Optical losses and absorption cross-section of silicon nanocrystals

 Si-rich silicon oxide and SiO₂ (SRSO)/SiO₂ multilayer (ML) samples were grown by reactive magnetron sputtering and then annealed at high temperature to induce the formation of Si-nc with mean size of 3-4 nm and density of about 3.5×10¹⁸ cm⁻³ as deduced from high resolution TEM micrographs. Refractive index and thickness have been determined by m-line measurements, which have shown a birefringence of about 1.5% due to the ML structure. Rib-loaded waveguides have been fabricated to measure propagation losses in the visible-infrared range. The analysis of the different contributions to optical losses such as Mie scattering and scattering due to waveguide roughness has allowed us to isolate the contribution due to the absorption losses and thus to extract the absorption cross-section at different wavelengths. Values of about 3.5×10⁻¹⁸ cm² have been found at 830 nm, increasing with decreasing of the wavelength.     

Low loss optical channel waveguides for the infrared range using niobium based hybrid sol-gel material

In this work, we report the fabrication of single-mode Nb₂O₅ based hybrid sol-gel channel waveguides. Nb₂O₅ based hybrid sol-gel material has been deposited by spin-coating on silicon substrate and channel waveguides have been fabricated by a UV direct laser writing process. Optical guided modes have been observed to confirm single-mode conditions and optical propagation loss measurements have been performed using the cut-back technique. Optical propagation losses were measured to be 0.8 dB/cm and 2.4 dB/cm at 1.31 μm and 1.55 μm respectively. These experimental results demonstrate low loss optical waveguiding within the infrared range and are very promising in view of material choice for the development of integrated optical devices for telecommunication.     

Fabrication of hollow optical waveguides in fused silica by three-dimensional femtosecond laser micromachining

We report on the fabrication of hollow optical waveguides in fused silica using femtosecond laser micromachining. We show that in such hollow waveguides, high-intensity femtosecond laser beams can be guided with low optical loss. Our technique, which was established earlier for fabrication of optofluidic structures in glass, can ensure a high smoothness at the inner surfaces of the hollow waveguides and provide the unique capability of fabrication of hollow waveguides with complex geometries and configurations. A transmission of ∼90% at 633 nm wavelength is obtained for a 62-mm-long hollow waveguide with an inner diameter of ∼250 μm. In addition, nonlinear propagation of femtosecond laser pulses in the hollow waveguide is demonstrated, showing that the spectral bandwidth of the femtosecond pulses can be broadened from ∼27.2 to ∼55.7 nm.     

Long range surface plasmon polariton waveguides at 1.31 and 1.55 μm wavelengths

We investigate characteristics of gold metal strip waveguides based on long range surface plasmon polaritons (LRSPPs) along thin metal strips embedded in a polymer for practical applications at the telecommunication wavelengths of 1.31 and 1.55 μm. Guiding properties of the gold strip waveguides are theoretically and experimentally evaluated with the limited thickness and width up to ∼20 nm and ∼10 μm, respectively. The lowest propagation loss of ∼1.4 dB/cm is obtained with a 14.5-nm-thick and 2-μm-wide gold strip at 1.55 μm. With a single-mode fiber, the lowest coupling loss of ∼0.4 dB/facet is achieved with a 14.5-nm-thick and 5-μm-wide gold strip at 1.55 μm. The lowest insertion losses are obtained 8-9 dB with 1.5 cm-long gold strips of a limited thickness and width at both the wavelengths. We demonstrate a 10 Gbps optical signal transmission via the LRSPP waveguide with a 14 nm-thick, 2.5 μm-wide, and 4 cm-long gold strip. These LRSPP waveguides have potential applications for optical interconnects and communications.     

Fabrication of highly ordered CuInSe2 films with hollow nanocones for anti-reflection

Highly ordered CuInSe₂ films with hollow nanocones were fabricated by electron beam evaporation and nanospheres lithograph. From the AFM analyses, polystyrene nanospheres with diameter of 220 nm are assembled regularly on glass substrates. After reaction ion etching under different powers and residues removal, different and new surface morphologies of substrates have been obtained, such as smooth nanocones and hollow nanocones. The diffuse reflection spectra demonstrate that films on the substrates with periodic nanopatterned structure have less reflection over wavelengths ranged from 200 nm to 2500 nm due to light trapping. Especially, reflection for hollow nanocone arrays has the larger suppression value than nanocone-patterned films, which proves that surface pattern of hollow nanocones has better anti-reflection effect. Furthermore, while hollow depth increases from 6 nm to 9 nm, its optical antireflective effect becomes remarkable. These results could yield new options for solar-cell design with higher energy conversion efficiency.     

High efficiency fiber coupling to silicon sandwiched slot waveguides

The design of a fiber coupler for high efficiency light coupling to silicon sandwiched slot waveguides is reported. The proposed fiber coupler is based on the inverted taper approach. Parameters have been optimized to maximize coupling efficiency for λ = 1550 nm and TM polarization. Maximum coupling efficiencies of 93% for a inverted taper length of 150 μm and a inverted taper tip width of 40 nm have been obtained by means of the overlap integral and 3D beam propagation method (BPM) simulations.     

Magnetization reversal dynamics in Au/Co/Au(111) ultrathin films: Effect of roughness of the buffer layer

We present a study of the magnetization reversal dynamics in ultrathin Au/Co/Au films with perpendicular magnetic anisotropy, for a Co thickness of 0.5, 0.7 and 1 nm. In these films, the magnetization reversal is dominated by domain nucleation for t_Co=0.5, 0.7 nm and by domain wall propagation for t_Co=1 nm. The prevalence of domain nucleation for the thickness range 0.5-0.7 nm is different from results reported in the literature, for the same system and for the same thickness range, where the magnetization reversal took place mainly by domain wall motion. We attribute this difference to the effect of roughness of the Au buffer layer on the morphology of the magnetic layer.     

Single mode ridge waveguides based on vinyltriethoxysilane hybrid sol–gel material

In this work, fabrication and characterization of a single-mode vinyltriethoxysilane (VTES) based hybrid organic–inorganic sol–gel ridge waveguide is reported. VTES based hybrid sol–gel material has been deposited by multiple spin-coating process on quartz substrate. Channel waveguides have been fabricated by UV photopolymerization followed by wet etching process. Optical guided mode was observed to ensure the single mode condition while cut-back technique was performed for loss propagation measurement. Optical propagation losses were measured to be 1.6 dB/cm at 1.31 μm wavelength. This value shows that this material is a promising choice for the development of integrated optical circuits for telecommunication field.     

Gold coatings on polyethyleneterephthalate nano-patterned by F2 laser irradiation

In this work we present periodic surface structures generated by linearly polarized F₂ laser light (157 nm) on polyethyleneterephthalate (PET). Atomic force microscopy was used to study the topological changes induced by the laser irradiation. The laser irradiation induces the formation of periodic ripple structures with a width of ca 130 nm and a height of about 15 nm in the fluence range 3.80-4.70 mJ/cm² and the roughness of the polymer surface increases due to the presence of these periodic structures. Subsequently, the laser modified PET foils were coated with a 50 nm thick gold layer by sputtering. After Au deposition on the PET foils with ripple structure, the roughness of surface decreases in comparison to PET with ripples without Au coating. For 50 nm thick Au layers, the ripple structure is not directly transferred to the gold coating, but it has an obvious effect on the grain size of the coating. With considerably thinner Au layers, the ripple structures are smoothened but preserved.     

A self-generated template route to hollow carbon nanospheres in a short time

A self-generated template route was reported to prepare hollow carbon nanospheres. The hollow spheres were obtained through the direct pyrolysis of ferrocene for 1 h. The external diameter of the hollow carbon nanospheres was 50-150 nm and the thickness of the wall was about 15 nm. A possible formation mechanism of the hollow carbon nanospheres was discussed.     

Optical loss measurements in femtosecond laser written waveguides in glass

The optical loss is an important parameter for waveguides used in integrated optics. We measured the optical loss in waveguides written in silicate glass slides with high repetition-rate (MHz) femtosecond laser pulses. The average transmission loss of straight waveguides is about 0.3 dB/mm at a wavelength of 633 nm and 0.05 dB/mm at a wavelength of 1.55 μm. The loss is not polarization dependent and the waveguides allow a minimum bending radius of 36 mm without additional loss. The average numerical aperture of the waveguides is 0.065 at a wavelength of 633 nm and 0.045 at a wavelength of 1.55 μm. In straight waveguides more than 90% of the transmission loss is due to scattering.     

Extrinsic surface scattering and intrinsic absorption loss of vinyl based hybrid organic–inorganic materials for optical waveguides applications

Hybrid photosensitive materials were prepared by using vinyltriethoxysilane (VTES), tetraethoxysilane (TEOS) and tetrabutoxytitanate (TTBu) precursors through sol–gel technique. The materials are intended for optical telecommunication applications. Thus, high optical transmissions at the second and third optical telecommunication windows are essential. Extrinsic surface scattering due to surface roughness and intrinsic absorption due to aliphatic CH and OH groups are always ascribable to the optical power attenuations at aforementioned optical telecommunication windows. Optical waveguide based on hybrid sol–gel materials were fabricated, characterized and analyzed in order to investigate the extrinsic and intrinsic sources of attenuation and their contributions. The fabricated samples were characterized for propagation loss, surface condition, and Fourier transform IR (FTIR) absorption spectra. Propagation loss were measured by means of cut-back method as 1.6 and 6.9 dB/cm at 1310 and 1550 nm wavelengths, respectively. Surface scattering loss was modeled based on measured rms roughness of 0.724 nm and turned out to contribute less than 0.01% of the total propagation loss. FTIR absorption spectra show the persistent existence of aliphatic CH and OH groups within the final hybrid sol–gel materials.     

Waveguiding and photoluminescence in Er-doped Ta2O5 planar waveguides

The optimization of erbium-doped Ta₂O₅ thin film waveguides deposited by magnetron sputtering onto thermally oxidized silicon wafer is described. Optical constants of the film were determined by ellipsometry. For the slab waveguides, background losses below 0.4 dB/cm at 633 nm have been obtained before post-annealing. The samples, when pumped at 980 nm yielded a broad photoluminescence spectrum (FWHM∼50 nm) centred at 1534 nm, corresponding to ⁴I_13/2-⁴I_15/2 transition of Er³⁺ ion. The samples were annealed up to 600 °C and both photoluminescence power and fluorescence lifetime increase with post-annealing temperature and a fluorescence lifetime of 2.4 ms was achieved, yielding promising results for compact waveguide amplifiers.     

Fabrication of fluorinated polyimide optical waveguides by micropen direct writing technology

A novel and cheap direct writing method based on the micropen has been developed to fabricate fluorinated polyimide stripe optical waveguides on Si/SiO₂ wafers. The overall design, starting material, micropen direct writing system and fabrication processes of the stripe optical waveguides are presented. The effects of the key direct writing parameters, such as the tip-to-substrate distance, extrusive gas pressure, writing speed and viscosity of the polyamic acid, on the dimension and morphology of the stripe optical waveguides are discussed in detail. After deposition by the micropen system and baking process, the fluorinated polyimide stripe optical waveguides with good morphology and surface quality can be fabricated using the optimal parameters. The propagation losses at the wavelength of 1.55 μm are in the range of 1.4-3.5 dB cm⁻¹ as characterized by different length combinations of the strip optical waveguides.     

Experimental investigation on submicron rib waveguide microring/racetrack resonators in silicon-on-insulator

In experiment, characteristics of silicon microring/racetrack resonators in submicron rib waveguides have been systematically investigated. It is demonstrated that only a transverse-electric mode is guided for a ratio of slab height to rib height h/H = 0.5. Thus, these microring/racetrack resonators can only function for quasi-transverse-electric mode, while they get rid of transverse-magnetic polarization. Electron beam lithography and inductively coupled plasma etching were employed and improved to reduce sidewall roughness for low propagation loss and high performance resonators. Then, the effects of waveguide dimensions, coupling region design, waveguide roughness, and oxide cladding for the resonators have been considered and analyzed.     

Low-loss, damage-resistant optical waveguides in Zn-diffused LiNbO3 by a two-step procedure

In the present work the fabrication of high-quality LiNbO₃ waveguides, based on Zn diffusion from vapor phase using a two-step diffusion process, has been demonstrated. The optical waveguides fabricated using this method have low propagation losses (less than 0.5 dB/cm measured at 633 nm), as a consequence of the high surface quality obtained. High photorefractive damage resistance is reported, and it has been related to the Zn incorporation into the LiNbO₃ crystals. Additional advantages of this waveguide fabrication method due to the low temperature of the process (<900 °C) are to prevent Li out-diffusion and to preserve ferroelectric domain structures. Received: 2 March 2000 / Accepted: 21 June 2000 / Published online: 13 September 2000     

Influence of surface effects on the performance of lead-niobium-germanate optical waveguides

Lead-niobium-germanate planar waveguides have been produced by pulsed laser deposition. The composition of the waveguides is found to be relatively weakly dependent on the laser fluence, while their surface morphology is affected dramatically. Smooth surfaces are obtained for a narrow fluence range centered at 2.0 J/cm², while particulates having typical diameters of <0.5 μm or droplets with typical diameters of <10 μm are observed at lower and higher fluences, respectively. The refractive index of the waveguides increases with fluence up to 2.1 at 2.0 J/cm², which is close to the value of the bulk glass, and remains constant at higher fluences. Propagation losses show instead a minimum (≈6.5 dB/cm) at 2.0 J/cm². The characteristics of the ablation process that leads to the ejection of solid particulates or molten droplets as well as the increase of the waveguides density on increasing the fluence are discussed to be responsible for the observed optical behavior.     

Numerical study of sub-wavelength plasmonic waveguide

The authors present an analysis of a plasmonic waveguide, simulated using a two-dimensional finite-difference time-domain technique. With the surface structures located on the surface of the metal, the device is able to confine and guide light waves in a sub-wavelength scale. And two waveguides can be placed within 150 nm (∼6% of the incident wavelength) that will helpful for the optoelectronic integration. Within the 20 μm simulation region, it is found that the intensity of the guided light at the interface is roughly two to four times the peak intensity of the incident light, and the propagation length can reach approximately 40 μm at the wavelength of 2.44 μm.     

A novel compact wavelength-division multiplexer using highly dispersive waveguide-to-waveguide coupling

A novel compact wavelength-division multiplexer using highly dispersive waveguide-to-waveguide coupling is designed, simulated and analyzed. The device consists of two very close single-moded waveguides that are periodically connected to form a mode-dependent dispersive grating. It is demonstrated that the wavelengths over the edges of the photonic band gap contributed by the grating can be separated in a very short propagation distance. Using the finite-difference time-domain method, the result shows that the wavelengths of 1570 and 1530 nm are separated by the grating in a coupler length of 57 μm which is much shorter than the required length of about 340 μm without grating assistance. The channel contrast of 20 dB and the insertion loss about 2 dB are achieved.     

Novel ultrasmall Si-nanowire-based arrayed-waveguide grating interleaver with spirals

A novel layout is presented to achieve an ultrasmall arrayed-waveguide grating (AWG) interleaver based on Si-nanowire waveguides. Spiral waveguides are inserted in the middle of arrayed-waveguide to obtain a large lightpath (required for the ultra-high diffraction order) in a small occupied area. A designed ultrasmall AWG interleaver with a free spectral range of 0.8 nm has a total size of only about 73 μm × 372 μm (0.027 mm²).