Nanoscale Grating Research Articles

Plasmonic grating assisted photo‐absorption enhancement in terahertz photoconductive antenna

AbstractA large aperture photoconductive antenna (PCA) with a plasmonic grating structure was fabricated and characterized. The intensity of the terahertz radiation from the plasmonic PCA was enhanced two‐fold compared with that from the conventional PCA without the plasmonic grating structure. By using an electrically isolated nano‐grating structure, the performance enhancement was found to be due to the plasmonic effect other than bias field enhancement. The photo‐absorption enhancement near the GaAs surface and the plasmonic grating interface was investigated to elucidate the terahertz radiation enhancement mechanism in the plasmonic PCA. The highly localized electric field near the nanoscale grating shows optical absorption at the GaAs surface was enhanced. The enhanced optical absorption increased the photo‐generated carrier density at the device surface, leading to enhanced intensity and bandwidth of the terahertz radiation from the plasmonic PCA.

Resonant Subwavelength and Nano-Scale Grating Structures for Biosensing Application: A Comparative Study.

Resonant-based sensors are attractive optical structures due to the easy detection of shifts in the resonance location in response to variations in the analyte refractive index (RI) in comparison to non-resonant-based sensors. In particular, due to the rapid progress of nanostructures fabrication methods, the manufacturing of subwavelength and nano-scale gratings in a large area and at a low cost has become possible. A comparative study is presented involving analysis and experimental work on several subwavelength and nanograting structures, highlighting their nano-scale features’ high potential in biosensing applications, namely: (i) Thin dielectric grating on top of thin metal film (TDGTMF), which can support the excitation of extended surface plasmons (ESPs), guided mode resonance, or leaky mode; (ii) reflecting grating for conventional ESP resonance (ESPR) and cavity modes (CMs) excitation; (iii) thick dielectric resonant subwavelength grating exhibiting guided mode resonance (GMR) without a waveguide layer. Among the unique features, we highlight the following: (a) Self-referenced operation obtained using the TDGTMF geometry; (b) multimodal operation, including ESPR, CMs, and surface-enhanced spectroscopy using reflecting nanograting; (c) phase detection as a more sensitive approach in all cases, except the case of reflecting grating where phase detection is less sensitive than intensity or wavelength detection. Additionally, intensity and phase detection modes were experimentally demonstrated using off-the-shelf grating-based optical compact discs as a low-cost sensors available for use in a large area. Several flexible designs are proposed for sensing in the visible and infrared spectral ranges based on the mentioned geometries. In addition, enhanced penetration depth is also proposed for sensing large entities such as cells and bacteria using the TDGTMF geometry.

Resonant Subwavelength and Nano-Scale Grating Structures for Biosensing Application: A Comparative Study

Resonant Subwavelength and Nano-Scale Grating Structures for Biosensing Application: A Comparative Study

Reconfigurable cavity-based plasmonic platform for resonantly enhanced sub-bandgap photodetection

Sub-bandgap photodetection based on plasmonic excitations represents a promising route for expanding the spectral range of photodetectors, enabling, for instance, silicon-based devices to be employed at telecom wavelengths. This approach harnesses internal photoemission, where hot carriers are generated via nonradiative plasmonic decay and are subsequently emitted from the metal to a semiconductor, yielding a photocurrent not spectrally limited by the bandgap. However, many schemes based on this approach suffer from low responsivities that hinder their uptake in real-world technologies. Here, we demonstrate a cavity-based platform for both enhancing the generated photocurrent and providing a means for dynamic reconfiguration of the operating wavelength. The proposed device is composed of an optical cavity where one of the mirrors is patterned with a nanoscale grating and interfaced at the other side with a semiconductor. Fabry–Pérot resonances supported by the cavity provide resonant excitation of plasmonic modes at the metal/semiconductor interface, leading to augmented hot-carriers and photocurrent generation compared to the non-resonant case. By employing this cavity-grating geometry, we experimentally demonstrate a fivefold increase in photocurrent due to the presence of cavity resonances. Electromechanical reconfiguration of the photodetector cavity length is also achieved, illustrating dynamic control over the detection wavelength. This cavity-based architecture is compatible with a variety of plasmonic nanostructures, including nanoparticles and nanoantennas, thus providing a flexible means of significantly increasing the photoresponse and hence bringing on-chip plasmonic hot-carrier technologies closer to realization for sub-bandgap photodetection, energy harvesting, and sensing.

CPLD-Based Displacement Measurement System for Nanoscale Grating Ruler

With the continuous development of science and technology, industrial production has higher and higher requirements for precision. Many high-precision measurement technologies emerge as the times require, and nanoscale grating ruler displacement measurement technology is one of them. As a kind of precision sensor, nanoscale grating ruler has important application in displacement measurement system. CPLD has the advantages of high integration and fast programming speed, which is often used to control the displacement measurement system of nanoscale grating ruler. The purpose of this paper is to deeply explore the measurement effect and related application principle of nanoscale grating ruler displacement measurement system based on CPLD technology. A set of nanoscale grating ruler displacement measurement system is designed based on CPLD technology. The output signal of grating ruler is programmed by CPLD. The x-axis displacement of the experimental platform controlled by stepping motor is measured, and the measured data are recorded by carrying out analysis and research. The results show that compared with the traditional phase difference measurement system, the measurement accuracy of the system based on CPLD is improved by 24.7%, the robustness of the measurement system is improved by 18.6%, and the measurement speed is increased by 27.3%. Therefore, this kind of nanoscale measurement precision grating ruler displacement measurement control system based on CPLD has three characteristics: high measurement accuracy, strong anti-interference ability, and high measurement motion efficiency, which can effectively meet the requirements of grating ruler displacement measurement system for high-precision manufacturing technology.

Resonant-Like Field Enhancement by Nanoscale Grating-Coupled Propagating Surface Plasmons and Localized Surface Plasmons in the Mid-Infrared Range: Implications for Ultrafast Plasmonic Electron Sources

We investigated electron emission induced by an intense mid-infrared (MIR) field from a nanoscale aluminum-coated grating. The total photoelectron yields clearly show a resonant-like behavior when frustrated diffraction occurs near the Wood anomaly. This result indicates that a strong near field, which is formed by the localized surface plasmons (LSPs) at the ridge of the grating, can be enhanced by resonantly produced propagating surface plasmons (PSPs) owing to the phase matching between the diffracted light and PSPs. The observed photoelectron spectra can be reproduced well by a simple one-dimensional (1D) model of a near field that contains two parameters: the field enhancement factor, α, and the ridge radius, r0. In addition, we show that the resonant-like photoemission was attributed to the interference of the near fields produced by the LSPs and PSPs in the nanoscale grating structure. These results demonstrate that the nanoscale structure is useful for ultrafast plasmonic electron sources.

Highly sensitive detection of dopamine using a graphene functionalized plasmonic fiber-optic sensor with aptamer conformational amplification

Surface plasmon resonance (SPR) optical fiber sensors can be used as cost-effective and relatively simple-to-implement alternatives to well established bulky prism configurations for high sensitivity in-situ biomedical measurements. The miniaturized size and remote operation ability offer a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo. The biosensor configuration presented here utilizes a nano-scale metal-coated tilted fiber Bragg grating (TFBG) imprinted in a commercial single mode fiber core with no structural modifications. A unique feature of our TFBG sensor is the use of a single layer graphene coating over the gold-coated fiber surface, functionalized with a selective DNA aptamer for highly sensitive detection of target molecules. We have demonstrated the capture of dopamine molecules by the DNA aptamer, resulting in aptamer well-defined conformational changes in response to dopamine surface affinities. This process amplifies the surface refractive index modulation over the fiber surface to enable precise dopamine concentration measurement in real time via monitoring of the surface plasmon resonance signals. The sensor shows a linear response for dopamine concentration in the range from 10−13 M to 10−8 M with a lower limit of detection of 10−13 M. This limiting concentration is lower than the concentration fluctuations of dopamine in the human brain. The sensor works with minimal cross-sensitivities because of the core mode calibration inherent in the TFBG. Integration of the TFBG with a hypodermic needle should allow similar measurements in vivo, presenting an appealing solution for rapid, low power consumption and highly sensitive detection of analytes at low concentrations in medicine as well as in chemical and environmental monitoring.

Fiber Grating-Assisted Surface Plasmon Resonance for Biochemical and Electrochemical Sensing

Surface plasmon resonance (SPR) optical fiber sensors can be used as a cost-effective and relatively simple-to-implement alternative to well established bulky prism configurations for in situ high sensitivity biochemical and electrochemical measurements. The miniaturized size and remote operation ability offer them a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo . Grating-assisted and polarization control are two key properties of fiber-optic SPR sensors to achieve unprecedented sensitivities and limits of detection. The biosensor configuration presented here utilizes a nanoscale metal-coated tilted fiber Bragg grating (TFBG) imprinted in a commercial single-mode fiber core with no structural modifications. Such sensor provides an additional resonant mechanism of high-density narrow cladding mode spectral combs that overlap with the broader absorption of the surface plasmon for high accuracy interrogation. In this paper, we briefly review the principle, characterization and implementation of plasmonic TFBG sensors, followed by our recent developments of the “surface” and “localized” affinity studies of the biomolecules for real life problems and the electrochemical actives of electroactive biofilms for clean energy resources.

Investigation on the special Smith-Purcell radiation from a nano-scale rectangular metallic grating

The special Smith-Purcell radiation (S-SPR), which is from the radiating eigen modes of a grating, has remarkable higher intensity than the ordinary Smith-Purcell radiation. Yet in previous studies, the gratings were treated as perfect conductor without considering the surface plasmon polaritons (SPPs) which are of significance for the nano-scale gratings especially in the optical region. In present paper, the rigorous theoretical investigations on the S-SPR from a nano-grating with SPPs taken into consideration are carried out. The dispersion relations and radiation characteristics are obtained, and the results are verified by simulations. According to the analyses, the tunable light radiation can be achieved by the S-SPR from a nano-grating, which offers a new prospect for developing the nano-scale light sources.

Plasmonic nanostructured metal-oxide-semiconductor reflection modulators.

We propose a plasmonic surface that produces an electrically controlled reflectance as a high-speed intensity modulator. The device is conceived as a metal-oxide-semiconductor capacitor on silicon with its metal structured as a thin patch bearing a contiguous nanoscale grating. The metal structure serves multiple functions as a driving electrode and as a grating coupler for perpendicularly incident p-polarized light to surface plasmons supported by the patch. Modulation is produced by charging and discharging the capacitor and exploiting the carrier refraction effect in silicon along with the high sensitivity of strongly confined surface plasmons to index perturbations. The area of the modulator is set by the area of the incident beam, leading to a very compact device for a strongly focused beam (∼2.5 μm in diameter). Theoretically, the modulator can operate over a broad electrical bandwidth (tens of gigahertz) with a modulation depth of 3 to 6%, a loss of 3 to 4 dB, and an optical bandwidth of about 50 nm. About 1000 modulators can be integrated over a 50 mm(2) area producing an aggregate electro-optic modulation rate in excess of 1 Tb/s. We demonstrate experimentally modulators operating at telecommunications wavelengths, fabricated as nanostructured Au/HfO2/p-Si capacitors. The modulators break conceptually from waveguide-based devices and belong to the same class of devices as surface photodetectors and vertical cavity surface-emitting lasers.

Terahertz generation using plasmonic photoconductive gratings

A photoconductive terahertz emitter based on plasmonic contact electrode gratings is presented and experimentally demonstrated. The nanoscale grating enables ultrafast and high quantum efficiency operation simultaneously, by reducing the photo-generated carrier transport path to the photoconductor contact electrodes. The presented photoconductor eliminates the need for a short-carrier lifetime semiconductor, which limits the efficiency of conventional photoconductive terahertz emitters. Additionally, the photo-absorbing active area of the plasmonic photoconductive terahertz emitter can be increased without a significant increase in the capacitive loading to the terahertz radiating antenna, enabling high quantum-efficiency operation at high pump power levels by preventing the carrier screening effect and thermal breakdown. A plasmonic photoconductive terahertz emitter prototype based on the presented scheme is implemented and integrated with dipole antenna arrays on a semi-insulating In0.53Ga0.47As substrate. Emitted terahertz radiation is characterized in a terahertz time-domain spectroscopy setup, measuring a terahertz pulse width of 590 fs full-width at half maximum in response to 150 fs pump pulses at 925 nm.

Surface Morphological Effects on Subwavelength Filter of Nanoscale Grating for Near Infrared Biosensing

We modeled the effects of surface roughness (SR) on a near-infrared guided-mode resonance (GMR) biosensor. A power spectral density function was used to describe the SR with spatial autocorrelation integration through a rigorous coupled-wave analysis (RCWA), and a peak wavelength value (PWV) shift in reflectance was observed under transverse electric (TE) and transverse magnetic (TM) polarization. Sub-nanometer SR caused significant PWV shifts, leading to invalid spectral recognition. In addition to random roughness, we studied the PWV reflectance shift for SR models such as sinusoidal, rectangular, and effective medium types. All SR models showed a larger PWV shift for TM polarization than for TE polarization. Owing to SR, PWV reflectance decreased abruptly for TM polarization at small angular deviations from normal incidence. However, the symmetrically splitting of the reflectance spectra for TE polarization implied a high angular tolerance to fabrication errors. Compared to vertical sidewall roughness in our prior work, the SR is much more deleterious for GMR biosensing and should be appropriately controlled.

Surface Morphological Effects on Subwavelength Filter of Nanoscale Grating for Near Infrared Biosensing

We modeled the effects of surface roughness (SR) on a near-infrared guided-mode resonance (GMR) biosensor. A power spectral density function was used to describe the SR with spatial autocorrelation integration through a rigorous coupled-wave analysis (RCWA), and a peak wavelength value (PWV) shift in reflectance was observed under transverse electric (TE) and transverse magnetic (TM) polarization. Sub-nanometer SR caused significant PWV shifts, leading to invalid spectral recognition. In addition to random roughness, we studied the PWV reflectance shift for SR models such as sinusoidal, rectangular, and effective medium types. All SR models showed a larger PWV shift for TM polarization than for TE polarization. Owing to SR, PWV reflectance decreased abruptly for TM polarization at small angular deviations from normal incidence. However, the symmetrically splitting of the reflectance spectra for TE polarization implied a high angular tolerance to fabrication errors. Compared to vertical sidewall roughness in our prior work, the SR is much more deleterious for GMR biosensing and should be appropriately controlled.

Laser-Focused Atomic Deposition for Nanascale Grating

Laser-focused atomic deposition is a technique with which nearly resonant light is used to pattern an atom beam. To solve the problem that the result of laser-cooled atoms cannot be monitored during the 30-min depositing time, we present a three-hole mechanically precollimated aperture apparatus. A 425 nm laser light standing wave is used to focus a beam of chromium atoms to fabricate the nanoscale grating. The period of the grating is 213±0.1 nm, the height is 4 nm and the full width at half miximum is 64±6 nm.

High-speed, efficient metal - semiconductor - metal photodetectors

Design principles and the fabrication technique of highly efficient, high-speed photodetectors based on MSM nanostructures are developed. To efficiently confine light in the region of the strong field as well as to decrease light losses due to reflection from the diode contacts, use is made of a nanoscale interdigital diffraction grating and a multilayer Bragg grating. Measurements of the reflection coefficients and the quantum efficiency for a multilayer structure are in good agreement with theoretical estimates. A record-high quantum efficiency (QE = 46 %) is obtained for high speed MSM photodetectors. The detector has a high spectral selectivity (Δλ1/2 = 17 nm) at a wavelength of 800 nm. Taking into account the diode capacitance and the drift time of photogenerated carriers, the performance of the detectors under study is ~ 500 GHz. The low level of the dark current density in the structures under study (j="1" pA μm-2) makes it possible to realise on their basis highly sensitive, high-speed selective detectors of optical radiation.

Effect of Post Exposure Bake of Hydrogen Silsesquioxane on the Nanoscale Grating Pattern

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