# One Dimensional Polymeric Organic Photonic Crystals for DFB Lasers

Authors: F. Scotognella; A. Monguzzi; M. Cucini; F. Meinardi; D. Comoretto; R. Tubino
Journal: International Journal of Photoenergy
 (2008)

---

## Abstract

We present a very simple method to realize a one-dimensional photonic crystal (1D PC), consisting of a dye-doped polymeric multilayer. Due to the high photonic density of states at the edges of the photonic band-gap (PBG), a surface emitting distributed feedback (DFB) laser is obtained with this structure. Furthermore, the incidence angle dependence of the PBG of the polymeric multilayer is reported.


---

## Body

## 1. Introduction

The use of
polymers and dye-doped polymers for the fabrication of photonic crystal is
currently being actively investigated. Photonic crystals, which consist of a
periodic dielectric lattice with a periodicity in the range of optical wavelengths,
are intensively studied in order to achieve control of light propagation. This
property, which can be used to confine, manipulate, and guide photons, should
allow the creation of all-optical integrated circuits and optoelectronic
devices [1–3]. In a medium, the
periodicity of the refractive index can be in one, two, and three dimension and
there are many examples of one-, two-, and three-dimensional photonic crystal
fabricated with several materials [4, 5].
While one-dimensional photonic crystals are inherently structurally simpler
than two- and three-dimensional analogues, only few investigations have been
reported on multilayer stacks of materials, known also as Bragg stacks. These
Bragg stacks can be useful for various photonic and optoelectronic applications.
In 1994, Dowling etal. [6] proposed and
analyzed a surface-emitting laser consisting of a one-dimensional photonic
crystal (1D PC) in which a gain material was incorporated. Similar to a Fabry-Pérot
type laser with a single longitudinal mode, this distributed feedback (DFB)
type laser can oscillate at one of the photonic band edges, where photon
density is significantly enhanced [6]. Although inorganic DFB laser is
commercially available, a small amount of papers reports the use of polymeric
1D PC for this application [7].
Furthermore, some properties, like incidence angle dependence of the photonic
band-gap (PBG) of these dye-doped structures, are usually not investigated.In this paper,
we describe the modeling, fabrication, and optical characterization of a
dye-doped one-dimensional photonic crystal, consisting in alternated stacks of
polyvinylcarbazole (PVK) and cellulose acetate (CA), where each CA stack is
doped with Rhodamine 6G (R6G). The laser emission of the dye in the photonic
structure and the angular dependence of the PBG are also measured and
discussed.


## 2. Modeling

In order to
predict the photonic properties of the structure (Figure1(a)) which we want to
fabricate with the two polymers, we use two-computational tools: MIT photonic
band-gap and Comsol Multiphysics. MIT Photonic Bandgap is a freeware developed
by S. G. Johnson. The program calculates ab initio the band structure of a
periodic dielectric structure. The inputs for the calculation are the
refractive index of the two materials (in our case nPVK=1.683 and
nCA=1.475 [7, 8]) and the ratio between their thicknesses  (dPVK/dCA).
Although the maximum width of a PBG with a 1D PC is obtained by stacking layers
with the same optical path (nPVKdPVK≈nCAdCA), we want to
fabricate a 1D PC where the dye-doped CA layer is 3/4 of the unit cell in order
to increase the absorption of the pump power by the dye. The program assumes
that the structure is infinite in all directions. In Figure 1(b), the calculated
dispersion relation and the photonic density of states for the polymeric 1D PC are
shown. The photonic density of state is very high in proximity of photonic band
edges. The photonic band gap (violet in the figure) is narrow, and this is due
to the relative low refractive index contrast of the two polymers (≈1.14).
Nevertheless, for a DBF laser application, this fact is negligible while the
zero group velocity at the photonic band edge is the main feature. The organic
dye which we choose to dope the CA layer is Rhodamine 6G, that emits between
550 and 600 nm. In order to obtain a PBG centered at about 600 nm (and a
photonic band edge at 590 nm), the thicknesses of the PVK layer and the
dye-doped CA layer have to be 50 and 150 nm, respectively. Comsol Multiphysics
is a commercially available software that solves numerically second-order
differential equations. The inputs for this calculation are similar to the
previous program. But in this case, we can decide the number of layers. In Figure 1(c) an e.m. wave with wavelength of 600 nm (the center of the PBG) is
reflected when it passes through a 39-layers 1D PC.(a) Dye-doped 1D PC, (b)
dispersion relation of the 1D PC (with an infinite number of layer) with MIT photonic
band-gap, and (c) propagation of an e.m. wave (λ = 600 nm) through a 39-layers 1D PC.
(a)(b)(c)


## 3. Experimental

Following [7],
polymers solutions of PVK (Acros Organics, Janssen-Pharmaceuticalaan 3, Geel, Belgium) and CA (Acros Organics) are prepared. The solvents
are chlorobenzene (Sigma
Aldrich, St Louis, Mo, United States) and diacetone alcohol (Acros Organics) for PVK and CA,
respectively. The number-average molecular weights of PVK and CA are 90 000 and 100 000, respectively. So, we have selected a concentration of 24,4 and 53,46 g/L. The CA layer is 0.5 w% doped by R6G. In order to obtain the multilayer, we have
tried with dip coating, but with this technique a deposited polymer film stays too much in contact with the solvent of the other polymer and consequently the solvent of the other polymer can slightly dissolve the film. We can overcome this problem with the spin coating technique, where about 99% of the solvent is ejected in the first second of rotation. A substrate of glass (Carlo Erba, Via Generale Carlo Ferraris, Erba Como, Lombardy, Italy) has been used
and the rotation speeds for PVK and for CA on the glass substrate are 3800 and 4100 rpm and the already coated layers 3600 and 4000 rpm, respectively. In this way, a (PVK/CA)9PVK multilayer is obtained. After each deposition, a
baking of 5 minutes at 80°C has done. We can neglect the refractive index of the substrate because his thickness is 2 mm, and it cannot affect the photonic properties of the
multilayer. The transmittance spectrum has been measured with a Cary Varian 50 spectrophotometer (bandwidth of 1 nm). For the laser emission measurement, a pulsed second harmonic of an Nd:YAG laser
(Quanta System, Via Iv Novembre, 116, Solbiate Olona (VA), Italy) has been used as a pump with a repetition rate and duration of 1 Hz and 7 nanoseconds, respectively. The sample has been obliquely pumped with laser emission of the dye normal to the surface (in the direction of the refractive index periodicity). The photoluminescence was revealed by Jobin Yvon CCD camera (bandwidth of 1 nm). The angular dependence transmittance spectra have been measured with an optical set-up based on fiber optical coupled to an Avantes 2048 compact spectrometers (spectral resolution 1.6 nm) working in the range 250–1100 nm. White light was provided by a tungsten-halogen/deuterium combined lamp.


## 4. Results and Discussion

In Figure2, the
transmission spectrum of the (PVK/CA)9PVK multilayer, where CA
layers are 0.5 w% doped by R6G, is shown. The PBG is centered at about 610 nm
and the photonic band edge at 590 nm. The band-gap is shifted by 10 nm with
respect to theoretical predictions; this is due to the difficult control of the thickness of each deposited stack. In the spectrum, we see also the absorption of the dye around 532 nm and the second order PBG. The depth of the PBG is about 60%. In Figure 3, the photoluminescence spectra of the R6G-doped 1D PC, at different pump energy density, are reported. The photoluminescence signal is revealed normally to the surface of the multilayer. The region between 540 and 570 nm shows the luminescence peaks of the dye, while between 570 and 630 nm
remarkable effects arise from the superimposition of the luminescence of the dye with the photonic structure. In the range of energy where the photonic band-gap is present, the luminescence of the dye is suppressed, while, at the energy corresponding to the first (higher energy) photonic band edge, the emission of the dye exhibits a narrow peak arising from the enhancement of the emission in
the region of the high density of states.Figure 2
Transmission spectrum of the (PVK/CA)9PVK multilayer. The green oval indicates the Rhodamine 6G absorption, while the orange oval indicates photonic band-gap.
At about 400 nm, the higher order photonic band-gap is presentFigure 3
Photoluminescnce spectra of the surface emitting DFB laser made with the dye-doped 1D PC upon increasing the pump energy.Figure4 shows
the trend of the transmission spectrum of the dye-doped 1D PC versus the
incident angle θ. In the range between 520 and 540 nm, the
absorption of R6G is shown, and it, obviously, does not change, whereas the
photonic band-gap, the higher order photonic band-gap, shows a red
shift of the order of 150 nm. The higher order PBG shows a red shift toward the
region of PVK absorption.Figure 4
Incidence angle dependence of the transmission spectrum of the dye-doped 1D PC.From these data,
we can obtain the refractive index and thickness of the multilayer by solving the equations system [9]:
(1)mλBragg=2Dneff2−sin2θ,W=4π|nPVK−nCAnPVK+nCA|,εeff−1εeff+2=xPVKεPVK−1εPVK+2+(1−xPVK)εCA−1εCA+2,
where the first
equation  is Bragg-Snell equation, the second is the expression for the PBG width for a 1D PC and the third is the Lorentz-Lorentz equation. In these equations, m is the diffraction order, λBragg is the
wavelength where the PBG is centered, xPVK is the fraction, the unit
cell fraction of the polymer PVK, neff is the effective refractive
index, θ is the incidence angle measured off from the normal, nPVK(CA) and εPVK(CA) are, respectively, the refractive index and the
dielectric constant of PVK (CA). Solving the system, we calculate the
refractive indexes (n) and the thicknesses (d) of the two layers and we can
compare these with the values that we have found in literature and from simulations
(Table 1). The results of theoretical simulations and the literature data are
in excellent agreement with the equation system solutions.Table 1
Refractive indexes and
thicknesses of the polymers in the 1D PC. Comparison with literature and theoretical predictions.
Spectroscopic
dataLiterature
dataTheoretical sim.nd  (nm)nd  (nm)PVK1.66848.471.68350CA1.470149.401.475150


## 5. Conclusion

In this work, we
have modeled, fabricated, and optically characterized a dye-doped polymeric 1D
PC. The modeling has been useful to predict the photonic properties of a
structure made with the chosen materials. Comsol Multiphysics uses finite
elements algorithms and permits to study finite structures, so the prediction
of the interaction between e.m. waves and aperiodic and disordered structures
with this program is also possible. We have found that the spin coating
deposition technique is good for easy fabrication of polymeric photonic
crystals. The fabricated dye-doped 1D PC, with the photonic band edge centered
in proximity of the laser dye emission, is a low threshold surface emitting DFB
laser. Bleaching of the dye is observed at high pump energy density. We intend
to overcome this problem by doping of the structure with novel laser dye, for
example, organic complexes of lanthanides [10] in which the optically pumped
excited state of the ligand is quickly depopulated by an efficient energy
transfer process to the emitting ion.Furthermore, the
incidence angle dependence of the PBG of the polymeric multilayer has been measured.
This behavior allows us to determine independentlyn and d and will be used for
controlling the spontaneous emission of the dye in the 1D PC.



---

*Source: 389034-2008-06-15.xml*