Circular polarization multiplexing terahertz metasurface based on dual-phase hybrid design and phase profile design method

Abstract

The present invention discloses a circularly polarized multiplexed terahertz metasurface and phase profile design method based on a dual-phase hybrid design. These metasurfaces, belonging to the field of metasurfaces, include periodically arranged metaatomic units, each comprising a silicon substrate and an asymmetric silicon resonator disposed on the silicon substrate. The asymmetric silicon resonator is a cross-displacement structure that introduces dislocation and internal rotation. By employing the aforementioned circularly polarized multiplexed terahertz metasurface and phase profile design method based on a dual-phase hybrid design, a controllable linear polarization conversion effect in the transmitted wave is achieved by breaking the mirror symmetry of the silicon resonator while retaining C2 symmetry. This achieves polarization-dependent dual-function optical manipulation, combining high performance, dynamic adjustability, and integration, providing innovative design ideas for the next generation of compact optical systems.

Description

Technical Field

[0001] The present invention relates to the field of metasurface technology, and in particular to a circular polarization multiplexing terahertz metasurface based on dual-phase hybrid design and a phase profile design method. Background Art

[0002] The wavefront is a key property of electromagnetic beams, determining many of the laws governing their transmission and interaction with matter. The manipulation and modification of optical wavefronts is a crucial topic in modern optics and photonics. The determining factor of an optical wavefront is the relative phase of its components. The primary challenge is how to introduce phase delay while ensuring similar amplitudes between components.

[0003] Initially, refractive elements such as spherical lenses exploited the accumulated optical path differences of light components traveling through a uniform bulk material to introduce phase differences, thereby achieving functions such as focusing and deflection. The advent of diffractive optical elements such as gratings and Fresnel zone plates has greatly expanded the control dimensions of optical wavefronts, to some extent alleviating the reliance of refractive optical devices on specific materials and the operating wavelength limitations. For example, gratings enable better wavelength-dependent wavefront control, while zone plates extend the operating frequency range to include X-rays.

[0004] The digital and programmable evolution of optical equipment is another important direction. Digital micromirrors (DMDs) and spatial light modulators (SLMs) have become relatively common wavefront control systems. By using pixelated tunable optical units, optical wavefront conversion can be performed in real time and in a reconfigurable manner to achieve important applications such as adaptive optics and computational holography.

[0005] Over the past decade, the vigorous development of nanophotonics and metasurface research has brought new opportunities for optical wavefront manipulation principles, technologies, and devices. Metasurfaces focus on optical effects at the subwavelength scale, using artificially designed meta-atoms to control local and non-local wave components. Their powerful light wave manipulation capabilities provide more possibilities for optical wavefront design and transformation devices. In particular, metasurfaces for wavefront transformation are particularly important in the terahertz band. Metasurfaces have unprecedented advantages in manipulating the local phase of light beams, such as compressed mode volume, controllable wavelength sensitivity, and subwavelength-scale linear or circular birefringence. This in turn gives wavefront manipulation devices planar structures, artificially constructed dispersion properties, and the integration of multiple polarization channels.

[0006] Multiplexing devices for circularly polarized light waves are an important part of micro-nano optics. Natural materials such as chiral molecules and magnetic materials can provide weak circular birefringence or circular dichroism, but they are difficult to use for efficient spin-dependent wavefront manipulation. The use of metasurfaces for circularly polarized wavefront control was originally based on the Pancharatnam-Berry (PB) phase, which uses the orthogonal evolution of circularly polarized waves in polarization space to obtain additional phases, but it exhibits conjugated characteristics for the two components. In 2015, Professor Arbabi combined the linear birefringence effect of metaatoms with the PB phase to complete the independent wavefront design of orthogonal circularly polarized waves; subsequently, scholars reported the manipulation of orthogonal polarization components based on local interference between metaatoms; in 2020, the scheme was extended to the amplitude control of arbitrary orthogonal polarization components; later it was extended to the complex amplitude manipulation of orthogonal components; it can be seen that the above phase manipulation schemes are all based on non-chiral metaatoms. In 2020, researchers proposed a multilayer twisted structure in the microwave band and demonstrated a chirality-assisted phase. By combining this with a resonant phase, they achieved a four-channel functional demonstration. In 2021, they reported a planar chiral dielectric metasurface and realized wavefront manipulation of orthogonal circularly polarized light. In the same year, researchers also proposed non-Hermitian properties in metal-insulator-metal (MIM) metasurfaces and achieved spin-dependent reflected wave manipulation based on topological protection near singular points. However, similar metasurface solutions have not yet been reported in the terahertz band, and chiral metadevices with simple structures and convenient designs for spin-multiplexed wavefront manipulation remain to be explored. Summary of the Invention

[0007] The purpose of the present invention is to provide a circular polarization multiplexing terahertz metasurface based on dual-phase hybrid design and a phase profile design method to solve the above technical problems.

[0008] To achieve the above objectives, the present invention provides a circularly polarized multiplexed terahertz metasurface based on a dual-phase hybrid design, comprising periodically arranged superatomic units, each of which comprises a silicon substrate and an asymmetric silicon resonator disposed on the silicon substrate. The asymmetric silicon resonator is a cross-dislocation structure that introduces dislocation and internal rotation.

[0009] Preferably, the height of the silicon substrate H1 = 600 μm;

[0010] The height H2 of the asymmetric silicon resonator is 400 μm, the major axis width W1 is 60 μm, the characteristic length L is 180 μm, and the minor axis width W2, the notch depth D, and the internal rotation angle θ are all variables.

[0011] Preferably, the arrangement period P of the super-atom units is 300 μm.

[0012] The phase profile design method of a circular polarization multiplexing terahertz metasurface based on a dual-phase hybrid design includes the following steps:

[0013] S1. Determine the structural parameters of the meta-atom based on dual-phase hybridization design. Through the structural parameters of the superpositive meta-atom, break the mirror symmetry of the silicon resonator while retaining the C2 symmetry, thereby achieving a controllable linear polarization conversion effect;

[0014] S2. Calculate the chiral phase response and establish the relationship between the total phase, the chiral phase, and the PB phase;

[0015] S3. Combining the calculation results of step S1 and step S2, the spin-decoupled beam deflection and vortex beam generation are achieved by adjusting the design parameters of the metasurface.

[0016] Preferably, in step S1, the Jones matrix R of the superatom is expressed as follows:

[0017]

[0018] Where R represents the rotation matrix under the circular polarization basis; and denote the additional phase of the co-polarization component respectively; The phase differences when converting right circularly polarized light to right circularly polarized light, right circularly polarized light to left circularly polarized light, left circularly polarized light to right circularly polarized light, and left circularly polarized light to left circularly polarized light respectively; and represent the total phases obtained for the left-hand circular polarization component and the right-hand circular polarization component, respectively.

[0019] Preferably, in step S2, the chiral phase response calculation formula is as follows:

[0020]

[0021] Where, and represent the total phase shifts of left-handed circularly polarized light and right-handed circularly polarized light in the transmitted wave, respectively; and represent the additional chiral phases of left-handed circularly polarized light and right-handed circularly polarized light, respectively.

[0022] Preferably, in step S3, the transmitted left-handed circularly polarized light and right-handed circularly polarized light are respectively provided with a linear phase gradient and a space-independent constant phase distribution to achieve deflection of the right-handed circularly polarized light, and the phase The contour expression is as follows:

[0023]

[0024] Where x is the position coordinate and k is a constant;

[0025] Alternatively, the two circularly polarized wave components are made to generate focused vortex beams with topological charges of 1 and -1 respectively, and the focal length f = 8 mm. The terahertz vector beam is generated according to the coherent superposition principle. At this time, the phase profile expression of the circularly polarized component in polar coordinates is as follows:

[0026]

[0027] Where, and represent the phase distribution of right-handed circularly polarized light and left-handed circularly polarized light, respectively; λ represents the wavelength; and r represents the radial distance in the polar coordinate system.

[0028] Therefore, the present invention adopts the above-mentioned circular polarization multiplexing terahertz metasurface and phase profile design method based on dual-phase hybrid design, which has the following beneficial effects:

[0029] 1. By designing the linear phase gradient of right-handed circularly polarized light (RCP), beam deflection is achieved (similar to a prism effect). At the same time, a quadratic phase distribution is applied to left-handed circularly polarized light (LCP) to achieve beam focusing (similar to a lens effect). A single optical element can achieve both deflection and focusing functions without the need for complex optical path design or mechanical adjustments, simplifying the system structure and reducing costs.

[0030] 2. By adjusting the polarization state of the incident light or the parameters of the metasurface, the deflection angle and focus position can be dynamically switched, which is suitable for reconfigurable optical systems (such as lidar and adaptive imaging);

[0031] 3. Independent phase distributions are designed for RCP and LCP, resulting in control efficiencies exceeding 90% for both RCP and LCP, significantly outperforming traditional diffractive optical elements and suitable for polarization-multiplexed communications and dual-channel imaging.

[0032] 4. The use of ultra-thin metasurfaces (micrometer-level thickness) to replace traditional lens / prism combinations, combined with polar coordinate phase design, has the advantages of small device size and light weight. It can be integrated into chip-level optical systems (such as terahertz on-chip systems) and is compatible with CMOS processes, making it suitable for large-scale production.

[0033] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a schematic diagram of the structure of a metasurface unit of a circular polarization multiplexing terahertz metasurface based on a dual-phase hybrid design according to the present invention;

[0035] Figure 2The chiral phase verification results of the meta-atom in the simulation experiment; (a) is the simulation result of the circularly polarized transmission amplitude of the meta-atom (D = 28 μm, W2 = 51 μm); (b) is the simulation result of the transmission phase of the meta-atom (D = 28 μm, W2 = 51 μm); (c) is the local electric field distribution of the chiral meta-atom under circularly polarized wave excitation; (d) and (e) are the experimental results of the circularly polarized transmission amplitude and transmission phase of the chiral metasurface sample; (f) is a local scanning electron microscope (SEM) photo of the chiral metasurface sample;

[0036] Figure 3 : The source analysis diagrams of the chiral phase and PB phase in the simulation experiment; (a) is the orthogonal linear polarization conversion caused by the symmetry breaking of the superatom when D = 0 μm, that is, the source analysis diagram of the chiral phase; (b) is the source analysis diagram of the chiral phase when D = 10 μm; (c) is the source analysis diagram of the chiral phase when D = 20 μm; (d) is the orthogonal linear polarization conversion caused by the in-plane rotation of the chiral superatom when θ = 0 deg, that is, the source analysis diagram of the PB phase; (e) is the orthogonal linear polarization conversion caused by the in-plane rotation of the chiral superatom when θ = 15 deg, that is, the source analysis diagram of the PB phase; (f) is the orthogonal linear polarization conversion caused by the in-plane rotation of the chiral superatom when θ = 30 deg, that is, the source analysis diagram of the PB phase;

[0037] Figure 4 The evolution diagram of the chiral phase and PB phase in the parameter space and polarization space of the simulation experiment; (a)-(d) are the amplitude and phase distribution diagrams of the LCP and RCP components in the D-W2 parameter space; (e) and (f) are the amplitude distribution of the LCP component and the additional PB phase distribution caused by the in-plane rotation angle;

[0038] Figure 5 A diagram showing the spin-dependent wavefront manipulation function of the chiral metasurface in a simulation experiment; (a) and (b) are the electric field distributions of the transmitted wave under the spin-decoupled beam deflection design; (c)-(f) are the amplitude and phase distributions of the LCP and RCP waves in the focal plane under the spin-decoupled focused vortex beam generation design;

[0039] Figure 6 Figure 3: Experimental verification diagram of the wavefront manipulation function of the chiral metasurface in the simulation experiment; (a) and (b) are the simulation results of the x and y polarization components of the electric field, respectively; (c) is a local scanning electron microscope (SEM) photograph of the metasurface sample; (d) and (e) are the measured results of the electric field components in the x and y directions of the transmitted vector beam, respectively; (f) is a schematic diagram of the two-dimensional electro-optical sampling terahertz time-domain spectroscopy test system.

[0040] Reference numerals

[0041] 1. Silicon substrate; 2. Asymmetric silicon resonator. DETAILED DESCRIPTION

[0042] In order to make the purposes, technical solutions and advantages disclosed in the embodiments of the present invention clearer, the embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the embodiments of the present invention and are not intended to limit the embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. Examples of the embodiments are shown in the accompanying drawings, where the same or similar numbers throughout represent the same or similar elements or elements with the same or similar functions.

[0043] It should be noted that the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or server that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or devices.

[0044] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0045] like Figure 1 As shown, a circularly polarized multiplexed terahertz metasurface based on a dual-phase hybrid design comprises periodically arranged superatomic units. The superatomic units comprise a silicon substrate and an asymmetric silicon resonator 2 disposed on the silicon substrate 1. The asymmetric silicon resonator 2 has a cross-displacement structure that introduces dislocation and internal rotation. The silicon substrate 1 has a height H1 of 600 μm; the asymmetric silicon resonator 2 has a height H2 of 400 μm, a major axis width W1 of 60 μm, a characteristic length L of 180 μm, and variable minor axis width W2, notch depth D, and internal rotation angle θ. The superatomic units have an arrangement period P of 300 μm. Both the silicon substrate 1 and the asymmetric silicon resonator 2 are made of high-resistivity silicon, with a dielectric constant ε of 11.9.

[0046] The phase profile design method of a circular polarization multiplexing terahertz metasurface based on a dual-phase hybrid design includes the following steps:

[0047] S1. Determine the structural parameters of the meta-atom based on dual-phase hybridization design. Through the structural parameters of the superpositive meta-atom, break the mirror symmetry of the silicon resonator while retaining the C2 symmetry, thereby achieving a controllable linear polarization conversion effect;

[0048] In step S1, the Jones matrix R of the superatom is expressed as follows:

[0049]

[0050] Where R represents the rotation matrix under the circular polarization basis; and denote the additional phase of the co-polarization component respectively; The phase differences when converting right circularly polarized light to right circularly polarized light, right circularly polarized light to left circularly polarized light, left circularly polarized light to right circularly polarized light, and left circularly polarized light to left circularly polarized light respectively; and represent the total phases obtained for the left-hand circular polarization component and the right-hand circular polarization component, respectively.

[0051] S2. Calculate the chiral phase response and establish the relationship between the total phase, the chiral phase, and the PB phase;

[0052] In step S2, the chiral phase response is calculated as follows:

[0053]

[0054] Where, and represent the total phase shifts of left-handed circularly polarized light and right-handed circularly polarized light in the transmitted wave, respectively; and represent the additional chiral phases of left-handed circularly polarized light and right-handed circularly polarized light, respectively.

[0055] S3. Combining the calculation results of step S1 and step S2, the spin-decoupled beam deflection and vortex beam generation are achieved by adjusting the design parameters of the metasurface.

[0056] In step S3, the transmitted left-handed circularly polarized light and right-handed circularly polarized light are respectively provided with a linear phase gradient and a space-independent constant phase distribution, so as to realize the deflection of the right-handed circularly polarized light, and its phase The contour expression is as follows:

[0057]

[0058] Where x is the position coordinate and k is a constant;

[0059] Alternatively, the two circularly polarized wave components are made to generate focused vortex beams with topological charges of 1 and -1 respectively, and the focal length f = 8 mm. The terahertz vector beam is generated according to the coherent superposition principle. At this time, the phase profile expression of the circularly polarized component in polar coordinates is as follows:

[0060]

[0061] Where, and Represent the phase distribution of right-handed circularly polarized light and left-handed circularly polarized light respectively; λ represents the wavelength; r represents the radial distance in the polar coordinate system

[0062] Simulation experiment

[0063] First, verify the chiral phase of the superatom: Figure 2 (a) and (b) show the transmission amplitude t of right circularly polarized light to left circularly polarized light at the operating frequency f = 0.6 THz. RL The transmission amplitude t of left circularly polarized light to right circularly polarized light LR The magnitudes are similar, but the phase difference is about 135°. Figure 2 As shown in (c), under the LCP (left-handed polarized light) and RCP (right-hand circularly polarized light) mechanisms, the local electric field distribution and transmission state generated in the metaatom are different, which is also the source of the phase difference.

[0064] In the experiment, the metasurface of the present invention was prepared by ultraviolet lithography and ICP (Inductively Coupled Plasma) etching process, and the circular polarization coefficient of transmission was tested by polarization-resolved terahertz time-domain spectroscopy system. The results are as follows: Figure 2 As shown in (d) and (e), there is a slight offset between the experimentally measured and simulated resonance frequencies, but the transmission amplitudes are relatively close, and the measured phase difference is about 100°. Figure 2 As shown in (f), the difference in phase difference may be caused by imperfect sample preparation (the etching depth of 400 μm may introduce shape errors).

[0065] In order to further explain the origin of the chiral phase and PB phase, the linear polarization conversion phenomenon caused by the mirror symmetry breaking and azimuthal rotation of the metaatom was analyzed, and the results were as follows: Figure 3 As shown. Figure 3 (a)-3(c) shows that as D increases, the x-polarized transmitted wave component t generated by the incident y-polarized component in the linear polarization component xy The transmitted wave component t of the y-polarization generated by the incident x-polarization in the linear polarization component yx The proportion of Figure 3 As shown in Figures (d)-3(f), the linear polarization conversion efficiency differs significantly when the metaatom is rotated in-plane (D = 20 μm, W2 = 90 μm). The linear polarization conversion effect induced by these two operations introduces different additional phases into the circularly polarized wave component. Any combination of circular polarization phases can be achieved by adjusting the rotation angle and in-plane geometry.

[0066] Then, by changing the in-plane geometric size and azimuth angle of the chiral superatom, the evolution law of the chiral phase and PB phase in the parameter space and polarization space is explored, and the results are as follows: Figure 4 First, the circular polarization transmission coefficient of the meta-atom with different values ​​of D and W2 is calculated, and the amplitude and phase (t LR and t RL ),Depend on Figure 4 As shown in (a)-4(d), the transmission amplitudes of the two polarization components are greater than 0.7 in most areas, but the phase changes are completely different. The geometric size has a strong influence on the phase of the RCP wave. Near the lowest point of the transmission amplitude, a phase gradient similar to the angular direction appears, while the phase change of the LCP wave is very weak. Therefore, this difference can be compensated by introducing the PB phase. Figure 4 As shown in (e) and (f), when the in-plane rotation angle θ increases from 0 to 180°, the LCP wave (t LR ) remains almost unchanged, while its phase value changes linearly, which satisfies the PB phase law very well.

[0067] By utilizing the arbitrary phase combination of spin decoupling, two all-silicon metasurfaces are designed in this embodiment to demonstrate the wavefront manipulation function of circular polarization multiplexing of the transmitted terahertz beam. The first design makes the transmitted RCP wave and LCP wave have a linear phase gradient and a spatially independent constant phase distribution, respectively, and the results are as follows: Figure 5 As shown in (a) and (b), the transmitted LCP-RCP component is deflected to the left, while the propagation direction of the RCP-LCP component remains unchanged. The second design is to allow the two circularly polarized wave components to generate focused vortex beams with topological charges of 1 and -1 respectively (design focal length f = 8mm). According to the coherent superposition principle, a terahertz vector beam can be generated. The results are shown in Figure 5 As shown in (c)-(f), the vortex beam characteristics of the present invention are demonstrated.

[0068] Finally, the vector beam test is used as an experimental verification. Figure 1 The same metasurface sample in the experiment. According to the principle of coherent superposition generation of vector beams, the superposition of orthogonal circularly polarized vortex beams with topological charges of 1 and -1 and relative phase of 0 will generate a radial vector beam, and a dark stripe will appear in the intensity pattern of its linear polarization component. Therefore, a two-dimensional electro-optical sampling terahertz time-domain spectroscopy test system is used, and after Figure 6(f) As can be seen, the entire system is supported by an amplified femtosecond laser source (Spectra-Physics, USA, center wavelengths of 800nm, spot diameter of 8mm, repetition rate of 1kHz). The pump light is expanded by a concave lens (L1) and then irradiated onto the ZnTe crystal at the emitting end, exciting a horizontally polarized terahertz wave (Ex) based on the optical rectification effect. The terahertz wave is collimated by an off-axis parabolic mirror, and the beam is transmitted through the sample and irradiated onto the ZnTe crystal at the receiving end, forming a field-induced birefringence effect. At the same time, the probe pulse laser is also reflected by the beam expansion module and the semi-transparent and semi-reflective mirror (BS) onto the receiving crystal. After transmission in the crystal, a slight polarization change occurs during reflection, that is, a phase difference is generated between the orthogonal linear polarization components. This phase difference is proportional to the amplitude of the terahertz electric field transmitted in the crystal, which is a single electro-optical sampling effect. A half-wave plate (HWP) and an optical polarizer (P) are placed between the delay module and the detection laser beam expander. The polarizer after the convex lens (L3) is used to adjust and purify the polarization state of the detection laser beam to measure different terahertz electric field components. The laser beam after electro-optical modulation is finally imaged by the optical imaging module. By adjusting the polarization state of the detection light, different terahertz electric field components can be sensed, and finally the circularly polarized terahertz electric field is calculated. ) The terahertz signal in the actual focal plane (f = 8.3 mm) was measured, and the intensity distribution of the electric field at the operating frequency f = 0.6 THz was calculated by Fourier transform, as shown in the figure. Figure 6 As shown, it can be seen that the experimental results are in good agreement with the simulation results, thus proving the effectiveness of the present invention.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A phase profile design method for circularly polarized multiplexed terahertz metasurface based on dual-phase hybrid design, characterized by: The following steps are involved: S1. Determine the structural parameters of the meta-atom based on dual-phase hybridization design. Through the structural parameters of the superpositive meta-atom, break the mirror symmetry of the silicon resonator while retaining the C2 symmetry, thereby achieving a controllable linear polarization conversion effect; In step S1, the Jones matrix of the superatom The expression is as follows: （1）； Where, represents the rotation matrix in the circular polarization basis; and denote the additional phase of the co-polarization component respectively; 、 、 、 The phase differences when converting right circularly polarized light to right circularly polarized light, right circularly polarized light to left circularly polarized light, left circularly polarized light to right circularly polarized light, and left circularly polarized light to left circularly polarized light respectively; and denote the total phases obtained for the left-hand circular polarization component and the right-hand circular polarization component, respectively; S2. Calculate the chiral phase response and establish the relationship between the total phase, the chiral phase, and the PB phase; S3, combining the calculation results of step S1 and step S2, realizing spin-decoupled beam deflection and vortex beam generation by adjusting the design parameters of the metasurface; The circularly polarized multiplexed terahertz metasurface based on dual-phase hybrid design includes periodically arranged superatomic units, which include a silicon substrate and an asymmetric silicon resonator arranged on the silicon substrate. The asymmetric silicon resonator is a cross-dislocation structure that introduces dislocation and internal rotation.

2. The phase profile design method according to claim 1, wherein: In step S2, the chiral phase response is calculated as follows: （2）； Where, and represent the total phase shifts of left-handed circularly polarized light and right-handed circularly polarized light in the transmitted wave, respectively; and represent the additional chiral phases of left-handed circularly polarized light and right-handed circularly polarized light, respectively.

3. The phase profile design method according to claim 2, wherein: In step S3, the transmitted left-handed circularly polarized light and right-handed circularly polarized light are respectively provided with a linear phase gradient and a space-independent constant phase distribution, so as to realize the deflection of the right-handed circularly polarized light, and its phase The contour expression is as follows: （3）； Where, is the position coordinate, is a constant; Alternatively, the two circularly polarized wave components generate focused vortex beams with topological charges of 1 and -1, respectively, and the focal length , a terahertz vector beam is generated according to the coherent superposition principle. At this time, the phase profile expression of the circular polarization component in polar coordinates is as follows: （4）； Where, and represent the phase distribution of right-handed circularly polarized light and left-handed circularly polarized light, respectively; Indicates wavelength; Represents radial distance in polar coordinates.

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