New methodology for optical sensing and analysis

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Linköping Studies in Science and Technology

Licentiate Thesis No. 1090

LiU-TEK-LIC-2004-19

New methodology

for optical sensing and analysis

Laboratory of Applied Optics

Department of Physics and Measurement Technology

Linköping University

SE-581 83 Linköping, Sweden

Linköping, 2004

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Preface

This thesis describes the research I have done, and partly will do, during my time as a PhD student in the laboratory of Applied Optics at Linköping University. Due to circumstances beyond the scope of this book, this incorporates three quite different projects. The first two, involving gas sensing and measuring on paper with ellipsometry, have been discontinued, whereas the third one, measuring fluorescence with a computer screen and web camera, is in full progress and will be until I complete my studies.

Thus the purpose of this work also has several aspects. Partly, it describes performed research and its results, as well as theoretical background. On the other hand, it provides practical and theoretical background necessary for future work. While the three projects are truly quite different, each of them has certain things in common with each of the other. This is certainly also true for the necessary theory. Two of them involve spectroscopic ellipsometry, for example, while another pair needs knowledge of color theory, etc. This makes it impossible to separate the projects, despite of their differences. Hopefully, these links between the different projects, connecting the different chapters, will make this work whole and consistent in its own way.

Finally, I need to thank several people that were important for the completion of this work. First my supervisors for the different projects, Kenneth Järrendahl, Hans Arwin, Daniel Filippini and Ingemar Lundström. Without them, none of this would ever have existed. My colleagues at Applied Optics, especially my fellow PhD students and friends, Michal, Alexander and Linda, for making my time at IFM more enjoyable. The many friends I have made here in Linköping in my korridor, among exchange students and their peer students, in Chorus Lin and probably some more, for making my life in Linköping such a great pleasure. Finally my family, for being so close to me while so far away and for undying support, especially at times when it was a little harder.

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Included papers

Paper 1

Improvement of porous silicon based gas sensors by polymer modification

J.W.P. Bakker, H. Arwin, G. Wang and K. Järrendahl

Published: phys. stat. sol. (A) 197, 378-381 (2003)

Abstract

Gas sensing was performed using spectroscopic ellipsometry and porous silicon films. Modification of the porous layer by polymer deposition showed an increase in sensitivity to organic solvent vapor of up to 135%. The increase in sensitivity is strongly dependent on polymer concentration. At high concentrations, too much polymer is deposited, presumably blocking the pores, causing a decrease in sensitivity. At sufficiently low concentrations, the polymer causes a strong increase in sensitivity. This is assumed to be caused by the polymer being deposited inside the pores, where its interaction with the vapor influences the sensitivity. At very low concentration, the sensitivity approaches values obtained without polymer modification. The sensitivity increase is different for different vapors, pointing to possible selectivity enhancement.

Author’s contribution:

All experimental work, analysis and writing.

Paper 2

Determination of refractive index of printed and unprinted paper using spectroscopic ellipsometry

J.W.P. Bakker, G. Bryntse and H. Arwin

In press: Thin Solid Films

Abstract

An attempt is made to address the basic physical properties of printed and unprinted paper surfaces by using spectroscopic ellipsometry in the range 300 - 900 nm to determine the effective complex-valued refractive index <N>. Some simulations to address the effect of structural properties have also been done and a qualitative comparison with some other methods, in particular Brewster angle measurements, has been made.

Unprinted paper and paper printed in different colors have been studied. The measured absorption properties matched the colors of the used inks well. The effects of roughness on the determined spectra of <N> are discussed. Simulations show that compared to other methods, like Brewster-angle reflectometry, spectroscopic ellipsometry provides a more accurate value of <N> especially in wavelength regions were the color pigments show absorption.

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Paper 3

Computer screen photo-assisted spectroscopic fluorimetry

J.W.P. Bakker, D. Filippini and I. Lundström

In manuscript

Abstract

Fluorescence measurements are demonstrated using a computer screen as a programmable light source and a web camera as detector; resulting data shows clear correlation with traditional spectroscopic fluorimetry, enabling the evaluation of fluorescence based assays in a platform for home based diagnostics and other low cost applications.

All experimental work, analysis and writing. Included papers

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Chapter 1 Introduction

Three projects are summarized by the papers at the end of this thesis. The first paper deals with gas sensing with porous silicon using ellipsometry. The second paper describes spectroscopic ellipsometry measurements on paper and the third and last fluorescence measurements using a computer screen and web camera.

The following chapters give an overview of the theoretical background for the scientific work described in the papers. Since all the papers involve optics, it starts with a brief overview of relevant elementary optics in chapter 2. After that, in chapter 3, theoretical and practical aspects of ellipsometry are described, a technique which is used for the first two projects. Continuing with optics, chapter 4 describes fluorescence, which is only relevant for one project: the fluorimetry project described in paper 3.

After chapter 4, the elementary physics is left behind and some more practical subjects are treated. Color theory, dealing with analyzing and reproducing color and human perception of color, is important for the printed paper in the second project and even more so for the computer screen photo-assisted measurement, where both the computer screen and the web camera are based on principles described in this chapter. How the color theory is applied in practice, as well as some physical background, can be read in the two following chapters about computer screens and CCD photo detectors. The last two chapters, describing porous silicon and paper, are relevant for projects 1 and 2, respectively.

Prospects for future work are given in chapter 10, focusing on the last project, since this is still a work in progress. The papers, finally, show the scientific output produced so far. The first paper has been published, the second is currently in press, the last one is in the last stages of manuscript writing.

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Chapter 2 Elementary Optics

2.1 Interaction of electromagnetic waves with matter

The Maxwell equations [1] give a macroscopic description of the propagation of electromagnetic waves and their interaction with matter:

(2.1) Ñ × =Dr r

(2.2) Ñ × =Br 0 (2.3) Ñ ´ = -Hr rJ i Dwr

(2.4) Ñ ´ =Er i Bwr

where_r D is the dielectric displacement,r r the charge density,B the magnetic induction,r H the magnetic field,J the current density andr E the electric field.r

The behavior of matter when subjected to electromagnetic fields is described by the constitutive equations,

(2.5) Br=mm0Hr

(2.6) Dr= ~ee0Er

(2.7) Jr= ~sEr

wherem is the relative magnetic permeability, ~e the relative dielectric function and ~s the conductivity.

By inserting (2.6) and (2.7) into (2.3), we obtain: (2.8) Ñ ´ = - æ + è çç ö_ø÷÷ = -r r r H iw i s E i E we e e wee ~ ~ 0 0 0

whereenow is defined as an effective value, containing both dielectric and conduction effects. In most cases the contribution of conduction will however be small ande e» ~. If the displacement fieldD is not able to follow the oscillations in the electric fieldr E asr

described by equation (2.6), damping of the oscillation will occur. This can be described mathematically by introducing an imaginary part ine:

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Similarly, the concept of refractive index n can be expanded to include absorption by including an imaginary part in the form of the extinction coefficient k. A capital N is used for this complex variant:

(2.10) N= +n ik

The Fresnel equations describe the reflection and transmission of a plane electromagnetic wave incident at an angle j₀ on a planar interface between two materials with complex refractive indices N0and N1,

(2.11) r E E N N N N s rs is = = -+ 0 0 1 1 0 0 1 1 cos cos cos cos j j j j (2.12) t E E N N N s ts is = = + 2 0 0 0 0 1 1 cos cos cos j j j (2.13) r E E N N N N p rp ip = = -+ 1 0 0 1 1 0 0 1 cos cos cos cos j j j j (2.14) t E E N N N p tp ip = = + 2 0 0 1 0 0 1 cos cos cos j j j

where rs, rp and ts, tp are the Fresnel reflection and transmission coefficients,

respectively, E is the magnitude of the electric field andj₁is the angle of refraction. j0, j1, N0 and N1 are also related through Snell’s law of refraction, N₀sinj₀ =N₁sinj₁.

Chapter 2, Elementary Optics

f

0

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Material 1

Material 2

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0

N

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is

H

E

rs

H

E

ip

E

rp

H

E

tp

H

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Figure 1 The (a) s-component and (b) p-component of polarized light incident on a planar interface

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The Fresnel equations are split up in reflection and transmission coefficients for the s- and p-polarized components of the light. For the s-polarized component, the electric field is normal to the plane of incidence (Fig. 1a) and for the p-polarized component, the electric field is parallel to the plane of incidence (Fig. 1b). As any incident wave can be written as a superposition of these two components, these equations are sufficient for all possible polarization states.

2.2 Reflection from layered structures

The above described theory only holds for a so-called two-phase system (Fig 2a), where the interaction takes place at the interface of two materials with different indices of refraction. For layered structures, all the involved interfaces need to be taken into account. In a three-phase system (Fig 2b), this would mean summing up light reflected at interface 01 and light transmitted at interface 01, reflected at interface 12 and transmitted at interface 10 and so on for higher order reflections. For multilayer structures, this becomes an impossible task, but a matrix formalism can be used to solve this problem. This formalism is called the Abelés formalism [2] or scattering matrix method [3].

A matrix equation E(z1)=SE(z2)is defined as follows: (2.15) E z E z S S S S E z E z + -+ -é ë ê ê ù û ú ú= éëê ù û ú ( ) ( ) ( ) ( 1 1 11 12 21 22 2 2) é ë ê ê ù û ú ú

where E+ and E- are the complex field vectors of the forward and backwards traveling waves, respectively, for planes at positions z1and z2. The scattering matrix S

contains contributions from all the interfaces and layers and thus describes the whole system. From the scattering matrix, effective reflection coefficients Rpand Rs, can be

calculated, so that:

Chapter 2, Elementary Optics

f

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Ambient

Substrate

Film

a

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(2.16) E E R R E E rp rs p s ip is æ è çç ö_ø÷÷ =æ_èçç ö_ø÷÷æ è çç ö_ø÷÷ 0 0 for isotropic media.

2.3 Effective medium approximation

When doing optical analysis, one frequently encounters mixtures of materials with known optical properties for the constituents. If the local variations of the optical properties are of a much smaller scale than the wavelength of the light, the mixture can be modeled as a continuum. The optical properties of the mixture can be calculated from the optical properties of the constituents. For this purpose, the effective medium approximation (EMA) method has been developed. Several different EMA models have been developed, optimized for different microstructures. Only the Bruggeman EMA model is treated here, since it has been proven fairly successful for the applications described in this work.

The Bruggeman EMA assumes spherical unit cells for all constituents in the mixture. For n materials with volume fraction fi, and dielectric functioneithe effective

dielectric functioneeff can then be defined using the following equation:

(2.17) fi i eff i eff i n e e e e -+ = =

å

₂ 0 1 and condition (2.18) fi i n =

å

= 1 1

This model is frequently used to describe both surface roughness [4] and porosity [5]. In both cases the material is described as a mixture of the substrate material and void.

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Chapter 3 Ellipsometry

Ellipsometry [3] is an optical technique for determining the optical properties and microstructure of surfaces and thin films. It is based on measuring the polarization change that occurs when light is reflected by or transmitted through the surface or film. Reflection ellipsometry will be considered in the remainder of this text. The technique has two main advantages. First, it is a non-destructive measurement ‘from a distance’, which makes it very suitable for in-situ real-time measurements. Second, because the measured variable is polarization change, it is essentially insensitive to drift in the intensity of the light source and the spatial resolution for determining film thickness is not limited by the diffraction limit, enabling changes in layer thickness in the order of Ångströms to be detectable.

3.1 Principles

Ellipsometry measures the change of polarization of light when it reflects of a surface. The polarization state of the incident wave can be defined as:

(3.1) ci ip is E E =

and the polarization state of the reflected wave: (3.2) cr rp rs E E =

The polarization change upon reflection can then be defined as the ratio of these states: (3.3) r c

c = r

i

or, expressed with Fresnel reflection coefficients: (3.4) r =R

R

p s

This is a complex quantity, and is usually expressed as: (3.5) r = tan YeiD

With R_p =R e_p idrp_{and R} _{R e}

s s irs

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(3.6) Y = R R p s and D =d_rp -d_rs

ThusY and D are the differential changes upon reflection in phase and amplitude, respectively, of the components of the electrical field vector parallel and perpendicular to the plane of incidence. These are the variables that are obtained from an ellipsometry measurement. Depending on the amount of information needed, they can be obtained as a function of wavelength, angle of incidence, time, or a combination of these.

3.2 Measurement

To conduct an ellipsometric measurement, one first needs an incident beam with known polarization, which is reflected on the sample, after which the change in polarization needs to be determined. A very common setup is the PCSA system, which stands for Polarizer Compensator Sample Analyzer. This is the order of the components between the light source and the detector. The light source is usually a laser or other monochromatic light source. The polarizer and analyzer are both linear polarization filters and the compensator is a quarter wave plate, with which an arbitrary elliptical polarization can be obtained by inducing a phase shift between the x- and y-components of the polarization vector. When the angles of the polarizer and analyzer are set in such a way that no light hits the detector (nulling),Y and D can be calculated from these angles.

For spectroscopic ellipsometry, nulling the system for every wavelength would become very tedious and time consuming. Therefore a Rotating Analyzer Ellipsometer (RAE) is used in this case (Fig. 3). The analyzer in this system rotates at a

constant speed, while the polarizer is kept in a fixed position. The linearly polarized incident light (the compensator is often omitted in this system) will generally be converted to elliptically polarized light, which gives a sinusoidal signal on the detector due to the rotating analyzer. By a Fourier analysis of the signal, the ellipsometric anglesY and D can then be determined.

Chapter 3, Ellipsometry light source monochromator polarizer rotating analyzer detector sample

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3.3 Analysis

Generally, the desired properties of the measured sample cannot be directly calculated from the measuredY and D. For this reason, ellipsometry is called an indirect technique. For any sample with known properties, however, the outcome of an ellipsometry measurement can be predicted using the matrix formalism mentioned in section 2.2. This calculated result can then be compared to the measured data and if needed adjusted to better fit the measured data.

The comparison and model tuning is performed as an iterative fitting process by a computer. In short the procedure is as follows. First, a measurement is performed to collect information about the sample in the form of ellipsometric angles. In principle the more information the better, since this gives a better basis for the mathematical fit. More information can be obtained by measuring at multiple wavelengths or multiple angles of incidence or both (variable angle spectroscopic ellipsometry). Second, a model is built up using what is known about the measured sample to define a layered structure with layer thicknesses and optical properties as close to the real values as possible. In case of materials with known optical properties, database values from earlier measurements can be used in the model. Third, some of the parameters (film thickness, optical properties) in the model are defined as variables, to be changed in the fitting process. Fourth, the mean square error (MSE) value is calculated, which is the standard deviation of the values calculated with the model from the measured values. Fifth, the parameters defined as variables are changed in order to decrease the MSE value. Step four and five are repeated until a minimum value of the MSE is reached.

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Chapter 4 Fluorescence

When molecules absorb ultraviolet or visible light, they are elevated to an excited electronic state. Generally, this excess energy will be dissipated as heat. Under certain conditions, however, some of the excess energy can be re-emitted as light of a different wavelength. This process is called photoluminescence [6]: luminescence for the general effect of molecules emitting light in a deexcitation process, photo- because the excitation is caused by photons (as opposed to, for example, chemiluminescence). Fluorescence, which plays an important role in this work, is a form of photoluminescence. For better understanding, a more detailed look at the process of excitation and emission is necessary.

4.1 Excitation and emission

What happens to the energy after absorption, can be depicted in a so-called Jablonski diagram (Fig.4 ). To know what kind of emission is seen, one needs to look at the molecular multiplicity M:

(4.1) M=2S+1

where S is the spin quantum number of the molecule, defined as the sum of the net spin of all electrons in the molecule. For most organic molecules, S=0, since the number of electrons is even. The multiplicity is then equal to 1 and the state is referred to as a singlet state. While in an excited state, it is possible for one electron to change its spin. Then S = + =1

2 1

2 1 and M= × + =2 1 1 3. This is called a triplet state. The states are

numbered by level of excitation: S0, S1, S2, etc. for the singlet states and T1, T2, etc for the triplet states (there is no T0, since in the ground level all spins are paired).

S0 S2 S1 T1 IC ISC VR VR VR

Absorption Absorption Fluorescence Phosphorescence

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When a photon is absorbed by the molecule, it is excited to a higher singlet state (see Fig. 4). Let us assume the molecule is excited to S2. It will immediately dissipate excess vibrational energy in the form of heat by collision with surrounding molecules. This process is called vibrational relaxation (VR). At the same time, a transition can occur from a low vibrational level in S2to a higher vibrational level with the same energy in S1. This is called internal conversion (IC). After this, the molecule can further relax to the lowest vibrational level in S1. These processes occur on a short time scale (~10-12 sec). Further vibrational relaxation to the S0level is however usually relatively slow. This enables relaxation by emission of a photon. This process, deexcitation from S1to S0by emission of a photon is called fluorescence. If a further transition occurs from S1 to T1by a process called intersystem crossing (ISC), the relaxation from T1to S0by photon emission is called phosphorescence

4.2 Fluorescence in practice

Quenching

Some practical considerations should be noted when dealing with fluorescence spectroscopy. It is important to know that fluorescence can be inhibited by a deactivation process known as quenching. Several mechanisms are known to cause quenching [10], but all involve radiationless deexcitation by interaction with other (quencher) molecules. This can be used to ones advantage to turn fluorescence on or off at will, but should of course be noted when designing experiments, to make sure no accidental unwanted quenching occurs.

Polarization

When polarized light is used to excite fluorescent molecules, absorption will be most likely to occur for molecules which have their absorption transition vectors aligned parallel to the polarization of the exciting light. Depending on the mobility of the molecules and the lifetime of the excited state, the polarization of the emitted light may vary. The degree of polarization of the emitted light can be defined as

(3.2) p I I I I f f f f = -+ ^ ^ || ||

Where If ||and If^are components of the polarization of the emitted light parallel and

perpendicular, respectively, to the polarization axis of the exciting light. This value can vary from -0.33 to +0.5 [10]. The degree of polarization will be higher for higher molecular weight of the fluorophore, higher solvent viscosity and longer excited state lifetime. This effect can be utilized for a fluorescent polarization (FP) measurement, which gives information about molecular orientation and mobility. This can be of value in studies of interaction of organic molecules, for example. It may also prove to be useful to filter out computer screen light in the CSPT setup (see paper 3), by using crossed polarizers.

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Chapter 5 Color theory and color reproduction

There are different ways to define color. The definition from physics will involve the wavelength of light within the limits of the visible spectrum (~350-750 nm). When dealing with spectroscopic measurements, for example spectroscopic fluorimetry, wavelength is the parameter that is used and color is hardly even mentioned. When using computer screens, web cameras and other devices optimized for the human eye, other definitions need to be used. For a better understanding of this, a closer look at the human eye is necessary.

5.1 The eye

Construction

The eye is built up as a fluid filled sphere, enclosed by three specialized layers [7]. From outermost to innermost, these layers are (see Fig. 5)

1. The sclera/cornea

2. The choroid/ciliary body/iris 3. The retina

The outermost layer consists for the most part of the sclera, a tough layer made of connective tissue, which is visible as the white part of the eye. At the front, where light enters the eye, the outer layer is transparent and is called the cornea. The second layer, the choroid, contains many blood vessels that nourish the retina. This also becomes specialized towards the front and forms the ciliary body and the iris. The ciliary body produces nutrients for the cornea and lens, which both lack blood supply. The iris

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controls the amount of light which enters the eye by changing the size of the opening (pupil) through which the light enters. The innermost layer is the retina, consisting of a pigmented layer and a nervous tissue layer.

Retina, rods, cones

The important part of the eye for understanding color vision is of course the retina, where the actual light detection takes place. There are two kinds of light sensitive cells in the retina, rods and cones (see Fig. 6). Rods have a much higher sensitivity than cones, but have no wavelength selectivity in the visible spectrum. They are only used for vision in dim lighting conditions. The cones however are subdivided in three different kinds, all with slightly different light receptors, making them sensitive to different parts of the spectrum.

5.2 Color vision

How the information from the different cones is processed to create a color image is still not completely understood. Several theories [8], differing in complexity and explanatory value, have been developed over the years. Three theories are described below. A schematic overview of the theories is given in Fig. 7.

Chapter 5, Color theory and color reproduction

Figure 6 schematic view of the retina

S

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Figure 7 Schematic view of different color vision theories: a. Young-Helmholtz theory, b. Hering theory and c. Opponent-process theory.

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Young-Helmholtz theory

This theory, developed in the nineteenth century, is also called the retinal approach, component theory or trichromatic theory. It builds on the assumption of the existence of three kinds of receptors, sensitive to red, green and blue light. These receptors are then directly coupled to the brain, where the color is reconstructed from the relative intensities of the different receptors. Note that this theory assumes more or less unique red, green and blue responses of the receptors, not the broad response that we now know the receptors in the eye actually have. Though this model may seem to fit fairly well to what we know about the eye, it cannot give a satisfactory explanation for several perceptual phenomena, like for example color blindness.

Hering Theory

This theory, developed in the 1870s, is also called opponent theory. It postulates three different kinds of receptors, which are each sensitive to two opponent colors: one to red and green, one to blue and yellow and one to black and white. These colors are called opponent because reddish green or blueish yellow for example are not possible. By a process described as assimilation or disassimilation, the red-green receptor (for example) will then emit a signal representing the redness or greennes of the detected light. The white-black receptors function slightly different, since whitish black (gray) is actually possible. Here successive contrast plays a role, that is black will be perceived when a dark area is next to a light area, because the white-black receptors give an opposite response.

This theory can explain a few phenomena that Young-Helmholtz cannot, but it lacks both completeness and physiological support. For example, it can explain one kind of red-green color blindness, but not the existence of two different kinds. Also, no color sensitive substance that can produce two separate effects by assimilation and disassimilation has ever been found.

Opponent-process theory

This theory, combining elements of the Young-Helmholtz and the Hering theory, is also known as zone theory or Hurvich-Jones theory. Like the Young-Helmholtz theory, it assumes the existence of three different receptors, sensitive to different wavelengths. The sensitivity maxima lie at 450, 530 and 560 nm. Thus they can be called red, green and blue receptors. However, since (unlike the receptors in the Young-Helmholtz theory) they are each sensitive to a very broad wavelength spectrum, it is better to call them long- (L), medium- (M) and short-wavelength (S) receptors.

In this theory, the opponent idea is incorporated in the nerve cells. This can be realized by constantly firing nerve cells, which are either inhibited, causing them to fire less, or stimulated, causing them to fire more. Each kind of receptor stimulates or inhibits different nerve cells, as shown in Fig. 7. Black is perceived by lateral inhibition: stimulation on one part of the retina will cause inhibition on adjacent parts. The opponent-process theory can accurately explain color blindness as well as several other perceptual phenomena.

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5.3 Color reproduction

To reproduce a color, it first needs to be sampled by a light sensitive device and then reconstructed, e.g. on a screen or printed on paper. Generally, an exact reproduction of the spectral distribution of the original light is however not necessary, since the purpose of the reproduction is to recreate an image for viewing by humans. It then suffices if the color looks identical to the human eye. Since the human eye does not sample with a very high spectral resolution, but only divides the visible spectrum in three parts by different receptors, recreating the right combination of stimuli for these receptors will give the desired effect. However, this also means that different spectral distributions can be identified as the same color. A classical example of this effect, called metamerism, is the formation of white light, which is possible by combining monochromatic red, green and blue light as well as by a continuous distribution of all wavelengths in the visible spectrum. In printing, this same effect can cause colors that appear the same under certain lighting conditions to be different when the illumination changes. To avoid misunderstandings, standard lighting conditions have therefore been defined for color matching. For certain specialized applications, however, spectral analysis will still be necessary. Especially when colors need to be recreated independent of lighting conditions, like accurate digitization of art work or generation of realistic virtual environments, trichromatic analysis does not suffice. Special techniques have been developed for these purposes, see for example [9].

Additive vs. subtractive color reproduction

Color reproduction can globally be subdivided in two different processes, additive and subtractive color reproduction. The additive process uses the primary colors red, green and blue (hence the abbreviation RGB) and combines them to produce new

colors. Secondary colors are produced by combining any two primary colors: red and green for yellow, green and blue for cyan and red and blue for magenta. Adding all three colors produces white, absence of all produces black. By varying relative intensities of the combined colors, other colors and shades can be produced. Any television, computer screen, digital projector, etc. uses this technique to reproduce colors. Pigments in printing ink or photographs, however, do not add color but instead absorb, or subtract, a part of the spectrum of the incident light. In this case subtractive color reproduction is used. Here one starts with white, i.e. white light reflecting on

Chapter 5, Color theory and color reproduction

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white paper, and subtracts red, green and blue to produce different colors. This subtraction is achieved by using the opposite colors of the colors to be subtracted. Thus to subtract red, a pigment reflecting green and blue and absorbing red is used (cyan). Likewise, yellow, reflecting red and green, is used to subtract blue and magenta, reflecting red and blue, to subtract green. Additive and subtractive color reproduction are illustrated in Fig. 8 Note that not CMY, for Cyan, Magenta and Yellow, but CMYK is the most frequently used model. Here the K stands for a Key color, normally black, which is added to enhance contrast, since a mixture of the three colors produces a grayish black.

Color fusion

Besides spectral resolution, spatial resolution is also an issue for color reproduction. To reproduce a color out of, say, a combination of colors, one would have to find a way to combine these colors on one position in space, i.e. a light spot on a screen or an ink spot on paper. To produce many colors, this would imply the need to produce light sources with a large variety in colors or a large stack of ink mixtures with different colors. Fortunately, one can use an effect called color fusion to solve this problem. Instead of combining the colors in one spatial position, they are now shown separately as small spots close to each other. If the combination of viewing distance and the size of the image color elements is such that the human eye cannot distinguish the separate spots due to lack of spatial resolution, a single color will be seen, which is a combination of the colors in the separate spots. This effect is known as color fusion (also known as additive coloration)

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Chapter 6 Computer screens

The most important output peripheral of a computer is certainly the screen. Nearly everything communicated to the user is displayed as visual information. New improvements are constantly emerging to make the screens more user friendly, with higher resolutions and refresh rates, more viewing comfort, etc. Paper 3 describes a new way of using the computer screen by exploiting it as a light source for optical measurements, in particular fluorescence spectroscopy, in a technique named computer screen photo-assisted technology (CSPT). This chapter describes the technique behind the two leading technologies for computer screens, CRT and LCD.

6.1 CRT

A cathode ray tube (Fig. 9) is based on an electron beam which is scanned over a surface covered with phosphors, which are excited by the electrons and in turn emit light. Its core is an evacuated ‘bottle’ with an electron gun in the narrow end. In color screens there are three separate electron beams, one for each color. These three beams are focused onto three phosphors with different colors within one picture element (pixel). These three phosphors together give the pixel its color using color fusion (see section 5.3)

In the electron guns, electrons are released from the cathode by heating and subsequently focused and accelerated towards the anode, which is located close to the phosphor screen. The deflection yoke creates a modulated magnetic field, directing the electron beam towards the desired spot on the screen. The electron beam is scanned from left to right (as seen from the front) and from top to bottom, lighting up the phosphor dots in sequence as it goes. The refresh rate (frequency with which the whole screen is scanned, also called vertical scanning frequency, VSF) is dependent on the resolution (number of pixels per unit area), and the horizontal scanning frequency (HSF). For modern CRT computer screens, the refresh rate is usually between 75 and 100 Hz.

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6.2 LCD

Liquid Crystal Displays (LCD) do not emit light in the pixels themselves, but instead control the emission of a backlight separately for each pixel. The backlight emits white light, color is produced by using color filters for the separate color elements in each pixel. Colors are again produced by color fusion (section 5.3). The liquid crystals in the screen consist of rod-like molecules, which are aligned to a finely grooved surface on both sides of the screen (see Fig.10 ). The grooves are aligned perpendicularly on either side, causing the molecules to twist their alignment from one direction to the other between the two boundaries. Polarized light passing these twisted molecules will twist its polarization direction according to the orientation of the

molecules. Thus, with a polarization filter on each side, the light will pass when the polarizers are crossed, since the polarization is rotated 90 degrees by the molecules. When a voltage is applied between the two boundaries, the molecules will prefer alignment with the potential, giving them a vertical alignment (Fig. 10, Right). Now light passes between the polarization filters without changing its polarization, causing it to be blocked by the second polarization filter. Thus the transmission of light can be controlled.

LCD screens usually suffice with a lower refresh rate. Since the light is not turned off for every pixel shortly after setting it, like in a CRT screen, it can operate flicker free even at low frequencies. Low response time of the liquid crystals used to be a problem. However, with the current state of the art this does not pose a practical problem anymore for most applications.

Chapter 6, Computer screens

Figure 10 The principle of an LCD device. In absence of voltage (left) the molecules (and thus the light) are twisted, causing it to pass. With a voltage, the light keeps its polarization and is blocked.

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Chapter 7 CCD

For recording light, any technique involving light being absorbed and leading to a detectable physical effect can in principle be used. For traditional photography, light sensitive chemicals constitute this effect. In digital recording, however, a chemical conversion does not suffice, since an electric signal is needed. Charged coupled devices (CCD’s) are the most common choice in this case.

7.1 Physical principle

A CCD [11] photodetector is based on the photoelectric effect [12]. It consists of an array of metal-insulator-semiconductor (MIS) capacitors, usually of the form metal-oxide-silicon (MOS). When the silicon is hit by photons, it ‘releases’ electrons (the electrons are excited to the conduction band). These electrons are collected in the MOS capacitors and after a certain exposure time, the charge buildup is measured. Since the number of released electrons is proportional to the intensity of the incident light, this measurement corresponds to an intensity measurement. Electrons can also be released by thermal effects, creating the so called dark current. This limits the exposure time, because the signal is masked by the thermal noise for long exposures. For low intensities, the amplifier gain can be raised to get a stronger signal.

7.2 Readout

Each MOS capacitor functions as a small well in which the released electrons are trapped. During the recording phase the wells are isolated from each other, preventing electrons to move between wells. In the readout phase, however applying an appropriate voltage to a cell will ‘deepen’ its potential well, causing electrons from an adjacent cell to be transferred into it. This coupling between the cells gives the CCD its name. This phenomenon is used to sequentially read all the pixels by moving them in an ordered fashion (see Fig. 11). The charges are moved in lines towards a readout register, where the cells are

moved one by one to an amplifier, which converts the charge into a voltage, which is in turn converted by an AD-converter, enabling it to be processed by a computer. Moving the charges is achieved by applying a controlled series of clock pulses. For a detailed description, see [11].

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7.3 Color

Though CCD image sensors have a certain spectral response curve, they are essentially insensitive to color. Therefore, filters need to be applied to be able to detect colors. Usually filters are applied to each cell on the chip separately, creating an array of (usually) red, green and blue cells. Thus, two colors are never recorded at exactly the same position. For closely spaced cells, this poses no problem in practice, since color fusion (see section 5.3) will ensure that this is not detectable to the human eye. Another technique, which can provide better quality (resolution), but is much more costly, is to use a separate detector for each color, using a beam splitter to separate the incoming light. Triplets of color values are usually combined as individual picture elements (pixels), each pixel containing three values varying from 0-255, giving the intensity of red, green and blue light for that pixel.

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Chapter 8 Porous silicon

Porous silicon was discovered in the mid fifties [13]. Since then it has had an undying attention in scientific literature. Much work has been done to better understand this complex material. The attention became even greater after reports of room-temperature photoluminescence [14] and electroluminescence [15]. Exploitation of the material as a gas sensor, as described in this work, is only one of many applications currently under investigation. Both its versatility and its relative ease of fabrication make it applicable in many different fields. One of these fields, gas sensing (see also [16]), is explored in paper 1. This chapter gives additional background information concerning fabrication, structure and analysis of the material.

8.1 Fabrication

The most common fabrication method for porous silicon is electrochemical etching or anodization. This method is fast and simple and provides a large control over the etching process. A schematic overview of the etching setup is shown in Fig. 12. The silicon sample is submerged in a solution

containing hydrofluoric acid (HF), ethanol and water. The ethanol is used because it is assumed to facilitate the removal of hydrogen, which is a byproduct of the etching process. This should result in more uniform porous layers [17]. A galvanostat is used to maintain a constant current during the etching. A constant voltage would not be advantageous, since the resistivity of the sample changes during the etching process, because porous silicon has a higher resistivity than crystalline silicon. Silicon is connected to the positive pole of the galvanostat (hence the name anodization) and a gold wire is used as counter electrode. A reaction pathway for the formation of porous silicon has been suggested by Lehmann and Gösele [18]. According to this model, holes are required to initiate the etching process. For this reason, p-doped silicon needs to be used or holes need to be formed in another way, for example by ultraviolet radiation.

Galvanostat

+

-Silicon

Gold

wire

HF/Ethanol/H

0

₂

Figure 12 overview of the setup for etching porous silicon

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8.2 Structure and analysis

A scanning electron microscopy (SEM) picture of a porous silicon sample is shown in Fig. 13. The dark spots on the picture are the pores, which are of the order of about 5-10 nm in size. The properties of the porous material, such as layer thickness, porosity, pore size and shape, etc, can vary largely depending on etching conditions such as HF concentration, doping concentration, crystal orientation, current density, etc [19].

For the experiments described in this work, pore sizes of 10 nm or less are used. Since these are small compared to the wavelength of the visible light used in the ellipsometry measurements, the material can be treated as a homogeneous medium using an effective medium approximation (EMA, see section 2.3). Designing a good model for a material as complex as porous silicon is however far from trivial. By using a multilayer model [5] with several EMA layers with varying porosity to incorporate a porosity gradient normal to the surface, a reasonable fit can be obtained. For gas sensing purposes, however, extensive modeling is not necessary, since not the absolute properties of the sample are important, but the relative change which arises when gas adsorbs to the surface.

Chapter 8, Porous silicon

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Chapter 9 Paper

9.1 Paper science and research

Even though we are fully submerged in the digital age, paper is still a very important medium. Most people cannot (yet) imagine a world without printed books, newspapers, photos, etc. Because of this, much research is being done on different aspects of paper. The chemistry of paper production [20] comprises a wide range of specializations. Research is done to improve paper quality, increase production efficiency, make production more environmentally friendly, etc. Research on the optical properties of paper is of course an important aspect (see for example [21]). Paper 2 in this work describes optical analysis of paper using spectroscopic ellipsometry, with a special focus on gloss.

9.2 Gloss

Obviously, when paper is used for printing text, images or both, the optical properties of the paper and the ink are of utmost importance, since they determine the readability and/or image quality to a great extent. One very important property is gloss [22]. Gloss deals with the ‘shininess’ of the paper and is determined by measuring the specular reflectance of the paper. Several standards have been developed to measure gloss (see for example [23], [24]). Depending on the application, different gloss characteristics of paper are required. For high quality images, a high gloss value is desirable. Unfortunately, local gloss variations become a critical issue and can have a negative effect on the quality of the image. Much is still unknown about the influence of microstructural and chemical properties of paper on its optical properties. New techniques are therefore constantly explored to gain knowledge in this field.

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Chapter 10 Prospects

Two important tasks remain in the near future for the CSPT fluorescence project. One is to increase sensitivity by filtering out the computer screen light, the other is to miniaturize the experimental setup, making measurements on smaller volumes possible and providing a more convenient format. These two goals are to be realized in a completely new measurement setup. The current plan is to produce small wells using photolithography (see Fig. 14), which will contain the fluorescent substance to be measured. For creating the wells, a photoresist called SU8 will be used, with which layers of up to 2 mm can be obtained within a single processing step. After finishing the setup, computer screen light will enter from below (as seen in the illustration) and the detector will be placed facing the top.

This new design will incorporate some of the ideas developed when designing the old setup, as well as some new ones. The walls of the wells will be coated (by evaporation) with aluminum (or some other highly reflective material), so that they will act as mirrors, reflecting the light towards the detector. The aluminum will also serve as an integrated mask, allowing light to enter only in the wells, where it is needed. To prevent possible quenching of

the fluorescence by the metal, it should not be the inner wall of the well, therefore a coating of photoresist will be applied (more standard photoresist than SU8 can be used here, since no thick layer is required). Note that the procedure shown in Fig. 14 is just a tentative plan, which has not been tested yet.

If the developed setup now is placed in a container, preventing unwanted light from entering, most aspects of the original setup (see paper 3) are present. One important aspect is missing: the mask which prevents unabsorbed computer screen light from entering the detector. The current plan is to solve this in another way. If the exciting light is linearly polarized, the emitted light will at most be partially polarized (see section 4.2). So if a polarizer is placed under the glass and one, with the polarization axis perpendicular to the first one, between the well and the detector, then in principle all the computer screen light will be blocked, while nearly half (depending on the degree of

Glass substrate Al deposition

SU8 depostition SU8 exposure and developing

Al deposition Photoresist deposition

Photoresist exposure

and developing Al etching

Figure 14 Possible deposition procedure for creating wells for the CSPT measurement. Note that the figure is only schematic, not on scale.

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polarization) of the emitted light will be able to pass. Note that the first polarizer is not needed when using an LCD screen, since the light coming from such a screen is already linearly polarized (see section 6.2). One important consideration when using this technique is that in the deposited polymers tension can occur, which can cause the polarization of transmitted light to change. Therefore it is important that the computer screen light does not pass through the polymer. This is the reason why the photolithography is designed in such a way that there is no polymer present on the bottom of the well and is also an additional motivation for the integrated aluminum mask.

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References

[1] E. Hecht, Optics, second edition, Addison Wesley (1987) [2] G.E. Jellison, Thin Solid Films 234, 416 (1993)

[3] R.M.A. Azzam and N. M. Bashara, Ellipsometry and polarized light, North-Holland (1987)

[4] D.E. Aspnes, J.B. Theeten and F. Hottier, Phys. Rev. B 20, 3292 (1979) [5] L.A.A. Pettersson, S. Zangooie, R. Bjorklund and H. Arwin, Mat. Res. Soc.

Symp. Proc, Vol. 431, 259 (1996)

[6] G.D. Christian and J. E. O’Reilly, Instrumental analysis, second edition, Allyn and Bacon, Inc. (1978)

[7] L. Sherwood, Human physiology from cells to systems, third edition, Wadsworth (1997)

[8] G.G. Field, Color and its reproduction, second edition, GATF (1999) [9] N. Gat, Proc. SPIE 4056, 50 (2000)

[10] C.A. Parker, Photoluminescence of solutions with applications to photochemistry and analytical chemistry, American Elsevier, (1968)

[11] S.M. Sze, Physics of semiconductor devices, second edition, John Wiley & Sons (1981)

[12] M. Karplus, R.N. Porter, Atoms and molecules: an introduction for students of

physical chemistry, Benjamin/Cummings (1970)

[13] A. Uhlir, Bell Syst. Tech. J. 35, 333 (1956) [14] L.T. Canham, Appl. Phys. Lett. 57, 1046 (1990)

[15] A. Halimaoui, C. Oules, G. Bomchil, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon and F. Muller, Appl. Phys. Lett. 59, 304 (1991)

[16] H. Arwin, G. Wang and R. Jansson, phys. stat. sol. (a) 197, 518 (2003) [17] G. Bomchil, R Herino, K. Barla and J.C. Pfister, J. Electrochem. Soc. 130, 1611

(1983)

[18] V. Lehmann and U. Gösele, Appl Phys. Lett. 60, 1863 (1991) [19] L. Canham (ed), Properties of porous silicon, INSPEC (1997)

[20] J.C. Roberts (ed), Paper Chemistry, second edition, Chapman & Hall (1996) [21] H. Granberg, Optical response from paper: angle-dependent light scattering

measurements, modeling, and analysis, PhD thesis, Royal institute of

technology, Sweden (2003)

[22] M.C. Béland, Gloss variation of printed paper: Relationship between

topography and light scattering, Ph.D. Thesis, Royal Institute of Technology,

Sweden (2001)

[23] Gloss assessment of plane paper and board surfaces by means of reflectometer values, DIN 54502, March 1992

New methodology for optical sensing and analysis

Linköping Studies in Science and Technology

Licentiate Thesis No. 1090

LiU-TEK-LIC-2004-19

New methodology

for optical sensing and analysis

Laboratory of Applied Optics

Department of Physics and Measurement Technology

Linköping University

SE-581 83 Linköping, Sweden

Linköping, 2004

Preface

Included papers

Table of contents

Chapter 1

Introduction

Chapter 2

Elementary Optics

2.1 Interaction of electromagnetic waves with matter

f

f

f

f

Material 1

Material 1

Material 2

Material 2

a

b

N

N

N

N

E

H

E

H

H

E

E

H

E

H

E

H

2.2 Reflection from layered structures

f

f

f

f

Ambient

Ambient

Substrate

Substrate

Film

a

b

N

N

N

N

N

2.3 Effective medium approximation

å

å

Chapter 3

Ellipsometry

3.1 Principles

3.2 Measurement

3.3 Analysis

Chapter 4

Fluorescence

4.1 Excitation and emission

4.2 Fluorescence in practice

Chapter 5

Color theory and color reproduction

5.1 The eye

5.2 Color vision

S

M