Frantisek Uherek

Influence of surface oxidation on plasmon resonance in monolayer of gold and silver nanoparticles

Surface plasmon resonance of gold and silver nanoparticle (NP) layers is investigated by the experiment as well as simulations. Although the good agreement was found for gold NP film, a significant mismatch in the resonance energy for silver NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in silver NPs. As an alternative to the NP size-dependent Drude model, the analysis based on effective medium approximation for refractive index of Ag-Ag2O material system is carried out and compared with the core-shell model. Both Mies model and numerical simulation results illustrate shift of the surface plasmon resonance due to silver NP surface oxidation.Surface plasmon resonance of gold and silver nanoparticle (NP) layers is investigated by the experiment as well as simulations. Although the good agreement was found for gold NP film, a significant mismatch in the resonance energy for silver NP film was observed. The deviation was assigned to the presence of silver oxide (Ag2O) in silver NPs. As an alternative to the NP size-dependent Drude model, the analysis based on effective medium approximation for refractive index of Ag-Ag2O material system is carried out and compared with the core-shell model. Both Mies model and numerical simulation results illustrate shift of the surface plasmon resonance due to silver NP surface oxidation.

Light emission from InGaN quantum wells grown on the facets of closely spaced GaN nano-pyramids formed by nano-imprinting

InxGa1-xN/GaN quantum wells have been grown on the {1011} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.

Plasmonic properties of Au-Ag nanoparticles: Distinctiveness of metal arrangements by optical study

The core-shell arrangement of binary compound plasmonic nanoparticles (NPs) is usually verified by plasmonic extinction spectra, since microscopy-based methods cannot provide analysis of many NPs. Here, we discuss possible scenarios of different metal arrangements: (i) core-shell model, (ii) bimetallic model, and (iii) mixture of pure metal NPs. The possibility of distinguishing individual cases is discussed in accordance with numerical simulations and an alternative characterization is suggested.

Design of a tunable RCE photodetector for the 1550 nm wavelength range

We present the design of a InGaAs/InP tunable RCE photodetector for the wavelength of 1550 nm. A coupled resonator with two Bragg mirrors has been used to get a broader resonator mode, so that the whole tuning range is enhanced by the cavity. The conditions for the efficient operation of a highly selective tunable photodetector are discussed.

Intense Cr:forsterite-laser-based supercontinuum source

Supercontinuum pulses covering the range from 1100 to 1700 nm with energies >1.0  mJ and excellent beam quality are generated via nonlinear spectral broadening of Cr:forsterite (1240 nm, 110 fs) pulses in pressurized molecular nitrogen. Our spectra, which extend over more than half an octave, offer an attractive alternative to intense few-cycle pulse synthesis in the 1-2 μm range and lend themselves as an important add-on to Cr:forsterite laser technologies.

Ridge-waveguide InAs/GaAs lasers

We report on the reduction of current spreading of InAs/GaAs lasers by using ridge-waveguide geometry. For the investigation of current spreading, lasers with various ridge widths 5, 10, 20 and 50 /spl mu/m are prepared. The laser properties are experimentally studied by the light output versus current characteristics (L-I) and near field patterns (NFP). Threshold current densities (J/sub th/) as a function of ridge width are compared to oxide-stripe geometry lasers with the same material structure. Finally, lasers with ridge-waveguide geometry show a reduction of total J/sub th/ for all ridge widths. Furthermore, the fundamental zero-order mode TEM/sub 00/ is achieved for 10 /spl mu/m ridge width.

Design, simulation, evaluation, and technological verification of arrayed waveguide gratings

Abstract. We present the design, simulation, evaluation, and technological verification of various low-index optical demultiplexers based on arrayed waveguide gratings (AWGs). When designing such optical demultiplexers, a set of input geometrical parameters must be first calculated. They are essential to create AWG layout that will be then simulated using commercial photonics tools. However, these tools do not support or support only partially such a fundamental calculation. Therefore, a new stand-alone tool called AWG-Parameters was developed, which strongly reduces the time needed for the design. From the calculated geometrical parameters, the AWG layouts were created and simulated using three commercial photonic tools: Optiwave, (Ottawa, Ontario, Canada), Apollo Photonics, (Ancaster, Ontario, Canada), and R-Soft, (Pasadena, California). The designs were also technologically verified. The simulated/measured transmission characteristics were evaluated by our newly developed AWG-Analyzer tool. This tool provides calculations of AWG transmission parameters, which are also missing in commercial photonic tools. Additionally, the tool provides clear definitions of calculated transmission parameters together with their textual and graphical representations. Finally, the transmission characteristics and parameters achieved from different photonic tools were compared with each other and discussed in detail. The simulated results were also compared with the measurements. Very good agreement was achieved between theoretical (AWG-Parameters tool), simulated (commercial photonic tools), and fabricated AWG transmission parameters.

Design, simulation and evaluation of AWG based demultiplexers

We present the design, simulation and evaluation of low-index 8-channel, 100 GHz optical demultiplexer based on arrayed waveguide gratings (AWG). The AWG structure was designed using AWG-Parameters tool and the layout was created and simulated using three commercial photonic tools: Optiwave, Apollo Photonics and R-Soft. The design was also technologically verified. The output simulated/measured transmission characteristics were evaluated by AWG-Analyzer tool. The transmission parameters, calculated from simulated/measured characteristics, show very similar results however, the best agreement was achieved between the measurement and the Optiwave simulation.

Performance evaluation of 2D wavelength/time optical CDMA system with optical hard-limiters by numerical modeling

We present the modification of a decoder for 2D wavelength/time (W/T) optical code division multiple access (O-CDMA) system by optical hard-limiters. The performance improvement of the system using the proposed decoder modification is shown by numerical simulation of the bit error probability as a function of active user number and weight of codeword for different decoder topologies. The influence of the non-ideal properties of the optical hard-limiters used on the bit error probability in the system is studied. The performance evaluation shows that the dependence of bit error probability versus the codeword weight has a local minimum, which allows an optimum codeword weight to be chosen for achieving the minimum bit error probability in a 2D W/T O-CDMA system for given system parameters.

Performance analysis of an optical CDMA system with non-ideal optical hard-limiters

Abstract Performance analysis of an incoherent time spread on–off keying optical Code Division Multiple Access (O-CDMA) system with non-ideal optical hard-limiters is reported in this paper. For the first time we made an approximation of the properties of an implemented passive optical hard-limiter. It was named as a non-ideal optical hard-limiter and its non-ideal parameter was defined. Shortened optical orthogonal codes generated by an extended set technique achieved multiple accesses in the investigated O-CDMA system. Performance improvement of O-CDMA system for the cases of O-CDMA receiver without any, with one and with two optical hard-limiters is shown by simulation of the signal-to-interference ratio and the bit error probability. The influence of non-ideal properties of the optical hard-limiter upon the performance of O-CDMA system is studied.