The role of exciton-exciton interaction on nonlinearities in GaN microdisks S. Shojaei a,b and F. Troiani b and A. Asgari a and M. Kalafi a and G. Goldoni b a Photonics-Electronics Group, Research Institute for Applied Physics and Astronomy(RIAPA), Tabriz, Iran; b Istituto Nazionale per la Fisica della Materia (INFM) and Dipartimento di Fisica, Universit` a degli Studi di Modena e Reggio Emilia, Via Campi 213/A, 41100 Modena, Italy ABSTRACT Large built-in piezoelectric fields in nitride nanostructures, because of their wurtzite structure, induce a spatial seperation between confined electrons and holes and lead to formation of electric dipoles. This paper investigates the effects of exciton-exciton interaction as a dipolar interaction in a GaN/Al x Ga 1-x N microdisk. We show how this interaction result in biexciton binding energies in the meV energy range. Also we study the effect of disk radius on exciton binding energy. Results show that the exciton binding energy in smaller disks, is larger than the bigger one. Keywords: Biexciton, Binding energy, Piezo-electric field 1. INTRODUCTION Nitride semiconductors have recently attracted much interest because of their potentialities in semiconductor electronics 1 and optoelectronics. Their wide direct band gap makes them suitable for the implementation devices emitting in the blue/UV spectral region. 2–7 single-photon sources 8 and low-threshold lasers 9 has been recently demonstrated in GaN quantum dots and microdisks, respectively. The wurtzite structure that characterizes most III-nitride systems, additionally implies some relevant differences with respect to, e.g., GaAs-based structures. 10 These include the presence of large built-in piezo-electric fields, which lead to a spatial separation of electrons and holes in GaN quantum wells and dots. Such relative displacement causes the spontaneous formation of large electric dipoles, which can in turn induce a long-range (dipole-dipole) coupling between photogenerated excitons. It was shown 11 in view of electro-optical properties of GaN/AlN coupled quantum dots, that the most interesting thing is exciton-exciton dipole interaction in first neighbors dots, since this will allow to implement a scheme for quantum information processing on nitride based QDs, exploiting excitons resonantly excited by femtosecond laser pulses. 12 In order to achieve analogous effects in commonly used III-V quantum dots, strong external fields have to be applied instead of built-in fields. It was suggested that the biexcitonic shift due to dipole interaction and resulting optical nonlinearities can be used for designing a semiconductor based fully-optical quantum information processing strategy and for imple- mentation of conditional quantum gates. 13, 14 More recently, it was shown that nonlinearities arising from the polariton-polariton interactions within a photonic dot could result in the observation of nonclassical properties in the emitted light (photonic blockade). 15 If the photonic confinement volume is small enough, the presence of just one polariton can block the resonant injection of a second one, since the polariton-polariton interaction shifts the resonance frequency by an amount of the order of the linewidth or even larger. The motivation of this work is the contribution of coloumb induced interaction between two excitons, on non- linearities, in semiconductor microcavities and microdisk lasers. 16, 17 In order to explain whatever happened in foregoing systems, we introduce model hamiltonian for excitons and photons in confined systems. Theoretical treatment can be started by recalling the quantum Hamiltonian model describing a quantum well exciton stongly coupled to planar microcavity photon mode. This treatment can be symplified by adopting a two band descrip- tion for the electronic system in order to describe a typical direct gap semiconductor such as GaN, GaAs, so in Further author information: S. Shojaei: E-mail: saeid.shojaei@unimore.it, skype: saeed.shojaei Nonlinear Optics and Applications III, edited by Mario Bertolotti, Proc. of SPIE Vol. 7354, 73541I · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.820538 Proc. of SPIE Vol. 7354 73541I-1