N.F. van Hulst

The effect of higher-order dispersion on slow light propagation in photonic crystal waveguides

We have studied the dispersion of ultrafast pulses in a photonic crystal waveguide as a function of optical frequency, in both experiment and theory. With phase-sensitive and time-resolved near-field microscopy, the light was probed inside the waveguide in a non-invasive manner. The effect of dispersion on the shape of the pulses was determined. As the optical frequency decreased, the group velocity decreased. Simultaneously, the measured pulses were broadened during propagation, due to an increase in group velocity dispersion. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. The asymmetry increased as the group velocity decreased. The asymmetry of the pulses is caused by a strong increase of higher-order dispersion. As the group velocity was reduced to 0.116(9)·c, we found group velocity dispersion of -1.1(3)·106 ps2/km and third order dispersion of up to 1.1(4)·105 ps3/km. We have modelled our interferometric measurements and included the full dispersion of the photonic crystal waveguide. Our mathematical model and the experimental findings showed a good correspondence. Our findings show that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion.

Near-field scanning optical microscopy in liquid for high resolution single molecule detection on dendritic cells

Clustering of cell surface receptors into micro‐domains in the plasma membrane is an important mechanism for regulating cellular functions. Unfortunately, these domains are often too small to be resolved with conventional optical microscopy. Near‐field scanning optical microscopy (NSOM) is a relatively new technique that combines ultra high optical resolution, down to 70 nm, with single molecule detection sensitivity. As such, the technique holds great potential for direct visualisation of domains at the cell surface. Yet, NSOM operation under liquid conditions is far from trivial. In this contribution, we show that the performance of NSOM can be extended to measurements in liquid environments using a diving bell concept. For the first time, individual fluorescent molecules on the membrane of cells in solution are imaged with a spatial resolution of 90 nm. Furthermore, using this technique we have been able to directly visualise nanometric sized domains of the C‐type lectin DC‐SIGN on the membrane of dendritic cells, both in air and in liquid.

Single-molecule pump-probe experiments reveal variations in ultrafast energy redistribution

Single-molecule pump probe (SM2P) is a novel, fluorescence-based technique that allows the study of ultrafast processes on the single-molecule level. Exploiting SM2P we have observed large variations (from 1 ps to below 100 fs) in the energy redistribution times of chemically identical molecules in the same sample. Embedding the molecules in a different matrix or changing the excitation wavelength does not lead to significant changes in the average redistribution time. However, chemically different molecules exhibit different characteristic redistribution times. We therefore conclude that the process measured with the SM2P technique is dominated by intramolecular energy redistribution and not intermolecular transfer to the surrounding matrix. The matrix though is responsible for inducing conformational changes in the molecule, which affect the coupling between electronic and vibrational modes. These conformational changes are the main origin of the observed broad distribution of redistribution times.

An Evanescent Field Optical Microscope

An Evanescent Field Optical Microscope (EFOM) is presented, which employs frustrated total internal reflection on a highly localized scale by means of a sharp dielectric tip. The coupling of the evanescent field to the sub‐micrometer probe as a function of probe‐sample distance, angle of incidence and polarization has been characterized quantitatively both experimentally and theoretically. The coupling efficiency of light into the tip agrees with a description based on complex Fresnel coefficients. By scanning the tip images have been obtained of non‐conducting dielectric samples, periodic gratings and non periodic structures, containing both topographic and dielectric information which clearly demonstrate the capacity of the evanescent field optical microscope for nanometer scale optical imaging. The effect of field gradient, tip‐sample distance, polarization direction and tip artifacts on the images has been investigated. Recent results are presented.

Propagation and localization of quantum dot emission along a gap-plasmonic transmission line

Plasmonic transmission lines have great potential to serve as direct interconnects between nanoscale light spots. The guiding of gap plasmons in the slot between adjacent nanowire pairs provides improved propagation of surface plasmon polaritons while keeping strong light confinement. Yet propagation is fundamentally limited by losses in the metal. Here we show a workaround operation of the gap-plasmon transmission line, exploiting both gap and external modes present in the structure. Interference between these modes allows us to take advantage of the larger propagation distance of the external mode while preserving the high confinement of the gap mode, resulting in nanoscale confinement of the optical field over a longer distance. The performance of the gap-plasmon transmission line is probed experimentally by recording the propagation of quantum dots luminescence over distances of more than 4 μm. We observe a 35% increase in the effective propagation length of this multimode system compared to the theoretical limit for a pure gap mode. The applicability of this simple method to nanofabricated structures is theoretically confirmed and offers a realistic way to combine longer propagation distances with lateral plasmon confinement for far field nanoscale interconnects.

The effect of higher order dispersion on slow light propagation in photonic crystal waveguides

We have studied the dispersion of femtosecond pulses in a photonic crystal waveguide. We found that slow propagating pulses were asymmetrically broadened, due to higher order dispersion. With decreasing group velocity, the asymmetry increased.

Local Probing of Higher-Order Dispersion in Photonic Crystal Waveguides

One of the exciting features of photonic crystals is that light may travel at very low group velocities at specific optical frequencies. In photonic crystal waveguides, such low group velocities are also possible. We have studied the propagation of femtosecond pulses in these waveguides, with a phase-sensitive and time-resolved near-field microscope. With this microscope, we visualized the pulses as they propagate in the waveguide. Moreover, we monitored the pulse envelope in time, at various positions in the structure, in order to study the pulse dispersion. We found, that the pulses indeed propagate more slowly as we approached the Brillouin zone boundary by decreasing the optical frequency. As the group velocity is reduced, a broadening of the pulses was observed. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. This asymmetric broadening can be attributed to an increase in higher-order dispersion. Our findings show, that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion, even at moderate group delays