Philip J. Harding

Dynamical ultrafast all-optical switching of planar GaAs/AlAs photonic microcavities

The authors study the ultrafast switching-on and -off of planar GaAs/AlAs microcavities. Up to 0.8% refractive index changes are achieved by optically exciting free carriers at λ = 1720 nm and pulse energy Epump = 1.8±0.18 μJ. The cavity resonance is dynamically tracked by measuring reflectivity versus time delay with tunable laser pulses, and is found to shift by as much as 3.3 linewidths within a few picoseconds. The switching-off occurs with a decay time of ∼ 50 ps. They derive the dynamic behavior of the carrier density and of the complex refractive index. They propose that the inferred 10 GHz switching rate may be tenfold improved by optimized sample growth.

Broadband sensitive pump-probe setup for ultrafast optical switching of photonic nanostructures and semiconductors

We describe an ultrafast time resolved pump-probe spectroscopy setup aimed at studying the switching of nanophotonic structures. Both femtosecond pump and probe pulses can be independently tuned over broad frequency range between 3850 and 21,050 cm(-1). A broad pump scan range allows a large optical penetration depth, while a broad probe scan range is crucial to study strongly photonic crystals. A new data acquisition method allows for sensitive pump-probe measurements, and corrects for fluctuations in probe intensity and pump stray light. We observe a tenfold improvement of the precision of the setup compared to laser fluctuations, allowing a measurement accuracy of better than DeltaR=0.07% in a 1 s measurement time. Demonstrations of the improved technique are presented for a bulk Si wafer, a three-dimensional Si inverse opal photonic bandgap crystal, and z-scan measurements of the two-photon absorption coefficient of Si, GaAs, and the three-photon absorption coefficient of GaP in the infrared wavelength range.

Nanophotonic hybridization of narrow atomic cesium resonances and photonic stop gaps of opaline nanostructures

We study a hybrid system consisting of a narrow-band atomic optical resonance and the long-range periodic order of an opaline photonic nanostructure. To this end, we have infiltrated atomic cesium vapor in a thin silica opal photonic crystal. With increasing temperature, the frequencies of the opals reflectivity peaks shift down by >20% due to chemical reduction of the silica. Simultaneously, the photonic bands and gaps shift relative to the fixed near-infrared cesium D 1 transitions. As a result the narrow atomic resonances with high finesse (ω/Δω=8×10 5 ) dramatically change shape from a usual dispersive shape at the blue edge of a stop gap, to an inverted dispersion line shape at the red edge of a stop gap. The line shape, amplitude, and off-resonance reflectivity are well modeled with a transfer-matrix model that includes the dispersion and absorption of Cs hyperfine transitions and the chemically reduced opal. An ensemble of atoms in a photonic crystal is an intriguing hybrid system that features narrow defectlike resonances with a strong dispersion, with potential applications in slow light, sensing, and optical memory

Instantaneous switching of photonic cavity structure by electronic Kerr nonlinearity

We present time-resolved reflectivity spectra of optically switched GaAs/AlAs photonic structures. We show for the first time instantaneous tuning of the optical properties, using a surprisingly large electronic Kerr effects at femtosecond times.

Photonic interactions of resonant cesium atoms and opal photonic crystals

We present the first-ever experiments on resonant atoms, Cesium vapor, in photonic crystals. The atomic transitions are strongly modified by photonic band structures of opal. Results are interpreted with an improved transfer-matrix model.

Nonsingle exponential switching dynamics of a GaAs/AlAs microcavity

Time and frequency resolved reflectivity and transmission spectra of an optically switched GaAs/AlAs microcavity are presented. We observe non-single exponential behavior of the resonance frequency, in contrast to previous expectations.

Competing ultrafast switching mechanisms in silicon woodpile photonic crystals near telecom wavelengths

The interest to optically switch photonic crystals is gathering momentum due to the inherent high speed of the process. While the speed of switching conventional transistors is limited by heat dissipation, no such limit exists for photonic systems in absence of absorption. Recently, several groups have switched Si photonic crystals via free carrier excitation [1–3]. This switching mechanism is popular because of the large change in refractive index and its high possible repetition rate. However, electronic Kerr switching offers the ultimate potential to switch instantaneously, with a repetition rate limited only by the duration of the pump pulse, instead of free carrier relaxation. Kerr switching could potentially boost the repetition rate from GHz to beyond THz.

Photonic Interactions of Resonant Cesium Atoms and Opal Photonic Crystals

We present the first experiments on resonant atoms, Cesium vapor, in photonic crystals. The atomic transitions are strongly modified by photonic band structures of opal. Results are interpreted with an improved transfer-matrix model.

Femtosecond versus Picosecond All-Optical Switching of 3D Silicon Photonic Crystals near Telecom Wavelengths

This work presents non-degenerate pump-probe experiments on a silicon woodpile photonic crystal. The pump tunes through half the electronic band gap (EG) of Si, and the probe through the blue stop gap edge, as shown by the boxes. The crystal is switched by exciting free carriers via two-photon absorption, and the dynamics of the stop gap is probed at time Deltat after the pump.