Broadband Half-wave Plate Research Articles

Broadband terahertz metasurface with three switchable functions based on complementary structure

In this work, we propose a switchable multifunctional metasurface based on complementary VO2-graphene structure. Tuning the state of VO2 and the Fermi level of graphene, the metasurface can act as broadband half-wave plate (HWP), quarter-wave plate (QWP) or absorber in the same frequency range of 0.65–1.27THz, respectively. In the V1G0 state, the metasurface behaves as a broadband HWP to realize linear-to-linear polarization conversion with the polarization conversion ratio >0.9 in the range of 0.64–1.37THz. In the V0G1 state, the metasurface acts as a broadband QWP achieving linear-to-circular polarization conversion with the ellipticity below −0.9 in the range of 0.65–1.27THz. In the V1G1 state, the metasurface is a broadband absorber with the absorption over 0.9 in the 0.63–1.32THz range. In addition, the physical mechanisms of the metasurface in different states are analyzed through the field and current distributions. Such a switchable multi-functional metasurface can simultaneously achieve high efficiency polarization conversion and absorption, which has potential applications in the integrated terahertz devices.

Numerical Simulations of Circular Dichroism and Polarization Conversion in VO2-Based Terahertz Metamaterials

Metamaterials with actively tunable functionalities are highly desirable for applications of advanced optoelectronic devices. In this paper, we theoretically present a metamaterial with diversified functionalities by availing of the phase transition characteristics of vanadium dioxide (VO2) in terahertz frequency regions. The research results demonstrate that the function of the designed metamaterial can be switched from giant circular dichroism (CD) to a reflecting broadband half-wave plate (HWP) and a quarter-wave plate (QWP). When VO2 is in the isolating state, the metamaterial exhibits a quite distinct transmission efficiency for circularly polarized lights, thus resulting in a maximum CD value ~0.97 at the resonant frequency. When VO2 is operating in the metallic state, the metamaterial performs like a broadband HWP, in which the nearly perfect linear polarization conversion can be achieved at the frequency range from 3 to 7 THz. Moreover, the structure can play a role of a high-efficiency QWP that can simultaneously convert the incident linear polarized light to left-handed and right-handed circularly polarized light. The calculated ellipticity indicates a good polarization conversion at the frequency of 2.4 THz and 7.4 THz, respectively. The physical mechanism of the discussed features and effects can be explained by exploring the electric field distributions. Furthermore, the structural parameters also exert great influences for achieving giant CD and HWP as well as QWP. The proposed metamaterial may offer a new approach for designing metamaterial devices with multi-functions in THz regions.

Function switchable broadband wave plate based on the Au-VO2 hybrid metasurface.

In recent years, the integration of active materials into a metasurface to achieve tunable devices has attracted much attention. Here, we design an Au-VO2 hybrid metasurface, which can switch between quarter-wave plate and half-wave plate due to the phase transition of VO2. At 298 K, the proposed structure acts as a quarter-wave plate in the 0.87-1.2 THz band, achieving the mutual conversion between linear polarization and circular polarization. Raising the temperature to 358 K, it works as a broadband half-wave plate in the range of 0.65-1.45 THz, with the reflective chirality preservation of circular polarization and the cross-polarization conversion of linear polarization. In the above cases, the response efficiencies are both above 90%. The switchable multifunction results from the tunable geometric phase of the metasurface, where the elaborately designed Au and VO2 blocks separately bring the phase of π/2. Furthermore, the electric field and current density distributions are employed to explain the physical mechanisms leading to the different functions. Such an active broadband metasurface is expected to find applications in tunable and multifunction devices manipulating the polarization and phase of terahertz waves.

Magnetically active terahertz wavefront control and superchiral field in a magneto-optical Pancharatnam-Berry metasurface.

Nowadays, the manipulation of the chiral light field is highly desired to characterize chiral substances more effectively, since the chiral responses of most molecules are generally weak. Terahertz (THz) waves are related to the vibration-rotational energy levels of chiral molecules, so it is significant to actively control and enhance the chirality of THz field. Here, we propose a metal/magneto-optical (MO) hybrid Pancharatnam-Berry (PB) phase structure, which can serve as tunable broadband half-wave plate and control the conversion of THz chiral states with the highest efficiency of over 80%. Based on this active PB element, MO PB metasurfaces are proposed to manipulate THz chiral states as different behaviors: beam deflector and scanning, Bessel beam, and vortex beam. Due to the magnetic-tunablibity, these proposed MO PB metasurfaces can be turned from an "OFF" to "ON" state by changing the external magnetic field. We further investigate the near-field optical chirality and the chirality enhancement factors in far field of the chiral Bessel beam and vortex beam, achieving the superchiral field with the highest chiral enhancement factor of 40 for 0th Bessel beam. These active, high efficiency and broadband chiral PB metasurfaces have promising applications for manipulation the THz chiral light and chiroptical spectroscopic techniques.

Demonstration of the broadband half-wave plate using the nine-layer sapphire for the cosmic microwave background polarization experiment

We report the development of the achromatic half-wave plate (AHWP) at millimeter wave for cosmic microwave background polarization experiments. We fabricate an AHWP consisting of nine A-cut sapphire plates based on the Pancharatnam recipe to cover a wide frequency range. The modulation efficiency and the phase are measured in a frequency range of 33 to 260 GHz with incident angles up to 10 deg. We find the measurements at room temperature are in good agreement with the predictions. This is the most broadband demonstration of an AHWP at millimeter wave.

A different optical composition for a broadband linear polarization rotator

We suggest and experimentally demonstrate a broadband polarization rotator. Our device consist of a Fresnel’s rhomb retarder and a composite broadband half-wave plate with relative rotation α between their fast polarization axis. Such a optical device rotate the polarization plane of a linearly-polarized light at angle 2α over a wide range of wavelengths.

Broadband phase shift engineering for terahertz waves based on dielectric metasurface

Broadband terahertz (THz) phase shift engineering and zero-dispersion waveplates based on dielectric metasurface have been investigated, of which structure is a periodical rectangular scattering units on silicon substrates. By designing proper geometric parameters of metasurface structure, the value, dispersion and bandwidth of the phase shift curves can be effectively manipulated. Based on this, the broadband half waveplate (HWP) and quarter waveplate (QWP) have been designed and fabricated. The experimental results show that the HWP can work in the broad range of 0.7–1.35 THz with the polarization conversion ratio (PCR) of close to 100% and the transmission of over 70%. And the QWP can operate in the range of 0.7∼0.85THz with the PCR of over 90% and the transmission of over 70%. The method of phase shift engineering based on dielectric metasurfaces and these broadband zero-dispersion waveplates have great potential in promoting the performance of THz application systems.

All-dielectric metasurfaces for circularly polarized beam-splitters with high conversion efficiency and broad bandwidth

Metasurface can arbitrarily control the polarization states of incident light and has attracted signicant attention. However, the transmission eciency of cross-polarization conversion of transmissive type metallic metasurfaces for polarized beam-splitters reported to date are low, especially the transmission eciency of cross-polarization conversion by single-layer metallic metasurfaces has a low theoretical upper limit of 25%, whose low value severely works against the application requirements. In this work, we propose two beam-splitters based on single-layer anomalous silicon metasurfaces, which can realize high conversion eciency and broad bandwidth wavefront control. A broadband half-wave plate and the high-eciency beam splitting of quarter-wave plate with circularly polarization beam are both achieved. Moreover, with big incident angles, the metasurface still maintains high-eciency performance.

Vanadium Dioxide Integrated Metasurfaces with Switchable Functionalities at Terahertz Frequencies

AbstractIntegration of switchable and diversified functionalities into a single metasurface has become an emerging research area that requires dealing with formidable challenges, especially for terahertz (THz) frequencies. Here, polarization‐insensitive and switchable THz metasurfaces are proposed with diversified functionalities that exploit insulator‐to‐metal transition in vanadium dioxide (VO2). The simulations demonstrate that the designed metasurface can be switched from a broadband absorber to a reflecting broadband halfwave plate (HWP). At room temperature, the metasurface efficiently absorbs normally incident waves ranging from 0.562 to 1.232 THz with the total absorption exceeding 90%. Once the temperature is high enough and VO2 is in its fully metallic state, the metasurface becomes a broadband HWP reflecting over 60% of the incident power with the linear polarization conversion efficiency exceeding 95% within the bandwidth of 0.49 THz. Moreover, the broadband performance is sustained over a wide range of incident angles. To extend the functionalities, metasurface supercell made of several VO2 antennas is integrated, consequently achieving directional polarization conversion when VO2 is in its fully metallic state while maintaining broadband absorption at insulating state. The proposed switchable metasurfaces are expected to enable advanced research and smart applications related to other tunable and diverse functionalities at THz frequencies.

A broadband reflective-type half-wave plate employing optical feedbacks

We propose and demonstrate a type of a broadband half-wave plate that operates in the reflective mode. It consists of a metal grating embedded in a dielectric slab and placed on top of a grounded metal surface. We theoretically show that owing to the optical feedback effect which originates from the wave reflections at the air-dielectric interface, the proposed half-wave plate exhibits a broadened and flattened response when comparing to the case where the feedback effect is absent. Such a prediction is validated using both numerical and experimental works carried out on a half-wave plate designed at 10 GHz. Moreover, our theoretical analysis also reveals that the half-wave plate has an interesting feature of broad angular response. Taking advantage of these features, we experimentally demonstrate that the proposed device can function as a freely tunable linear polarization converter with polarization conversion residues less than −20 dB in a wide frequency band, under the condition that the incident angle is as large as 45 degrees.

高功率单频LBO 腔内倍频461 nm 钛宝石激光器

A high-power single-frequency 461 nm light by intracavity doubling of Ti:sapphire laser is designed and built. An astigmatically compensated double-folded resonator with six mirrors is employed to give two tight-focus regions, where Ti:sapphire and doubling crystals are located respectively. A type-I critical phase-matched LBO, an optical diode composed with a TGG crystal and a broadband half wave plate (HWP) are placed inside the resonator for the intracavity frequency doubler and forcing the laser to operate unidirectionally. Three birefringent filters with different thicknesses of 1, 2, 4 mm and a temperature controlled etalon with thickness of 0.25 mm are inserted in the cavity to tune the fundamental wavelength of 461 nm Ti∶sapphire laser. With the pump power of 14 W and an optimized LBO crystal with the length of 7.3 mm, the tunable single-frequency second harmonic output of 1.02 W at 460.86 nm is obtained. The peak-to-peak fluctuation of the power is less than ±1.3% within 3 h. The measured beam quality of M 2 and the tuning range of blue laser are better than 1.59 and 20 nm, respectively.

Intra-cavity spectral shaping based on optical rotatory dispersion in a broadband Ti : S regenerative amplifier

We theoretically present a passive method for intra-cavity spectral shaping in a broadband Ti : S regenerative amplifier on optical rotatory dispersion (ORD). Besides a polarization-dependent Ti : S gain crystal, implementing the method requires a broadband half-wave plate and a wedge pair made of an optical rotator crystal for an adjustable ORD. Our calculations show inducing proper ORD allows the amplifier to boost an 800 nm pulse from sub-nJ up to 3.18 mJ with a bandwidth of 94.5 nm without peak wavelength shift or spectral narrowing. For a certain ORD, this method allows us to get tunable broadband amplification with almost constant output pulse energy by simply rotating the half-wave plate. In spite of its flexibility in controlling the output spectrum of the amplifier, this method is very simple to align.

Broadband high-efficiency half-wave plate: a supercell-based plasmonic metasurface approach.

We design, fabricate, and experimentally demonstrate an ultrathin, broadband half-wave plate in the near-infrared range using a plasmonic metasurface. The simulated results show that the linear polarization conversion efficiency is over 97% with over 90% reflectance across an 800 nm bandwidth. Moreover, simulated and experimental results indicate that such broadband and high-efficiency performance is also sustained over a wide range of incident angles. To further obtain a background-free half-wave plate, we arrange such a plate as a periodic array of integrated supercells made of several plasmonic antennas with high linear polarization conversion efficiency, consequently achieving a reflection-phase gradient for the cross-polarized beam. In this design, the anomalous (cross-polarized) and the normal (copolarized) reflected beams become spatially separated, hence enabling highly efficient and robust, background-free polarization conversion along with broadband operation. Our results provide strategies for creating compact, integrated, and high-performance plasmonic circuits and devices.

Realizing full visible spectrum metamaterial half-wave plates with patterned metal nanoarray/insulator/metal film structure

Abrupt phase shift introduced by plasmonic resonances has been frequently used to design subwavelength wave plates for optical integration. Here, with the sandwich structure consisting of a top periodic patterned silver nanopatch, an in-between insulator layer and a bottom thick Au film, we realize a broadband half-wave plate which is capable to cover entire visible light spectrum ranging from 400 to 780 nm. Moreover, when the top layer is replaced with a periodic array of composite super unit cell comprised of two nanopatches with different sizes, the operation bandwidth can be further improved to exceed an octave (400-830 nm). In particular, we demonstrate that the designed half-wave plate can be used efficiently to rotate the polarization state of an ultra-fast light pulse with reserved pulse width. Our result offers a new strategy to design and construct broadband high efficiency phase-response based optical components using patterned metal nanoarray/insulator/metal structure.

Geometrical optics of a twist-oriented nematic liquid crystal

We have systematically analyzed the spatial dynamics of the polarization state of light in smoothly inhomogeneous anisotropic media. As a medium of this kind, we have considered a nematic liquid crystal with twist orientation. A system of coupled equations has been solved with respect to the Cartesian components of the electric-field component using a rotating coordinate system. We have shown that, upon propagation of light in this medium, the state of the polarization experiences oscillations the spatial frequency of which depends on the thickness of the specimen. Our analysis also has shown that, at an appropriate choice of the thickness, this twist cell behaves as a broadband half-wave plate.

High-pulse-energy, linear optical parametric oscillator with narrow and symmetrical far field

A new method to obtain a narrow and symmetrical far field from a high-pulse-energy optical parametric oscillator (OPO) with a linear resonator has been tested. The OPO employs two identical nonlinear crystals that are cut for type II phase matching, rotated such that their walk-off planes are orthogonal, and separated by a broadband half-wave plate. The OPO has a simple geometry, can be double-pass pumped, is wavelength tunable and operates stably with high conversion efficiency. The method has been demonstrated in a KTP-based OPO pumped at 1064 nm and a BBO-based OPO pumped at 532 nm, with output pulse energies up to 60 mJ and 75 mJ, respectively.

Broadband plasmonic half-wave plates in reflection

We demonstrate, both numerically and experimentally, that metal-insulator-metal configurations in which the top metal layer consists of a periodic arrangement of nanobricks, thus supporting gap-surface plasmon resonances, can be designed to function as reflective broadband half-wave plates. Using gold as the metal, the constructed wave plates in the near-infrared regime show scalability, bandwidth of ~20% of the design wavelength, and theoretical reflectivity above 85%, while a reflectivity of ~50% is experimentally measured.

Broadband Birefringent Half-Wave Plate

We proposed a means of generating a broadband birefringent half-wave plate by sandwiching two A-plates, which can be partitioned into several regions. Each region differs in the wavelength of its incident light. The two A-plates have their slow axes in each region set at angles corresponding to the transmitted axis of the entry polarizer. By applying spherical trigonometry, and the 2×2 Jones matrix and Müller matrix methods, the input polarization state can be transformed to the same antipodal point. There exist some relations among the slow-axis angle and phase retardation value of the A-plates that should be satisfied. The software whose trade name is LCD Master is used to simulate the trajectory of the polarization state on a Poincare sphere. An undulation of Stokes parameters is observed, which may be suppressed by matching the refractive indices of the two media. An experiment on the stacking of commercial A-plates is conducted to further verify the feasibility of the proposed configuration. Finally, a method of fabricating a broadband birefringent half-wave plate is proposed. The transmittance spectra of a light-emitting diode (LED), a red, green, blue (RGB) color filter, and a broadband half-wave plate under a parallel polarizer are multiplied respectively in each region and resynthesized together. The resulting transmittance spectrum gives better suppression of light leakage over the visible range than that in the case of a single half-wave plate.