We present a simulation method for light emitted in uniaxially anisotropic light-emitting thin film devices. The simulation is based on the radiation of dipole antennas inside a one-dimensional microcavity. Any layer in the microcaviy can be uniaxially anisotropic with an arbitrary orientation of the optical axis. A plane wave expansion for the field of an elementary dipole inside an anisotropic medium is derived from Maxwell’s equations. We employ the scattering matrix method to calculate the emission by dipoles inside an anisotropic microcavity. The simulation method is applied to calculate the emission of dipole antennas in a number of cases: a dipole antenna in an infinite medium, emission into anisotropic slab waveguides and waveguides in liquid crystals. The dependency of the intensity and the polarization on the direction of emission is illustrated for a number of anisotropic microcavities.
©2011 Optical Society of America
Optically anisotropic materials exhibit different material properties depending on the orientation of the electric field. Traditionally they are used in polarizing beam splitters  and most importantly in liquid crystal displays (LCDs) which were responsible for the breakthrough of flat-panel displays in a large number of applications: TV's, mobile phones, all types of electronic devices,... The anisotropic properties of liquid crystals can be controlled by applying a moderate voltage over the device or by other external means. In the last years more and more liquid crystal based optical devices have been reported outside of their use in displays, such as: liquid crystal based lasers , polymerized liquid crystal devices , tunable waveguide devices , switchable/tunable filters ,... Many other applications have been demonstrated or suggested, a more complete overview can be found in a review paper .
Anisotropic materials are also used in other types of photonic devices such as organic light emitting diodes (OLEDs) which are attracting a lot of attention for high quality light sources  and displays . OLEDs feature some unique properties like: low power consumption, potential for cheap manufacturing, excellent colors, new design possibilities, ... Anisotropy has been reported in OLEDs using polymer molecules , whose long chains are naturally anisotropic, but recently also in small molecule OLEDs . OLEDs made with polymerized liquid crystals have also been fabricated .
With the growing number and increasing diversity of applications for optically anisotropic materials, there is a need for accurate simulation and design tools for such applications. In this article we present a simulation method to calculate the radiation of an electrical dipole antenna inside a thin anisotropic film or in a stack of uniaxially anisotropic layers. This method simulates light emission, taking into the account the anisotropy of the materials, inside liquid crystal devices, anisotropic OLEDs and many other thin film devices. Thin layer architectures are often encountered in devices made by spincoating, deposition from vapor phase or liquid filling of cells.
Our method is based upon the decomposition of the dipole field into a superposition of plane and evanescent waves. The radiation pattern of the dipole inside the layer structure is modified by reflection, transmission and interferences of these waves by interfaces inside the layer stack. Simulation of one-dimensional thin film stacks using plane wave decomposition was pioneered by Lukosz  and has been applied in many fields  for stacks containing only isotropic materials. Wasey describes an extension of this model to take in account anisotropic layers but only allows anisotropic materials with optical axes perpendicular to the layer interfaces . The method we present here can simulate the radiation of an electric dipole antenna inside a stack of one-dimensional anisotropic layers with arbitrary orientation of the optical axes. Earlier methods are restricted to stacks of isotropic materials or to stacks of uniaxial anisotropic materials with specific orientations of the optic axis . Other methods such as the finite-difference-time-domain (FDTD) method or rigorous coupled wave analysis (RCWA) can be used to model the radiation from non-planar structures at the expense of more computation resources [15, 16].
In section 2 the plane wave decomposition of the dipole field in an infinite medium and the alterations of the dipole field caused by layered stacks are described. The method described in section 2 is then applied to examples of anisotropic devices in section 3. First the radiation of dipole in a homogeneous anisotropic medium is compared to that of a dipole in an isotropic layer enclosed by an anisotropic medium. The radiation of dipoles in an anisotropic waveguiding layer bound by a metal electrode is discussed with particular attention to the polarization and propagation direction of the waveguided modes. The effects of coupling between the ordinary and extra-ordinary waves is also illustrated. Finally we calculate the radiation of a dipole into the modes of a waveguide in a liquid crystal. We summarize our results in section 4.
2. Calculation method
2.1. Physical background & notations
We consider the problem of a radiating elementary electrical dipole inside a stack of uniaxial anisotropic one-dimensional layers. The problem of a classical elementary electrical dipole antenna is equivalent to the emission of photons by an ensemble of excited states decaying through an electric dipole transition . The coordinate system is chosen so that the x- and y-axis are parallel to the layer stack and the z-axis is normal to the stack. The orientation of the dipole moment p is defined by an inclination angle ν (with respect to the z-axis) and an azimuth angle ζ in the xy-plane. We choose the origin of the xyz-system to coincide with the location of the dipole. Each uniaxial medium i inside the layer stack is characterized by the orientation of the optical axis c i (extra-ordinary polarization) and the two eigenvalues of the dielectric tensor and , where and are the eigenvalues respectively in a plane perpendicular (ordinary polarization) and parallel to c i. The orientation of c i is also determined by an inclination angle and azimuth angle. A sketch of the coordinate system and layer stack is presented in Fig. 1 .
Any monochromatic electric field E can be written as a superposition of plane and evanescent waves. The electric field of a single plane or evanescent wave is written as:
Because of Snell's law waves with given kx and ky only couple to waves with the same kx and ky in the other media. This transverse part of k can be grouped to . In an anisotropic medium the value for kz depends on the medium and the polarization of the plane wave. In uniaxial materials the electric field of the ordinary polarization is perpendicular to c, the extra-ordinary polarization has an electric field component parallel to c. The wave-vector can also be written as a sum of a part perpendicular and a part parallel to c. The amplitude of the wave-vector in medium i is given by the z-component can be found with . λ is the wavelength of the light in vacuum and ni is the refractive index. For uniaxial anisotropic media there is a different refractive index for the ordinary waves ni,o and extra ordinary waves ni,e. The value of ni,o is the same for every direction in the medium but ni,e depends on the direction of propagation. Throughout this paper the subscripts ∥, ⊥ and z are used to denote components respectively parallel to c, perpendicular to c and parallel to z.
A plane wave with travels in a certain direction, the inclination angle of propagation is found with , the azimuth angle is found with . When , is an imaginary number and the wave is an evanescent wave that decays exponentially with z. In non-absorbing media no power is dissipated by evanescent waves. No direction of propagation can be associated with evanescent waves since these only have a non-zero field in a small area. Evanescent waves represent the near-field of the dipole.
2.2. Radiation of elementary dipoles in a homogeneous anisotropic medium
In the following paragraphs the plane wave decomposition of the dipole field is derived. Reference  rewrites Maxwell's equations as uncoupled differential equations for 4 scalar Hertz potentials Θ, Ψ, Π and Φ. An uncoupled differential equation is obtained for Θ and Ψ (Eq. (74) and 75 in ):
A solution for Θ and Ψcan be found by performing a spatial and temporal Fourier transform on Eq. (3) to (7). Using the relations: , and , an expression for Θ and Ψ in the Fourier domain can be easily derived:
From the scalar Hertz potentials Ψand Θ the two remaining scalar Hertz potentials Φ and Π can be found (by performing the Fourier transform of Eq. (76) and 77 in ):
Taking the Fourier transform of Eq. (80) in  then allows to calculate E in the Fourier domain from the scalar Hertz potentials Θ and Ψ and source term u:
The electric field in xyz-coordinates is then found by the inverse Fourier transform of Eq. (13) along kx, ky and kz.
Only the inverse Fourier transform along kz is explicitly calculated to obtain the plane wave decomposition E( k x, k y ). The integral of the field in Eq. (13) over kz can be split in two terms and separately solved using contour integration. The first term of Eq. (13) has two poles:
and . These poles can either be purely real () or contain an imaginary part ().
The poles of the second term of Eq. (13) are given by:Eq. (14) is performed by choosing a contour so that for , when z>0 this is the lower half of the complex kz plane. In case the poles are purely real a small loss term is added to and and after integration the limit is taken. For each term only one pole lies inside the contour and only this pole is evaluated when performing the integration. For z>0 and purely real poles, the poles inside the contour are:
In this way expressions are found for E (for z>0):19]. The results from both approaches are in agreement. For the field in (20), is independent of the direction of c, this is called the ordinary polarization. For the field in (21), does depend on the direction of c and E e is called the extra-ordinary polarization. From the expression of E, H can be determined using , bearing in mind that the value of kz depends on the polarization.
The power flux through a plane with constant z radiated by a dipole is then found by integrating the z-component of the Poynting vector S (unit W/m2) over that plane:
The * denotes complex conjugation. E and H in Eq. (22) are two separate double integrals over dkx dky. When integrating over a plane of constant z we can bring both E and H under the same double integral. Because of orthogonality an extra factor is needed in the new integrand K:
The integrand K (unit W.m2) can be split in an ordinary Ko and an extra ordinary part Ke. K is calculated for the field in a plane with z>0 (K+) or a plane with z<0 (K-). The total power radiated by a dipole F (in W) is:
From Eq. (20) and (21) it is also possible to derive the unit vectors of the ordinary and extra-ordinary eigenmodes of a uniaxial medium. The electric field of a mode is found by multiplying the unit vector with a complex amplitude. Looking at Eq. (20) and (21) it is clear that the ordinary field cannot have a component along c but the extra-ordinary field can. In an anisotropic medium the electric fields of the eigenmodes are no longer perpendicular to each other (unlike in an isotropic medium).
2.3. Radiation of dipole inside an anisotropic microcavity
The radiation pattern of a dipole antenna can be significantly altered by placing the dipole inside an optical microcavity . Interferences between reflections at the different layer interfaces cause variations in the local density of states at the location of the dipole antenna. As a result the angular emission pattern of the dipole is modified, this is also called the Purcell effect. Such microcavities are often made by depositing a series of thin films. The lateral dimensions of the films are much larger than their thicknesses and so the microcavities are in good approximation one-dimensional.
2.3.1. A dipole in an isotropic microcavity
For isotropic microcavities a model for dipole radiation based on plane wave decomposition was developed by Lukosz  and has been applied to simulate a wide range of applications . The starting point of the Lukosz method is the plane wave decomposition of the field of a dipole antenna in an infinite medium . The electric field of the dipole in an infinite medium is then altered by interference from the reflections of the various interfaces of the microcavity. A schematic can be seen in Fig. 2 .
The combined reflections lead to the following formula for the amplitudes of the TE and TM waves in an isotropic microcavity based on the amplitude of the field in an infinite medium (found with Eq. (20) and (21)) and the complex field reflection coefficients of the top and bottom parts of the cavity and :
and include reflections within the top and bottom stacks and phase delays by propagation over distances and respectively. The superscript +/− marks waves that respectively have a Poynting vector S with positive or negative z-component . In Eq. (27) and (28) the numerator expresses the effect of wide angle interference, i.e. interference of the upward emitted wave with the reflection of the downward emitted wave (or vice versa). The denominator embodies multiple beam interference: it sums all reflections between the top and bottom parts of the cavity. and may be calculated as described in . Since and are different for the transverse magnetic (TM) and transverse electric (TE) polarization, a separate calculation is done for each polarization.
2.3.2. A dipole in an anisotropic microcavity
For an anisotropic microcavity a similar method can be applied to calculate the radiation of a dipole in the cavity. The eigenmodes of anisotropic materials are two linearly polarized waves, the ordinary (o) and the extraordinary (e) wave (instead of the TE and TM polarization). The normalized fields of the eigenmodes are given by Eq. (25) and (26). The complex amplitude of each mode in an infinite medium is given by Eq. (20) and(21). The amplitude of modes travelling in the –z direction is found with the corresponding expression for z<0. The polarization state of the light is determined by the complex amplitudes of the ordinary and extra-ordinary waves and their difference in phase. One must be careful to simulate all changes in polarization that occur during propagation and reflection or transmission in an anisotropic cavity. In anisotropic media e- and o-waves are coupled when reflection or transmission at an interface takes place, therefore the reflection coefficients have to be replaced by reflection matrices. Equation (27) and (28) should be replaced by:
The reflection matrices are:Eq. (29) and (30) become identical to (27) and (28) respectively.
2.3.3. The scattering matrix method
To calculate the reflection matrix we employ the scattering matrix method introduced by Ko . Other methods, such as the Berreman 4x4 matrix method , can also be applied but we have chosen the scattering matrix method because it is numerically more stable when dealing with evanescent waves and total internal reflection.
The scattering matrix method calculates a 4x4 scattering matrix that relates incoming waves in layer i traveling upward and in the top layer N traveling downward with the outgoing waves in the top layer traveling upward and in the layer i traveling downward . is a block matrix of four 2x2 matrices. A sketch of the input and output waves in the scattering matrix method is shown in Fig. 3 .
Once the matrix is determined, the reflection matrix and the transmission matrix can be identified. and link the outgoing waves to and .
An analogous procedure is used to determine and . The fields emitted to the outside layer (N or 0) can be calculated from :
Starting from the corresponding magnetic field () and Poynting vector (Eq. (22)) is calculated.
The scattering matrix of an entire stack can be built by starting from which is equal to the unity matrix and then adding extra layers step by step. Ko provides a formula for calculating the scattering matrix of a stack with an additional layer when and the matrix are known . relates the fields above the j + 1/j interface to the fields above the interface j/j-1.
The relation between and is:
We construct in the following way (a sketch is shown in Fig. 4 ). First the complex amplitudes of the input waves propagate from the j-1/j interface to the j/j + 1 interface, this causes a phase change proportional to the thickness of the layer dj, expressed by the matrix :
At the interface j/j+1 all transverse fields have to be constant, this is expressed by the following boundary conditions: , , and . From the complex amplitudes of the four eigenmodes in medium j + 1 at the j/j + 1 interface and the unit fields of the four eigenmodes (given by Eq. (25) and (26)), the field components , , and at the j/j+1 interface can be calculated. The matrix that links the amplitudes of the four eigenmodes in medium j+1 with the four field components , , and at the j/j+1 interface is written as .
The matrix elements of the first two rows of are the x- and y components of Eq. (25) and (26). The corresponding magnetic field (per unit electric field) is found with (unit ), the x- and y-component are the respective matrix elements of the third and fourth row.
In summary (see Fig. 4), relates the amplitudes of the eigenmodes in medium j at the j-1/j interface to the amplitude of the eigenmodes in medium j at the j+1/j interface, (or ) relates the amplitudes of the four eigenmodes to the four transverse field components in medium j (or j+1) and relates the amplitudes of the eigenmodes at the j-1/j interface in medium j to the amplitudes of the eigenmodes at the j+1/j interface in medium j+1. The boundary condition at the j+1/j interface can then be expressed as:
This equation has to hold for any input amplitudes , so that:Eq. (42) and (41), this equation allows to calculate for every layer. is found with Eq. (37)-(40) after successively adding all layers. and are identified with Eq. (33) and (34).
3. Calculation examples
In this section we demonstrate the abilities of the method described in section 2 by applying it to three problems: the radiation of a dipole in an infinite medium, the emission inside a multilayer structure containing an anisotropic layer and one-dimensional waveguides in liquid crystal. For these calculations the method is implemented as a computer program.
3.1. Emission from a thin isotropic film into an anisotropic medium
As a reference we calculate the total power F radiated by a dipole emitting with a wavelength in an infinite anisotropic medium for various orientations of the dipole moment. We consider a dipole oriented along the x, y or z-axis (respectively called px, py and pz) inside an anisotropic medium (, ) with . We calculate F by evaluating the integral in Eq. (24). We then also calculate F for a dipole situated in the middle of an isotropic layer (n=1) with thickness d which is bound on top and bottom side by a semi-infinite anisotropic medium with the same properties as described above. F is then calculated as a function of d (a schematic is shown in Fig. 5 ). The dipole moment is chosen so that this dipole radiates a power of 1 Watt in vacuum. In Fig. 6 F is shown as a function of the thickness d of the isotropic layer. For a dipole in a homogeneous anisotropic medium an analytical formula for F (Watt) can be found that depends on the refractive indices of the material and the angle ρ between p and c:
Figure 6 shows F in an anisotropic medium for dipoles px, py and pz. The solid lines represent F calculated with the anisotropic plane wave decomposition. The dashed lines show F for a dipole in an isotropic layer of different thicknesses. The results of the simulation for a dipole in a homogeneous anisotropic medium are the same as the results of the analytical formula (Eq. (45)). The calculation with a dipole in an isotropic layer between anisotropic media agrees with the analytical solution for . The reflections at the layer interfaces lead to interference effects, which are a function of the thickness d of the layer and of the projections of the wave vector k x and k y. The constructive and destructive interferences in the emission pattern, lead, after integration, to an oscillating behavior of the emitted power F. As the layer thickness increases from 100nm to the µm range the number of minima and maxima in the integration increases which causes a damping of the oscillation. For very thick isotropic layers, the anisotropic material no longer influences F.
3.2. Emission and waveguiding in a thin anisotropic layer
As an example of an anisotropic multilayered structure we calculate the radiation pattern of a dipole into a film of anisotropic material deposited on a thick glass substrate and covered with a metal mirror. The configuration (depicted in Fig. 7 ) is an optically thick glass substrate covered with a thin isotropic emitting layer and an anisotropic layer with higher refractive index than the substrate (the parameters are for the liquid crystal 5CB) and an Aluminum mirror. The following material parameters are used: , , , , . The orientation of 5CB is either or . An isotropic waveguiding layer () with the same thickness was taken as a reference. The thicknesses of the layers are 5nm, 1000nm and 200nm for the emitting layer, 5CB and Al respectively. The dipole is situated in the middle of the emitting layer. K is calculated for an ensemble of randomly oriented dipoles by averaging . All simulations are done for the emission wavelength .
The radiation of a dipole is investigated by considering the power density per interval . The unit of Kis W.µm2, so that after integration over (unit µm−2, see Eq. (24)), we obtain the total power emitted by the dipole. Again we have chosen the dipole so that 1 Watt would be emitted by the dipole in vacuum. expresses the inclination angle in which the wave travels in a medium, if a wave can no longer propagate in the medium i. For waveguided modes is the effective index of propagation of that mode.
Figure 8 is a plot of as a function of for the isotropic reference. In Fig. 8a) is shown for the entire range of , Fig. 8b) shows a close-up of the radiation for . For plane waves can enter the glass substrate and is a smooth function. Interference between the waves emitted towards the glass and reflections of waves emitted towards the Al (wide angle interference) causes minima and maxima in the vs. curve. These plane waves radiate energy to the outside of the device, and are sometimes called leaky modes. Waves with are trapped inside the waveguiding layer by total internal reflection. In this region the distribution is characterized by a discrete number of waveguided modes. In a completely lossless structure waveguided modes have zero width. In our simulation the modes have a finite width because of absorption in the aluminum layer. For even larger () the waves are evanescent in the waveguiding layer and no power is radiated (in a loss-free medium). For the isotropic reference, modes exist for both polarizations up to , as can be seen in the close-up Fig. 8b).
In Fig. 9 is shown for an anisotropic waveguiding layer with (a total overview is shown in Fig. 9a), Fig. 9b) is a close up of the waveguiding range). For a continuous distribution of radiated waves is seen, with an interference pattern similar to the isotropic reference. For the presence of waveguided modes is polarization dependent because of the anisotropy. For both ordinary and extra-ordinary polarized modes exist. For ordinary polarized waves are evanescent and hence no ordinary polarized modes are present. However extra-ordinary polarized waves are not evanescent, so extra-ordinary polarized modes are seen. Once no more modes are observed, because both ordinary and extra-ordinary waves are now evanescent. Since the layer stack and the orientation of the optical axis are invariant for rotation around the z-axis, the angular distribution is the same for every azimuth angle of propagation ϕ.
In a structure which is no longer invariant to rotation around the z-axis, the radiation of the dipole will also depend on the azimuth angle ϕ. Figure 10 and Fig. 11 show for a waveguiding layer with . Figure 10a) and Fig. 11a) show an overview of the entire range for the o- and e-polarized waves respectively. Figure 10b) and Fig. 11b) are close ups of the waveguiding regions for o- and e-polarization. In the region a continuous function of radiated waves is seen as in the previous cases. For the o-polarization waveguided modes appear in the range. For e-polarized waves the behavior of waveguided modes depends on the azimuth angle of propagation. Extra-ordinary waves propagating in the yz-plane () have an electric field mainly parallel to c and waveguided modes occur for . Extra-ordinary waves propagating in the xz-plane () have an electric field perpendicular to c and experience a refractive index equal to . In the xz-plane waveguiding only occurs between . For e-waves with an azimuth angle the effective index of refraction is between and and and waveguided modes are seen for .
3.3. Coupling between ordinary and extra-ordinary waves
In an anisotropic multilayered structure the o- and e-waves are coupled by reflection and transmission at the interfaces in the cavity. This coupling influences the polarization and direction in which a dipole emits. We have simulated the emission from a dipole in the following layer structure (see Fig. 12 ): an optically thick glass substrate with a 1000nm thick anisotropic layer, a 5nm anisotropic emitting layer and another 1000nm anisotropic layer, above that there is a thick layer of air. The optical axes of the anisotropic layers are parallel to the xy-plane. The optical axes of the anisotropic layers on the glass and air side have an azimuth angle respectively and , for the emitting layer . The refractive indices of the materials and the wavelength are the same as in the previous example. A small absorption term is added to the refractive indices of 5CB, ne,5CB and no,5CB, so that waveguided modes have a non-zero width.
Figure 13 shows , and as a function of (with azimuth angle of propagation) for a dipole along the x-axis. We can distinguish 3 regions in the emission pattern. For is continuous and positive, however the flux of o-waves through this plane is negative. A net flux of radiation of the ordinary waves towards the emitter is possible because of coupling between e- and o-waves. The dipole emits both e- and o-waves, but a fraction of the e-waves couples to o-waves (and vice-versa) upon reflection, which makes it possible to find a net negative flux . In the range waves cannot escape through the anisotropic/air interface because of total internal reflection. In this region is positive and is negative. The net flux is positive but small because the absorption in the anisotropic layer is small and most emitted light is reflected back. For waveguided modes are found for both e- and o- polarization. There is still emission in the ordinary polarization for values of because the evanescent o-waves couple to the waveguided e-waves.
3.4. Anisotropic waveguides
Waveguides in liquid crystals are another example of anisotropic microcavities. A one-dimensional waveguide in a liquid crystal can be created by applying a voltage to a liquid crystal cell . The orientation of the liquid crystal is locally altered by the applied electric field and in this way a refractive index profile is created for e-waves inside the cell. A dipole antenna inside the liquid crystal can emit light to the outside world and into the waveguide. We calculate the radiation pattern of randomly oriented dipoles inside a one dimensional waveguide, with the configuration sketched in Fig. 14 . A thick layer of 5CB is sandwiched between two optically thick substrates. The inclination angle α of 5CB is assumed to follow a Gaussian profile around the center (z=0) of the LC layer. The profile of the inclination angle becomes . The full width at half maximum of this profile is determined by σ (). The Gaussian profile is built up as a stack of homogeneous 100nm thick layers with the inclination angle of each layer given by the Gaussian distribution for the middle of that layer. The refractive indices of 5CB and wavelength () are the same as in the previous example. Again a small absorption term is added to the refractive indices of 5CB and so that waveguided modes have a non-zero width.
A typical vs. plot for plane waves propagating in the xz-plane is shown in Fig. 15a ). In the region waveguided modes occur for both o- and e-polarization. These are waves trapped in the entire LC-layer between the two glass substrates. For only e-polarized modes occur. These are waveguided by the refractive index profile created by the varying orientation of the LC inside the cell. Figure 15b) shows the effective indices of refraction for the waveguided modes as a function of the FWHM of the Gaussian profile. For a small FWHM the waveguide is single-mode, as the FWHM increases, higher order modes arise.
In this paper we presented a simulation method for light emission inside uniaxial anisotropic thin film devices. In this method the electric field of an elementary dipole inside an anisotropic layer is written as a superposition of plane and evanescent waves. A scattering matrix formalism for plane and evanescent waves is used to calculate the emission inside one-dimensional microcavities. The dipole emission is calculated for a number of interesting examples. A comparison between the emission from an anisotropic material and the emission from an isotropic layer embedded in anisotropic material was made. We illustrated the dependence of waveguided modes in anisotropic layers on the polarization and the direction of propagation with respect to the optical axis of the material. As a third example we calculated the modes of a one dimensional waveguides in an inhomogeneous liquid crystal and the effective indices of refraction for such modes.
The work leading to these results has received funding from the IWT (Flemish Institute for Science and Technology). The authors acknowledge the Belgian Science Policy (project IAP 6/10-photonics@be). The work leading to these results has received funding from the European Community’s Seventh Framework Programme under grant agreement no. FP7-224122 (OLED100.eu).
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