We examine the multiple exciton population dynamics in PbS quantum dots by ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We simultaneously probe the first three excitonic transitions over a broad spectral range. Transient
spectra show the presence of first order bleach of absorption for the 1S_h-1S_e transition and second order bleach along with photoinduced absorption band for 1P_h-1P_e transition. We also report evidence of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures of carrier multiplication (multiexcitons for the single absorbed photon) from analysis of the first and second order bleaches, in the limit of low absorbed photon numbers (<N_{abs}>~ 10^-2), at pump energies from two to four times the semiconductor band gap. The multiexciton generation efficiency is discussed both in terms of a broadband global fit and the ratio between early- to long-time transient absorption signals.. Analysis of population dynamics shows that the bleach peak due to the biexciton population is red-shifted respect the single exciton one, indicating a positive binding energy.
We describe the strong optomechanical dynamical interactions in ultrahigh-Q/V slot-type photonic crystal cavities. The dispersive coupling is based on a mode-gap photonic crystal cavities with light localization in an air mode with 0.02(lambda/n)3 mo
dal volumes while preserving optical cavity Q up to 5 x 106. The mechanical mode is modeled to have fundamental resonance omega_m/2pi of 460 MHz and a quality factor Qm estimated at 12,000. For this slot-type optomechanical cavity, the dispersive coupling gom is numerically computed at up to 940 GHz/nm (Lom of 202 nm) for the fundamental optomechanical mode. Dynamical parametric oscillations for both cooling and amplification, in the resolved and unresolved sideband limit, are examined numerically, along with the displacement spectral density and cooling rates for the various operating parameters.
We investigate the nonlinear response of photonic crystal waveguides with suppressed two-photon absorption. A moderate decrease of the group velocity (~ c/6 to c/15, a factor of 2.5) results in a dramatic (30x) enhancement of three-photon absorption
well beyond the expected scaling, proportional to 1/(vg)^3. This non-trivial scaling of the effective nonlinear coefficients results from pulse compression, which further enhances the optical field beyond that of purely slow-group velocity interactions. These observations are enabled in mm-long slow-light photonic crystal waveguides owing to the strong anomalous group-velocity dispersion and positive chirp. Our numerical physical model matches measurements remarkably.
We demonstrate experimentally an air-slot mode-gap photonic crystal cavity with quality factor of 15,000 and modal volume of 0.02 cubic wavelengths, based on the design of an air-slot in a width-modulated line-defect in a photonic crystal slab. The o
rigin of the high Q air-slot cavity mode is the mode-gap effect from the slotted photonic crystal waveguide mode with negative dispersion. The high Q cavities with ultrasmall mode volume are important for applications such as cavity quantum electrodynamics, nonlinear optics and optical sensing.
We report the observations of spontaneous Raman scattering in silicon photonic crystal waveguides. Continuous-wave measurements of Stokes emission for both wavelength and power dependence is reported in single line-defect waveguides in hexagonal latt
ice photonic crystal silicon membranes. By utilizing the Bragg gap edge dispersion of the TM-like mode for pump enhancement and the TE-like fundamental mode-onset for Stokes enhancement, the Stokes emission was observed to increase by up to five times in the region of slow group velocity. The results show explicit nonlinear enhancement in a silicon photonic crystal slow-light waveguide device.
We show that a scalable photonic crystal nanocavity array, in which single embedded quantum dots are coherently interacting, can perform as an universal single-operation quantum gate. In a passive system, the optical analogue of electromagnetically-i
nduced-transparency is observed. The presence of a single two-level system in the array dramatically controls the spectral lineshapes. When each cavity couples with a two-level system, our scheme achieves two-qubit gate operations with high fidelity and low photon loss, even in the bad cavity limit and with non-ideal detuning and decoherence.
We propose and demonstrate the digital resonance tuning of high-Q/Vm silicon photonic crystal nanocavities using a self-limiting atomic layer deposition technique. Control of resonances in discrete steps of 122 +/- 18 pm per hafnium oxide atomic laye
r is achieved through this post-fabrication process, nearly linear over a full 17 nm tuning range. The cavity Q is maintained in this perturbative process, and can reach up to its initial values of 49,000 or more. Our results are highly controllable, applicable to many material systems, and particularly critical to matching resonances and transitions involving mesoscopic optical cavities.
By virtue of a silicon high-Q photonic crystal nanocavity, we propose and examine theoretically interactions between a stationary electron spin qubit of a semiconductor nanocrystal and a flying photon qubit. Firstly, we introduce, derive and demonstr
ate for the first time the explicit conditions towards realization of a spin-photon two-qubit phase gate, and propose these interactions as a generalized quantum interface for quantum information processing. Secondly, we examine novel single-spin-induced reflections as direct evidence of intrinsic bare and dressed modes in our coupled nanocrystal-cavity system. The excellent physical integration of this silicon system provides tremendous potential for large-scale quantum information processing.
