Molecular isomerization and fragmentation of polyatomic molecules controlled by inner-valence recollision-ionization

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets

Two and One Dimensional Light Propagation in Layer-by-Layer Deposited Colloidal Nanocrystal Waveguides

Optical waveguides containing high concentrations of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two-and one dimensional waveguiding in these structures is demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes indicate that the losses are dominated by surface roughness. The deposition on the structured samples does not lead to any additional losses. This fact and the exceptionally high content of nanocrystals make these structures highly suitable for photonic applications like laser or optical amplifiers. Introduction Colloidal semiconductor nanocrystals (NCs) [1] -[3] give a widely size tunable room temperature luminescence Sample Preparation The NC WGs are fabricated by controlled deposition of CdTe NC/polymer films onto glass substrates by the layer-by-layer assembly method. This makes use of the alternating adsorption of (sub)monolayers of positively charged poly(diallyldimethylammonium chloride) (PDDA) molecules and negatively surface-charged CdTe NCs, 60 S. Pichler et al. each provided in aqueous solutions. The total film thickness is controlled by the number of PDDA/CdTe NC bilayers and by the deposition time for each monolayer. For a film with 40 bilayers we obtain a typical thickness of 120 nm. Since the effective thickness of a PDDA molecule (approx. 1 nm) is several times smaller than that of the used CdTe NCs with a diameter of approximately 3.2 nm For the fabrication of 1D WGs, the NC/polymer bilayer films were deposited on substrates which were patterned with grooves, with a width of 5, 10, 20 and 40 µm and a length of 2 cm. The grooves with a depth of 450 nm were wet chemically etched by a buffered HF solution whereby a 30 nm thick Cr layer was used as etch mask (see inset of Results and Discussion The waveguiding properties of the NC/polymer films are studied by propagation loss measurements, making use of the NC luminescence. In particular, an Ar-ion laser was used to excite the NCs from a direction perpendicular to the sample surface while the photoluminescence (PL) emitted in lateral direction is collected from the sample edge by a microscope objective. The PL spectra are recorded as a function of the distance z, measured between the excitation spot, which is moved by a mirror, and the edge of the sample (see inset in The luminescence spectra observed for the NCs in the grooves To show that the WG losses are dominated by the surface roughness, we investigated 1D WGs with various layer thicknesses and widths. Reducing the thickness from 40 to 10 NC/PDDA bilayers results in an increase of the loss coefficient by a factor of 2. This is almost independent on the width of the substrate grooves, varied between 5 and 40 µm. Conclusion In summary the high potential of layer-by-layer deposited NC/PDDA films for applications in optical devices is demonstrated. In these films, deposited on planar as well as on patterned glass substrates two and one dimensional waveguiding is observed with penetration length of several centimeters

Selective control over fragmentation reactions in polyatomic molecules using impulsive laser alignment

We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.FWN – Publicaties zonder aanstelling Universiteit Leide

Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives

This paper discusses the fundamentals, applications, potential, limitations, and future perspectives of polarized light reflection techniques for the characterization of materials and related systems and devices at the nanoscale. These techniques include spectroscopic ellipsometry, polarimetry, and reflectance anisotropy. We give an overview of the various ellipsometry strategies for the measurement and analysis of nanometric films, metal nanoparticles and nanowires, semiconductor nanocrystals, and submicron periodic structures. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about process control, geometry factors, anisotropy, defects, and quantum confinement effects of nanostructures

Demonstration and control of the fast-absorption recovery times of the InGaN/GaN quantum well saturable absorbers

10.1117/12.756401Proceedings of SPIE - The International Society for Optical Engineering6824-PSIS

Two- and One-Dimensional Light Propagations and Gain in Layer-by-Layer-Deposited Colloidal Nanocrystal Waveguides

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230 cm(-1) from the amplified spontaneous emission. This high value makes the authors' waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.</p

Highly Efficient Molecular Ionization Probed by Few-cycle Laser Pulses

Field ionization of hydrocarbon molecules to high charge states is studied as a function of laser pulse duration, peak intensity and molecular alignment. Results are in agreement with the recently proposed mechanism of multi-bond enhanced ionization

Absorption recovery time reduction in InGaN/GaN quantum well saturable absorbers

10.1063/1.2924412Journal of Applied Physics10310-JAPI

Two- and one-dimensional light propagations and gain in layer-by-layer-deposited colloidal nanocrystal waveguides

Optical waveguides containing high percentages of colloidal nanocrystals have been fabricated by layer-by-layer deposition on planar and patterned glass substrates. The two- and one-dimensional waveguidings in these structures are demonstrated by propagation loss experiments. The experimental results obtained for various film thicknesses and widths of the waveguide stripes together with simulations of the light propagation indicate that the losses are dominated by surface roughness. The variable stripe length method is used to determine the optical gain of 230 cm–1 from the amplified spontaneous emission. This high value makes the authors' waveguide structures very promising for applications in amplifiers and lasers with reduced threshold powers.</p