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

Ge doping influence on CdTe postmelting effect

DTA measurements of CdTe and CdTe +1(2;4;6) at% Ge specimens were performed in the 1140 K < melting point < 1440 K temperature range. The influence of the heating rate and the free space above the melt values on the registered thermal effects was critically investigated. The CdTe melting point as high as 1379±1 K was recorded in some experiments. A series of additional effects has been observed while CdTe and CdTe-Ge were heating, cooling or CdTe-Ge melts holding at 1428±1  K, 1 h. They are interpreted either as due to non-equilibrium crystallization, evaporation-condensation events, and dissociation-association processes or caused by the occurrence of a high-temperature cadmium telluride polymorphic modification

Growing the high-resistive Cd₁₋xZnxTe single crystals from a vapor phase

A modified vapor phase growth method to obtain high-resistive Cd1-xZnxTe single crystals (0 < x < 0.13) is presented. The single crystals (about 25 cm⁻³ in size) with natural faceting were grown by vapor transport in silica ampoules with a special shape using a polycrystalline ingot as initial source material. It is shown that minimization of plastic deformation effect in preparation of the most structurally perfect crystals is possible by a way of heat removal from the crystallization front by radiation. The growth of high-resistive material required careful preparation of the initial charge with close to stoichiometric composition. The obtained crystals were successfully tested for creating the room temperature X-ray and gamma-ray detectors

In and InSe doping influence on CdTe postmelting effect

The influence of small In or InSe (up to 10 mol. %) additions to the CdTe melt on additional endothermic effects (AEE) positions in DTA thermograms was studied. It was observed, that both AEEs at 1392 K, typical for pure CdTe melt, and “own" at Tm+(9-10) K appear in CdTe+In melt heating curves during thermocycling. The melting of CdTe-InSe alloys occurs step-by-step without “own" postmelting effects. The solid CdTe dissociation enthalpy near Tm (ΔHdiss=287±22 kJ/mol) and the CdTe fusion enthalpy (ΔHf=43.85±1.15 kJ/mol) were estimated on the base of the obtained DTA data

Structural changes in molten CdTe

Shear viscosity (h) measurements of CdTe and CdTe + 2 at% In melts were performed using a cup viscometer up to 1403 K. The h(T) dependencies obtained during slow heating and cooling (Vh/c = = 10-15 K/h) show hysteresys near a melting point. According to the η(T) dependencies data drastic changes the melts structure occurred at 1376 K both during the heating and cooling of the melts

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