High Index Thin Film Research Articles

Enhancing Sensitivity of Mid-infrared Waveguide Spectroscopy with a High-Index Thin Film

Enhancing Sensitivity of Mid-infrared Waveguide Spectroscopy with a High-Index Thin Film

Large-area narrowband Fabry–Pérot interferometers for long-wavelength infrared spectral sensing

This paper presents a proof-of-concept for microelectromechanical system (MEMS)-based fixed cavity Fabry–Pérot interferometers (FPIs) operating in the long-wavelength infrared (LWIR, 8 to 12 μm) region. This work reports for the first time on the use of low-index BaF2 thin films in combination with Ge high-index thin films for such applications. Extremely flat and stress-free ∼3-μm-thick free-standing distributed Bragg reflectors (DBRs) are also presented in this article, which were realized using thick lift-off of a trilayer structure fabricated using Ge and BaF2 optical layers. A peak-to-peak flatness was achieved for free-standing surface micromachined structures within the range of 10 to 20 nm across large spatial dimensions of several hundred micrometers. Finally, the optical characteristics of narrowband LWIR fixed cavity FPIs are also presented with a view toward the future realization of tunable wavelength MEMS-based spectrometers for spectral sensing. The measured optical characteristics of released FPIs agree with the modeled optical response after taking into consideration the fabrication-induced imperfections in the free-standing top DBR such as an average tilt of 15 nm and surface roughness of 25 nm. The fabricated FPIs are shown to have a linewidth of ∼110 nm and a suitable peak transmittance value of ∼50 % , which meets the requirements for their utilization in tunable MEMS-based LWIR spectroscopic sensing and imaging applications requiring spectral discrimination with narrow linewidth.

Fiber-integrated phase-change reconfigurable optical attenuator

We report on the experimental demonstration of an optical-fiber-integrated, nonvolatile transmission switching device. The operating mechanism exploits a cavity resonance spectral shift associated with an induced change in the refractive index of a high-index thin film on the polished side facet of the fiber. In the present case, a thermally induced amorphous-crystalline structural transition in a 500 nm layer of germanium antimony telluride at a distance of 500 nm from the core-cladding interface of an SMF-28 single-mode fiber delivers resonant transmission contrast >0.5 dB/mm at 1315 nm. Contrast is a function of active layer proximity to the core, while operating wavelength is determined by layer thickness—varying thickness by a few tens of nanometers can provide for tuning over the entire near-infrared telecoms spectral range.

Use of a broadband monitoring system for the determination of the optical constants of a dielectric bilayer.

This paper extends a method previously applied to the determination of the optical constants of a high-index thin film to a dielectric bilayer. This method is based on the time recording of the spectral transmittance of the stack during its deposition with the help of an in situ broadband monitoring system.

Sensitivity enhancement of polymeric Mach–Zehnder interferometers by use of thin high-index films

A simple yet highly-effective means for sensitivity enhancement of polymer-based effective index sensors using tailored high-index thin films is presented. Simulation and experimental results show that an appropriately-placed PTFE/Ta2O5 thin film can lead to enhanced sensitivity of asymmetric Mach–Zehnder interferometers to external refractive index changes of factors between 3 and 8. The films are deposited on the polymer using ion beam sputtering (IBS) at low temperatures (<80°C) thereby avoiding detrimental heating effects on the substrate. It is shown that the limit of detection of the interferometer sensors is enhanced by a factor 20 (down to Δn≤10−4) using these films.

Polarization-driven self-organization of silver nanoparticles in 1D and 2D subwavelength gratings for plasmonic photocatalysis

One of the main challenges in plasmonics is to conceive large-scale, low-cost techniques suitable for the fabrication of metal nanoparticle patterns showing precise spatial organization. Here, we introduce a simple method based on continuous-wave laser illumination to induce the self-organization of silver nanoparticles within high-index thin films. We show that highly regular and homogeneous nanoparticle gratings can be produced on large areas using laser-controlled self-organization processes. This very versatile technique can provide 1D and 2D patterns at a subwavelength scale with tunable features. It does not need any stabilization or expensive devices, such as those required by optical or electron lithography, and is rapid to implement. Accurate in-plane and in-depth characterizations provide valuable information to explain the mechanisms that lead to pattern formation and especially how 2D self-organization can fall into place with successive laser scans. The regular and homogeneous 2D self-organization of metallic NPs with a single laser scan is also reported for the first time in this article. As the reported nanostructures are embedded in porous TiO2, we also theoretically explore the interesting potential of organization on the photocatalytic activity of Ag-NP-containing TiO2 porous films, which is one of the most promising materials for self-cleaning or remediation applications. Realistic electromagnetic simulations demonstrate that the periodic organization of silver nanoparticles can increase the light intensity within the film more than ten times that produced with randomly distributed nanoparticles, leading as expected to enhanced photocatalytic efficiency.

Misfit Strain Relaxation of Ferroelectric PbTiO3/LaAlO3 (111) Thin Film System.

Ferroelectric thin films grown on high index substrates show unusual structural and switching dynamics due to their special strain states. Understanding the misfit relaxation behavior is crucial to facilitate the high index thin film growth with improved quality. In this paper, ferroelectric PbTiO3 thin films were grown on LaAlO3 (111) substrates by pulsed laser deposition technique. The microstructures were investigated by combinations of conventional and aberration-corrected transmission electron microscopy. Diffraction contrast analysis and high resolution imaging reveal that high density interfacial dislocations were distributed at the interfaces. These dislocations have mixed character with Burgers vectors of a <110> and line directions of <112>. The edge components of the dislocations, with the Burgers vectors parallel to the interface, accommodate the lattice mismatch and are the main contributor to the misfit relaxation of this system. The formation mechanism of these dislocations is proposed and discussed to elucidate the novel mismatch relaxation behavior of <111> oriented perovskite films.

Surface and waveguide collection of Raman emission in waveguide-enhanced Raman spectroscopy.

We demonstrate Raman spectroscopy on a high index thin film tantalum pentoxide waveguide and compare collection of Raman emission from the waveguide end with that from the waveguide surface. Toluene was used as a convenient model analyte, and a 40-fold greater signal was collected from the waveguide end. Simulations of angular and spatial Raman emission distributions showed good agreement with experiments, with the enhancement resulting from efficient collection of power from dipoles near the surface into the high-index waveguide film and substrate, combined with long interaction length. The waveguide employed was optimized at the excitation wavelength but not at emission wavelengths, and full optimization is expected to lead to enhancements comparable to surface-enhanced Raman spectroscopy in robust low-cost metal-free and nanostructure-free chips.

Light Trapping Textures Designed by Electromagnetic Optimization for Subwavelength Thick Solar Cells

Light trapping in solar cells allows for increased current and voltage, as well as reduced materials cost. It is known that in geometrical optics, a maximum 4n^2 absorption enhancement factor can be achieved by randomly texturing the surface of the solar cell, where n is the material refractive index. This ray-optics absorption enhancement limit only holds when the thickness of the solar cell is much greater than the optical wavelength. In sub-wavelength thin films, the fundamental questions remain unanswered: (1) what is the sub-wavelength absorption enhancement limit and (2) what surface texture realizes this optimal absorption enhancement? We turn to computational electromagnetic optimization in order to design nanoscale textures for light trapping in sub-wavelength thin films. For high-index thin films, in the weakly absorbing limit, our optimized surface textures yield an angle- and frequency-averaged enhancement factor ~39. They perform roughly 30% better than randomly textured structures, but they fall short of the ray optics enhancement limit of 4n^2 ~ 50.

Guided-mode resonant grating filter with an antireflection structured surface.

We describe a new structure of guided-mode resonant grating (GMRG) filters with low sideband reflectance. This GMRG filter consists of a high-index thin film on an antireflective structured surface called "moth-eye structure." Since the high-index film undulates along the surface structure, the film acts as a modulated optical waveguide. An incident light wave satisfying a resonant condition is reflected by the GMRG filter, and nonresonant light waves pass through the filter. This GMRG filter is valid for reducing reflection of nonresonant light waves in a wide spectral range. The resonant reflection of this new filter was investigated by numerical calculation based on an electromagnetic grating analysis. In the case of a triangular antireflective surface structure whose thickness is 2x greater than its period, the sideband reflectance for nonresonant light waves was lower than 0.5% for TM-polarized light in a wide range of wavelengths.

A composite optical waveguide-based polarimetric interferometer for chemical and biological sensing applications

A new polarimetric interferometer has been developed on the basis of the phase difference between transverse electric (TE)/sub 0/ and transverse magnetic (TM)/sub 0/ modes in a composite optical waveguide (OWG). The composite OWG consists of a single-mode potassium ion-exchanged planar waveguide overlaid with a high-index thin film that has two tapered ends and supports only the TE/sub 0/ mode. Applying tapered velocity coupling theory, we found that the TE/sub 0/ and TM/sub 0/ modes coexisting in the potassium ion-exchanged layer were separated in the thin film region of the composite OWG: the TE/sub 0/ mode was coupled into the thin film while the TM/sub 0/ mode was confined in the potassium ion-exchanged layer. Interference occurs between TE- and TM-polarized output components when a single output beam is passed through a 45/spl deg/-polarized analyzer. The phase difference /spl phi/ between both orthogonal output components is very sensitive to the superstrate index n/sub c/ in the thin film region. Our experimental results indicate that a slight change of /spl Delta/n/sub c/=3.71/spl times/10/sup -6/ results in the phase-difference variation of /spl Delta//spl phi/=1/spl deg/ for a 5-mm-long TiO/sub 2//K/sup +/ composite OWG with a 34-nm-thick TiO/sub 2/ film. Such a simple polarimetric interferometer can be applied to chemical or biological sensors by modifying the upper film surface of the composite OWG with a chemically or biologically active substance.

Optical constants in the ultraviolet region of evaporated high index thin films

A suitable method to determine the optical constants of high index thin films is essential for developing high efficiency dielectric thin film devices in theuv region from 240 nm to 400 nm. A quick and accurate method is established to determine these constants. Using this method the optical losses, refractive index, absorption coefficient and extinction coefficient of ZrO2 films prepared by the method of reactive evaporation were evaluated in theuv region.

Temperature-dependent dispersion of cadmium sulfide thin film in a slab-type optical waveguide

A previously reported optical guided-wave method for determining the optical constants of high-index thin films has been used to measure the temperature-dependent dispersion in vacuum-evaporated cadmium sulfide thin films. Measurements were carried out over the spectral range from 4579 to 6328 Å using Ar and He–Ne lasers at 18 K, 83 K, and room temperature in an optical cryostat.