A low-threshold nanolaser based on hybrid plasmonic waveguides at the deep subwavelength scale

A novel nanolaser structure based on a hybrid plasmonic waveguide is proposed and investigated. The coupling between the metal nanowire and the high-index semiconductor nanowire with optical gain leads to a strong field enhancement in the air gap region and low propagation loss, which enables the realization of lasing at the deep subwavelength scale.By optimizing the geometric parameters of the structure, a minimal lasing threshold is achieved while maintaining the capacity of ultra-deep subwavelength mode confinement. Compared with the previous coupled nanowire pair based hybrid plasmonic structure, a lower threshold can be obtained with the same geometric parameters. The proposed nanolaser can be integrated into a miniature chip as a nanoscale light source and has the potential to be widely used in optical communication and optical sensing technology.     

Radiation losses due to discontinuities in asymmetric three-layer optical waveguides

In this paper, the radiation losses caused by film thickness and refractive index discontinuities in asymmetric slab waveguides are examined theoretically. A technique developed by Mahmoud and Beal for dealing with the continuous part of the radiation spectrum is used. Several numerical examples which show the radiation loss for various waveguide configurations are presented and discussed and, where appropriate, comparisons with other related work made. In the case of asymmetric thickness steps, the radiation losses are found to be appreciably higher than those computed for symmetric guides. The radiation losses for guides having the same thickness were found to decrease as the guide thickness increased. Once the TE₁ mode become guided, the losses decreased rapidly. However, if the two guides had different thickness then it was found that the losses exhibited a minimum.     

Quantitative measurement of low propagation losses at 1.55 mum on planar photonic crystal waveguides

The Fabry-Perot resonance technique has been used to determine the propagation losses of planar photonic crystal (PC) waveguides. The structures are patterned into a GaInAsP confining layer on an InP substrate. Losses as low as 11 dB/mm have been measured on a guiding structure with three missing rows. The influence of the PC guide width and air-filling factor is demonstrated.     

Low-loss transmission band in photonic crystal waveguides with sharp cutoff at a frequency below the bandgap

We present TE transmission measurements of photonic crystal waveguides with high hole radius to period ratio r/Λ = 0.388. This geometry introduces a unique low loss transmission band in addition to the traditional PhC guiding band and very sharp transmission edges for devices with a length of 50 μm or longer. Finite difference time domain and plane wave expansion simulations confirm the results and show that the sharpness of the cutoffs can be explained by the spectral shape of the guiding mode in the band diagram.     

Excitation dependent losses and temperature increase in various hollow waveguides at 10.6 μm

Theoretical and experimental studies have been made on excitation dependent transmission properties of various hollow waveguides at 10.6 μm. A heat problem of the waveguide has been experimentally treated when high powered CO₂ laser light is launched.     

Relationship between field shift and phase constant change in two-dimensional three-layered dielectric or hollow waveguides due to uniform bends

It is shown theoretically that the field shift x and phase constant change in two-dimensional three-layered dielectric or hollow waveguides bent uniformly with large bending radiusR are related by x = 2R/ ₀, where ₀ is the axial phase constant. The relationship predicts that the field distributions of the TE₀ or TM₀ mode shift toward the outward direction of bending, whereas those of other TE _n or TM _n (n=1,2,...) modes shift inward in a hollow waveguide. Characteristic features in dielectric waveguides are also described.     

Neutrino oscillations at reactors: What is next?

We briefly review previous and future reactor experiments aimed at searches for neutrino masses and mixing. We also consider the new idea to seek small mixing-angle oscillations in the atmospheric-neutrinomass-parameter region at Krasnoyarsk.     

Survey of Plasmonic Nanoparticles: From Synthesis to Application

The unique properties of plasmonic nanostructures have fuelled research based on the tremendous amount of potential applications. Their tailor‐made assemblies in combination with the tunable size and morphology of the initial building blocks allow for the creation of materials with a desired optical response. In this respect, it is crucial to synthesize nanoparticles with a defined shape that are at the core of such developments. Moreover, the interaction of individual nanoparticles with an incident electromagnetic field cannot only be influenced by their structure, but in fact, also by their spatial arrangement to each other. To harvest such opportunities, a profound theoretical understanding of these interactions is required as well as concise strategies to create such ordered assemblies. A quantitative evaluation of their optical properties can only be conducted when discrete structures of high uniformity can be achieved. As a consequence, separation steps have to be applied in order to obtain materials of high purity and uniformity. This also allows for an easier structural characterization of the nanoparticles and their assembled superstructures. In this progress report, an overview about the current development in this field of research is provided.     

Time-shift quantum key distribution: Sensitivity to losses

The sensitivity to losses of a recently proposed protocol of time-shift quantum key distribution with the use of decoy states is studied. An attack with discrimination of all states is analyzed. It is established that the use of decoy states ensures the security of the protocol at a high level of losses up to 11.7 dB.     

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) ~ c/10 to a waveguide in which it is v(g) ~ c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.     

Plasmonic nanoparticle monomers and dimers: from nanoantennas to chiral metamaterials

We review the basic physics behind light interaction with plasmonic nanoparticles. The theoretical foundations of light scattering on one metallic particle (a plasmonic monomer) and two interacting particles (a plasmonic dimer) are systematically investigated. Expressions for the effective particle susceptibility (polarizability) are derived, and applications of these results to plasmonic nanoantennas are outlined. In the long-wavelength limit, the effective macroscopic parameters of an array of plasmonic dimers are calculated. These parameters are attributable to an effective medium corresponding to a dilute arrangement of nanoparticles, i.e., a metamaterial where plasmonic monomers or dimers have the function of “meta-atoms”. It is shown that planar dimers consisting of rod-like particles generally possess elliptical dichroism and function as atoms for planar chiral metamaterials. The fabricational simplicity of the proposed rod-dimer geometry can be used in the design of more cost-effective chiral metamaterials in the optical domain.     

Wave reflection and transmission due to defects in infinite structural waveguides at high frequencies

In waveguide structures, waves may be partially reflected by local non-uniformities such as cracks and other defects. The reflection and transmission characteristics associated with the presence of a discontinuity may be used, in principle, to give some indication of both the location and size of the defect. A combined spectral element and finite element (SE/FE) method has been used previously to investigate the effects of local non-uniformities at relatively low frequencies. However, for analysis at higher frequencies, where complex deformation of the waveguide occurs, it is necessary to extend this approach. Such high frequency analysis is necessary if small defects are to be located within the waveguide cross-section. In order to investigate wave propagation at higher frequencies, a combined spectral super element and finite element (SSE/FE) method is presented. This method allows the transmission, reflection and wave conversion at discontinuities to be determined for complex waveguides. As an example of the use of this method, wave reflection and transmission in rails are estimated at frequencies between 20 and 40 kHz for various notional sawcut-like defects of progressively increasing size. This shows the feasibility of the approach for realistic waveguides. However, from these simulations it is shown that defects have to be quite large before they can be detected using a single transducer position on the rail cross-section using train-induced vibration.     

Evaluation of losses and group effective refractive index of Er3+:Ti:LiNbO3 optical waveguides

In this paper, we report some experimental results concerning several types of loss measurements of Er³⁺:Ti:LiNbO₃ optical waveguides by using optical methods. Using the Fabry-Pérot interferometry method, we evaluated the attenuation coefficient of an optical waveguide resonator for a laser radiation having λ = 1.55 μm. We also evaluated the insertion losses, polarization dependent losses, and coupling with external losses. Additionally, from the transmitted spectra of the symmetrical monomode Fabry-Pérot optical waveguide resonator, we were able to evaluate the value of the group effective refractive index.