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Switchable Ultrathin Quarter-wave Plate in Terahertz Using Active Phase-change Metasurface.

Wang D, Zhang L, Gu Y, Mehmood MQ, Gong Y, Srivastava A, Jian L, Venkatesan T, Qiu CW, Hong M - Sci Rep (2015)

Bottom Line: In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface.After the transition to metal phase, the quarter-wave plate operates at 0.502 THz.At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, 117576, Singapore.

ABSTRACT
Metamaterials open up various exotic means to control electromagnetic waves and among them polarization manipulations with metamaterials have attracted intense attention. As of today, static responses of resonators in metamaterials lead to a narrow-band and single-function operation. Extension of the working frequency relies on multilayer metamaterials or different unit cells, which hinder the development of ultra-compact optical systems. In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface. Before the phase transition, VO2 behaves as a semiconductor and the metasurface operates as a quarter-wave plate at 0.468 THz. After the transition to metal phase, the quarter-wave plate operates at 0.502 THz. At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light. This work reveals the feasibility to realize tunable/active and extremely low-profile polarization manipulation devices in the terahertz regime through the incorporation of such phase-change metasurfaces, enabling novel applications of ultrathin terahertz meta-devices.

No MeSH data available.


Switchable THz QWP design and fabrication results.(a) Experimental switching schematic of the THz QWP. A linear normal incident THz wave polarized at θ = 45° to the two slots is converted into a circularly polarized light. Through the VO2 phase transition controlled by a resistive heater, the operating frequency of the QWP can be switched between f1 = 0.468 THz and f2 = 0.502 THz. The top left inset is a microscope image of one unit cell in the fabricated samples. The scale bar is 50 μm. The following are the geometrical parameters: P = 150, Lx = 90, Ly = 124, lx = 9, ly = 5 and w = 9 μm, respectively. The top right inset is the simulated ellipticities of the output THz waves, indicating that at both f1 and f2 the output THz waves are circularly polarized. (b) Schematic backside view of the resistive heater with a square aperture (6 × 6 mm2) milled at the center to allow THz to pass through. (c) Measured electrical conductivity of fabricated VO2 films at different temperatures during the heating and the cooling cycles. The fabricated films exhibit stable electrical conductivity switching between 300 and 400 K during either the heating or the cooling cycles.
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f1: Switchable THz QWP design and fabrication results.(a) Experimental switching schematic of the THz QWP. A linear normal incident THz wave polarized at θ = 45° to the two slots is converted into a circularly polarized light. Through the VO2 phase transition controlled by a resistive heater, the operating frequency of the QWP can be switched between f1 = 0.468 THz and f2 = 0.502 THz. The top left inset is a microscope image of one unit cell in the fabricated samples. The scale bar is 50 μm. The following are the geometrical parameters: P = 150, Lx = 90, Ly = 124, lx = 9, ly = 5 and w = 9 μm, respectively. The top right inset is the simulated ellipticities of the output THz waves, indicating that at both f1 and f2 the output THz waves are circularly polarized. (b) Schematic backside view of the resistive heater with a square aperture (6 × 6 mm2) milled at the center to allow THz to pass through. (c) Measured electrical conductivity of fabricated VO2 films at different temperatures during the heating and the cooling cycles. The fabricated films exhibit stable electrical conductivity switching between 300 and 400 K during either the heating or the cooling cycles.

Mentions: Figure 1a shows a schematic of the switchable QWP, which is composed of ultrathin asymmetric cross-shaped resonator arrays with VO2 pads inserted at the end of the cross-shaped resonators. The complementary metasurfaces present high transmission coefficients at the resonance frequencies due to the extraordinary optical transmission effect with specific phase delays3940. It only allows the resonant EM wave to pass through, which eliminates the interference of the non-resonant EM wave. In our design, two slots in the QWP are perpendicular to each other with a slight difference in length. The fundamental resonance in each slot is able to present a maximum phase shift of 180° between the transmitted and the incident light. Therefore, birefringence can be introduced by controlling the length of the slots in the asymmetric cross-shaped resonators. The switching property of the QWP is controlled by a resistive heater as a proof of concept to manipulate the VO2 phase transition at different temperatures, which can also be realized by optical pumping41. When VO2 pads act as a semiconductor at 300 K, phase difference between two orthogonal slots can reach 90° at f1 = 0.468 THz, while the transmission coefficients in these two slots are the same. At this frequency, when the incident THz wave is polarized at θ = 45° to the two slots, the device operates as a QWP. Through the phase transition, free carriers in the VO2 pads increase, resulting in the rise of the electrical conductivity. The VO2 pads behave like a metal at 400 K. This leads to a shortened effective length of cross-shaped resonators and the QWP operates at f2 = 0.502 THz. Therefore, this THz QWP can switch its operating frequency between two states through the VO2 phase transition. The top right inset is the simulated ellipticities of the output THz wave at 300 and 400 K. It is observed that the ellipticites at f1 = 0.468 and f2 = 0.502 THz are close to 1, indicating a circular polarization of the output THz waves. The top left inset in Fig. 1a shows a microscope image of one unit cell in the fabricated metasurfaces.


Switchable Ultrathin Quarter-wave Plate in Terahertz Using Active Phase-change Metasurface.

Wang D, Zhang L, Gu Y, Mehmood MQ, Gong Y, Srivastava A, Jian L, Venkatesan T, Qiu CW, Hong M - Sci Rep (2015)

Switchable THz QWP design and fabrication results.(a) Experimental switching schematic of the THz QWP. A linear normal incident THz wave polarized at θ = 45° to the two slots is converted into a circularly polarized light. Through the VO2 phase transition controlled by a resistive heater, the operating frequency of the QWP can be switched between f1 = 0.468 THz and f2 = 0.502 THz. The top left inset is a microscope image of one unit cell in the fabricated samples. The scale bar is 50 μm. The following are the geometrical parameters: P = 150, Lx = 90, Ly = 124, lx = 9, ly = 5 and w = 9 μm, respectively. The top right inset is the simulated ellipticities of the output THz waves, indicating that at both f1 and f2 the output THz waves are circularly polarized. (b) Schematic backside view of the resistive heater with a square aperture (6 × 6 mm2) milled at the center to allow THz to pass through. (c) Measured electrical conductivity of fabricated VO2 films at different temperatures during the heating and the cooling cycles. The fabricated films exhibit stable electrical conductivity switching between 300 and 400 K during either the heating or the cooling cycles.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4595731&req=5

f1: Switchable THz QWP design and fabrication results.(a) Experimental switching schematic of the THz QWP. A linear normal incident THz wave polarized at θ = 45° to the two slots is converted into a circularly polarized light. Through the VO2 phase transition controlled by a resistive heater, the operating frequency of the QWP can be switched between f1 = 0.468 THz and f2 = 0.502 THz. The top left inset is a microscope image of one unit cell in the fabricated samples. The scale bar is 50 μm. The following are the geometrical parameters: P = 150, Lx = 90, Ly = 124, lx = 9, ly = 5 and w = 9 μm, respectively. The top right inset is the simulated ellipticities of the output THz waves, indicating that at both f1 and f2 the output THz waves are circularly polarized. (b) Schematic backside view of the resistive heater with a square aperture (6 × 6 mm2) milled at the center to allow THz to pass through. (c) Measured electrical conductivity of fabricated VO2 films at different temperatures during the heating and the cooling cycles. The fabricated films exhibit stable electrical conductivity switching between 300 and 400 K during either the heating or the cooling cycles.
Mentions: Figure 1a shows a schematic of the switchable QWP, which is composed of ultrathin asymmetric cross-shaped resonator arrays with VO2 pads inserted at the end of the cross-shaped resonators. The complementary metasurfaces present high transmission coefficients at the resonance frequencies due to the extraordinary optical transmission effect with specific phase delays3940. It only allows the resonant EM wave to pass through, which eliminates the interference of the non-resonant EM wave. In our design, two slots in the QWP are perpendicular to each other with a slight difference in length. The fundamental resonance in each slot is able to present a maximum phase shift of 180° between the transmitted and the incident light. Therefore, birefringence can be introduced by controlling the length of the slots in the asymmetric cross-shaped resonators. The switching property of the QWP is controlled by a resistive heater as a proof of concept to manipulate the VO2 phase transition at different temperatures, which can also be realized by optical pumping41. When VO2 pads act as a semiconductor at 300 K, phase difference between two orthogonal slots can reach 90° at f1 = 0.468 THz, while the transmission coefficients in these two slots are the same. At this frequency, when the incident THz wave is polarized at θ = 45° to the two slots, the device operates as a QWP. Through the phase transition, free carriers in the VO2 pads increase, resulting in the rise of the electrical conductivity. The VO2 pads behave like a metal at 400 K. This leads to a shortened effective length of cross-shaped resonators and the QWP operates at f2 = 0.502 THz. Therefore, this THz QWP can switch its operating frequency between two states through the VO2 phase transition. The top right inset is the simulated ellipticities of the output THz wave at 300 and 400 K. It is observed that the ellipticites at f1 = 0.468 and f2 = 0.502 THz are close to 1, indicating a circular polarization of the output THz waves. The top left inset in Fig. 1a shows a microscope image of one unit cell in the fabricated metasurfaces.

Bottom Line: In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface.After the transition to metal phase, the quarter-wave plate operates at 0.502 THz.At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, 117576, Singapore.

ABSTRACT
Metamaterials open up various exotic means to control electromagnetic waves and among them polarization manipulations with metamaterials have attracted intense attention. As of today, static responses of resonators in metamaterials lead to a narrow-band and single-function operation. Extension of the working frequency relies on multilayer metamaterials or different unit cells, which hinder the development of ultra-compact optical systems. In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface. Before the phase transition, VO2 behaves as a semiconductor and the metasurface operates as a quarter-wave plate at 0.468 THz. After the transition to metal phase, the quarter-wave plate operates at 0.502 THz. At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light. This work reveals the feasibility to realize tunable/active and extremely low-profile polarization manipulation devices in the terahertz regime through the incorporation of such phase-change metasurfaces, enabling novel applications of ultrathin terahertz meta-devices.

No MeSH data available.