Effect of the Materials’ Properties in the Design of High

Transmittance and Low FWHM SiO

/TiO

Thin Film Optical Filters

for Integration in a Malaria Diagnostics Device

Mariana S. Costa

, Vitória Baptista

1,2,3 b

, Graça Minas

, Maria I. Veiga

2,3 d

and Susana O. Catarino

Microelectromechanical Systems Research Unit (CMEMS-UMinho), School of Engineering, University of Minho,

Campus de Azurém, Guimarães, Portugal

Life and Health Sciences Research Institute (ICVS), School of Medicine, University of Minho,

Campus de Gualtar, Braga, Portugal

ICVS/3B’s – PT Government Associate Laboratory, Braga/ Guimarães, Portugal

Keywords: Malaria Diagnostics Device, Optical Filters, Optical Reflectance, Refractive Index, TFCalc.

Abstract: Malaria is an infectious disease, highly prevalent in world regions with lacking healthcare conditions.

Nowadays, malaria diagnostic methods in these endemic regions are mainly based on microscopy and rapid

diagnostic tests by immunochromatographic assays. Here, it is presented an optical diagnostic method, based

on reflectance spectrophotometry, through hemozoin (Hz) quantification, towards an innovative non-invasive

malaria diagnostic device. Therefore, a set of optical filters, with high transmittance and low full width at half

maximum (FWHM) at specific wavelengths, is designed for being integrated in the device. These allow the

full reconstruction of the optical reflectance spectrum, able to distinguish between healthy and infected

samples, with a detection limit up to 12.5 parasites/μl of red blood cells. This work presents the design,

performance simulation, and optimization of 16 highly selective narrow band-pass optical filters, based on

multilayer stacks of SiO

/TiO

thin films. The optical properties of the thin films layer materials, in particular

the refractive indexes, are the main focus in this study. Three different reflective indexes were evaluated and

the results showed that, for all the simulated conditions, each filter is sensitive to a single wavelength with a

FWHM < 25 nm and peak transmittance intensity > 90%, but slight variations were observed for the different

refractive indexes. The simulation results proved that these 16 optical filters designs are extremely sensitive

to the material properties, although they are the best option regarding the required optical response, assuring

feasibility and being adequate for the fabrication process.

1 INTRODUCTION

Malaria is a life-threatening and parasitic infectious

disease, with a worldwide impact. This disease is a

leading cause of death in many malaria-endemic

regions such as Western Pacific, South, and Central

America, sub-Saharan Africa, South East Asia, and

the Eastern Mediterranean. Simultaneously, malaria

imported infections, in non-endemic areas amongst

returning travellers from endemic regions, are

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https://orcid.org/0000-0002-8962-0710

significantly increasing. In 2018, there were still 87

countries and regions with ongoing malaria

transmission, and malaria resulted in an estimated

228 million cases and 405 000 deaths (Mer et al.,

2020).

Nowadays, there are available many methods for

malaria diagnosis, which are based on the detection

of the parasites in the blood. Examples of these

methods are clinical diagnosis, optical microscopy,

molecular diagnosis by polymerase chain reaction

Costa, M., Baptista, V., Minas, G., Veiga, M. and Catarino, S.

Effect of the Materials’ Properties in the Design of High Transmittance and Low FWHM SiO2/TiO2 Thin Film Optical Filters for Integration in a Malaria Diagnostics Device.

DOI: 10.5220/0010193900210031

In Proceedings of the 14th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2021) - Volume 1: BIODEVICES, pages 21-31

ISBN: 978-989-758-490-9

(PCR) or loop-mediated isothermal amplification

(LAMP) and, more recently, several commercially

available rapid diagnostic tests (RDT). Clinical

diagnosis is the traditional method, based on the

analysis of the patients’ symptoms, which is

performed by a medical specialist. Therefore, the risk

associated with the subjectivity of this method may

lead to misdiagnosis and wrong treatment (Orish et

al., 2016). Optical microscopy allows the

quantification and distinction of species but requires

laboratorial equipment and qualified technicians,

which can lead to a subjective interpretation of the

results. This method is inexpensive yet difficult to

implement in remote endemic regions, allowing for a

detection limit between 50 to 100 parasites/µl of red

blood cells (Kasetsirikul et al., 2016). RDT have

comparable sensitivity and can be used in remote

locations. However, RDT are expensive and do not

allow the quantification of parasites, only allowing

their identification. The detection limit of this method

is 100 parasites/µl of red blood cells (Varo et al.,

2020). In terms of sensitivity and specificity, the best

method is the molecular diagnosis using PCR or

LAMP, with a detection limits of 1 to 5 parasites/µl

and ≤ 1 parasites/µl of red blood cells, respectively.

Nevertheless, these techniques need to be performed

in equipped laboratories. Thus, they are not used as

routine diagnosis on the field, being applied only for

research purposes (Gitta et al., 2020). Since all these

methods require the collection of blood samples and

disposable reagents and/or consumables (Silva et al.,

2017), new portable and low-cost diagnosis methods

and devices have been developed, aiming for

innovative solutions with no need for blood samples

and, therefore, non-invasive.

The malaria disease is caused by Plasmodium

parasites that are transmitted to humans via the bite of

infected female Anopheles mosquito vectors. If the

disease is not treated timely, progression to severe

disease with organ dysfunction and death may occur

(Krampa et al., 2020). The presence of malaria

parasites on human blood leads to a set of

morphological and biochemical alterations on the red

blood cells (RBCs). One of the main phenomena is

the degradation of haemoglobin (Hb), which is an

essential nutrient for Plasmodium metabolism during

its intracellular development. Hb degradation leads to

the release of the toxic heme group. Then, the parasite

detoxicates and produces crystal particles of heme,

called hemozoin (Hz). Hz, being the final product of

the Hb catabolism, accumulates in the RBCs as the

infection proliferates, while the Hb concentration

decreases. Additionally, human healthy blood does

not have any Hz and its concentration increases as the

disease advances.

Since Hz and Hb have different characteristic

optical spectra, mainly in the visible range, with

different absorbance and reflectance peaks, and those

spectra are modified according to the Hb and Hz

concentrations in blood (related to the presence or

absence of malaria infection), it is possible to identify

the presence of malaria parasites through optical

reflectance spectrophotometry as well as predict the

infection stage (Baptista et al., 2020; Catarino et al.,

2020; Silva et al., 2017).

Therefore, optical spectrophotometry, based on

absorbance or reflectance, has been arising as an

alternative solution for the improvement of the

existing malaria diagnostic methods and devices.

(Wong Kee Song, 2005). The research team proposes

the implementation of a non-invasive optical device

based on optical reflectance for the identification and

quantification of malaria parasites, aiming for a

detection limit up to 12.5 parasites/µl of red blood

cells. The device must contain optical band-pass

filters, specific for selected wavelengths, with high

transmittance and low full width at half maximum

(FWHM), to allow the reconstruction of the optical

reflectance spectra of the samples from a set of

discrete reflectance values. The optical device will

include a white light source, that emits light directed

to the sample, since the reflected sample’s spectrum

contains the specificity of the sample composition.

This reflected light reaches an array of photodiodes,

which converts the reflected light into electric

currents. The designed optical filters (16 filters, as

will be following detailed) will be responsible for

selecting and filtering the light at different

wavelengths, helping to reconstruct the spectrum of

each sample. Also, these filters must be multilayer

thin films, able to be deposited, during fabrication, on

top of a 16 silicon photodiodes’ array, included in the

optical device. Since it is well-known that thin-film

optical filters are highly sensitive to the properties of

their materials, as well as their thickness, the main

motivation for this work is to study the spectral

response of different dielectric materials, with

different optical properties, aiming for their full

characterization before fabrication. Therefore, in this

work, the spectral response of 16 thin-film band-pass

optical filters will be simulated, considering different

refractive indexes (from different databases), and

their performance will be assessed and discussed.

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

2 DESIGN OF THIN-FILM

OPTICAL FILTERS

Figure 1 presents the original continuous reflectance

spectra of healthy RBCs and parasite infected RBCs,

at different parasitaemia, measured with a

spectrophotometric top-bench setup, comprised by a

200 W Quartz Tungsten Halogen light source (model

66881, Oriel Newport), optical-fibre probes, a cuvette

sample holder and an AvaSpec-ULS2048XL EVO

spectrometer (Avantes). Data were collected using

the AvaSoft 8.11 software. Barium sulphate was used

as a reference for reflectance measurements. The

plasmodium falciparum samples were cultured at Life

and Health Sciences Research Institute from

University of Minho (ICVS) (Baptista et al., 2020).

Figure 1: Reflectance spectra of healthy RBCs and RBCs

with early (rings) and late (trophozoites) with different

parasitaemia, from 12.5 to 50 parasites/all of the RBCs.

From this previous work of the research group

(Baptista et al., 2020) (represented in Figure 1), it was

selected a group of 16 specific relevant spectral

bands, in the optical spectrum. This set of

wavelengths will allow to reconstruct the continuous

reflectance spectra of the samples, containing healthy

RBCs and RBCs with early (rings) and late

(trophozoites) with parasitaemia ranging from 12.5 to

50 parasites/RBCs (Baptista et al., 2020). These

selected spectral bands are: 400, 435, 520, 590, 610,

620, 630, 640, 650, 660, 670, 680, 700, 720, 740 and

800 nm. In the following section, the methods for the

design of 16 thin-film multilayer optical filters for the

above referred narrow spectral bands will be

described.

2.1 Multilayer Structure Selection

The proposed narrow band-pass optical filters are

based on a multilayer thin-film structure, taking

advantage of the constructive or destructive

combinations that produce, simultaneously, passing

and rejection optical bands. The filters’ structures are

similar to a Fabry-Perot interferometer structure,

which consists of two flat parallel mirrors separated

by a layer with a pre-defined thickness, called

resonance cavity (Minas et al., 2004).

Figure 2 presents the working principle of a

multilayer optical filter, where the multilayer

structure, composed by two parallel mirrors and

visible resonance cavity, is visible. According to the

literature in the optical field, to obtain a good filter

performance using a Fabry-Perot interferometer

structure, the total number of layers must be 9 or 11.

After the simulations, it was chosen the 11 layers per

filter structure (Minas et al., 2006). The mirrors can

be dielectric films, featuring low energy absorption

rates and high transmittance at specific wavelengths,

comprised by five layers each with, alternatively,

high (H) and low (L) refractive index materials. The

resonance cavity of the filter has a multiple-beam

interference that causes a very high optical

transmission at a narrow band of wavelengths around

a wavelength for which the cavity is a multiple of one-

half wavelength thick. Furthermore, considering a

simple approach for the design of the optical filters

(an interference of first order and a light incidence

angle of 0º), Figure 2 presents two expressions for the

calculation of the resonance cavity and mirror

thicknesses, dt and ds, respectively. Finally, λ is the

transmitted wavelength and n is the refractive index

of the resonant cavity material (Pimenta et al., 2015).

Figure 2: Multilayer optical filter structure.

Effect of the Materials’ Properties in the Design of High Transmittance and Low FWHM SiO2/TiO2 Thin Film Optical Filters for

Integration in a Malaria Diagnostics Device

Therefore, in this optical filter structure,

considering similar films on both mirrors, the

thickness of the resonance cavity determines the

tuned wavelength. Taking advantage of that, 16 filters

were computationally designed and simulated using

the TFCalc 3.5 software, based on finite elements

methods, and supplied by Software Spectra Inc. As

previously referred, those filters were centred at 16

specific spectral bands in the visible optical spectrum:

400, 435, 520, 590, 610, 620, 630, 640, 650, 660, 670,

680, 700, 720, 740 and 800 nm.

2.2 Materials Selection

The parallel mirrors of the Fabry-Perot optical filters

are composed by multilayer stacks made with

dielectric thin films, which can produce high

transmission and low absorption losses. Since the

multilayer structure should consist in thin films with

low and high refractive indexes, silicon dioxide

(SiO

) and titanium dioxide (TiO

), with low and high

refractive indexes, respectively, were selected as the

materials to comprise the filters. These dielectrics

materials are rigid, extremely difficult to remove

from the substrate, compatible with CMOS

fabrication and commonly deposited by Ion Beam

Deposition (IBD), which is the process typically used

for fabrication of optical filters. Additionally, SiO

was selected because of its almost constant

dependence of the refractive index, within the visible

range in the light spectrum, while TiO

was selected

due to fabrication constraints since its deposition

process is well-characterized (Pimenta et al., 2015).

3 SIMULATION OF THIN FILMS

OPTICAL FILTERS

Regarding the simulations, the TFCalc 3.5 software

was used for the computational design and simulation

of the 16 optical filters, characterizing them in terms

of transmittance peak and FWHM. The intensity of

the transmitted peaks should be as high as possible,

with at least twice the intensity of any noise peak that

might appear in the considered spectral range.

Regarding the FWHM, a value around 10 nm is

acceptable for the intended application.

In order to optimize the design and future

fabrication processes, the optical filters were initially

divided into three spectral regions: UV/VIS (400 nm

– 435 nm), VIS (520 nm – 620 nm) and VIS/IR (620

nm – 800 nm). In order to centre the optical filters at

different spectral bands by adjusting only the

thickness of the resonant cavity, made from SiO

, the

thicknesses of both mirrors' films were maintained

equally. The reference wavelengths chosen for each

region were 420 nm, 550 nm and 680 nm,

respectively. Besides the properties of the films’

materials, the simulation also considered the

properties of the substrate (glass), exit medium

(glass), reference wavelength, and incident medium

(air), which may influence the filters optical response.

In this way, three different experiments of

numerical simulations were performed, to assess the

best performance of the 16 optical filters, which most

closely resembles the performance of the filters that

will be achieved after fabrication. Between the

different sets of simulations, the values of the

refractive indexes (n) of the multilayer materials

(SiO

and TiO

) were modified, and different material

databases were evaluated. Table 1 presents the

refractive indexes of SiO

and TiO

at the 16 expected

spectral bands, according to two databases (Sopra

S.A. and refractiveindex.info (DeVore, 1951;

“https://refractiveindex.info/,” n.d.; Malitson, 1965),

as it will be explained in the following sections.

3.1 Ideal Refractive Indexes

The first design and simulations were performed

using the refractive indexes provided by the SOPRA

database (Table 1), since these values are considered

ideal for both materials, and only depend on the

optical wavelength. The Sopra S.A. company

(France) made its optical database available, as well

as the conversion to TFCalc format (Baptista et al.,

2020).

Table 2 shows the combination of layer thickness

for each optical filter designed in TFCalc, optimized

for the highest transmittance, for each relevant

wavelength. As observed in the table, the narrow

optical filters were divided into four regions: 400 –

435 nm, 520 – 620 nm, 630 – 720 nm and 740 – 800

nm, each group with similar layer thicknesses for the

mirrors, which will facilitate the design of masks and

future fabrication of the thin film optical filters.

In the designed filters, the two mirrors are

symmetrical and consist of five layers of TiO

and

SiO

, alternately, where layers of the same material

have the same thickness for each wavelength. This

separation of the 16 wavelengths in four sections will

be repeated in the next sets of simulations. Finally, for

each wavelength, the resonance cavity has a different

thickness, which was adjusted to achieve the highest

possible optical transmittance.

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

Table 1: Refractive indexes of SiO

and TiO

according to Sopra S.A. and refractiveindex.info databases.

Wavelength

Refractive Index (n)

SiO

TiO

Sopra S.A. refractiveindex.info Sopra S.A. refractiveindex.info

400 1.4701 1.4841 3.2861 2.3379

435 1.4668 1.4810 3.2192 2.2690

520 1.4613 1.4759 3.0000 2.1824

590 1.4584 1.4733 2.9100 2.1456

610 1.4577 1.4727 2.8894 2.1379

620 1.4574 1.4724 2.8800 2.1344

630 1.4571 1.4722 2.8750 2.1311

640 1.4568 1.4719 2.8700 2.1280

650 1.4565 1.4717 2.8600 2.1251

660 1.4563

1.4715

2.8500 2.1223

670 1.4560 1.4713 2.8444 2.1197

680 1.4558 1.4711 2.8400 2.1172

700 1.4553 1.4707 2.8300 2.1126

720 1.4549 1.4703 2.8200 2.1084

740 1.4544 1.4700 2.8100 2.1046

800 1.4533 1.4692 2.7900 2.0951

Table 2: Optical filters in the UV/Vis, Vis and Vis/IR regions and respective layer thicknesses, with the combinations SiO

/TiO

using SOPRA refractive index (RC: Resonance Cavity).

Peak of high transmittance per λ (nm)

400 435 520 590 610 620 630 640 650 660 670 680 700 720 740 800

Layer thickness (nm)

TiO

33 46 60 70

SiO

73 94 117 121

TiO

33 46 60 70

SiO

73 94 117 121

TiO

33 46 60 70

SiO

(RC)

127 151 168 219 235 243 196 204 211 219 226 233 247 262 248 291

TiO

33 46 60 70

SiO

73 94 117 121

TiO

33 46 60 70

SiO

73 94 117 121

TiO

33 46 60 70

3.2 Approximation of the Real

Refractive Indexes

Then, a new group of 16 filters was designed and

simulated, using the refractive indexes of the

materials obtained from the refractiveindex.info

database. This database is considered closer to the

real values of the refractive indexes of the materials,

contrarily to the ideal values from Sopra S.A.

Table 3 shows the combination of the layer

thickness values for each optical filter designed in

TFCalc, optimized for the highest transmittance,

using the refractiveindex.info database.

In comparison with Table 2, it is possible to verify

that these refractive indexes values lead to an increase

on the thickness of almost all filters (selected to

achieve the highest transmittance at the intended

wavelengths). However, in the 400 nm and 435 nm

band-pass filters, the SiO

layer is similar to the

Effect of the Materials’ Properties in the Design of High Transmittance and Low FWHM SiO2/TiO2 Thin Film Optical Filters for

Integration in a Malaria Diagnostics Device

previous one, while in the 630 nm and 800 nm band-

pass filters, those thicknesses decrease.

3.3 Impact of the Layer Thickness on

the Refractive Indexes of the

Materials

Finally, a group of 16 optical filters was designed

considering the variation of the refractive index of the

materials (TiO

2 and SiO2) with the thickness of each

layer (Table 4). In this set of simulations, the

refractive indexes were gathered from interpolation

of the data obtained from previous studies of the

research group (Pimenta et al., 2015), where the

refractive index of each material was experimentally

measured for a specific layer thickness. In the

reported data, each refractive index slightly decreases

with the increase of wavelength (in the 400 nm – 800

Table 3: Optical filters in the UV/Vis, Vis and Vis/IR regions and respective layer thicknesses, with the combinations SiO

/TiO

, using the refractive indexes from refractiveindex.info database (RC: Resonance Cavity).

Peak of high transmittance per λ (nm)

400 435 520 590 610 620 630 640 650 660 670 680 700 720 740 800

Layer thickness (nm)

TiO

44 64 78 96

SiO

73 98 114 120

TiO

44 64 78 96

SiO

73 98 114 120

TiO

44 64 78 96

SiO

(

)

125 159 149 218 238 248 192 201 211 218 231 240 260 280 234 293

TiO

44 64 78 96

SiO

73 98 114 120

TiO

44 64 78 96

SiO

73 98 114 120

TiO

44 64 78 96

Table 4: Optical filters in the UV/Vis, Vis and Vis/IR regions and respective layer thicknesses, with the combinations SiO

/TiO

, using experimental refractive indexes from Pimenta et al. (2016) (RC: Resonance Cavity).

Peak of high transmittance per λ (nm)

400 435 520 590 610 620 630 640 650 660 670 680 700 720 740 800

Layer thickness (nm)

TiO

44 58 70 90

SiO

73 94 115 120

TiO

44 58 70 90

SiO

73 94 115 120

TiO

44 58 70 90

SiO

(RC)

114 144 151 211 229 238 185 194 203 211 220 228 245 263 210 262

TiO

44 58 70 90

SiO

73 94 115 120

TiO

44 58 70 90

SiO

73 94 115 120

TiO

44 58 70 90

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

nm range) and is higher as the thickness of the layer

increases, for both materials. For example, in the SiO

layer, the 73 nm thickness film has refractive indexes

around 1.47 (for all wavelengths range, despite their

small variations within the optical spectrum), while

for the 120 nm thickness film, it has refractive

indexes around 1.51. The same happens with the TiO

layer, i.e., for a 44 nm thickness, the refractive

indexes are between 2.6 and 2.3, while for the 90 nm

thickness layers, the refractive indexes are between

2.7 and 2.4, depending on the wavelength.

Therefore, Table 4 presents the combination of the

layers' thickness values for each filter, designed in

TFCalc, optimized for the highest transmittance when

the variation of the refractive index with the thickness

is considered.

4 RESULTS AND DISCUSSION

This section presents the main results of the optical

filter simulation for the three groups of refractive

indexes. The results of each set of designed optical

filters simulations are presented through the

transmittance spectra for the 16 optical filters.

Figure 3 represents the spectra of the 16 narrow-

band optical filters when the SOPRA refractive

indexes are considered in the simulations. As seen in

Figure 3 plot, the percentage of transmittance is very

high (almost 100%) and the FWHM value is lower

than 10 nm for all filters. This simulation represents

the ideal properties of the materials. However, the

resultant spectra will not represent a reliable

approximation of the filters’ performance after

fabrication. Figure 4 presents the simulated spectra of

the

16 narrow-band optical filters when refractive

Figure 3: Transmittance vs wavelength, for the matrix of 16 optical filters, using Sopra S.A. refractive indexes. The structure

of the filters is shown in Table 2.

Figure 4: Transmittance vs wavelength, for the matrix of 16 optical filters, using refractive indexes from refractiveindex.info

database. The structure of the filters is shown in Table 3.

Effect of the Materials’ Properties in the Design of High Transmittance and Low FWHM SiO2/TiO2 Thin Film Optical Filters for

Integration in a Malaria Diagnostics Device

indexes from refractiveindex.info database are

considered in the simulations.

These results are closer to reality, and describe

better the materials' behaviour, which will be

essential during fabrication. When compared to the

previous results, while the transmittance is still high

(> 90%), the FWHM is significantly worse since it is

higher than 10 nm for all designed filters.

Finally, Figure 5 presents the spectra of the 16

optical filters, when the refractive index of the thin

film materials, used for simulation, is dependent on

the thickness of the layer. This design, when

compared to the previous one (Figure 4), shows better

results regarding the FWHM, since all filters present

a FWHM < 10 nm or around this value. Considering

the transmittance, their values are above 90%,

similarly to the ones from Figure 4.

In all the presented designs, the simulation results

confirm that multilayer stacks of 11 layers comprised

of Si0

/Ti0

thin films and a SiO

layer for the

resonance cavity are a good option for the design of

the thin film and narrow-band optical filters,

regarding their optical characteristics. Furthermore,

these designs are feasible for future fabrication

processes. The performance of the optical filters

could be improved by increasing the number of layers

in the dielectric mirrors, but the complexity of the

fabrication process would also increase (Minas et al.,

2004).

Moreover, those simulations allowed to conclude

that each band-pass filter for a specific spectral band

has a high transmittance, close or exceeding 90%

(which is explained by the theoretical refractive

indexes), and the FWHM average is around 10 nm

when the experimental refractive indexes, dependent

on the thickness of the layers, are considered.

Additionally, these results show that slight variations

in the refractive indexes imply significant

modifications in the thin films filters to achieve high

transmittance at the desired spectral band, and this

must be taken into account during the fabrication

processes.

4.1 Example of Application in the

Malaria Parasites Reflectance

Spectra

In order to assess if the designed and simulated

optical filters would be able to detect differences in

optical reflectance spectra, for the intended malaria

diagnostics applications, the transmittance data of the

designed 16 optical filters (based on the

refractiveindex.info database) were multiplied by a

typical reflectance spectrum of healthy RBCs and by

spectra of malaria infected RBCs, at different

parasitaemia (spectra previously presented in Figure

1). Figures 6 (a) and (b) present two examples of the

spectra obtained from the multiplication between

RBCs reflectance spectrum (Figure 1) and all the

simulated optical filters transmittance data.

Particularly, Figure 6 (a) presents the data for healthy

RBCs and Figure 6 (b) presents the data for RBCs

with late stage trophozoites containing a parasitaemia

of 50 parasites/μl of red blood cells. It is possible to

observe that the optical filters spectra are still visible

and distinguishable, with slight variations in the

peaks’ amplitude, resultant from the effect of the

reflectance spectra. Finally, Figure 6 (c) presents the

spectra resultant from the superposition of the healthy

and infected RBCs reflectance spectra with the area

of transmittance of all optical filters, i.e., at each

wavelength, the plot presents the cumulative effect of

all the filters whose transmittance spectra fall on that

Figure 5: Transmittance vs wavelength, for the matrix of 16 optical filters, using experimental refractive indexes from Pimenta

et al. (2016). The structure of the filters is shown in Table 4.

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

specific wavelength. For instance, while at 435 nm

only one filter allows light to be transmitted (the 435

nm filter), at 620 nm, all filters from 590 nm and 640

nm allow some light to be transmitted, increasing the

total amount of light to be received by the photodiode

array. The high number of optical filters presented in

the 570 nm – 750 nm range leads to an increase in the

total amplitude of the plot in that region, Figure 6 (c).

The results show that, between 400 nm and 550 nm,

there is no notorious difference between healthy and

malaria-infected RBCs. However, in the 600 nm –

650 nm range, there is a significant increase in the

obtained spectra, and the slope between 500 nm and

650 nm is higher as the parasitaemia decreases. From

these results, it is possible to infer that the system may

be able to distinguish between healthy and malaria

infected samples, enhancing its potential as a

diagnostic tool.

5 CONCLUSIONS

The paper describes the design, performance

simulation and optimization of 16 narrow band-pass

optical filters with multilayers based on thin films of

SiO

/TiO

, aiming an optical reflectance device for

non-invasive

malaria

diagnostics.

These

filters

were

(

)

(

)

(c)

Figure 6: (a) Optical transmittance spectra (%) obtained from the multiplication between the healthy RCBs reflectance

spectrum and the optical filters transmittance spectra; (b) Optical transmittance spectra (%) obtained from the multiplication

between the RBCs with late stage trophozoites with a parasitaemia of 50 parasites/μl reflectance spectrum and the optical

filters transmittance spectra; (c) Optical transmittance spectra (a.u.) resultant from the superposition of the healthy and

infected RBCs reflectance spectra (Figure 1) with the area of transmittance of all 16 optical filters.

Effect of the Materials’ Properties in the Design of High Transmittance and Low FWHM SiO2/TiO2 Thin Film Optical Filters for

Integration in a Malaria Diagnostics Device

characterized taking into account the materials of the

layers, as well as the requirements for high optical

transmittance and low FWHM. The design and

simulation of thin-film optical filters are a

challenging process. Several variables must be

simultaneously controlled to obtain optical filters

centred at the desired wavelengths, such as the thin-

films’ materials, thickness of each layer, refractive

indexes of the materials and multilayer structure.

Additionally, the simulated results showed that slight

variations in the refractive indexes imply significant

modifications in the thin films filters to achieve high

transmittance at the desired spectral band, a feature

that must be taken into account during the fabrication

processes. A compromise between the variation of the

materials' refractive indexes and the filter's

performance was achieved.

Despite some deviations in the simulated results

for the different experiments of the optical filters

(mainly in the FWHM values), their performance was

successfully evaluated, since it is possible to obtain

high transmittance for each of the selected

wavelengths. Also, the simulation results proved that

these 16 optical filters designs are extremely sensitive

to the material properties. However, the simulation

results also show that these filters are a good option

regarding the required optical response, assuring

feasibility and being suitable for the fabrication

process, showing high potential to be integrated into

the intended optical reflectance device for malaria

diagnosis.

Finally, the results from the combination of the

samples’ reflectance and the optical filters’

transmittance spectra, also show the potential of the

presented system to distinguish between samples of

different malaria parasites’ concentration with high

sensitivity, up to a limit of 12.5 parasites per

microliter of RBCs. This value is comparable to

current diagnostic methods and detection devices.

Besides that, the proposed optical diagnosis

methodology device is new, easily implemented, non-

invasive and does not need specialised laboratorial

equipment or facilities. Following the promising

simulation results, future developments will include

the deposition processes to fabricate the 16 optical

filters.

ACKNOWLEDGEMENTS

This work was supported by Project NORTE-01-

0145-FEDER-028178 funded by NORTE 2020

Portugal Regional Operational Program under

PORTUGAL 2020 Partnership Agreement through

the European Regional Development Fund and the

Fundação para a Ciência e Tecnologia (FCT), IP. V.

Baptista thanks FCT for the SFRH/BD/145427/2019

grant. Maria Isabel Veiga thanks FCT for her contract

funding provided through DL 57/2016 (CRP).

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Effect of the Materials’ Properties in the Design of High Transmittance and Low FWHM SiO2/TiO2 Thin Film Optical Filters for

Integration in a Malaria Diagnostics Device