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

Microfluidic integration of photonic crystal fibers for online photochemical reaction analysis

Liquid-filled hollow-core photonic crystal fibers (HC-PCFs) are perfect optofluidic channels, uniquely providing low-loss optical guidance in a liquid medium. As a result, the overlap of the dissolved specimen and the intense light field in the micronsized core is increased manyfold compared to conventional bioanalytical techniques, facilitating highly-efficient photoactivation processes. Here we introduce a novel integrated analytical technology for photochemistry by microfluidic coupling of a HC-PCF nanoflow reactor to supplementary detection devices. Applying a continuous flow through the fiber, we deliver photochemical reaction products to a mass spectrometer in an online and hence rapid fashion, which is highly advantageous over conventional cuvette-based approaches

Observation of a stronger-than-adiabatic change of light trapped in an ultrafast switched GaAs-AlAs microcavity

We study the time-resolved reflectivity spectrum of a switched planar GaAs-AlAs microcavity. Between 5 and 40 ps after the switching (pump) pulse we observe a strong excess probe reflectivity and a change of the frequency of light trapped in the cavity up to 5 linewidths away from the cavity resonance. This frequency change does not adiabatically follow the fast-changing cavity resonance. The frequency change is attributed to an accumulated phase change due to the time-dependent refractive index. An analytical model predicts dynamics in qualitative agreement with the experiments, and points to crucial parameters that control future applications.Comment: Discussed effect of probe bandwidth. Included functional forms of n(z) and R(z

Flying particle sensors in hollow-core photonic crystal fibre

Optical fibre sensors make use of diverse physical effects to measure parameters such as strain, temperature and electric field. Here we introduce a new class of reconfigurable fibre sensor, based on a 'flying-particle' optically trapped inside a hollow-core photonic crystal fibre and illustrate its use in electric field and temperature sensing with high spatial resolution. The electric field distribution near the surface of a multi-element electrode is measured with a resolution of similar to 100 mu m by monitoring changes in the transmitted light signal due to the transverse displacement of a charged silica microparticle trapped within the hollow core. Doppler-based velocity measurements are used to map the gas viscosity, and thus the temperature, along a hollow-core photonic crystal fibre. The flying-particle approach represents a new paradigm in fibre sensors, potentially allowing multiple physical quantities to be mapped with high positional accuracy over kilometre-scale distances

Child maltreatment : prevalence and risk factors

Hoe hangt kindermishandeling met gehechtheid samen, en wat zijn de risicofactoren die de kans op kindermishandeling vergroten? Het proefschrift van Eveline Euser geeft antwoord op deze vragen, en presenteert de laatste stand van zaken met betrekking tot preventie van mishandeling. Kindermishandeling en gehechtheid Alle mishandelde kinderen zijn gehecht aan hun ouders, zelfs als die ouders de daders zijn. De meta-analyse in dit proefschrift laat echter zien dat mishandelde kinderen vaker onveilig en gedesorganiseerd gehecht zijn, in vergelijking tot kinderen die opgroeien in risicogezinnen (bv. in armoede of met een drugsverslaafde ouder) maar niet mishandeld worden. Kinderen met meer dan vier risicofactoren lopen echter wel evenveel gevaar gedesorganiseerde gehechtheid te ontwikkelen. Verrassend is dus dat de opeenstapeling van sociaal-economische risico’s en kindermishandeling beide een negatieve invloed hebben op de kwaliteit van de gehechtheidsrelatie. Preventie en interventie op het gebied van kindermishandeling. In Nederland zijn momenteel geen wetenschappelijk onderbouwde interventies beschikbaar die specifiek tot doel hebben om kindermishandeling te bestrijden. Evidence-based programma's zullen daarom ontwikkeld moeten worden en getoetst in gecontroleerde experimenten. De huidige praktijk in ons land is dat preventieve opvoedingsprogramma’s die eigenlijk ontworpen zijn voor andere problematiek of in andere landen zijn beproefd (bijv. Triple P, een programma gericht op gedragsproblemen) worden ingezet bij Nederlandse gezinnen met een hoog risico op mishandeling. Uit onderzoek zal nog moeten blijken of deze preventieprogramma’s ook effectief zijn in het voorkomen van kindermishandeling. Nu worden opvoedingsprogramma’s soms overhaast breed uitgerold zonder dat er grondige experimentele evaluaties van eigen bodem voorhanden zijn. De preventie van kindermishandeling is te belangrijk om aan het toeval te worden overgelaten.LEI Universiteit LeidenFSW - Gezinsopvoeding - Ou

Long-term consequences of differences in early growth : epidemiological aspects

The content of this thesis is of two sorts: in one part, three topics about the early origins of adult disease are addressed, preceded by three related methodological studies which form the other part. The thesis starts with a systematic review of the literature about the growth of infants born preterm. Next, three specific methodological issues related to early origins of adult disease studies are addressed. A: the most favorable regression model for analyzing and interpreting the effect of both prenatal and subsequent postnatal growth on adult health outcomes. B: the efficiency of reliability studies in the context of a multi-centre study. And C: the correct and clear assessment of reliability in case of log transformed outcomes. These methods are used ind the second part, in which three topics about the effects of prenatal and early postnatal growth on adult health outcomes are addressed, namely: A the association between birth weight and adult renal function in non-premature subjects. B: The association between birth weight and the adult metabolic syndrome, and its separate components in the same population. And finally, C: the association between early growth and adult body composition in subjects born very preterm.Novo Nordisk, Pfizer, Ferring, Ipsen Farmaceutica, Eli Lilly Nederland, Friso kindervoeding, VygonUBL - phd migration 201

Fluorescence-based remote irradiation sensor in liquid-filled hollow-core photonic crystal fiber

We report an irradiation sensor based on a fluorescent “flying particle” that is optically trapped and propelled inside the core of a water-filled hollow-core photonic crystal fiber. When the moving particle passes through an irradiated region, its emitted fluorescence is captured by guided modes of the fiber core and so can be monitored using a filtered photodiode placed at the fiber end. The particle speed and position can be precisely monitored using in-fiber Doppler velocimetry, allowing the irradiation profile to be measured to a spatial resolution of ~10 $\mu$m. The spectral response can be readily adjusted by appropriate choice of particle material. Using dye-doped polystyrene particles, we demonstrate detection of green (532 nm) and ultraviolet (340 nm) light.Richard Zeltner acknowledges funding from the Cluster of Excellence “Engineering of Advanced Materials” (www.eam.uni-erlangen.de) at the University of ErlangenNuremberg.This is the author accepted manuscript. The final version is available from AIP Publishing via http://dx.doi.org/10.1063/1.495359