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Z-directional time-averaged power flow on the XY plane at 1 THz for the right-handed helically grooved wire. Lower panel is the plots (solid line is the upper enveloping line) of power varying along the line indicated by the horizontal white dashed line located at y¼ Rþ 10 mm away from the wire outline. The head portion of the curve is truncated for clarity, which corresponds to the giant oscillation caused by input excitation. The measured pitch wave is approximately 4.1 mm (indicated by the vertical dashed lines). Scale bar is 500 mm. The total length of the wire is 10 mm, and other geometry parameters are identical to the above case.
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Chiral spoof surface plasmon polaritons in the terahertz region, resulting from a coherent superposition of TM and HE modes, can be generated by normally exciting helically corrugated metal wire with a linearly polarized Gaussian light. The handedness of the chiral SPP depends on the handedness of the helical groove. The chirality of the surface wa...
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Context 1
... β is the wave number of different Eigenmodes. Snapshots of Z-direction time-averaged power flow in the XZ plane for a longer right-handed case (L ¼10 mm, calculated using Remcom XFDTD) are presented in Fig. 4. It is obvious that a right-handed single- stranded spiral pattern is formed around the structured wire. By plotting the power energy distribution along the line at X¼ Rþ10 mm, denoted by a white dashed line, we can measure the spiral pitch, which is approximately 4.1 mm long and agrees with the calculated one derived from P ¼2π/(β 1 À ...
Context 2
... frequency f 0 ¼1 THz, 1.2 THz, 1.3 THz. Fig. 5 shows the spatial map of C in vertical planes of Z¼3000 mm, 5000 mm, 9000 mm, and 10,010 mm. Note that the vertical planes of Z¼ 10,010 mm show the map of degree of circular polarization on the outside facet, which is 10 mm away from the end of a wire whose total length is 10,000 mm, as shown in Fig. 4. From the maps listed in panel (a), we can see that two peaks exist with opposite signs that correspond to left-circular and right- circular polarization, respectively. Despite the presence of a non- zero Ez component, a high degree of circular polarization is obtained (C 1 at two peaks ≈ ± ). Interestingly, the two opposite ...
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Surface plasmon polaritons (SPPs) are electromagnetic waves that have attracted significant interest owing to their subwavelength confinement and the strong field enhancement that they provide. Yet in the terahertz (THz) frequency region of the spectrum, which is well below the plasma frequency of metals, these surface waves are characterized by ex...
Citations
... The polarization manipulation of terahertz (THz) waves is one of the most important aspects in the design of THz devices [5][6][7][8][9][10]. Its robustness and chirality have made it useful for many optical applications [11], including biomolecule sensing, drug detection [12], and 6G wireless communication [13]. Polarization converters can be used easily to modify the polarization states of THz waves, as the above-mentioned signal features. ...
... T HE polarization manipulation of terahertz (THz) wave is one of the most significant functions in the development of THz devices [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Among various THz waves with different polarization states, the THz circularly polarized wave, for its robustness and chirality [15], has been manifested in a wide range of optical applications, such as 6G wireless communication, drug detection and biomolecules sensing [16][17][18]. However, the states of most electromagnetic waves generated from THz emitters are linearly polarized [19], and the function of corresponding polarization converters are just linear to cross [20][21]. ...
Terahertz polarization converter has drawn great attention for its properties of wavefront control in recent years. However, the low transmission efficiency of these converters remains a major obstruction to be resolved. A promising solution is the all-dielectric metasurface, based on which, this study proposes a high-efficiency transmissive terahertz linear-to-circular polarization converter. The simulations show the transmission efficiency of the converter is beyond 88% with an ellipticity < -0.8 from 0.555 to 0.737 THz, and the maximum efficiency even reaches 100% at 0.660 THz. Furthermore, the converter can also be actively manipulated by combining with dynamic material. The frequency sensitivity is 98 GHz/100K, and the modulation depth of ellipticity at 0.725 THz exceeds 76%. The excellent performances of these converters offer more potentials for terahertz devices.
... A recently developed metallic nanowire is a promising candidate for resolving these challenges [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] . The nanowire can produce chiral light and possesses a nanometersized geometry that is comparable to that of sub-10-nm chiral biomolecules. ...
Chiral surface plasmon polaritons (SPPs) produced by plasmonic nanowires can be used to enhance molecular spectroscopy for biosensing applications. Nevertheless, the switchable stereoselectivity and detection of various analytes are limited by a lack of switchable, chiral SPPs. Using both finite-element method simulations and analytic calculations, we present a graphene-coated chalcogenide (GCC) nanowire that produces mid-infrared, chiral SPPs. The chiral SPPs can be reversibly switched between “on” (transparent) and “off” (opaque) by non-volatile structural state transitions in the dielectric constants of the chalcogenide glass Ge2Sb2Te5. Furthermore, by controlling the Fermi energy of the graphene-coating layer, the nanowire can output either non-chiral or chiral SPPs. A thermal-electric model was built to illustrate the possibility of ultrafast on/off switching of the SPPs at the terminus of the nanowire. Finally, we show that a selective, lateral sorting of sub-10-nm enantiomers can be achieved via the GCC nanowire. Chiral nanoparticles with opposite handedness experience transverse forces that differ in both their sign and magnitude. Our design may pave the way for plasmonic nanowire networks and tunable nanophotonic devices, which require the ultrafast switching of SPPs, and provide a possible approach for a compact, enantiopure synthesis.
... The metallic nanowires can control the propagation of surface plasmon polaritons (SPPs), 9−17 which may produce the chiral SPPs. 18,19 Substantial efforts are now focused on developing the metallic nanowires with a broad tunability of SPPs. 20−24 It could offer numerous useful applications, for example, controlling the demultiplexing functions in plasmonic nanocircuits and optical signal distribution for different routing. ...
Plasmonic nanowire was found to generate chiral surface plasmon polaritons (SPPs), with the perspective of enhancing molecular spectroscopy for biosensing applications. However, the lack of chiroptical switches exhibits significant limitations in detecting a multitude of various analytes with a high sensitivity and in asymmetric catalysis to provide a switchable stereoselectivity. Here, we numerically and analytically propose a graphene-coated Ge2Sb2Te5 (GST225) nanowire to solve this problem. We highlight that the chiral SPPs propagating along the nanowire can be reversibly switched between “on” (transparent) and “off” (opaque) as transiting the state of GST225 core between amorphous and crystalline. Moreover by changing the Fermi energy of the graphene coating layer, the hybrid nanowire can produce either achiral or chiral SPPs at the output of nanowire. A thermal-electric model is put forward to study the temporal variation of the temperature of the GST225 core. Transiting the structural phase of GST225 in nanosecond was theoretically demonstrated. Our proof of concept permits the preparation of circularly polarized light source with a fast switching “on/off” function. We foresee its potential applications for tunable nanophotonic devices, plasmonic nanowire networks, on-chip biosensing, etc., where the active controlling of SPPs is necessary.
... Note that if the working frequency is out the range of the circularpolarization band, the propagating wave will be mixed with other SSPP mode (e.g., TM mode or HE−1 mode), and thus, the pure circular polarization will get destroyed. Readers can find the details regarding the evolution of polarization for different frequencies wave propagating on such waveguide in [28]. ...
... Although all the mentioned figures are considering the HE+1 mode, it can be inferred that the opponent mode, i.e., HE−1 mode, has an opposite direction on rotation and polarization. It is mentioned that the excited HE−1 mode is always accompanied by the TM0 mode because the working frequency covers both dispersion curves of the two modes; thus, the distribution of the electromagnetic field and polarization degrees for the frequencies outside the circular polarization band behave complicated [28]. It is worth stressing that the desired frequency can be controlled by adjusting the geometry size of the structure. ...
We report the characteristics of a right-handed helically grooved metal wire waveguide. For the helically grooved waveguide, the normal degenerate HE1 spoof surface plasmon polariton (SSPP) mode could be decomposed into two opposite circularly polarized modes, i.e., HE-1 and HE+1 SSPP modes, due to the chirality of helical grooves. The dispersion characteristics of the nondegenerate HE + 1 SSPP modes were numerically obtained, and their spiral electromagnetic distributions were also presented. It was found that the HE + 1 modes were circularly polarized; they were also vortex modes and possessed an orbit angular monument. In addition, the focusing characteristics of such waves near the waveguide end were investigated. The distinct advantages, such as a relatively long focusing length and the circular polarization of the chiral HE + 1 SSPP modes, make the helically grooved metal wire waveguide a potential for near-field circular dichroism detection.
In undergraduate courses on classical electromagnetism, it is taught that a perfect conductor expels the electromagnetic (EM) field, and hence its surface is not able to support the propagation of bound EM waves. However, when the surface of a perfect conductor is structured at a length scale much smaller than the operating wavelength, geometrically induced surface EM modes can be supported. Owing to their similarities with the surface plasmon polaritons (SPPs) in the optical regime, these surface EM modes were named spoof surface plasmons. The concept of spoof surface plasmons has opened up a new line of research within plasmonics with the aim of transferring all the potentialities of SPPs in the optical regime to lower frequencies (microwave, terahertz, and midinfrared regimes) in which a metal behaves as a quasiperfect conductor. In recent years, several research groups have extended this concept from planar surfaces to waveguides, and eventually to resonators, covering the entire range of structures studied in standard plasmonics. This review provides a detailed perspective on the recent developments in spoof surface plasmon photonics from both the fundamental and applied sides.
Directly predicting the line positions of samples in the terahertz (THz) band is of significant importance for their THz identification. However, it is really a challenge to gain accurately the line positions by means of theoretical calculation, because the calculation typically involves various parameters, such as level energy and transition moment, which usually we hardly get directly. Based on the classical forced vibration model of dipoles, we propose a quantitative expression, i.e. the forced radiation intensity of molecular electric dipoles, which intend to predict the line positions of absorption peaks in the THz fingerprint spectra of a sample. We verified our expression by 9 recognized frequencies selected from the fingerprint spectra of water vapor in the THz band. Both the line positions and intensities of the absorption peaks of water vapor we calculated by the expression are well consistent with the experimental measurements. The line positions we calculated are also more accurate and comprehensive than that of water clusters simulated from Density Functional Theory (DFT). Our findings further support the theory of coherent superposition to advance a new method to exactly analyze the generation mechanism of molecular THz-fingerprint spectroscopy of a sample.
Throughout the 19th and 20th century, chirality has mostly been associated with chemistry. However, while chirality can be very useful for understanding molecules, molecules are not well suited for understanding chirality. Indeed, the size of atoms, the length of molecular bonds and the orientations of orbitals cannot be varied at will. It is therefore difficult to study the emergence and evolution of chirality in molecules, as a function of geometrical parameters. By contrast, chiral metal nanostructures offer an unprecedented flexibility of design. Modern nanofabrication allows chiral metal nanoparticles to tune the geometric and optical chirality parameters, which are key for properties such as negative refractive index and superchiral light. Chiral meta/nano-materials are promising for numerous technological applications, such as chiral molecular sensing, separation and synthesis, super-resolution imaging, nanorobotics, and ultra-thin broadband optical components for chiral light. This review covers some of the fundamentals and highlights recent trends. We begin by discussing linear chiroptical effects. We then survey the design of modern chiral materials. Next, the emergence and use of chirality parameters are summarized. In the following part, we cover the properties of nonlinear chiroptical materials. Finally, in the conclusion section, we point out current limitations and future directions of development.
