Engineering Department at BUITEMS. Currently he is also student of Master of Science (Electronic Engineering) at BUITEMS where he is working in the area optical filter design. E-mail: [email protected].
Rabia Zafar Ali (b. 1991) received a Bachelor's degree in Electronic Engineering from Baluchistan University of Information Technology, Engineering and Management Sciences (BUITEMS), Pakistan in year 2013. She was awarded ICT R&D scholarship for her undergraduate studies. Currently she is student of Master of Science (Electronic Engineering) at BUI-TEMS where she is working in the area optoelectronics. E-mail: [email protected].
Code of State Categories Scientific and Technical Information (in Russian - GRNTI)): 29.31.26. Received September 7, 2016. The final version - November 3, 2016.
