Group velocity of the probe light in semiconductor–metal molecules
Zhi-Hong Xiao, HuiFang Zhang, HongZhen Lin
Results in Physics
The group velocity of light is investigated in a hybrid nanostructure comprised of semiconductor quantum dot (SQD) and metal nanoparticle (MNP). In the presence of MNP, the hybrid absorption of the system can be eliminated and forms a broad transparency window even though related dipole transition is not forbidden. Owing to the presence of MNP, there is the formation of the steep dispersion at the transparency regime, thus, the controllable group velocity of light can be implemented. The group
... elocity of light is changed from subluminal to superluminal via adjusting the inter-particle distance between SQD and MNP. Nanomaterials incorporating semiconductor quantum dot (SQD), metal nanoparticle (MNP), dye molecular, and metal surface have recently received great attention. For instance, the hybrid structure composed of semiconductor quantum dots (SQDs) and metal nanoparticles (MNPs) is focused due to its unique physical properties [1-9]. SQDs have many advantages, such as bright photo-luminescence, tunable color, and high photostability. Putting a SQD in the vicinity of a proper MNP, the SQD and the MNP are coupled by Coulomb interaction and the coupling strength depends on the geometry of the hybrid structure. In such hybrid structure, many new optical effects are investigated, such as exciton/plasmonic induced transparency [5, 8] , photonic induced diffraction grating , etc. In this letter, we studied the group velocity of light in a SQD-MNP hybrid structure based on exciton/plasmonic induced transparency. When a proper coupling field is applied, the hybrid absorption of the system dramatically decreases until it vanishes in the presence of the MNP, and there is the steep dispersion at a transparency regime. Thus, the controllable group velocity of the probe light can be implemented. Further analysis shows that the light propagation can be changed from subluminal to superluminal by coherent exciton-plasmon interaction. Fig. 1a describes a hybrid nanostructure composed of a spherical semiconductor quantum dot (SQD) with radius b and a spherical metal nanoparticle (MNP) of radius a. e b is the background dielectric constant. e s and e m are the dielectric constants of SQD and MNP, respectively. D p and D c denote the detuning given by D p = x 12 À x p and D c = x 23 -x c , respectively. The SQD and MNP are separated by a distance R. Fig. 1b shows the energy scheme of the system. The plasmonic excitations of the MNP are a continuous spectrum; the excitations of the SQD are excitons with discrete energy levels. The interband transition j2i $ j3i is excited by a strong coupling field with frequency x c and the Rabi frequency is given by X c ¼ l 23 E c = h, and a weak probe field with frequency x p drives the interband transition j1i $ j2i, and the Rabi frequency X c ¼ l 12 E c = h, where l 12 and l 23 are the transition dipole moments of the SQD. The induced polarization of the probe light will be P ¼ b vE p , and R ( k, where k is the wavelength of the probe field. Moreover, we use the dipole approximation, assuming a, b < R, where . Reðx S 2 a e effm Á e effs Á R 6 e effm ¼ ð2e b þ e m Þ=3e b ; e effs ¼ ð2e b þ e s Þ=3e b ; r ¼ ðe m À e b Þ=ð2e b þ e m Þ: The external applied fields are parallel to the major axis of the system (S a = 2). Used parameters are l 12 ¼ 0:67nm;c 21 ¼ 1ns À1 ; c 31 ¼ 3ns À1 ; b ¼ 1; s ¼ 6; N ¼ 3 Â 10 21 m À3 .