Pedestal Waveguides Research Articles

Enhanced optical gain assisted by the plasmonic effects of Au nanoparticles in Nd³⁺ doped TeO₂-ZnO waveguides produced with the pedestal architecture

Investigation of the signal enhancement of Nd3+ codoped TeO2-ZnO pedestal waveguides, at 1064 nm, due to Au nanoparticles deposited over the core is presented for the first time. Nd3+ doped TeO2-ZnO thin film was obtained by RF Magnetron Sputtering deposition. The resulting core with 500 nm height and widths in the 4–40 μm range, exhibited low roughness average in all area measured (0.48 ± 0.04) nm. Minimum propagation losses of 2.2 dB/cm were observed for waveguide width of 40 μm whereas an increase took place for smaller ones. Scanning electron microscopy (SEM) allowed the waveguide structure inspection and transmission electronic microscopy (TEM) the Au nanoparticles evaluation. The results showed that the Au nanoparticles contributed up to 75 % of relative gain enhancement, under 808 nm excitation. This increase was due to the local field growth in the proximity of the nanoparticles that enhances the density of excited Nd3+. The internal gain that considers the propagation losses reached positive values for larger core widths (above 8 μm).The present study opens possibilities for optical amplifiers with low propagation losses based on different metal-dielectric composites, as well as other waveguide-based devices.

Theoretical research of chalcogenide pedestal waveguide gas sensor

AbstractA rectangular waveguide with a small external confinement factor (γ) limits the gas sensing performance. The fabrication process of a suspended waveguide with large γ is complex. Pedestal waveguide can be fabricated by wet etching on the basis of a rectangular waveguide, which makes full use of the target analyte area under the core layer and increases γ. The TM mode chalcogenide pedestal waveguide was optimized to obtain a large γ. The γ of the optimized TM mode pedestal waveguide is 55.7% and the limit of detection of methane (CH4) for a 1 cm‐long waveguide is 1.67%. The fabrication tolerance of the pedestal waveguide was analyzed. The pedestal radius error has a great influence on sensing performance. This work provides a method for the design and performance analysis of a pedestal waveguide sensor.

Monolithic Germanium-Tin Pedestal Waveguide for Mid-Infrared Applications

Germanium-tin (GeSn) is a CMOS-compatible group-IV material. Its growth, however, is plagued by the tendency of Sn segregation and the generation of defects within the GeSn layer when it is grown on the lattice-mismatched substrate. Thus far, thin GeSn has been reported for use in a direct-band gap for near-mid infrared light source and photodetector. In this communication, we report the growth of high quality single-crystalline GeSn (∼ 1 μm) with low compressive stress (−0.3%) and low defects (3 × 107 /cm2) on Ge buffer on Si substrate. The as-grown GeSn is then fabricated into pedestal waveguide of width 1.25 μm. An estimated propagation loss of 1.81 dB/cm and bending loss of 0.19 dB/ bend are measured at 3.74 μm. In the absence of Ge-O absorption peaks at 820 and 550 cm−1, under optimal fabrication and measurement condition, the proposed GeSn waveguide might possibly support light propagation for wavelength beyond 25 μm.

Radiation pressure and electrostriction induced enhancement for Kerr-like nonlinearities in a nanoscale silicon pedestal waveguide

We report an enhancement of effective nonlinear-index coefficient from the combination of radiation pressure and electrostriction in an air-cladding silicon pedestal waveguide. The effective nonlinear-index coefficient was experimentally measured by characterizing spectral broadening with incident pulses in the pedestal waveguide. Larger spectral broadening was observed in the pedestal waveguide compared to a conventional silicon waveguide. Theoretically, the primary enhancement of the effective nonlinear-index coefficient can be calculated by using Kramers–Kronig relations, which is associated with the change in absorption from stimulated Brillouin scattering in the pedestal waveguide. The calculations were consistent with experimental results. By tailoring the pillar width of the pedestal waveguide, about 27.6% enhancement in the effective nonlinear-index coefficient of the pedestal waveguide was possible at the center wavelength of 1555 nm.

Low-loss pedestal Ta2O5 nonlinear optical waveguides.

In this work, we investigate a pedestal tantalum oxide (Ta2O5) material platform for integrated nonlinear optics (NLO). In order to achieve low propagation losses with this material, pedestal waveguides with Ta2O5 cores were designed. The nonlinear refractive index n2 of this new platform was obtained by measuring the amount of spectral broadening due to self-phase modulation (SPM) of 23 fs optical pulses at 785 nm propagating through the waveguides. In this manner, a nonlinear index of (5.8 ± 2.0) × 10-19 m2W-1 was found for this material, which is in good agreement with values reported in related works where strip waveguides were used for a similar purpose. Furthermore, due to the pedestal configuration, propagation losses as low as 1.6 dB·cm-1 for narrow waveguides and 0.1 dB·cm-1 for large waveguides were obtained. Finite element method (FEM) mode analysis was performed to calculate the mode characteristics, as well as the effective areas of the waveguides. The high nonlinear and linear refractive indices, wide bandgap and low propagation losses make this platform ideal for applications extending from the visible into the mid-IR regions of the optical spectrum. Due the large gap, Ta2O5 should have low two photon absorption at the near-IR as well.

Improving performance in ytterbium-erbium doped waveguide amplifiers through scattering by large silicon nanostructures

Optical waveguide amplifiers have seen a growing interest in the last years due to their applications in telecommunication. This paper reports a notable increase of the relative gain of Yb3+/Er3+ codoped Bi2O3–GeO2 waveguides by introducing disorder in the form of silicon nanostructure as scattering centers. A photoluminescence enhancement of about 10 times for the 520 nm and 1530 nm emission bands is observed in the waveguides when the silicon nanostructures are introduced. Increase of the Yb3+/Er3+ effective absorption, due to the scattering provided by the silicon nanostructures, and decrease of [Bi+], caused by the introduction of silicon, are proposed as likely causes for the luminescence and gain enhancement. The pedestal waveguides were fabricated by RF-sputtering followed by optical lithography and reactive ion etching. RF-sputtering of silicon together with Yb/Er and Bi2O3–GeO2 glass, followed by heat treatment, produced Yb3+/Er3+ codoped Bi2O3–GeO2 waveguides with silicon nanostructures of size 25–30 nm. The resulting relative gain reached 5.5 dB/cm at 1542 nm representing an enhancement of 50% with respect to waveguides without silicon nanostructures. This strategy of introducing appropriate disorder may open an avenue for designing and manufacture of novel photonic devices in this emerging field of integrated optics.

A new fabrication process of pedestal waveguides based on metal dielectric composites of Yb3+/Er3+ codoped PbO-GeO2 thin films with gold nanoparticles

This work reports the signal enhancement of Yb3+/Er3+ codoped PbO-GeO2 pedestal waveguides due to gold nanoparticles deposited over the core layer. The pedestal structure was obtained by conventional photolithography and plasma etching with a new procedure that does not use metallic hard-masks that normally introduce roughness, leading to light scattering. This new procedure brings advantages that benefit light guiding, reducing the propagation losses. Yb3+/Er3+ codoped PbO-GeO2 thin film was obtained by RF Magnetron Sputtering deposition and was used as core layer (410 nm height). In order to cover the core with gold nanoparticles the sputtering technique was used, followed by annealing at 400 °C during 1 h. The minimum propagation losses obtained were of 1.0 dB/cm at 1068 nm. Scanning Electron Microscopy (SEM) was employed for the waveguides structure inspection and transmission electronic microscopy (TEM) was used to verify the presence of gold nanoparticles on the waveguides. It was observed an enhancement of 180% for the relative gain that reached 7.8 dB/cm at 1530 nm, for an optical waveguide with 6 μm core width, under 980 nm excitation (pump power of 60 mW), attributed to the local field enhancement in the vicinity of the gold nanoparticles. The new fabrication process presented in this work opens possibilities for optical amplifiers with low propagation losses based on different metal dielectric composites, as well as other waveguide-based devices.

Loss reduction of silicon-on-insulator waveguides for deep mid-infrared applications.

We report that propagation loss of optical waveguides based on a silicon-on-insulator (SOI) material platform can be greatly reduced. Our simulations show that the loss, including SiO2 absorption and substrate leakage, but no scattering loss, is 0.024 and 0.53dB/cm in the deep mid-infrared at 4.8 and 7.1μm wavelengths, where the material absorption in SiO2 is 100 and 1000dB/cm, respectively. The loss becomes negligible, compared to scattering loss in Si waveguides. This is enabled by using the TE10 mode in a pedestal waveguide. We also show that the TE10 mode can be excited in the proposed waveguide by the fundamental mode with a coupling efficiency of >94%. Low propagation loss, high coupling efficiency, and fabrication-friendly design would make it promising for practical use of SOI devices in the deep mid-infrared.

Influence of gold nanoparticles on the 805 nm gain in Tm3+/Yb3+ codoped PbO-GeO2 pedestal waveguides

The production and characterization of pedestal waveguides based on PbO-GeO2 amorphous thin films codoped with Tm3+/Yb3+, with and without gold nanoparticles (NPs), are reported. Pedestal structure was obtained by conventional photolithography and plasma etching. Tm3+/Yb3+ codoped PGO amorphous thin film was obtained by RF Magnetron Sputtering deposition and used as core layer in the pedestal optical waveguide. The minimum propagation losses in the waveguide were 3.6 dB/cm at 1068 nm. The internal gain at 805 nm was enhanced and increased to 8.67 dB due to the presence of gold NPs. These results demonstrate for the first time that Tm3+/Yb3+ codoped PbO-GeO2 waveguides are promising for first telecom window and integrated photonics, especially for applications on fiber network at short distances.

Advances on the fabrication process of Er3+/Yb3+:GeO2–PbO pedestal waveguides for integrated photonics

The present work reports the fabrication, passive and active characterization of Yb3+/Er3+ codoped GeO2–PbO pedestal waveguides. We show the advances obtained in pedestal fabrication by comparing waveguides obtained under different processes parameters. The thin films were deposited on previously oxidized silicon wafers in Ar plasma at 5mTorr; pedestal waveguides, with 1–100μm width range were defined by conventional lithography procedure, followed by reactive ion etching (RIE). A comparison between the results of propagation losses and internal gain is presented in order to show that the improvement of fabrication process contributed to enhance the performance of the pedestal waveguides. Reduction of about 50% was observed for the propagation losses at 632 and 1068nm, whereas enhancement of approximately 50% was obtained for the internal gain at 1530nm (4 and 6dB/cm, for 70μm waveguide width), under 980nm excitation. The present results demonstrate the possibility of using Yb3+/Er3+ codoped GeO2–PbO as pedestal waveguide amplifiers.

Fabrication and optical characterization of pedestal micro-structures on DUV210 polymer: waveguides structures towards micro-resonators

The paper presents the global design, fabrication and optical characterizations of pedestal waveguides and 2D microresonators made of DUV210 polymer. These particular geometries are achieved thanks to specific deep-UV lithography procedures allowing sub-lambda development coupled to chemical etching able to shape such pedestal configuration on various optical microstructures. Scanning electron microscopy images have confirmed their features. Moreover, such families of pedestal structures were characterized experimentally, including the optical losses measurements for the waveguides and the optical resonance responses for two kinds of microresonators. Optical studies of single mode propagation losses measurements have been performed by a cut back method leading to values close to 20 dB/cm at a 635 nm wavelength. Additionally, resonant spectral analysis has been performed into pedestal rings and racetracks microresonators with a broadband laser source centered at 795 nm, demonstrating the presence of expected theoretical resonances. Free spectral range values of 2.86 nm and 2.51 nm have been measured for these new designed pedestal resonators on DUV210 related to quality factor values superior to 520 and 610 respectively.

Fabrication of Yb3+/Er3+ codoped Bi2O3–WO3–TeO2 pedestal type waveguide for optical amplifiers

We report, for the first time to our knowledge, experimental results on pedestal waveguides produced with Yb3+/Er3+ codoped Bi2O3–WO3–TeO2 thin films deposited by RF Sputtering for photonic applications. Thin films were deposited using Ar/O2 plasma at 5mTorr pressure and RF power of 40W on substrates of silicon wafers. The definition of the pedestal waveguide structure was made using conventional optical lithography followed by plasma etching. Propagation losses around 2.0dB/cm and 2.5dB/cm were obtained at 633 and 1050nm, respectively, for waveguides in the 20–100μm width range. Single-mode propagation was measured for waveguides width up to 10μm and 12μm, at 633nm and 1050nm, respectively; for larger waveguides widths multi-mode propagation was obtained. Internal gain of 5.6dB at 1530nm, under 980nm excitation, was measured for 1.5cm waveguide length (∼3.7dB/cm). The present results show the possibility of using Yb3+/Er3+ codoped Bi2O3–WO3–TeO2 pedestal waveguide for optical amplifiers.

Chip-scale Mid-Infrared chemical sensors using air-clad pedestal silicon waveguides

Towards a future lab-on-a-chip spectrometer, we demonstrate a compact chip-scale air-clad silicon pedestal waveguide as a Mid-Infrared (Mid-IR) sensor capable of in situ monitoring of organic solvents. The sensor is a planar crystalline silicon waveguide, which is highly transparent, between λ = 1.3 and 6.5 μm, so that its operational spectral range covers most characteristic chemical absorption bands due to bonds such as C-H, N-H, O-H, C-C, N-O, C=O, and C≡N, as opposed to conventional UV, Vis, Near-IR sensors, which use weaker overtones of these fundamental bands. To extend light transmission beyond λ = 3.7 μm, a spectral region where a typical silicon dioxide under-clad is absorbing, we fabricate a unique air-clad silicon pedestal waveguide. The sensing mechanism of our Mid-IR waveguide sensor is based on evanescent wave absorption by functional groups of the surrounding chemical molecules, which selectively absorb specific wavelengths in the mid-IR, depending on the nature of their chemical bonds. From a measurement of the waveguide mode intensities, we demonstrate in situ identification of chemical compositions and concentrations of organic solvents. For instance, we show that when testing at λ = 3.55 μm, the Mid-IR sensor can distinguish hexane from the rest of the tested analytes (methanol, toluene, carbon tetrachloride, ethanol and acetone), since hexane has a strong absorption from the aliphatic C-H stretch at λ = 3.55 μm. Analogously, applying the same technique at λ = 3.3 μm, the Mid-IR sensor is able to determine the concentration of toluene dissolved in carbon tetrachloride, because toluene has a strong absorption at λ = 3.3 μm from the aromatic C-H stretch. With our demonstration of an air-clad silicon pedestal waveguide sensor, we move closer towards the ultimate goal of an ultra-compact portable spectrometer-on-a-chip.

Single polarization transmission in pedestal-supported silicon waveguides

In this Letter, properties of a pedestal-supported silicon waveguide are investigated, showing that it supports single polarization transmission. The pedestal is fabricated easily through a wet-etching process on strip waveguides. Theoretical analysis shows that this property is due to the leakage of quasi-TM mode when the pedestal width is small. A polarization extinction ratio larger than 20 dB at 1550 nm is measured in the pedestal waveguide sample, demonstrating single polarization transmission property experimentally. Thanks to its large single polarization transmission bandwidth, robustness in fabrication tolerance, and simple fabrication process, pedestal waveguides will have potential applications as simple silicon-integrated polarizers.

Marcatili’s extended approach: comparison to semi-vectorial methods applied to pedestal waveguide design

This paper deals with a theoretical study of pedestal waveguides. An extension of the Marcatili method has been developed in order to adapt this analytical method to pedestal structures. Simulations are performed for two different T-pedestal waveguide (T-PW) configurations corresponding respectively to a high and a lower core to pedestal widths ratio (T-PW I and T-PW II). Each configuration is simulated considering two core widths (2 and 4 µm) and a core height ranging from 1 to 2 µm at a 670 nm wavelength. Then, this extended Marcatili method has been compared with a semi-vectorial finite difference method (SVFD) and a spectral method developed by Galerkin, both based on a numerical approach. The simulation of the T-PW structure with these three methods shows a good congruence since the relative differences between Marcatili's method and the numerical methods remain below 6%. Then, the three approaches are applied to study the modal birefringence minimization in the case of pedestal structures. Simulations are typically performed for waveguide height and width values ranging, respectively, around (1.6–2.6) µm and (1.8–6) µm, with pedestal widths ranging around (0.4–0.8) µm, at a 670 nm wavelength. The authors stress a specific property of pedestal configurations: by judiciously adjusting the dimensional parameters (core and pedestal width and core height), the birefringence can be completely screened out.

Universal coupling between metal-clad waveguides and optical ring resonators

We demonstrate excitation of whispering gallery modes in optical ring resonators using a gold-clad pedestal planar waveguide structure. The gold-clad structure provides a strong evanescent field for light-coupling into the resonator while enabling low transmission loss throughout much of the visible and near-infrared region. This is advantageous compared to the previously demonstrated anti-resonant reflecting optical waveguide (ARROW) structure, which can only transmit a narrow wavelength band. We show that the height of the pedestal waveguide can be designed to optimize the coupling conditions for the ring resonator. This technology enhances the practicality of optical ring resonators for sensing devices, laser systems, and many other important applications.

Radiation Modes and Roughness Loss in High Index-Contrast Waveguides

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We predict the scattering loss in rectangular high index-contrast waveguides, using a new variation of the classical approach of coupled-mode theory. The loss predicted by this three-dimensional (3-D) model is considerably larger than that calculated using previous treatments that approximate the true 3-D radiation modes with their two-dimensional counterparts. The 3-D radiation modes of the ideal waveguide are expanded in a series of cylindrical harmonics, and the coupling between the guided and radiation modes due to the sidewall perturbation is computed. The waveguide attenuation can then be calculated semianalytically. It is found that the dominant loss mechanism is radiation rather than reflection, and that the transverse electric polarization exhibits much larger attenuation than transverse magnetic polarization. The method also gives simple rules that can be used in the design of low-loss optical waveguides. The structural properties of sidewall roughness of an InGaAs/InP pedestal waveguide are measured using atomic force microscopy, and the measured attenuation is found to compare well with that predicted by the model. </para>