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Showing posts with label surface plasmon resonance. Show all posts
Showing posts with label surface plasmon resonance. Show all posts
The phase behavior of the reflected terahertz radiation (THz) under surface plasmon resonance (SPR) supported by doped graphene has been comprehensively investigated. For a TM–polarized wave, the dependence of the phase on the angle of incidence has a region with an abrupt jump–like change. We found in particular that the resonance phase dependence would change from step–like contour to Fano lineshape when the system passed through the optimum SPR conditions (i.e., R = 0) in terahertz regime. Monitoring the transformation could provide ultrahigh–sensitive label–free detection of biomolecules. Importantly, the characteristic of phase jumps as a readout response to achieve refractive index sensing that outperforms traditional terahertz–amplitude based attenuated total reflection (ATR) spectroscopy. The results demonstrated a high figure of merit (FOM) of up to 171 based on the terahertz phase information. Moreover, the sensing range could be tuned by changing the surface conductivity of graphene via high doping levels or with few–layer graphene. These terahertz phase response characteristics of graphene plasmon are promising for tunable ultra–sensitivity (bio)chemical sensing applications.
Photonic crystal fibers (PCFs), also known as microstructured optical fibers (MOFs), are a distinct class of optical fibers that are particularly suited to applications in the fields of sensing, biomedical imaging, time domain spectroscopy, security, DNA hybridization and cancer detection, and in optical communications. Unlike conventional optical fibers, PCFs provide high birefringence and controllable chromatic dispersion.
Solid-core PCFs experience large material loss that is not suitable for terahertz signal transmission, while hollow-core PCFs limit electromagnetic waves to short propagation distances and have high bending losses that are inversely proportional to the diameter and bending radius of the fiber. Because these undesirable features have slowed the acceptance of solid and hollow-core PCFs, porous-core fibers have been developed.
Our team at the University of Adelaide focuses on porous-core PCFs that contain an engineered number of microstructured air holes in the core, allowing the designer to control such global fiber parameters as air hole size, pitch distance (center-to-center distance between air holes), core diameter, and air-hole shape. In turn, operational parameters such as effective material loss, birefringence, dispersion, confinement loss, numerical aperture, and other modal properties can be obtained by design as dictated by application requirements.
PCFs as waveguides
The main function of a waveguide is to transmit electromagnetic radiation with the lowest possible transmission loss and near-zero dispersion at the desired wavelength. Over the last decade, a number of waveguide structures have been designed and studied for efficient and reliable transmission of electromagnetic waves.
FIGURE 1. Photonic crystal fiber (PCF) designs proposed to reduce transmission loss of electromagnetic (and especially terahertz) waves include a hybrid core in a regular hexagonal cladding (a), a hybrid core in a modified hexagonal cladding (b), a rotated hexagonal core in a circular cladding (c), and a diamond-shaped core in a kagome cladding (d).FIGURE 2. Highly birefringent PCF-based waveguides include a kagome cladding with elliptical air-hole core (a), a kagome cladding with rectangular air-hole core (b), and a rectangular air-hole cladding with an array of elliptical air holes in the core (c).
Initially, terahertz electromagnetic waves were guided by metallic waveguides. The various types of metallic waveguides included circular, parallel-plate, bare-metal wire, and slit waveguides. Unfortunately, metallic waveguides face a number of problems, including the fact that metal strips and slots create large Ohmic and attenuation losses; circular metallic waveguide substrates incur high dielectric losses; beam spreading in the unguided media cause divergence losses in parallel- plate waveguides; and radiative losses in the bare metallic waveguide occur because of weak confinement of the mode to the structure. A better option for terahertz transmission is the optical fiber dielectric waveguide.
Reducing transmission loss
Considering the advantage of PCFs, a number of waveguide structures have been designed for low-loss transmission of electromagnetic waves (see Fig. 1).
Dramatic reduction of the material absorption loss caused by the bulk material used in the background of a PCF is possible through (a) a hexagonal cladding and hybrid structured core; (b) a modified hexagonal cladding in which the air holes from each edge are removed, reducing loss and improving birefringence; (c) a circular-clad PCF waveguide with rotated hexagonal core; and (d) a kagome-clad PCF that reduces confinement loss 3–4X more than other competing PCF structures.
The latter kagome configuration reduces loss by a factor of 3–4X more than the other reported cladding structures. The reason being that kagome cladding has the ability to constrict more light inside the core and restrict light from going further towards the cladding.
High birefringence
For polarization-preserving applications, PCFs need to have asymmetry between the x- and y-polarization modes. As such, a number of asymmetrically structured waveguides have been proposed to obtain high birefringence, including elliptical and rectangular air holes inside a kagome lattice (see Fig. 2).
These elliptical and rectangular air holes in the core create large asymmetry between the x and y polarization mode, thus improving the birefringence. Note that it is also possible to generate birefringence using circular-shaped air holes—however, that also requires a structure that generates asymmetry between the polarization modes.
Birefringence and sensing
By replacing the ambient air in the holes of a PCF with various analytes, a PCF can be converted to a sensor. Sensitivity is improved by optimizing the modified total internal reflection (MTIR) mechanism of the PCF to improve the interaction of light with the surrounding PCF substrate materials.
For example, if we use water (refractive index of approximately 1.33) as an analyte inside the core hole instead of air (refractive index around 1.0), the light interaction with the analyte will be stronger because of strong MTIR due to the higher refractive index of the core than the cladding. In essence, core power fraction is increased and confinement loss is decreased. Note that relative sensitivity is proportional to core power fraction and therefore, as core power fraction increases, the relative sensitivity also increases (see Fig. 3).
FIGURE 3. Examples of PCFs used as sensors include a suspension-type cladding with porous core (a) and a rectangular airhole cladding with hollow core (b).FIGURE 4. Standard deviation in optical constants is compared to sample thickness. The optimum thicknesses, where the standard deviation is minimal, are indicated by arrowheads. By moving towards a thicker sample, the standard deviation rapidly increases to the point comparable to the optical constants’ values. The standard deviation at high frequency is more sensitive to the thickness increment, as the terahertz-ray magnitude at high frequencies is relatively low.FIGURE 5. A PCF-based surface plasmon resonance biosensor can be used in external sensing (a) or internal sensing (b) configuration.
So, why is PCF birefringence necessary for sensing?Considering ethanol as the chemical analyte, experiments show that measurement uncertainty increases with sample thickness because terahertz waves, when used as the interrogation wavelength, suffer increased absorption. However, if sample thickness is reduced too much, uncertainty also increases because of an insufficient interaction depth, so there is a tradeoff between these extremes and an optimal thickness can be calculated (see Fig. 4).
It has been proven that the optimal sample thickness to minimize measurement uncertainty is (2/α), where α is the absorption coefficient. Note that the absorption coefficient for ethanol is in the 20–80 cm-1 range in the 0.2–1.4 THz band. If you consider α= 20 cm-1, this will yield the worst-case largest thickness of 2/α= 2/20 = 0.1 cm = 1 mm.
Although 1 mm is a rather-large thickness value, the minima in the uncertainty vs. thickness curves are not narrow, but reasonably flattened—that is, halving the optimal thickness does not significantly degrade the uncertainty. As a result, we can reasonably halve the value of 1 mm and select 0.5 mm as a suitable sample thickness. When this 0.5 mm sample is then analyzed at around 1.4 THz, the path length of terahertz radiation is now effectively 0.05 × 80 = 4 absorption lengths. This corresponds to a 20 log e^-4 ∼=-35 dB attenuation, which is manageable.
Fiber-based terahertz heterodyne detection is known to be possible at power levels as low as 3 μW. This means that the input terahertz power into the fiber must be >160 μW, which is perfectly achievable. Because it is also well known that fiber-based heterodyne detection requires the polarization of the local oscillator to be aligned to the polarization being detected at the end of the fiber, polarization-preserving fibers must be used, solidifying the need for birefringence in PCF fiber-based terahertz sensing.
PCF sensor comparison
Characterizations of both porous-core and hollow-core PCFs for sensing show that hollow cores can improve performance in that more analytes can fill the core structure.3In a sensor structure with a suspension type cladding with a circular-air-hole based porous core, the core interfaces can create issues with sensing, depending on the glass materials used.
There is also the option in PCF sensing of using an external or an internal sensing mechanism (see Fig. 5). In an external sensing approach, the analyte channel is outside the plasmonic material and is relatively easy to fill. Conversely, an internal sensing approach with the analyte within the air holes is more complicated because filling microstructured tiny air holes with an analyte is difficult.
Using the surface plasmon resonance (SPR) effect between the metal and dielectric interface of a PCF can also create a SPRbased biosensor. By incorporating such metals as gold, copper, iron, and silver within the fiber core, SPR works when the electrons of the p-polarized light waves oscillate between the metal-dielectric interfaces. A tiny change of environmental refractive indices shifts the resonance wavelengths, enabling extremely sensitive SPR sensors for medical diagnostics and testing, antigen-antibody interactions, environmental monitoring, homeland security, and food safety.4
To date, PCF fabrication methods such as capillary stacking, drilling, and sol-gel methods create circular holes, while 3D printing, extrusion, and chemical vapor deposition (CVD) methods create noncircular and complex structures. As these methods improve, next-generation PCF designs will continue to progress in both sensing and transmission applications.
Flexibility in engineering holey structures and controlling the wave guiding properties in photonic crystal fibers (PCFs) has enabled a wide variety of PCF-based plasmonic structures and devices with attractive application potential. Metal thin films, nanowires, and nanoparticles are embedded for achieving surface plasmon resonance (SPR) or localized SPR within PCF structures. This paper begins with an outline of plasmonic sensing principles. This is followed by an overview of fabrication and experimental investigation of plasmonic PCFs. Reported plasmonic PCF designs are categorized based on their target application areas, including optical/biochemical sensors, polarization splitters, and couplers. Finally, design and fabrication considerations, as well as limitations due to the structural features of PCFs, are discussed.
The control of light-matter interaction in metasurfaces offers an unexplored potential for the excitation and manipulation of light. Here, we combine experimental terahertz time-domain spectroscopy and near-field scanning terahertz microscopy to demonstrate the role of reciprocal vectors in the transmission and plasmonic resonances of quasicrystal metasurfaces. An investigation of two-dimensional metasurface structures with different rotationally symmetric quasicrystal arrangements demonstrates that the transmission minima resulting from Wood’s anomaly are directly related to the surface plasmon resonances. We also find that the surface plasmon resonances of the quasicrystal metasurface were determined by the reciprocal vectors, which could be well explained by the coupling condition of the resonances, and the characteristic frequencies remain un-shifted under various slit sizes. Our findings demonstrate a new potential in developing novel plasmonic metasurfaces.
We experimentally observe the excitation of dark multipolar spoof localized surface plasmon resonances in a hybrid structure consisting of a corrugated metallic disk coupled with a C-shaped dipole resonator. The uncoupled corrugated metallic disk only supports a dipolar resonance in the transmission spectrum due to perfect symmetry of the structure. However, the dark multipolar spoof localized surface plasmon resonances emerge when coupled with a bright C-shaped resonator which is placed in the vicinity of the corrugated metallic disk. These excited multipolar resonances show minimum influence on the coupling distance between the C-shaped resonator and corrugated metallic disk. The resonance frequencies of the radiative modes are controlled by varying the angle of the C-shaped resonator and the inner disk radius, both of which play dominant roles in the excitation of the spoof localized surface plasmons. Observation of such a transition from the dark to radiative nature of multipolar spoof localized plasmon resonances would find potential applications in terahertz based resonant plasmonic and metamaterial devices.
We design terahertz (THz) surface-plasmon-resonance (SPR) sensors using a ferroelectric polyvinylidene fluoride (PVDF) thin layer for biological sensing. The reflectivity properties based on SPR are described using transfer matrix method (TMM) and numerically simulated using finite-difference time domain (FDTD) method. The sensing characteristics of the structure are systematically analyzed through the examination of the reflectivity spectrum. The results reveal that the pronounced SPR resonance peak has quasi-linear relationship with the refractive index variation of the material under investigation. Through analyzing and optimizing the structural parameters of the THz SPR sensor, we achieved the theoretical value of the refractive index detection sensitivity as high as 0.393 THz/unit change of refractive index (RIU) for a 20-μm-thick liquid sample with a 10-μm PVDF layer. This work shows great promise toward realizing a THz SPR sensor with high sensitivity for identifying the signatures of biological fluid sample.
Terahertz radiation, the no-man's land of the electromagnetic spectrum, has long stymied researchers. Optical technologies can finagle light in the shorter-wavelength visible and infrared range, while electromagnetic techniques can manipulate longer-wavelength radiation like microwaves and radio waves. Terahertz radiation, on the other hand, lies in the gap between microwaves and infrared, whether neither traditional way to manipulate waves works effectively. As a result, creating coherent artificial sources of terahertz radiation in order to harness it for human use requires some ingenuity.
Difficulties of generating it aside, terahertz radiationhas a wide variety of potential applications, particularly in medical and security fields. Because it's a non-ionizing form of radiation, it is generally considered safe to use on the human body. For instance, it can distinguish between tissues of different water content or density, making it a potentially valuable tool for identifying tumors. It could also be used to detect explosives or hidden weapons, or to wirelessly transmit data.
In a step towards more widespread use of terahertz radiation, researchers have designed a new device that can convert a DC electric field into a tunable source of terahertz radiation. Their results are published this week in the Journal of Applied Physics, from AIP Publishing.
This device exploits the instabilities in the oscillation of conducting electrons at the device's surface, a phenomenon known as surface plasmon resonance. To address the terahertz gap, the team created a hybrid semiconductor: a layer of thick conducting material paired with two thin, two-dimensional crystalline layers made from graphene, silicene (a graphene-like material made from silicon instead of carbon), or a two-dimensional electron gas. When a direct current is passed through the hybrid semiconductor, it creates a plasmon instability at a particular wavenumber. This instability induces the emission of terahertz radiation, which can be harnessed with the help of a surface grating that splits the radiation.
By adjusting various parameters—such as the density of conduction electrons in the material or the strength of the DC electric field—it is possible to tune the cutoff wavenumber and, consequently, the frequency of the resulting terahertz radiation.
"[Our work] demonstrates a new approach for efficient energy conversation from a dc electric field to coherent, high-power and electrically tunable terahertz emission by using hybrid semiconductors," said Andrii Iurov, a researcher with a dual appointment at the University of New Mexico's Center for High Technology Materials and the City University of New York. "Additionally, our proposed approach based on hybrid semiconductors can be generalized to include other novel two-dimensional materials, such as hexagonal boron nitride, molybdenum disulfide and tungsten diselenide."
Other labs have created artificial sources of terahertz radiation, but this design could enable better imaging capabilities than other sources can provide. "Our proposed devices can retain the terahertz frequency like other terahertz sources but with a much shorter wavelength for an improved spatial resolution in imaging application as well as a very wide frequency tuning range from a microwave to a terahertz wave," said Iurov.
