30 research outputs found
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
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. 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
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
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
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
Recommended from our members
Real-time Doppler-assisted tomography of microstructured fibers by side-scattering.
We introduce the concept of Doppler-assisted tomography (DAT) and show that it can be applied successfully to non-invasive imaging of the internal microstructure of a photonic crystal fiber. The fiber is spun at ~10 Hz around its axis and laterally illuminated with a laser beam. Monitoring the time-dependent Doppler shift of the light scattered by the hollow channels permits the azimuthal angle and radial position of individual channels to be measured. An inverse Radon transform is used to construct an image of the microstructure from the frequency-modulated scattered signal. We also show that DAT can image sub-wavelength features and monitor the structure along a tapered fiber, which is not possible using other techniques without cutting up the taper into several short pieces or filling it with index-matching oil. The non-destructive nature of DAT means that it could potentially be applied to image the fiber microstructure as it emerges from the drawing tower, or indeed to carry out tomography on any transparent microstructured cylindrical object