Published: August 31, 2011 r2011 American Chemical Society 19939 dx.doi.org/10.1021/jp206105r | J. Phys. Chem. C 2011, 115, 19939–19946 ARTICLE pubs.acs.org/JPCC Directional Enhancement of Spontaneous Emission in Polymer Flexible Microcavities L. Frezza, †,§ M. Patrini, ‡ M. Liscidini, ‡ and D. Comoretto* ,† † Dipartimento di Chimica e Chimica Industriale, Universit  a degli Studi di Genova, Via Dodecaneso 31, I-16146 Genova, Italy ‡ Dipartimento di Fisica “A. Volta”, Universit  a degli Studi di Pavia, Via Bassi 6, I27100 Pavia, Italy 1. INTRODUCTION Photonic crystal (PhC) structures are characterized by a periodic modulation of the dielectric function on a scale compar- able to the wavelength of interest. Such a modulation allows for the control of the photon dispersion relation and the creation of photonic band gaps (PBGs), that is, frequency ranges for which light propagation in one or more directions is forbidden. 1 Through a careful design of a PhC, it is also possible to achieve light confinement and extremely high electromagnetic field enhancement by inserting “ad hoc” structural/dielectric defects in the periodic structure. Since the pioneering work by Yablonovitch, 2 the control of light emission has been indicated as one of the most interesting and promising applications of PhC structures. 2,3 In particular, suppression, enhancement, or redistribution of spontaneous emis- sion have been and are subjects of intensive investigations. 4À8 In this regard, 1D photonic microcavities have been studied in the framework of spontaneous emission control and, more in gen- eral, lightÀmatter interaction. 9 A 1D PhC microcavity is con- stituted of a defect layer sandwiched between two multilayers (distributed Bragg reflectors, DBRs) that are dielectric mirrors, that is, 1D PhC (Figure 1a). Semiconductor and oxide periodic multilayers have been studied for over 30 years in the fields of linear and nonlinear optics. 10À12 Although these structures are simple regular stacks of layers of different materials, their growth/fabrication can be time- and cost-expensive depending on the material choice and the operation wavelength, which defines the typical layer thicknesses. In this scenario, polymers have been able to attract a growing interest because of their peculiar optical, mechanical, and proces- sing properties. As a matter of fact, polymer PhC can be prepared by exploiting spontaneous self-assembly of suitable block copoly- mers or by spin-coating. At present, block copolymers PhC find major application as color-responsive sensors in wet environ- ment, 13À21 whereas strictly photonic studies are performed with spin-cast structures. Indeed, enhancement of second-harmonic generation of thin polymer films embedded in a dielectric micro- cavity, 22 light-emitting diodes using all-polymer microcavities, 23 and distributed-feedback dye lasing in a polymer multilayer 24À26 have been demonstrated. Different fabrication methods of poly- mer DBR and planar microcavities have been investigated by several authors to achieve a suitable dielectric contrast, to retain an easy processability and chemical compatibility of macromo- lecular components, and to allow their chemical doping with the desired photoactive material. Fluorocarbon polymers have been used to obtain a relatively large dielectric contrast. However, the limited processability of such polymers requires a sophisticated deposition technique (ion-beam sputtering), which is not com- patible with low-cost devices and with processing of conjugated semiconductors or of other organic/hybrid active media. 27À29 The use of coextrusion methods, widely adopted in industrial plants, has been demonstrated to produce polymer multilayers 30 but has not been extensively applied for photonic applications. The spin-coating techniques are instead very powerful to obtain polymer DBR structures because they require only the use of macromolecules or colloids possessing a significant dielectric contrast, which could be dissolved in orthogonal solvents. 8,31À34 The possibility of fabricating high optical quality Received: June 28, 2011 Revised: August 31, 2011 ABSTRACT: We report on the control of spontaneous emission in flexible polymer 1D photonic crystal cavities fabricated by spin coating having a layer of poly(9,9-dioctylfluorenyl-2,7-diyl-co-1,4- benzo-(2,1 0 -3)-thiadiazole) (F8BT) as an active material. The optical properties of these full-polymer photonic crystals are systematically investigated by means of polarized angular-resolved transmittance and photoluminescence spectral measurements. We demonstrate strong directional emission enhancement when the emitter is located in the defect layer and resonantly coupled to the microcavity mode. The experimental results can be successfully reproduced with different theoretical optical models.