Receiver field of regard is one of the major problems for free space optical (FSO) communications. Drift or vibrations in transceiver orientations reduces effective communication time. The methods implemented to overcome this limitation, often require bulky optical and complex mechanical assemblies with feedback control, that are not suitable for long run operation in an airborne system. In this paper, we propose a novel receiver system that can effectively reduce the impact of pointing errors. The system is composed of two metalenses and one off-the-shelf conventional lens. The first metalens focuses optical beam incident at different angles on the aperture, at different locations on focal plane. The second metalens is placed on the focal plane of first metalens. After passing through the second metalens, the beams become parallel to optical axis of the receiver optical system. The parallel beams are collected by a suitable off-the-shelf aspheric lens and focused back on single detector that sits on a point on the optical axis. The system is designed and analyzed by physical optics theory. With 0.5 mm receiver aperture and 50 mm diameter aspheric lens, Zemax simulation shows that the system can collect +/-5-degree incident angle with detector diameter of 273μm. COMSOL frequency domain simulation with smaller diameter beam shows that the efficiency of the 2 metalens system is about 80%. Efficient metalens design and beam compression at detector plane are two key features of the proposed system. The system relaxes the strict requirement of aligning the transmitter and receiver unit in FSO communication.
Active tunability of optical leaky wave antenna is highly desired to enable greater control on light-matter interaction, sensing, and communication. Phase-changing materials can be integrated in optical antennas to enable such tunability. Among the phase-changing materials, vanadium dioxide (VO2) is the most useful as it shows the semiconductor to metal transition (68°C) very close to the room temperature. The phase transition in VO2 can be commonly induced by optical pulses or electrical joule heating. VO2 exhibits significant temperature-dependent electrical and optical coefficients even outside of the transition temperature making it suitable for both - fine and coarse tuning of the properties of optical devices depending on the temperature bias. In this work, we study optical leaky wave antenna consisting of a silicon nitride waveguide with periodic VO2 nanowire perturbations. We present the numerical analysis of different arrangements of the periodic perturbations. The antenna operates by the coupling between the evanescent mode of the waveguide and the nanowires. We show that, by selective joule heating of individual nanowires we can tune the optical property of corrugations and enable wider tuning range and higher degree of control on the radiated beam. We also include a comparative study to show tunability and performance of the antenna with different phase-changing materials like vanadium pentoxide (V2O5) and germanium-antimony-tellurium (GST). We show that, around the phase transition temperature of VO2, the directive gain of the antenna can be modulated by up to 25 dB and the radiation peak position can be tuned by up to 2.3°.
Phase-changing materials are promising due to their sharp temperature dependent characteristics and have high potential of being integrated in optical switching and sensing techniques. Among such materials, vanadium dioxide (VO2) is the most utilitarian because of its transition temperature being close to the room-temperature. VO2-based bolometers utilize the material’s large temperature coefficient of resistivity to detect infrared radiation. However, to achieve large sensitivity, the active radiation absorption area needs to be large enough that allows sufficient temperature buildup from incident radiation absorbed by VO2, thus requiring large pixel dimen- sion and degrading the spatial resolution of bolometric sensing. Moreover, the absorption by the VO2 material is not optimized for a specific frequency band in most of the applications. On the other hand, plasmonic nanos- tructures can be tuned and designed to selectively and efficiently absorb a specific band of the incident radiation for local heating and thermal imaging. In this work, we propose to incorporate plasmonic nanostructures with VO2 nanowires that amplifies the slope of impedance change due to the thermal variations to achieve a higher sensitivity. We present the numerical analysis of the mid-infrared electromagnetic radiation absorption by the proposed detector showing near-perfect absorption by the plasmonic absorbers. Besides, the thermal buildup and the nanowire resistance change is predicted for different substrate, as the substrate plays a big role in heat distribution. We show high sensitivity and ultra-low noise equivalent temperature difference (NEDT) by our novel bolometric detector. We also discuss the fabrication of the VO2.
Plasmonic structures have a wide variety of sensing applications because of their high field localization effect that leads to high sensitivity at lower powers. Specifically, plasmonic nanohole arrays are attractive platforms for sensing because of their easy alignment and measurement. In terms of fabricating these sensors, usually an adhesion layer is needed to ensure firm contact between the plasmonic metal layer and the substrate. Most fabrication efforts rely on titanium or chromium based metallic adhesion layers. However, the presence of the adhesion layer may hinder the plasmonic resonance by broadening the resonance and reducing the plasmonic field enhancement. This leads to degradation of sensing capabilities. We investigate the effect of tantalum, chromium, and titanium adhesion layers on plasmonic sensors made of nanohole arrays. Using the bulk refractive index data for metallic adhesion layers, we show that tantalum has the potential to show less damping effect compared to commonly used chromium and titanium. However, it still causes significant damping because of its high absorption, which becomes even larger for tantalum thin film according to our ellipsometry measurement results. We also propose here to use MgO dielectric adhesion layers to avoid the damping effect. Our investigation on MgO adhesion layers shows strong adhesion properties without scarifying sensor performance. Moreover, we will present an alternate sensor geometry that is less prone to damping by the adhesion layer and that can enhance the plasmonic resonance even if there is a metallic adhesion layer.
We propose a plasmo-thermomechanical mid-infrared detector operating at 4.3 μm wavelength. The design utilizes an array of the bimetallic fishbone nanowires that are suspended 50 nm above a 1.5 μm × 0.3 μm silicon nitride waveguide to create a leaky wave radiation. Moreover, the thermo-mechanically actuated nanowire will induce evanescent wave modulation that can be detected by the leaky wave or transmitted power of the waveguide. The antenna has a strip length of 1.77 μm and can yield an absorption coefficient of 42.4% with a period of 3.1 μm. Six unit cells are connected by a nanowire, and the fishbone-like nanowires are clamped at the two ends, leaving the center free to bend. The mid-infrared energy is absorbed by the resonant metallic antennas, resulting in a temperature increment. The mismatch of the thermal expansion coefficients of the bimetallic materials, gold and nickel, actuates the nanowire, and thus changes the gap between the nanowire and the waveguide. The deformation of the nanowire modulates the waveguide evanescent field, and hence alternates the transmitted power as well as the leak wave power. With a normal incident power of 4 μW/μm2 , the temperature in the center of the nanobridge can be increased over 135 K above the ambient temperature, leading to an elevation of 23.5 nm in the center and thus weakening the evanescent modulation strength. The difference of S21 caused by the gap change is 0.106. This methodology can be applied in other spectrums and the fabrication progress will be reported later.
Plasmonic nanostructures are highly used for sensing purposes since they support plasmonic modes
which make them highly sensitive to the refractive index change of their surrounding medium.
Therefore, they can also be used to detect changes in optical properties of ultrathin layer films in a
multilayer plasmonic structure. Here, we investigate the changes in optical properties of ultrathin
films of macro structures consisting of STT-RAM layers. Among the highest sensitive plasmonic
structures, nanohole array has attracted many research interest because of its ease of fabrication,
small footprint, and simplified optical alignment. Hence it is more suitable for defect detection in
STT-RAM geometries. Moreover, the periodic nanohole pattern in the nanohole array structure
makes it possible to couple the light to the surface plasmon polariton (SPP) mode supported by the
structure. To assess the radiation damages and defects in STT-RAM cells we have designed a
multilayer nanohole array based on the layers used in STT-RAM structure, consisting 4nm-
Ta/1.5nm-CoFeB/2nm-MgO/1.5nm-CoFeB/4nm-Ta layers, all on a 300nm silver layer on top of a
PEC boundary. The nanoholes go through all the layers and become closed by the PEC boundary on
one side. The dimensions of the designed nanoholes are 313nm depth, 350nm diameter, and 700nm
period. Here, we consider the normal incidence of light and investigate zeroth-order reflection
coefficient to observe the resonance. Our simulation results show that a 10% change in refractive
index of the 2nm-thick MgO layer leads to about 122GHz shift in SPP resonance in reflection
pattern.
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