Andras G. Pattantyus-Abraham

Site-Selective Optical Coupling of PbSe Nanocrystals to Si-Based Photonic Crystal Microcavities

A novel method for patterning optically active colloidal PbSe nanocrystals on Si surfaces is reported. Oleate-capped PbSe nanocrystals were found to adhere preferentially to H-terminated Si surfaces over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface of a silicon-on-insulator wafer prepatterned with photonic crystal microcavities. Aqueous HF was then used to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. This patterning technique allows for the selective deposition of PbSe nanocrystals at the main antinode of the silicon-based microcavities. More than a 10-fold photoluminescence enhancement due to the cavity-nanocrystal coupling was observed.

Emission spectrum of electromagnetic energy stored in a dynamically perturbed optical microcavity

An ultrafast pump-probe experiment is performed on wavelength-scale, silicon-based, optical microcavities that confine light in three dimensions with resonant wavelengths near 1.5 mum, and lifetimes on the order of 20 ps. A below-bandgap probe pulse tuned to overlap the cavity resonant frequency is used to inject electromagnetic energy into the cavity, and an above-bandgap pump pulse is used to generate free carriers in the silicon, thus altering the real and imaginary components of the cavitys refractive index, and hence its resonant frequency and lifetime. When the pump pulse injects a carrier density of ~ 5 x10(17) cm(-3) before the resonant probe pulse strikes the sample, the emitted radiation from the cavity is blue-shifted by 16 times the bare cavity linewidth, and the new linewidth is 3.5 times wider than the original. When the pump pulse injects carriers, and thus suddenly perturbs the cavity properties after the probe pulse has injected energy into the cavity, we show that the emitted radiation is not simply a superposition of Lorentzians centred at the initial and perturbed cavity frequencies. Under these conditions, a simple model and the experimental results show that the power spectrum of radiation emitted by the stored electromagnetic energy when the cavity frequency is perturbed during ring-down consists of a series of coherent oscillations between the original and perturbed cavity frequencies, accompanied by a gradual decrease and broadening of the original cavity line, and the emergence of the new cavity resonance. The modified cavity lifetime is shown to have a significant impact on the evolution of the emission as a function of the pump-probe delay.

Efficient coupling of photonic crystal microcavity modes to a ridge waveguide

The unidirectional coupling of a microcavity mode to a ridge waveguide in a slab photonic crystal structure was investigated for the first time. Experimental observation of the coupling efficiency for the signal coupled out of the structure is in good agreement with the result of three-dimensional finite-difference time-domain simulations. The coupling efficiency of the cavity mode to the output channel is ∼60%.

Energy dependence of electronic energy relaxation in poly(p-phenylenevinylene)

Femtosecond time-resolved photoluminescence spectroscopy is used to study the dynamics of optical emission from poly(p-phenylenevinylene) at 77 K over an energy range of 2.19–2.37 eV. All of the decay curves in this energy range can be described by the summation of two exponential functions. Analysis of the spectral dependence of the time constants extracted from these fits indicates that the transient photoluminescence at any given wavelength is due in general to the superposition of three transitions emanating from a single set of inhomogeneously broadened excitons. The three distinct transitions correspond to processes in which two, one, or zero phonons are excited in the host polymer during the emission event. By separating out these three contributions, it is determined that the effective lifetime of the inhomogeneously broadened excitons increases monotonically from 980 fs at 280 meV above the peak in the density of states (DOS), to 114 ps near the peak in the DOS.