Amplified Spontaneous Emission in Semiconductor‐Nanocrystal/Synthetic‐Opal Composites: Optical‐Gain Enhancement via a Photonic Crystal Pseudogap

Abstract

Interest in the possibility of compact, low-threshold lasers has motivated investigations of the emission of active materials embedded in photonic crystal (PC) structures. The sharp decrease in the group velocity of light at the photonic band edge increases the interaction time of the light with the gain medium, enhancing optical gain. [1,2] Such gain enhancements have been observed in a variety of PC systems, including onedimensional (1D), [3] two-dimensional (2D), [4] and three-dimensional (3D) [5] structures. We focus on opals, which are 3Darrangements of spheres packed in a face-centered cubic (FCC) lattice. While colloidal nanocrystals (NCs) have been infiltrated into opals [6] and such materials have demonstrated modification of the spontaneous emission of the NCs, [7,8] amplified spontaneous emission (ASE) from a colloidal NC/opal material has not been demonstrated before. We present novel materials consisting of a titania sol-gel/CdSe or PbSe NC nanocomposite [9,10] infiltrated into polystyrene opal PCs. We demonstrate that these sol-gel/NC/PC composites exhibit efficient ASE at decreased thresholds relative to the reference sol-gel/NC materials despite the lower volume loading of NCs in the photonic structure. This observation indicates an effective increase of the optical gain as a result of the influence of the first L-point gap edge in the opal. This L-point gap corresponds to the lowest energy gap for light traveling in the <111> direction within the FCC opal structure (referred to as simply the L-point gap). In this work, opal samples are prepared via the vertical deposition technique from polystyrene particles of both 269 and 600 nm in diameter.[11] The opal growth occurs in the <111> direction. Both the volume fraction of particles in the precursor solution and the particle size dictate the number of resulting opal layers. After slow evaporation on a vibration-isolation table, strong opalescence is observable in the samples, with a 40 to 50 % maximum reflectivity. These opals are sintered at 100°C for 2 h to prevent them from falling apart during infiltration and then combined with sol-gel/NC composites via controlled dipping and withdrawal.[12] Sol-gel/NC composites are very attractive infiltration materials because of their large NC volume fractions and stable photoluminescence (PL) quantum yields. While these materials do not provide sufficient index contrast to open a complete photonic bandgap, [13,14] the resulting L-point gap still modifies the emission properties of the NCs. Figure 1a shows a typical micrograph of an infiltrated 269 nm opal sample. The sol-gel/NC composite does not infiltrate into pores and cracks, leaving a seemingly uniform coating on the polystyrene surfaces. The L-point gap is manifested as a strong peak in the reflectance spectra. After infiltration with the CdSe NC/titania sol-gel composite, the normal incidence opalescence changes from green to bright red, closely matching the color of the raw sol-gel by eye. The dipping process used to combine the sol-gel/NC composi...

Spectral Redistribution in Spontaneous Emission from Quantum‐Dot‐Infiltrated 3D Woodpile Photonic Crystals for Telecommunications

“…A mixture of 0.8 g PbO, 2.86 g oleic acid, and technological grade 1-octadecene (ODE) (to a total mass of 16 g) was heated to 150°C. An injection solution of 0.64 g selenium, 6.0 g trioctylphosphine and technological grade ODE (to a total mass of 10.1 g) was swiftly injected and the system was then cooled to room temperature. The Synthesized QDs were purified by chloroform/methanol solution repeatedly and finally dissolved in toluene after filtered by a 0.2-lm filter.…”

“…A QD film with a thickness of ∼ 0.5-1 lm was formed outside the PC during the infiltration, thus providing a direct and convenient reference for our studies of QD emission inside the PC. Unlike incorporation techniques performed with composites, [6] this method allows the infiltration and washing processes to be repeated many times for a given sample, giving a reliable comparison between different QD loading levels in the PC. A concentration-dependent red-shift of the stop gap of the infiltrated 3D PC was observed, as shown in Figure 2a.…”

“…2a). Lead chalcogenide (PbSe, PbS, PbTe) nanocrystals have been identified to be useful for telecommunication photonic devices [6,13] due to their tuneable NIR optical properties. We utilized PbSe QDs with emission properties matching the bandgaps of the 3D woodpile PCs.…”

“…[7] Owing to their readily tuneable emission properties, nanocrystal quantum dots (QDs) have been incorporated into 3D opal PCs as an active medium. [3,6,8,9] Radiation dynamics and spectral modification of spontaneous emission [3] have been investigated with QDs emitting visible light. However, experimental study on emission control of QDs with 3D PCs in the near-infrared (NIR) wavelength region, which holds a key to next generation photonic devices for telecommunications, [10][11][12][13] has not yet been demonstrated.…”

“…[3][4][5][6] Understanding the inhibition and enhancement of spontaneous emission from active materials embedded in a three-dimensional (3D) PC is fundamentally important in the field of quantum optics, and could also lead to technology breakthroughs in next generation photonic devices such as highly efficient solar cells and low-threshold lasers. [7] Owing to their readily tuneable emission properties, nanocrystal quantum dots (QDs) have been incorporated into 3D opal PCs as an active medium.…”