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

Abstract

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. 4 These favorable characteristics provide a flexible platform for the development of integrated photonic devices based on NCs, including optical amplifiers and lasers operating at room temperature. The main difficulty in achieving NC-based lasing is the very efficient nonradiative Auger recombination, 5 so that laser operation is achieved so far only with pulsed excitation. 6 Strategies to improve the laser performance are ͑a͒ to increase the NC concentration in the active layer, 7 ͑b͒ the use of high-finesse optical feedback structures, 6 and ͑c͒ the optimization of the optical waveguides. While the properties of optical waveguides containing Si nanocrystals fabricated by implantation have been studied in detail, 8 for waveguides containing colloidal NCs only the principal operation has been demonstrated. 9 Therefore, here we present a systematic investigation of twodimensional ͑2D͒ and one-dimensional ͑1D͒ waveguides ͑WGs͒, optically activated by strongly luminescent colloidal CdTe NCs synthesized according to Ref. 10. To obtain structures with smooth surfaces and high NC volume fractions the layer-by-layer deposition technique 11 is applied. The waveguides exhibit propagation loss coefficients less than 1 cm −1 , depending on the thickness of the WGs. The losses are only weakly dependent on the width of the 1D WGs and are determined by the surface roughness. The amplified spontaneous emission, excited by femtosecond pulses, evidences a high optical gain of 230 cm −1 , showing that our WGs are highly suitable as active components in NC-based lasers and amplifiers.The NC WGs are fabricated by controlled deposition of CdTe NC/polymer films onto glass substrates by the layerby-layer assembly method. 11 This simple technique makes use of the alternating adsorption of ͑sub͒monolayers of oppositely charged molecules or nanoparticles. In particular, we use positively charged poly͑diallyldimethylammonium chloride͒ ͑PDDA͒ molecules in combination with negatively surface-charged CdTe NCs, each provided in aqueous solutions with appropriately adjusted pH values. 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 170 nm. Since the effective thick...

Core/Alloyed-Shell Quantum Dot Robust Solid Films with High Optical Gains

“…First, the utilization of large diameter QDs (∼8.2 nm) with a large absorption cross-section increases the absorption of pump light. 22,53,54 We estimate the absorption cross-section of our CdSe/Cd 1−x Zn x Se 1−y S y core/ alloyed-shell QDs to be σ ∼ 10 −14 cm 2 (based on the absorbance data of our cross-linked QD film), which is much larger than typical values of CdSe QDs that are usually around 10 −15 −10 −16 cm 2 . 22,46 Second, highly reflective substrates (silicon, with a 290−295 nm thick SiO 2 surface layer) result in the excitation laser to pass twice through the film.…”

Quantum Dots and Their Multimodal Applications: A Review

“…Several strategies have been proposed to improve the lasing, including increased Qdot concentrations in the active layer, use of improved optical feedback structures, and optimized optical waveguides. Two dimensional waveguides loaded with luminescent colloidal CdTe Qdots grown with a layer-by-layer deposition technique have been investigated [ 321 ]. The waveguides exhibited propagation loss coefficients of <1 cm −1 .…”

“…The losses depended weakly on the width of the waveguides but depend strongly on the surface roughness. High optical gain of ~230 cm −1 was demonstrated using femtosecond pulses [ 321 ], showing that this kind of Qdot waveguides could be suitable for lasers and optical amplifiers. Qdots embedded composites were also used to demonstrate photorefractivity and other nonlinear optoelectronic properties [ 222 , 322 ].…”

Influence of quantum dot concentration on Förster resonant energy transfer in monodispersed nanocrystal quantum dot monolayers

“…Additionally, the LbL technique enables the realization of complex layered structures at the nanoscale on substrates of any shape and morphology and is therefore a very powerful and flexible tool. [12][13][14][15] Thus, it is used for the preparation of many different types of structures and has, in particular, shown valuable results for the preparation of light emitting and guiding structures, [24][25][26][27] photovoltaic devices, [28][29][30] sensors, and detectors. [31][32][33] Artificially designed microparticles and nanoparticles with multiple functionality can be synthesized with the help of the LbL technique and have been proposed for applications in areas such as quantum-information processing, optoelectronics, or biotechnology.…”