Multi-wavelength add-drop filter with phase-modulated shifted Bragg grating.

We present, for the first time to the best of our knowledge, a multichannel add-drop operation with a phase-modulated shifted Bragg grating based filter. The device is realized in a silicon-on-insulator waveguide platform with TiO2 as a coating material to reduce the refractive index contrast. The operation is shown for three and five wavelength channels within the telecom C-band. A line width of 0.6 nm with an extinction ratio of 20 dB is achieved. The shifted Bragg grating is modulated maintaining a modal phase-matching condition for multiple wavelengths. The phase function is calculated with an iterative Fourier transform algorithm. The experimental results are in very good agreement with the design.

Silicon slot waveguide Fano resonator.

The growing interest for Fano resonators during the past decade is due to the narrow line shape observable in their optical spectra. The drastic phase shift occurring at the resonance yields a steep drop from a high to low amplitude. Fano resonances can be obtained by a combination of nanostructures. Such a system is extremely sensitive in terms of both geometrical parameters and environmental conditions. Here we study a complex arrangement of photonic crystal cavities and slot waveguides on a silicon chip. Our structure, composed of several cavities in parallel, has a particular response superimposing a shallow photonic bandgap and a resonance with a Fano line shape. It provides a low noise and a clear asymmetric resonance. We demonstrate it experimentally and show the potential of such a device for sensing. A sensitivity of 92 nm/RIU is measured.

Influence of an Al2O3 surface coating on the response of polymeric waveguide sensors.

The responses of a polymer ridge waveguide Young interferometer with and without a bilayer of Al2O3/TiO2, fabricated by atomic layer deposition, are studied and compared when applied as an aqueous chemical sensor. The phase shift of the guided mode, as a result of the change in refractive index of the cover medium, is monitored. The results indicate that the over-coating affects the linearity of the sensor response. The effect of concentration on the linearity of the sensor response is investigated by applying different concentrations of water-ethanol solution. Although the performance of the sensor is improved by the additional layers, the study reveals a non-monotonic behavior of the device. We show that it comes mainly from the adsorption of ethanol molecules on the surface of the films. Such an understanding of the platform is crucial for sensing of analytes involving polar molecules.

Multi-wavelength filtering with a waveguide integrated phase-modulated Bragg grating.

We present a multi-wavelength band-rejection filter on a titanium dioxide-coated silicon-on-insulator platform. The concept rests on the use of a finely tuned waveguide-based Bragg grating for which the periods are slightly varied from one to another. This phase-modulated Bragg grating enables precise customization of integrated waveguide filters. The number of rejection bands and the center-to-center separation between them are tailored by dividing the grating into several super-periods and coding an optimal phase function onto each super-period. The optimal phase function is obtained by employing an iterative Fourier transform algorithm. The design is supported by an experimental demonstration.

Strip-loaded waveguide on titanium dioxide thin films by nanoimprint replication.

We present a convenient, low-cost, and mass-production-compatible technique for the fabrication of strip-loaded waveguides. The technique is based on the atomic layer deposition of a slab waveguide, nanoimprinting of a strip, and integration of two structures by lamination. The guiding layer is chosen to be a 200 nm thick titanium dioxide film. The waveguide characteristics are determined by the use of ring resonators. The technique is demonstrated for titanium dioxide thin films, but it is applicable to any other material that meets the refractive index difference condition between the loading strip and the slab waveguide.

Strip-loaded horizontal slot waveguide.

We introduce a novel concept for an optical waveguide called a strip-loaded slot waveguide. It allows an extraordinary confinement of the field in a waveguide with an extremely tiny (vertical) cross section. Unlike conventional slot waveguides, the proposed configuration has potential for very low propagation losses. Its operation is demonstrated at telecommunication wavelengths and, in addition, it is fabricated by means of mass production compatible techniques: atomic layer deposition and nanoimprint replication. The possibility to fabricate the proposed structure with such low-cost techniques opens a new path for a variety of nanophotonics applications.

Add-drop filter based on TiO2 coated shifted Bragg grating.

We present a titanium dioxide coated shifted Bragg grating in a silicon-on-insulator platform enabling optical add-drop functionality. The device works on the basis of mode conversion due to shifted sidewall structure followed by mode splitting based on an asymmetric Y-coupler. We experimentally demonstrate the working principle of the device. A reflection bandwidth of 2.2 nm with 14 dB extinction ratio is obtained with a 300 µm long shifted Bragg grating. The performance of the device is also compared without the titanium dioxide coating. A scope of spectral tunability with titanium dioxide re-coating (0.8 nm per 1 nm re-coating) by atomic layer deposition is experimentally verified.

Hybrid layered polymer slot waveguide Young interferometer.

We demonstrate a polymer slot waveguide Young interferometer coated with a bilayer of Al2O3/TiO2. The approach enables relaxed dimensions of the polymer waveguide which simplifies the fabrication of the structure with a resolution of 50 nm. The layers were coated by an atomic layer deposition technique. The feasibility of the device was investigated by exploiting the interferometric structure as a bulk refractive index sensor operating at 975 nm wavelength for detection of an ethanol-water solution. A refractive index change of 1 × 10-6 RIU with a sensing length of only 800 µm was detected. The approach confirms the possibility of realizing a low cost device with a small footprint and enhanced sensitivity by employing the TiO2 rails in the sides of the slot waveguide.

Optical characterization of directly deposited graphene on a dielectric substrate.

By using scanning multiphoton microscopy we compare the nonlinear optical properties of the directly deposited and transferred to the dielectric substrate graphene. The direct deposition of graphene on oxidized silicon wafer was done by utilizing sacrificial copper catalyst film. We demonstrate that the directly deposited graphene and bi-layered transferred graphene produce comparable third harmonic signals and have almost the same damage thresholds. Therefore, we believe directly deposited graphene is suitable for the use of e.g. nanofabricated optical setups.

Slow-light enhanced electro-optic modulation with an on-chip silicon-hybrid Fano system.

We present the theoretical study of a coupled cavity system yielding a Fano response on a fully on-chip silicon platform hybridized with an electro-optic polymer. This novel Fano system is based on a slot waveguide Bragg grating geometry, enabling a huge enhancement of the electro-optic properties due to slow light effects at the resonance. The modulator shows a high resonance tunability of 1.75 nm/V and a low switching power of 0.63 V. Such a versatile system shows the promise for various nonlinear and active devices only by using suitable cover material.

Parabolic opening in atomic layer deposited TiO(2) nanobeam operating in visible wavelengths.

We investigate the feasibility of developing a one dimensional photonic crystal cavity on a TiO2 platform operating in the visible. The atomic layer deposition technique is used to finely adjust the parameters of the structure. We present the experimental demonstration of a nanobeam cavity with a quadratically tapered row of holes, in which a parabolic window is opened in order to facilitate the infiltration of gas, liquid, nonlinear material, or quantum emitters. The structure exhibits a photonic band gap between λ = 635 nm and λ = 690 nm and several resonances within the photonic band gap.

Optical identification based on time domain optical coherence tomography.

We present a novel method for optical identification, i.e., authenticating valuable documents such as a passport, credit cards, and bank notes, using optical coherence tomography (OCT). An OCT system can capture three-dimensional (3D) images and visualize the internal structure of an object. In our work, as an object, we consider a multilayered optical identification tag composed of a limited number of thin layers (10-100 µm thick). The thickness, width, and location of the layers in the tag encode a unique identification information. Reading of the tag is done using a time domain OCT (TD-OCT) system. Typically, a TD-OCT system requires continuous mechanical scanning in one or more directions to get a 3D volume image of an object. The continuous scanning implies a complicated optical setup, which makes an OCT system fragile and expensive. We propose to avoid the conventional scanning by (1) not requiring 3D imaging, and (2) utilizing the motion of the optical tag itself. The motion is introduced to the tag reader, for example, by a user, which replaces the need for conventional scanning. The absence of a conventional scanning mechanism makes the proposed OCT method very simple and suited for identification purposes; however, it also puts some constraints to the construction of the optical tag, which we discuss in this paper in detail.

Titanium dioxide slot waveguides for visible wavelengths.

We present the first, to our knowledge, experimental demonstration of a titanium dioxide slot waveguide operating in the visible range of light. Ring resonators based on slot waveguides were designed, fabricated, and characterized for λ≃650 nm. The fabrication method includes atomic layer deposition, electron beam lithography, and reactive ion etching. The required narrow slot widths of a few tens of nanometers were achieved by using a conformal atomic layer re-coating technique. This unique feature-size-reduction technique was applied after the final etching step.

One-dimensional photonic crystals with cylindrical geometry.

A one-dimensional photonic crystal (1DPC) consisting of a stack of alternate TiO(2) and Al(2)O(3) layers is deposited on the side wall of a glass rod by Atomic Layer Deposition. The stack is designed to sustain TE-polarized Bloch Surface Waves (BSW) in the visible spectrum at wavelengths shorter than 650 nm. Experimental evidence of light coupling and guiding capabilities of the 1DPC is provided together with a possible application for fluorescence-based remote sensors.

Polarization independent integrated filter based on a cross-slot waveguide.

We investigate an in-line band pass filter, working both for TE and TM polarizations, based on a cross-slot waveguide merged with a Bragg grating and an optical cavity. Different types of cavities (C2- and C4-symmetric) are presented in order to optimize the filtering and make the device dependent or independent on the polarization. We show a strong light confinement in an extremely small volume, which offers an advantage for further sensing applications. Moreover, we show how the inclusion of a silicon nanowire in the cavity helps the guiding and increases the amplitude of the resonance. In this study we make use of both the Fourier Modal Method and the Finite Difference Time Domain method to perform the numerical simulations.

Impact of ALD grown passivation layers on silicon nitride based integrated optic devices for very-near-infrared wavelengths.

A CMOS compatible post-processing method to reduce optical losses in silicon nitride (Si(3)N(4)) integrated optical waveguides is demonstrated. Using thin layer atomic layer deposition (ALD) of aluminum oxide (Al(2)O(3)) we demonstrate that surface roughness can be reduced. A 40 nm thick Al(2)O(3) layer is deposited by ALD over Si(3)N(4) based strip waveguides and its influence on the surface roughness and the waveguide loss is studied. As a result, an improvement in the waveguide loss, from very high loss (60 dB/cm) to low-loss regime (~5 dB/cm) is reported for a 220 nm x 500 nm Si(3)N(4) wire at 900 nm wavelength. This opens prospects to implement very low loss waveguides.

A merged photonic crystal slot waveguide embedded in ALD-TiO2.

We demonstrate the concept of a merged nanoscale photonic crystal slot waveguide that acts as a bandpass filter in the near infrared region of the spectrum. The device is based on the integration of a photonic crystal cavity in a slot waveguide on a silicon on insulator substrate. The device is further embedded in amorphous titanium dioxide using atomic layer deposition, which allows to reduce two-photon absorption losses and creates the possibility to combine nonlinear guided-wave optics resulting from the strong field confinement in the slot region with slow light effects in the photonic crystal cavity. Our approach is fully compatible with complementary metal oxide semiconductor technology and opens up new perspectives for the integration of all-optical signal processing functionalities in hybrid silicon nanophotonics platforms.

Horizontal slot waveguide channel for enhanced Raman scattering.

Herein we characterize and experimentally demonstrate a new type of a horizontal slot waveguide structure for remarkably enhanced Raman scattering detection in nanometer-scale void channels. As the measurement sensitivity is one of the key limiting factors in nanofluidic detection, it is essential to search advanced solutions for such detection. Combining an all dielectric resonance waveguide grating and a surface enhanced Raman scattering (SERS) substrate in a close proximity it is possible to create high electromagnetic field energy hot zones within an adjustable slot region. This results in a strong enhancement in Raman scattering. We show the theoretical principles and demonstrate, with rhodamine 6G molecules, an approximately 20-fold enhancement compared to a conventional SERS substrate within the corresponding slot arrangement. We foresee potential applications for the proposed approach in the fields of medical, biological and chemical sensing, where the high detection sensitivity is essential due to integration with nanofluidic devices.

Enhanced sensitivity in polymer slot waveguides by atomic layer deposited bilayer coatings.

The refractive index sensitivity of a polymer slot waveguide coated with a bilayer of Al2O3/TiO2 was investigated theoretically and optimized for biosensor applications. The influence of atomic-layer-deposition-coated thin high-refractive-index layers on the slot confinement factor and the homogeneous sensitivity of polymer slot waveguides with different geometries were simulated. The results were compared with those of an optimized noncoated polymer slot waveguide, both operating at visible wavelengths. The simulations reveal that the proposed structure offers a significant improvement in the confinement factor and the sensitivity. These calculations present guidelines for the design and fabrication of relatively sensitive polymer slot waveguide devices for low-cost biochemical sensor applications.

Effect of substrate overetching and heat treatment of titanium oxide waveguide gratings and thin films on their optical properties.

We investigate subwavelength titanium oxide (TiO2) resonance waveguide gratings (RWGs) and TiO2 thin films of thicknesses ~200 nm fabricated by atomic layer deposition (ALD), in both amorphous and crystalline phases on fused silica substrates. The TiO2 RWGs are fabricated by electron beam lithography, reactive ion etching, and ALD. The thin films and RWGs are characterized structurally by x-ray diffraction and scanning electron microscopy. The optical characterization of RWGs and optical constants of TiO2 films are studied by an ellipsometer. RWGs are designed for TE and TM modes in such a way that an overetch effect of the fused silica substrate can be investigated. Various RWG samples are prepared by gradually increasing the overetch depth and subsequently measuring the performance of the RWGs. A close agreement between the calculated and experimentally measured resonance wavelength spectral shifts is obtained; however, the magnitudes of the measured shifts are greater than calculated ones. A parallel study related to the measurement of the refractive indices and remeasuring the optical shifts of RWGs is carried out after a heat treatment of all the samples under study. The RWGs do not reveal significant spectral changes after the heat treatment; this is primarily due to a change in the surface chemistry by the redeposition of the reaction byproducts on the grating lines.