Spatial homogeneity of optically switched semiconductor photonic crystals and of bulk semiconductors

This paper discusses free carrier generation by pulsed laser fields as a mechanism to switch the optical properties of semiconductor photonic crystals and bulk semiconductors on an ultrafast time scale. Requirements are set for the switching magnitude, the time-scale, the induced absorption as well as the spatial homogeneity, in particular for silicon at lambda= 1550 nm. Using a nonlinear absorption model, we calculate carrier depth profiles and define a homogeneity length l_hom. Homogeneity length contours are visualized in a plane spanned by the linear and two-photon absorption coefficients. Such a generalized homogeneity plot allows us to find optimum switching conditions at pump frequencies near v/c= 5000 cm^{-1} (lambda= 2000 nm). We discuss the effect of scattering in photonic crystals on the homogeneity. We experimentally demonstrate a 10% refractive index switch in bulk silicon within 230 fs with a lateral homogeneity of more than 30 micrometers. Our results are relevant for switching of modulators in absence of photonic crystals

All-optical octave-broad ultrafast switching of Si woodpile photonic band gap crystals

We present ultrafast all-optical switching measurements of Si woodpile photonic band gap crystals. The crystals are spatially homogeneously excited and probed by measuring reflectivity over an octave in frequency (including the telecommunication range) as a function of time. After 300 fs, the complete stop band has shifted to higher frequencies as a result of optically excited free carriers. The switched state relaxes quickly with a time constant of 18 ps. We present a quantitative analysis of switched spectra with theory for finite photonic crystals. The induced changes in refractive index are well described by a Drude model with a carrier relaxation time of 10 fs. We briefly discuss possible applications of high-repetition-rate switching of photonic crystal cavities

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 1720 nm and a pulse energy of 1.8 micro Joules. 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 around 50 ps. The authors 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.Comment: 1.) Replaced figure 1 (linear reflectivity) with a more recent and improved measurement 2.) Included a Figure of Merit for switching and compared to other recent contributions 3.) Explained more precisely the effect of embedded Quantum Dots (namely no effect on measurement) 4.) Changed wording in a few place

Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals

We present ultrafast optical switching experiments on 3D photonic band gap crystals. Switching the Si inverse opal is achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of second order Bragg diffraction where the photonic band gap is predicted. We find good experimental switching conditions for free-carrier plasma frequencies between 0.3 and 0.7 times the optical frequency: we thus observe a large frequency shift of up to D omega/omega= 1.5% of all spectral features including the peak that corresponds to the photonic band gap. We deduce a corresponding large refractive index change of Dn'_Si/n'_Si= 2.0% and an induced absorption length that is longer than the sample thickness. We observe a fast decay time of 21 ps, which implies that switching could potentially be repeated at GHz rates. Such a high switching rate is relevant to future switching and modulation applications

Excitation of higher-order modes in optofluidic hollow-core photonic crystal fiber

Higher-order modes are controllably excited in water-filled kagomè-, bandgap-style, and simplified hollow-core photonic crystal fibers (HC-PCF). A spatial light modulator is used to create amplitude and phase distributions that closely match those of the fiber modes, resulting in typical launch efficiencies of 10–20% into the liquid-filled core. Modes, excited across the visible wavelength range, closely resemble those observed in air-filled kagomè HC-PCF and match numerical simulations. These results provide a framework for spatially-resolved sensing in HC-PCF microreactors and fiber-based optical manipulation

Photonic crystal fibre as an optofluidic reactor for the measurement of photochemical kinetics with sub-picomole sensitivity

Photonic crystal fibre constitutes an optofluidic system in which light can be efficiently coupled into a solution-phase sample, contained within the hollow core of the fibre, over long path-lengths. This provides an ideal arrangement for the highly sensitive monitoring of photochemical reactions by absorption spectroscopy. We report here the use of UV/vis spectroscopy to measure the kinetics of the photochemical and thermal cis-trans isomerisation of sub-picomole samples of two azo dyes within the 19-mu m diameter core of a photonic crystal fibre, over a path length of 30 cm. Photoisomerisation quantum yields are the first reported for "push-pull'' azobenzenes in solution at room temperature; such measurements are challenging because of the fast thermal isomerisation process. Rate constants obtained for thermal isomerisation are in excellent agreement with those established previously in conventional cuvette-based measurements. The high sensitivity afforded by this intra-fibre method enables measurements in solvents in which the dyes are too insoluble to permit conventional cuvette-based measurements. The results presented demonstrate the potential of photonic crystal fibres as optofluidic elements in lab-on-a-chip devices for photochemical applications

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 fs pump and probe pulses can be independently tuned over broad frequency range between 3850 and 21050 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 $\Delta$R= 0.07% in a 1 s measurement time. Demonstrations of the improved technique are presented for a bulk Si wafer, a 3D 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.Comment: 31 pages, 15 figure

Generating high fidelity structured light fields through an ultrathin multimode fibre using phase retrieval

Funding: The authors wish to thank the Engineering and Physical Sciences Research Council (EP/L015889/1, EP/T008369/1 and EP/L016567/1) for financial support during the period of this research. GSDG acknowledges support from a UK Future Leaders Fellowship (MR/T041951/1). KD acknowledges support from the Australian Research Council (FL210100099).Light transmission through a multimode fiber (MMF) has gained major importance for imaging and manipulation. The majority of phase retrieval algorithms used for a MMF implicitly assume light propagation to be described by a unitary operation, yet the transmission matrix of a multimode fiber is inherently non-unitary. It is demonstrated that this erroneous assumption can impede the performance of many commonly used MMF phase retrieval algorithms and demonstrate that the weighted Yang–Gu algorithm outperforms other phase retrieval algorithms in this scenario. Once accounted for, the non-unitary property of the transmission matrix can be leveraged to generate intricate intensity and phase patterns at the output of the fiber, and shape specific output fields. This is experimentally demonstrated by generating Laguerre–Gaussian beams that carry orbital angular momentum, and by forming images in planes offset from the distal end of the fiber facet.Peer reviewe

Long-range optical trapping and binding of microparticles in hollow-core photonic crystal fibre.

Optically levitated micro- and nanoparticles offer an ideal playground for investigating photon-phonon interactions over macroscopic distances. Here we report the observation of long-range optical binding of multiple levitated microparticles, mediated by intermodal scattering and interference inside the evacuated core of a hollow-core photonic crystal fibre (HC-PCF). Three polystyrene particles with a diameter of 1 µm are stably bound together with an inter-particle distance of ~40 μm, or 50 times longer than the wavelength of the trapping laser. The levitated bound-particle array can be translated to-and-fro over centimetre distances along the fibre. When evacuated to a gas pressure of 6 mbar, the collective mechanical modes of the bound-particle array are able to be observed. The measured inter-particle distance at equilibrium and mechanical eigenfrequencies are supported by a novel analytical formalism modelling the dynamics of the binding process. The HC-PCF system offers a unique platform for investigating the rich optomechanical dynamics of arrays of levitated particles in a well-isolated and protected environment.This work was supported by Max Planck Society. R. Z. acknowledges funding from the Cluster of Excellence "Engineering of Advanced Materials" at the Friedrich-Alexander University in Erlangen, Germany

Spectrofluorimetry with attomole sensitivity in photonic crystal fibres

We report the use of photonic crystal fibres (PCF) as spectrofluorimetric systems in which sample solutions are excited within the microstructure of the fibre. The use of intra-fibre excitation has several advantages that combine to enable highly sensitive measurements of fluorescence spectra and lifetimes: long path-lengths are achieved by the efficient guidance of the fundamental mode; sample volumes contained within the micron-scale structure are very small, only a few nanolitres per cm of path; collection and guidance of the emitted fluorescence is efficient and the fluorescence lifetime is unperturbed. Fluorophores in bulk solution can be studied in hollow core PCF, whereas the use of PCF with a suspended, solid core enables selective excitation of molecules in close proximity to the silica surface, through interaction with the evanescent field. We demonstrate the measurement of fluorescence spectra and fluorescence lifetimes in each of these excitation regimes and report the detection of attomole quantities of fluorescein