Transparent conductive oxides (TCOs) have emerged as materials for nanoscale electro-optic modulators. The free-carrier-induced epsilon-near-zero effect by applying a gate voltage is capable of achieving ultra-strong electro-absorption (EA) effect. As the EA rate has been theoretically proved to be dependent on the mobility of the TCO gate, we experimentally demonstrated a hybrid silicon-plasmonic EA modulator using high-mobility In2O3 as the gate. With an ultra-compact active region of only 5-μm long, we achieved a small voltage swing Vpp of only 2 V to obtain an EA rate of 1.2 dB / μm, resulting in very high energy efficiency of 110 femto-joule/bit. We also experimentally proved the correlation between the EA rate and the mobility of TCO materials through comparison with indium-tin-oxide gated modulators. In addition, the hybrid EA modulator is capable of covering more than 100-nm optical bandwidth in the telecommunication wavelength window, which is limited by the bandwidth of the grating coupler.
Surface-enhanced infrared absorption (SEIRA) is capable of identifying molecular fingerprints by resonant detection of infrared vibrational modes through the coupling with plasmonic modes of metallic nanostructures. However, SEIRA for on-chip gas sensing is still not very successful due to the intrinsically weak light-matter interaction between photons and gas molecules and the technical challenges in accumulating sufficient gas species in the vicinity of the spatially localized enhanced electric field, namely the “hot-spots”, generated through plasmonics. In this paper, we present a suspended silicon nitride (Si3N4) nano-membrane device by integrating plasmonic nano-patch gold antennas with metal-organic framework (MOF), which can largely adsorb carbon dioxide (CO2) through its nanoporous structure. Unlike conventional SEIRA sensing relying on highly localized hot-spots of plasmonic nanoantennas or nanoparticles, the device reported in this paper engineered the coupled surface plasmon polaritons in the metal-Si3N4 and metal-MOF interfaces to achieve strong optical field enhancement across the entire MOF film. We successfully demonstrated on-chip gas sensing of CO2 with more than 1,800× enhancement factors within a 2.7 μm thin film, and the detection limit was estimated to be about 52 ppm. This work proved the feasibility of developing a new type of on-chip SEIRA gas sensing using hybrid plasmonic-MOF nano-devices with miniaturized size and ultra-high sensitivity.
The ever-growing demands to compute information, store, and communicate generate a continuous driving force for transformative photonic technologies. On chip photonic integrated circuits (PICs) founded a roadmap for scaling down photonic modules to meet the challenges of bandwidth enhancement and power reduction. However, it also produces an even critical need for state-of-the-art devices and better materials. Lately, the material group of transparent conductive oxides (TCO) has appealed attentions for on-chip photonic components. In specific, indium tin oxide (ITO) has been found to have active large refractive index variations, which creates possibilities to realize high speed Electro-Optic (E-O) modulation of sub-diffraction device scales. This ITO based tapered plasmonic waveguide devices will combine the large E-O absorption effects of ITO with the localized SPPs and the ultra-strong optical field confinement. In this paper, we design the device which is fabricated on commercial silicon-on-insulator (SOI) platform integrated with a 3μm long, 300nm wide gold plasmonic slot waveguide, which can dynamically switch the optical transmission from high absorption mode by enhanced plasmonic E-O absorption to low loss mode. The active E-O modulation region consists of a metal-HfO2-ITO capacitor that can electrically switch the ITO into ENZ with ultra-high modulation strength of 2.62dB/μm in simulation and 3.5dB extinction ratio in experiment. We also demonstrated the EA modulator a relative uniform E-O modulation within 1530~1600nm wavelength.
Transparent conductive oxides (TCOs) are getting increasing attention due to their unique epsilon-near-zero (ENZ) effect. The optical properties of TCOs can be dramatically changed from dielectric-like to metallic-like by controlling the carrier concentration in the telecommunication wavelengths, resulting in a near zero permittivity. The carrier concentration can be electrically manipulated when TCOs are built in a metal-oxide-semiconductor structure. With applied electrical bias, an accumulation layer forms at the oxide/TCO interface. When the accumulation layer meets ENZ condition, optical field is concentrated in the very thin accumulation layer, and thus the light-matter interaction is greatly enhanced. Such property makes TCOs wonderful materials for building active electro-optical devices. To data, several TCOs based devices configurations have been reported, such as metal-insulator-metal (MIM) waveguide based modulator, Si waveguide based plasmonic MOS modulator, PlasMOStor, and TCOs based tunable metasurfaces. Here, we design and demonstrate a TCOs based tunable subwavelength grating for surface normal modulation in the telecommunication wavelengths. The device combines the TCOs MOS structure with plasmonic grating filter. When applying voltage, the light-matter interaction in the active TCOs region is further enhanced by the surface-plasmon resonances coupled guided-mode resonances (GMRs), which enables high efficient modulation for both transmission and reflection mode with only 10nm thick TCOs layer. At peak wavelength (~1.55 μm), the simulated modulation depth achieves as high as 32% for transmission mode and 56% for reflection mode.
Surface-enhanced infrared absorption (SEIRA) is a spectroscopic technique used to identify molecular fingerprints by resonant detection of infrared vibrational modes through coupling with the plasmonic modes of metallic nanostructures. Many reported works have demonstrated its capability to enhance the infrared absorption of solid or liquid samples. However, this technique has not been successfully applied to gas sensing yet due to the short light-matter interaction length and intrinsically weak absorption of gas compared to solid or liquid materials. Usually, IR gas sensing is conducted in a gas cell with a long absorption path. In the paper, we propose an integrated photonic device to expand the application of SEIRA to gas sensing by combining metal-organic framework (MOF) ZIF-8 (zeolitic imidazole framework) with plasmonic nanoantenna array. The device consists of an Au nanopatch array on sapphire substrate and is covered by a thin layer of MOF material. The MOF thin film, which is a new class of highly nanoporous material, serves as a gas absorber to selectively adsorb and concentrate CO2 from ambient environment into the thin layer, which has a high spatial overlap with the high intensity optical field of the plasmonic nanopatch antenna array. Namely, we can effectively increase the gas molecule concentration at the hot-spots for the SEIRA device. The experimentally demonstrated peak IR enhancement factor of the device for carbon dioxide sensing is over 1,100 times.
Photonic crystal slabs (PCSs), which generally consist of two-dimensional arrays of nanoholes in the top layer of a dual layer dielectric film, have been demonstrated as a promising platform for optical biosensing. Both the Fano resonance in a perfect PCS and the Lorentzian resonance in a micro-cavity resulted from an introduced defect in PCS have been studied. While, the use of resonance peak shift for detecting molecules owing to the change of the refractive index is a nonspecific biosensing technique. Biorecognition molecules, such as antibodies that can specific bond to interesting molecules, are conjugated on the PCS to improve the detection specificity. It is a widely adopted assumption that the conjugated molecules form into a uniform nanofilm in the PCS based biosensors, which covers either the entire surface of the dielectric layer or the entire sidewalls of nanoholes. However, the actual device performance is much lower than that obtained based on this assumption, which suggests the over-simplicity of the hypothesis above. It is of keen interest to reveal the actual arrangement and distribution of molecules on PCS for designing high-performance PCS biosensors. Here, we propose models and analysis of the distribution of nanofilms on PCS. We employed Raman scattering technique to experimentally reveal the actual various configurations of nanofilms, which support our theoretical modeling. The results obtained in this research can be essential for designing high-performance PCS based nanobiosensors.
Raman scattering spectroscopy is a unique tool to probe vibrational, rotational, and other low-frequency modes of a molecular system and therefore could be utilized to identify chemistry and quantity of molecules. However, the ultralow efficient Raman scattering, which is only 1/109 ~ 1/1014 of the excitation light due to the small Raman scattering cross-sections of molecules, have significantly hindered its development in practical sensing applications. The discovery of surface-enhanced Raman scattering (SERS) in the 1970s and the significant progress in nanofabrication technique, provide a promising solution to overcome the inherent issues of Raman spectroscopy. It is found that In the vicinity of nanoparticles and their junctions, the Raman signals of molecules can be significantly improved by an enhancement factor as high as 1010, due to the ultrahigh electric field generated by the localized surface plasmons resonance (LSPR), where the intensity of Raman scattering is proportional to the |E|4. In this work, we propose and demonstrate a new approach combining LSPR from nanocapsules with densely assembled silver nanoparticles (NC-AgNPs) and guidemode- resonance (GMR) from dielectric photonic crystal slabs (PCSs) for SERS substrates with robustly high performance.
We demonstrate an ultra-compact on-chip spectrometer for near-infrared (NIR) spectroscopy based on narrow-band band-pass filter array. Each individual filter consists of a plasmonic metallic grating with subwavelength period and extremely narrow slits on a quartz substrate, with a polymer cover layer as the waveguide layer. A narrow-band guided-mode resonance (GMR) associated with a surface-plasmon resonance (SPR) gives rise to the narrow-band transmission spectrum. Full width at half maximum (FWHM) of fabricated filter’s spectrum is measured to be from 7 to 13 nm, and the operation bandwidth of the entire filter array covers wavelength range over 270 nm from 1510 to 1780 nm. We measure the NIR absorbance spectrum of xylene using our filter array device to demonstrate its application as a spectrometer.
We demonstrate ultra-sensitive near-infrared (NIR) fiber-optic gas sensors enhanced by metalorganic framework (MOF) Cu-BTC (BTC=benzene-1,3,5- tricarboxylate), which is coated on a single-mode optical fiber. For the first time, we obtained high-resolution NIR spectroscopy of CO2 adsorbed in MOF without seeing any rotational side band. Real-time measurement showed different response time depending on the concentration of CO2, which is attributed to the complex adsorption and desorption mechanism of CO2 in Cu-BTC. The lowest detection limit of CO2 we achieved is 20 ppm with only 5-cm long Cu-BTC film.
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