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

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

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

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

Microcavity strongly doped with CdSe nanocrystals

The fabrication and the study of sol gel-microcavities strongly doped with CdSe nanocrystals are presented. The Fabry-Perot microcavities are formed by a Distributed Bragg Reflector covered with a CdSe doped active layer and a silver mirror. These microcavities are studied with reflectometry experiments. We observe that the cavity resonance peak is enlarged by a factor two when his spectral position corresponds to the first absorption line of the CdSe nanocrystals. Transfer matrix method calculation taking into account the CdSe absorption have been performed and are in good agreement with the experimental values. Copyright Springer-Verlag Berlin/Heidelberg 2004

Exciton nonlinearities and optical gain in colloidal CdSe/CdS dot/rod nanocrystals

We characterized the optical nonlinearities of CdSe nanocrystals surrounded by rod-like CdS shells with ultrafast measurements of time-resolved photoluminescence. We measured the exciton-exciton interaction to be, depending on structure details, attractive or repulsive, by as much as 31 meV, due to the unique band alignment in the CdSe/CdS. This feature makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, as it appears combined with 80% photoluminescence quantum yield, narrow size and shape distributions and the antenna effect of the CdS rod shell enhancing optical absorption by more than a factor 50 with respect to bare dots. © 2010 Materials Research Society

Exciton-Exciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals

Exciton-exciton interaction in dot/rod CdSe/CdS nanocrystals has proved to be very sensitive to the shape of nanocrystals, due to the unique band alignment between CdSe and CdS. Repulsive exciton-exciton interaction is demonstrated, which makes CdSe/CdS dot/rods promising gain media for solution-processable lasers, with projected pump threshold densities below 1 kW cm(-2) for continuous wave lasing