Integrated nanophotonic waveguide-based devices for IR and Raman gas spectroscopy

On-chip devices for absorption spectroscopy and Raman spectroscopy have been developing rapidly in the last few years, triggered by the growing availability of compact and affordable tunable lasers, detectors, and on-chip spectrometers. Material processing that is compatible with mass production has been proven to be capable of long low-loss waveguides of sophisticated designs, which are indispensable for high-light–analyte interactions. Sensitivity and selectivity have been further improved by the development of sorbent cladding. In this review, we discuss the latest advances and challenges in the field of waveguide-enhanced Raman spectroscopy (WERS) and waveguide infrared absorption spectroscopy (WIRAS). The development of integrated light sources and detectors toward miniaturization will be presented, together with the recent advances on waveguides and cladding to improve sensitivity. The latest reports on gas-sensing applications and main configurations for WERS and WIRAS will be described, and the most relevant figures of merit and limitations of different sensor realizations summarized

Producing Nanoscale Laser Spot and Its Applications

Driven by the exponential growth in the field of nanotechnology in the last few decades, there has been a huge impetus in the design and production of subwavelength nanoscale laser spot which has found wide range of applications in different fields such as nanofabrication and data storage, among others. Limited by the diffraction limit of light when using conventional optics, generally metallic nano-apertures and nano-antennas are used for producing sub-100 nm spots. Design of such types of nano structures typically involves the use of surface plasmons to effectively collect and concentrate light below the diffraction limit. This work discusses the design and performance of several types of nanostructures to produce a nanoscale hotspot and their applications in different fields are also studied both experimentally and numerically. First we discuss the bowtie aperture and its light focusing performance and enhancement in the near field. Experiments were conducted to validate its application in near field optical lithography. Using a massive array of bowtie apertures, we have performed scanning optical lithography experiments with high precision gap control mechanism with the help of an Interferometric Spatial Phase Imaging (ISPI) system. We successfully demonstrated simultaneous writing of more than one thousand patterns with resolution less than 50 nm. Further, a novel type of cross sectional ridge waveguide nanoscale aperture is introduced and designed. Rather than using a sequential fabrication technique, layer-by layer fabrication method is used to make these nanostructures which ensures a very fine feature capable of producing a very tiny hot spot. We illustrated the performance of these apertures by scattering near field scanning optical microscope (s-NSOM) which show good near field localization characteristics. Next, we look at another emerging application of these nanoscale hotspots, in the field of data storage, where heat assisted magnetic recording (HAMR) is widely thought to be one of the next generation technologies to achieve high density data storage. We studied the optical performance of several types of apertures and antenna, also called near field transducers (NFT) in the HAMR terminology, including the bowtie aperture, E antenna, triangular antenna, C-aperture and the lollipop antenna in the presence of the recording medium. Subsequent thermal performance of the recording medium and the NFTs are calculated and several thermal figures of merit are established. Some design strategies and simple modifications of the NFTs are discussed which aim at improving the performance of the NFT like introducing a taper in its geometry. Also, it was found that changing the working wavelength of these NFTs from the typical 800 nm to longer wavelengths can increase the thermal performance of the HAMR system. Other nanostructure designs capable of generating similar hotspots are explored further such as a split-ring resonator (SRR) type of nanostructure. In addition to plasmonic resonance peaks, SRR has been shown to possess LC-circuit type of resonance in the infrared and optical frequency range which can help in generating hot spots in different wavelength range. These nanostructures are fabricated and characterized with the help of s-NSOM. Apart from generating sub wavelength focused spots using nanostructures, it has been found that arrayed nanostructures can also be used to enhance the force at the nanoscale and one can achieve a larger electromagnetic force acting on the metallic sample than is possible without the nanostructure. In the final part of the work, we experimentally verify and measure the enhanced force on a metallic surface due to presence of resonant slots on the surface. Experiments are performed to measure the deflection of a thin membrane under the effect of an incident laser both with and without the slots and results are compared and it is found that depending on the dimension and geometry of the slots, enhanced pushing force as well as pulling force can be observed

High-aspect-ratio free-standing membrane waveguides for mid-infrared nanophotonics

Nanophotonic devices for optical sensing often display poor evanescent field interaction. We demonstrate a suspended thin-film waveguide with stronger light–analyte interaction than a free-space beam, as verified by detecting acetylene at 2566 nm

Trace gas detection through spectroscopy using thin-film waveguides with extraordinarily confined guided light

Spectroscopic measurements of acetylene are performed with chip-integrated thin-film membrane waveguides based on Ta2O5 platform in MIR. Results show an excellent fit with the theoretical database and promise outstanding sensing performance with an on-chip device

Free-standing tantalum pentoxide waveguides for gas sensing in the mid-infrared

Typical applications of integrated photonics in the mid-infrared (MIR) are different from near-infrared (telecom) range and, in many instances, they involve chemical sensing through MIR spectroscopy. Such applications necessitate tailored designs of optical waveguides. Both cross-sectional designs and processing methods of MIR waveguides have been a subject of extensive research, where material transparency and substrate leakage of guided modes have been the most common challenges. Both these challenges can be solved simultaneously with air-suspended waveguides. In this paper, tantalum pentoxide (Ta2O5, tantala) thin films deposited on silicon were tested for two different dry under-etching procedures, XeF2 and SF6 plasma, with both of them facilitating selective removal of silicon. We analyze the advantages and limitations of these two methods and optimize the processing for fabricating membranes with arbitrary length and cross-sectional aspect ratio over 300. The performance of these high-aspect-ratio membranes as a framework for single-mode waveguides is rigorously analyzed at 2566 nm wavelength. With tantala being transparent up to 10 µm wavelength, such waveguides are particularly well suited for gas sensing in MIR

Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy

Nanophotonic waveguides are at the core of a great variety of optical sensors. These structures confine light along defined paths on photonic chips and provide light–matter interaction via an evanescent field. However, waveguides still lag behind free-space optics for sensitivity-critical applications such as trace gas detection. Short optical pathlengths, low interaction strengths, and spurious etalon fringes in spectral transmission are among the main reasons why on-chip gas sensing is still in its infancy. In this work, we report on a mid-infrared integrated waveguide sensor that successfully addresses these drawbacks. This sensor operates with a 107% evanescent field confinement factor in air, which not only matches but also outperforms free-space beams in terms of the per-length optical interaction. Furthermore, negligible facet reflections result in a flat spectral background and record-low absorbance noise that can finally compete with free-space spectroscopy. The sensor performance was validated at 2.566 μm, which showed a 7 ppm detection limit for acetylene with only a 2 cm long waveguide

Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy

Nanophotonic waveguides are at the core of a great variety of optical sensors. These structures confine light along defined paths on photonic chips and provide light–matter interaction via an evanescent field. However, waveguides still lag behind free-space optics for sensitivity-critical applications such as trace gas detection. Short optical pathlengths, low interaction strengths, and spurious etalon fringes in spectral transmission are among the main reasons why on-chip gas sensing is still in its infancy. In this work, we report on a mid-infrared integrated waveguide sensor that successfully addresses these drawbacks. This sensor operates with a 107% evanescent field confinement factor in air, which not only matches but also outperforms free-space beams in terms of the per-length optical interaction. Furthermore, negligible facet reflections result in a flat spectral background and record-low absorbance noise that can finally compete with free-space spectroscopy. The sensor performance was validated at 2.566 μm, which showed a 7 ppm detection limit for acetylene with only a 2 cm long waveguide.</p