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Abstract
A liquid crystal elastomer (LCE) film is successfully deposited with a terahertz metamaterial using thermal evaporation via a programmed electronic shutter and high-efficiency cooling system. The transmittance of the metamaterial at its resonance frequency is monotonically increased from 0.0036 to 1.0 as a pump beam bends the LCE film, so the metamaterial has a large switching contrast of 277 at the frequency. The monotonic increase in the resonance transmittance arises from the constant resonance frequency of the metamaterial at the transmittance modulation and depicts that the metamaterial-deposited LCE film can continuously tune the transmitted intensity of a terahertz beam. The metamaterial-deposited LCE film has potential in developing continuously tunable intensity modulators with large switching contrasts for the application of terahertz imaging and terahertz communication. Therefore, the thermal evaporation expands the application of metamaterials and improves their optical properties.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Liquid crystals have been used to develop advanced terahertz devices such as phase shifters and wave plates [1–3]. Liquid crystal elastomer (LCE) films, which are made from LC monomers, azobenzene monomers and photo-initiators, have attracted considerable attention due to their excellent light-induced mechanical response [4,5]. The bending of an LCE film is based on the photoisomerization of its azobenzene polymer. The LCE film deforms (recovers) at the irradiation of a pump beam with a short (long) wavelength since the trans → cis (cis → trans) isomerization of the azobenzene polymer causes the contraction (extension) of its LC polymer [4]. In addition to the pump beam with the long wavelength, heat and thermal relaxation cause the extension of the LC polymer [4]. LCEs will be damaged or distorted after they are immersed into acetone, acid and alkali. The damage and distortion reduce the light-induced mechanical response of the LCE films. Therefore, it is difficult to deposit metallic patterns on LCE films via photolithography, lift-off process and wet etching [5]. A possible approach to deposit metallic patterns on LCE films is thermal evaporation through shadow masks. However, the LCE films will be damaged by the metallic vapor at the thermal evaporation. Therefore, it is of great interests that researchers want to develop an approach to deposit metallic patterns on LCE films.
Metamaterials, which comprise arrays of split ring resonators (SRRs), have been widely used to develop controllable intensity modulators because the localized electromagnetic resonance of the SRRs strongly absorbs incident electromagnetic waves. The key performance parameter for a controllable intensity modulator is to have a large switching contrast C since it can be used in protective optical circuitry and reconfigurable optical networks. C is given by an equation of C = (Tmax − Tmin)/Tmin, where Tmin and Tmax are the minimum and maximum transmittances of a metamaterial at a specific frequency, respectively [6]. The equation depicts that C will be significantly increased as Tmax and Tmin approach 1 and 0, respectively.
Controllable intensity modulators can be achieved by MEMS-based metamaterials [6–8], metamaterials that involve photoconductive materials [9–11], and metamaterials that are imbedded into liquid crystals [12]. The wirelines in the MEMS-based metamaterials connect the SRRs for the introduction of external voltage into the metamaterials, suppressing the localized electromagnetic resonance of the SRRs, and increasing Tmin. [6–8] Tmin in these metamaterials exceed 0.02 due to the wirelines. Therefore, separated SRRs can be used to develop controllable intensity modulators with high switching contrasts. Photoconductive materials have smaller carrier densities than noble metals, so the metamaterials which involve the photoconductive materials [9–11] have larger Tmin than the noble metamaterial [12]. Tmin in these metamaterials exceed 0.1 due to the photoconductive materials [9–11]. Therefore, noble metals can be used to develop controllable intensity modulators with large switching contrasts. The substrates, wirelines, buffer layers and ITO films in the controllable intensity modulators reduce Tmax due to Fresnel reflection and absorption [6–8]. If controllable intensity modulators don’t have these elements, the modulators will have high switching contrasts due to Tmax = 1. Large Tmin and small Tmax reduce the switching contrasts of the previous controllable intensity modulators [6–12], so the maximum switching contrast in the modulators is merely 22 [7].
This work uses a thermal evaporator with a programmed electronic shutter and high-efficiency cooling system to deposit a terahertz metamaterial on an LCE film. The metamaterial has a transmittance of 0.0036 at its resonance frequency due to the strong electromagnetic resonance of the separated silver SRRs. The resonance transmittance of the metamaterial is monotonically increased to 1.0 at the irradiation of a pump beam that bends the LCE film because the SRRs and bent LCE film are out of the spot of a terahertz beam. The terahertz metamaterial has a large switching contrast of 277, and can continuously tune the transmitted intensity of the terahertz beam due to the monotonic increase in the resonance transmittance. The metamaterial-deposited LCE film is a continuously tunable intensity modulator with a large switching contrast. As a result, the thermal evaporation expands the application of metamaterials and improves their optical properties
2. Experiment
An LCE film is made from mixing 75 mol % LC-monomer (RM105, Merck), 20 mol % crosslinker (RM257, Merck), 4 mol % azobenzene monomer (M11E, homemade material) and 1 mol % photo-initiator (IRG784, Ciba). The chemical structure and synthesis of M11E can be found elsewhere [13]. Two glass substrates are coated with polyvinyl alcohol layers, and the layers are rubbed by a nylon roller. The rubbed polyvinyl alcohol layers align the LC monomer, crosslinker and azobenzene monomer before photopolymerization.
An empty cell comprises the two glass substrates, which are separated by spacers with a thickness of 20 μm, and the cell is filled with the mixing. The mixing is heated to its isotropic phase, and then cooled to its nematic phase at a cooling rate of 0.1 °C/min. Following the cooling process, the mixing is irradiated with a linearly-polarized green light with an intensity of 10 mW/cm2 for 5 min. The LCE film is separated from the cell following the photopolymerization.
A shadow mask is obtained using low-pressure chemical vapor deposition, photolithography, dry etching and wet etching, and the detailed fabrication process for the shadow mask can be found elsewhere [14]. The shadow mask is put on the LCE film, and then they are placed in a thermal evaporator to deposit a 200-nm-thick silver layer on the film. We made a programmed electronic shutter, and set up it in the thermal evaporator. The shutter is used to prevent the heat which is transferred from the metallic vapor from accumulating in the LCE film. In addition, a cooling system efficiently dissipates the heat from the electrodes of the tungsten boat in the evaporator. A metamaterial-deposited LCE film is obtained following the removal of the shadow mask.
Figure 1(a) presents the geometrical dimensions of one of the SRRs in the metamaterial. The SRR has a linewidth (t), split gap (g), short side (s), long side (l), period in the x direction and period in the y direction of 6 μm, 20 μm, 50 μm, 60 μm, 70 μm and 80 μm, respectively. Figure 1(b) displays the reflective optical microscope image of the metamaterial-deposited LCE film. This image depicts that the metamaterial is successfully deposited on the LCE film. In other words, metallic patterns can be deposited on LCE films using the thermal evaporator with the programmed electronic shutter and high-efficiency cooling system.
Fig. 1. (a) Dimensions of SRR unit. (b) Optical microscope image of metamaterial-deposited LCE film.
Figure 2(a) presents the experimental setup of the unbent metamaterial-deposited LCE film. This film is pasted on a metal plate with a circular window using 3M tapes, and the centers of the metamaterial and window are aligned with each other. The LCE film has an area of 15 × 11 mm2; the SRR array has an area of 7 × 7 mm2, and the circular window has a diameter of 7 mm. The LCE film has fixed and free ends at the positions of x = 0 mm and 11 mm, respectively.
Fig. 2. Experimental setup of metamaterial-deposited LCE film that are (a) unbent and bent with respect to (b) edge line and (c) center line of metamaterial.
A blue beam with a wavelength of 450 nm from a pump laser is used to bend the metamaterial-deposited LCE film, and the pump beam is incident at an angle of 45° with respect to the normal surface of the metal plate. The transmittance of the metamaterial at its resonance frequency is switched under a full (half) window as the LCE film is bent with respect to the edge line (center line) of the metamaterial at x = 0 (5.5) mm, as presented in Figs. 2(b) and 2(c). The position of the bending line of the film is determined by the tapes. The bend angle θ (Θ) of the LCE film under the full (half) window is defined as an acute angle between the lines that are tangent to the bent film at the positions of x = 0 (5.5) mm and 11 mm, as displayed in Figs. 2(b) and 2(c).
The metal plate with the metamaterial-deposited LCE film and pump laser are placed in the chamber of a terahertz spectrometer (TPS 3000, TeraView) for studying the effect of the bending of the film on the resonance transmittance of the metamaterial. A terahertz beam that is polarized in a direction parallel to the x axis of Fig. 1(a) is normally incident to the metamaterial-deposited LCE film, and the beam has a spot size of 6 mm at the window. The pump beam is incident at an angle of 45° with respect to the normal surface of the metal plate during the spectral measurement of the metamaterial-deposited LCE film to prevent the photoconductive antennas of the spectrometer from being damaged.
3. Results and discussion
3.1 Light-induced bending response
Figure 3 presents the bend angles of the metamaterial-deposited LCE film that is bent with respect to the edge line of the metamaterial at various pump intensities. The bend angle increases with the increase in the pump intensity, and the bend angle approaches 90° at a pump intensity of 22.5 mW/cm2. Therefore, the bending of the LCE film can be controlled by the pump beam. The bent film recovers to its original state after the removal of the pump beam since the azobenzene monomer of the LCE film has a low concentration of 4 mol % in the film. The insets in Fig. 3 present the captures of the metamaterial-deposited LCE film that is exposed to the pump beam at θ = 0°, 30°, 50° and 70°, respectively. The window (white circle of Fig. 3) opens at θ > 0°, and the area of the opened region (red sector of Fig. 3) increases with the increase in the bend angle. This result depicts that part of a terahertz beam will pass straight through the opened region as the beam is normally incident to the window, and does not interact with the metamaterial-deposited LCE film at θ > 0°. The SRRs (blue polygon in Fig. 3) near the fixed end of the LCE film are out of the window because the LCE film is bent with respect to the edge line of the metamaterial, so they will not have electromagnetic resonance at θ > 0°.
Fig. 3. Bend angles of metamaterial-deposited LCE film that is bent with respect to edge line of metamaterial at various pump intensities. The insets present the photos of the metamaterial-deposited LCE film that is exposed to the pump beam at bend angles of θ = 0°, 30°, 50° and 70°. Part of a terahertz beam passes straight through the opened region (red sector) of the window (white circle). The SRRs (blue polygon) near the fixed end of the bent film are out of the window.
The time for the bending of the metamaterial-deposited LCE film from θ = 0° (90°) to θ = 90° (0°) is defined as its switching on (off) time in this work. A blue (green) pump beam with a wavelength of 450 (532) nm is incident at an angle of 45° with respect to the normal surface of the metal plate is used to bend (recovery) this film. The switching on (off) time of the film is one (three) second at the irradiation of the blue (green) pump beam with an intensity of 22.5 (40.0) mW/cm2. Therefore, the film has a response time of four seconds as θ increases from 0° to 90° and then decreases back to 0°. The space of the chamber is occupied by the blue pump laser, so the green pump source does not put in the chamber of the terahertz spectrometer. The bent LCE film in the chamber returns to its unbent configuration due to thermal relaxation, so it has a large switching off time of approximately 10 minutes following removal of the blue pump beam. Lee et. al. used 2-azo azobenzene monomer to develop photodriven cantilevers with a maximum oscillating frequency of 120 Hz [15]. A situation in which 2-azo azobenzene monomer is used in this work may fabricate a metamaterial-deposited LCE film with a response time of several milliseconds. Efforts are being made at the authors’ laboratory to fabricate metamaterial-deposited LCE films with small response times, the results of which will be published in the near future.
3.2 Terahertz spectra
Figure 4(a) presents the experimental spectra of the metamaterial-deposited LCE film that is bent with respect to the edge line of the metamaterial at various bend angles. The resonance transmittance of this film increases with the increase in the bend angle, and its resonance frequency remains at the same value. The transmittance modulation in Fig. 4(a) arises from the bending of the film. The film bending causes the rotation and movement of the SRRs of the metamaterial. When the LCE film is bent with respect to the edge line of the metamaterial, the SRRs are rotated with respect to the line. The rotation of the SRRs increases the incident angle of the terahertz beam on the surface of the bent film, reducing the electric field of the incident beam because only the projection of the incident field on the bent surface excites the electromagnetic resonance of the metamaterial. The reduced field inefficiently excites the electromagnetic resonance of the metamaterial. Therefore, the rotation of the SRRs increases the transmittance of the metamaterial-deposited LCE film at its resonance frequency. When the LCE film is bent with respect to the edge line of the metamaterial, the SRRs near the fixed end of the bent film move out of the path of the terahertz beam. As a result, the movement of the SRRs increases the transmittances of the metamaterial at all the frequencies of the resonance spectrum. The window opens with the rotation and movement of the SRRs, so a part of the terahertz beam passes straight through the opened region of the window. The opened region increases the transmittances of the metamaterial at all the frequencies of the resonance spectrum. Therefore, the transmittance modulation of the metamaterial-deposited LCE film arises from the film bending which involves the rotation and movement of the SRRs.
Fig. 4. (a) Experimental spectra of metamaterial-deposited LCE film that is bent with respect to (a) edge and (b) center lines of metamaterial at various bend angles. The inset in (a) [(b)] presents the switching of the resonance transmittance of the metamaterial under a full (half) window.
The transmittance of the metamaterial at the resonance frequency is 0.0036 (−24.43 dB) on a linear (decibel) scale because the separated silver SRRs have strong electromagnetic resonance at θ = 0° [12]. The transmittance of the metamaterial at the frequency is 1.0 (0 dB) on a linear (decibel) scale since all the SRRs and the bent LCE film are not exposed to the terahertz beam at θ = 90°. The switching contrast (277) of the metamaterial is larger than that (22) of the previous controllable intensity modulator [7]. Therefore, the metamaterial-deposited LCE film can be used to develop optically controllable intensity modulators with large switching contrasts for the application of terahertz imaging and terahertz communication. The resonance transmittance of the metamaterial is monotonically increased with the pump intensity since it has the constant resonance frequency at the transmittance modulation, as presented in Fig. 3 and Fig. 4(a). Therefore, the metamaterial-deposited LCE film can continuously tune the transmitted intensity of the terahertz beam.
Figure 4(b) presents the experimental spectra of the metamaterial-deposited LCE film that is bent with respect to the center line of the metamaterial at various bend angles. The resonance transmittance of the metamaterial increases with the increase in the bend angle due to the rotation of the SRRs. The transmittance modulation has a bigger difference in Fig. 4(a) than in Fig. 4(b) due to the bending line of the metamaterial-deposited LCE film. As the metamaterial-deposited LCE film is bent with respect to the edge line of the metamaterial, the film is out of the path of the incident terahertz beam at θ = 90°. The metamaterial-deposited LCE film does not interact with the incident terahertz beam, so its transmittance at 0.65 THz is zero dB at θ = 90°. As the metamaterial-deposited LCE film is bent with respect to the center line of the metamaterial, a half of the film is not bent at Θ = 90°. The half of the metamaterial-deposited film interacts with the incident terahertz beam, so its transmittance at 0.65 THz is −20 dB at Θ = 90°. The metamaterial-deposited LCE film has a larger resonance transmittance at θ = 90° than at Θ = 90°, and it has the same resonance transmittance at θ = 0° and Θ = 0°. Therefore, the transmittance modulation has a bigger difference as the metamaterial-deposited LCE film is bent with respect to the edge line of the metamaterial than to its center line.
The experimental results in Fig. 4(b) depict that the sensitivity of the resonance transmittance of the metamaterial to the bend angle of the LCE film is decreased as the bend axis of the LCE film is moved to the center line of the metamaterial. The experimental results in Fig. 4(a) and Fig. 4(b) present that the sensitivity of the resonance transmittance of the metamaterial to the bend angle of the LCE film can be tuned by moving its bend axis. Therefore, the metamaterial-deposited LCE film has potential to develop mass flow meters with various measuring ranges.
4. Simulation
Most computers cannot simulate the experimental spectra of Fig. 4(a) and Fig. 4(b) because a simulated model of a millimeter-scale bent LCE film on which micro-scale SRR units are deposited requires a lot of computer memory and computation time. An alternative method for studying the effect of the bending of the LCE film on the electromagnetic resonance of the metamaterial is to study the effect of the rotation of the SRRs on the electromagnetic resonance of the metamaterial because the bent LCE film rotates the SRRs with respect to the edge or center line of the metamaterial. The pump beam bends the LCE film, rotating the SRRs by various angles with respect to the edge line of the metamaterial. Therefore, the SRRs have larger rotation angles near the free end of the bent film than near its fixed end. The rotation angle ϕ of the SRRs near the free end of the bent film equals the bend angle θ of the LCE film since the latter is measured between the lines that are tangent to the bent film at the free (x = 0 mm) and fixed (x = 11 mm) ends, as presented in Fig. 2(b). Therefore, the dependence of the resonance transmittance of the metamaterial on θ in Fig. 4(a) can be considered as the dependence of the resonance transmittance of the metamaterial on ϕ.
A single SRR unit is used to verify the operating mechanism of the metamaterial-deposited LCE film that is bent with respect to the edge line of the metamaterial via Computer Simulation Technology (CST) software based on the finite element method. The insets in Fig. 5(a) and Fig. 5(b) present the simulated models of the SRR unit that is rotated by an angle of ϕ with respect to its center line and edge line at x’ = 20 μm, respectively. The SRR unit has the same geometrical structure as the SRR unit of Fig. 1(a), and is deposited on a 20-μm-thick LCE film with an area of 70 μm × 80 μm. The LCE film with the SRR unit is placed at an identical distance of 100 μm from the input port of a terahertz beam and the output port of a transmittance monitor before the rotation of the film, and the two ports remain in the same positions after the rotation of the film. The two ports have an area of 70 μm × 80 μm before the rotation of the film, and the area remains at the same value after the rotation of the film. The calculation region of the finite element method includes the whole LCE film before and after the rotation of the film, and the transverse area of the calculation region remains at the same value during the simulation. The terahertz beam in the simulation is polarized in a direction parallel to the x’ axis of Fig. 5(b). The electric and magnetic boundary conditions are used for the SRR unit in the simulation. The dielectric constant of the LCE film is 3.2 at 0.650 THz in the simulation, and was obtained from its time-domain spectrum.
Fig. 5. Simulated spectra of SRR unit that is rotated with respect to (a) its center line and (b) edge line at x’ = 20 μm at various rotation angles. (c) Angle-dependent resonance transmittance curves of SRR unit that is rotated with respect to its center line, edge line at x’ = 0 μm, edge line at x’ = 10 μm, and edge line at x’ = 20 μm. Insets in (a) and (b) present the simulation models of the SRR unit that is rotated by an angle ϕ with respect to its center line and edge line at x’ = 20 μm, respectively.
Figure 5(a) presents the simulated spectra of the SRR unit that is rotated with respect to its center line at various rotation angles. The resonance transmittance of the SRR unit in Fig. 5(a) is increased due to its rotation. The rotation of the SRR unit increases the incident angle of a terahertz beam on the surface of the rotated LCE film, reducing the electric field of the incident beam because only the projection of the incident field on the rotated surface excites the electromagnetic resonance of the SRR unit. The reduced field inefficiently excites the electromagnetic resonance of the SRR unit. Therefore, the rotation of the SRR unit increases its resonance transmittance. The input and output ports open with the rotation of the SRR unit, so a part of the terahertz beam passes straight through the opened region of the input and output ports. The opened region increases the transmittances of the SRR unit at all the frequencies of the resonance spectrum. As a result, the transmittance modulation in Fig. 5(a) is caused by the rotation of the SRR unit.
Figure 5(b) presents the simulated spectra of the SRR unit that is rotated with respect to its edge line at x’ = 20 μm at various rotation angles. The resonance transmittance of the SRR unit in Fig. 5(b) is increased due to its rotation and movement. The rotation of the SRR unit decreases the electric field of an incident terahertz beam on the surface of the rotated LCE film, suppressing the electromagnetic resonance of the SRR unit, and increasing its resonance transmittance. A part of the SRR unit moves out of the path of the terahertz beam when the LCE film is rotated with respect to the edge line of the SRR unit, so the movement of the SRR unit increases its transmittances at all the frequencies of the resonance spectrum. The input and output ports open with the rotation and movement of the SRR unit, so a part of the terahertz beam passes straight through the opened region of the input and output ports. The opened region increases the transmittances of the SRR unit at all the frequencies of the resonance spectrum. Therefore, the transmittance modulation in Fig. 5(b) arises from the rotation and movement of the SRR unit.
The inset of Fig. 5(a) reveals that the whole SRR unit is in the input and output ports as it is rotated with respect to its center line. Therefore, the simulated results in Fig. 5(a) presents the dependence of the resonance transmittance of the SRR unit on its rotation. The inset of Fig. 5(b) displays that the SRR unit moves out of the input and output ports at ϕ = 90° as it is rotated with respect to its edge line at x’ = 20 μm. Therefore, the simulated results in Fig. 5(b) presents the dependence of the resonance transmittance of the SRR unit on its rotation and movement. Therefore, the simulated results in Fig. 5(a) and Fig. 5(b) can be used to verify the operating mechanism of the metamaterial-deposited LCE film that is bent with respect to the edge line of the metamaterial.
The rotation of the SRR unit in Fig. 5(a) modulates its resonance transmittance. Besides the rotation of the SRR unit, its movement in Fig. 5(b) modulates its resonance transmittance. Therefore, the SRR unit has a larger resonance transmittance at ϕ > 0° as it is rotated with respect to the edge line at x’ = 20 μm than to the center line. The simulated results in Fig. 5(a) depict that the SRR unit has a slight shift in its resonance frequency. The slight frequency shift has been reported by Tao et. al. [16] The various resonance frequencies of the SRR unit that is rotated with respect to its center line is different from the constant resonance frequency of the metamaterial-deposited LCE film that is bent with respect to the edge or center line of the metamaterial. Figure 5(c) presents the angle-dependent resonance transmittance curves of the SRR unit that is rotated with respect to the center line, edge line at x’ = 0 μm, edge line at x’ = 10 μm, and edge line at x’ = 20 μm. The simulated results in Fig. 5(c) depict that the sensitivity of the electromagnetic resonance of an SRR unit to its rotation is increased as the rotation axis moves from the center line of the SRR unit to its edge line. As a result, the resonance transmittance of the metamaterial is more sensitive to the bend angle as the LCE film is bent with respect to the edge line of the metamaterial than to its center line.
The dependence of the resonance transmittance of the metamaterial-deposited LCE film on the incident angle of a terahertz beam can be achieved by the rotation of the single SRR unit because the incident angle of the beam is equivalent to the rotation angle of the unit, as presented in Figs. 5(a) − 5(c) [16].
Four SRR units are used to verify that the transmittance modulation has a bigger difference as the metamaterial-deposited LCE film is bent with respect to the edge line of the metamaterial than to its center line. The inset in Figs. 6(a) and 6(b) presents the simulated model of the four SRR units that are rotated with respect to their edge (center) line. The rotation of the four SRR units in the inset of Figs. 6(a) and 6(b) is called all (half) rotation. Each of the 4 units has the same geometrical structure as the SRR unit of Fig. 1(a), and is deposited on a 20-μm-thick LCE film with an area of 70 μm × 80 μm. The LCE film with the 4 units is placed at an identical distance of 200 μm from the input port of a terahertz beam and the output port of a transmittance monitor, and the two ports have an area of 140 μm × 160 μm. A terahertz beam in the simulation is polarized in a direction parallel to the x’ axis of the inset of Fig. 6(b).
Fig. 6. Simulated spectra of four SRR units that are rotated with respect to (a) their edge line and (b) center line at various rotation angles. The inset in (a) [(b)] presents the simulated model of the four SRR units under all (half) rotation. (c) Angle-dependent resonance transmittance curves of four SRR units under all and half rotations.
Figures 6(a) and 6(b) present the simulated spectra of the four SRR units that are rotated with respect to their edge (center) line at various rotation angles ϕ (Φ). Each of the spectra has a resonance peak at a frequency of approximately 0.65 THz as ϕ ranges from 0° to 75°, and has two coupling peaks near the resonance frequency. The simulated spectra in Fig. 6(a) depict that the transmittance of the four units at their resonance frequency increases with the rotation angle. Therefore, the simulated results of Fig. 6(a) agree with the experimental results of Fig. 4(a). The coupling peaks in the spectra of Fig. 6(a) arise from the electromagnetic interaction between the SRR units. The coupling peaks of SRR units will affect their resonance frequency and transmittance if the number of the units exceeds 4 (data not shown herein). In addition, we didn’t see any coupling peaks near the resonance frequency of the metamaterial-deposited LCE film. Therefore, a single unit is used in Fig. 4(a) to verify the operating mechanism of the metamaterial-deposited LCE film that is bent with respect to the edge line of the metamaterial. Figure 6(c) displays the angle-dependent resonance transmittance curves of the four SRR units under the all and half rotations. The sensitivity of the resonance transmittance of the four SRR units to their rotation angle is larger under the all rotation than under the half rotation. The simulated results in Fig. 6(c) agree with the experimental results of Fig. 4(a) and Fig. 4(b). As a result, the transmittance modulation has a bigger difference as the metamaterial-deposited LCE film is bent with respect to the edge line of the metamaterial than to its center line.
Five SRR units are used to make a scenario under which a metamaterial is gradually pushed out of the path of a terahertz beam. Each of the 5 units has the same geometrical structure as the SRR unit of Fig. 1(a), and is deposited on a 20-μm-thick LCE film with an area of 70 μm × 80 μm. The LCE film with the 5 units is placed at an identical distance of 100 μm from the input port of a terahertz beam and the output port of a transmittance monitor, and the two ports have an area of 350 μm × 80 μm. Figure 7(a) presents the simulated model of the 5 SRR units that are gradually pushed out of the input and output ports. Figure 7(b) displays the simulated spectra of 0 unit, 1 unit, 2 units, 3 units, 4 units and 5 units in the input and output ports. The simulated spectra in Fig. 7(b) depict that the transmittance of the SRR units at their resonance frequency of approximately 0.65 THz is increased as the number of the SRR units in the input and output ports is decreased. This result reveals that the resonance transmittance of a metamaterial is increased as the metamaterial is gradually pushed out of the path of a terahertz beam. In addition to a resonance peak, each of the simulated spectra in Fig. 7(b) has coupling peaks. The coupling peaks arise from the electromagnetic interaction between the SRRs. The coupling peaks of SRR units will affect their resonance frequency and transmittance if the number of the SRR units exceeds 5 (data not shown).
Fig. 7. Simulated model of five SRR units that are gradually pushed out of input and output ports. (b) Simulated spectra of 0 unit, 1 unit, 2 units, 3 units, 4 units and 5 units in input and output ports.
Figure 8 presents the dependence of the real and imaginary dielectric constants of an LCE film without a metamaterial on terahertz frequency. The real and imaginary dielectric constants of the film are 3.20 and 0.55, respectively. Therefore, the loss tangent of the LCE film is 0.172.
Fig. 8. Dependence of real and imaginary dielectric constants of LCE film without metamaterial on terahertz frequency.
5. Application
The metamaterial-deposited LCE film has a large switching contrast of 277 at the resonance frequency of the metamaterial. Therefore, this film can be used to develop intensity modulators for high-contrast terahertz imaging and frequency isolators for terahertz communication. As the metamaterial-deposited LCE film is placed in front of a terahertz detector, a feature of an objective should have a high-contrast terahertz image in real time because the unbent (bent) film can suppress (enhance) the transmitted intensity of a terahertz beam outside (inside) the feature. Terahertz waves are a candidate for future 6G cellular networks. Metamaterial-deposited LCE films with different resonance frequencies can be achieved by changing the dimensions of the SRRs of the metamaterials. The films at the irradiation of pump beams can allow terahertz waves with different frequencies to pass through or not to pass through. Therefore, these films can be developed into frequency isolators for preventing interference by channels.
The red curve of Fig. 5(c) reveals that the rotation of the SRR unit can module the transmitted intensity of a terahertz beam as its rotation axis is in the y’ axis. It is interesting that the dependence of the electromagnetic resonance of the SRR unit that is rotated with respect to the x’ axis on its rotation. The inset in Fig. 9 presents the simulation model of the SRR unit. A terahertz beam in the simulation is polarized in a direction parallel to the x’ axis of the inset of Fig. 9, and the other simulation parameters of Fig. 9 are the same as those of Figs. 5(a) − 5(c). Figure 9 presents the simulated spectra of this SRR unit at rotation angles ranging from 0° to 90°. The simulated results in Fig. 9 depict that the rotation of the SRR unit at the irradiation of a pump beam can manipulate the frequency of a broadband terahertz beam as it is rotated with respect to the x’ axis. Efforts are being made at the authors’ laboratory to fabricate optically controllable terahertz filters using metamaterial-deposited LCE films, the results of which will be published in the near future.
The metamaterial-deposited LCE film can be cut into small units with dimensions ranging from several hundreds of micrometers [17] to several millimeters using laser cutting machines. In addition, the pump source that bend the metamaterial-deposited LCE film can be replaced with a vertical cavity surface-emitting laser (VCSEL) chip. A metamaterial-deposited LCE unit can be integrated with its VCSEL chip, so the overall setup is compact.
Square metal patterns can be deposited on an LCE film using a thermal evaporator and shadow mask. The LCE film with the metal patterns should be cut into many small units with dimensions of several tens of micrometers using a laser cutting machine with a small linewidth. Each of the LCE units should have a practically infinite extinction ratio, and functions like an optically controllable shutter. The LCE units may be developed into a spatial light modulator since the LCE units have geometrical dimensions of several tens of micrometers. Efforts are being made at the authors’ laboratory to fabricate an LCE film with metal patterns, the results of which will be published in the near future.
The metamaterial-deposited LCE film has a large switching contrast of 277 at 0.65 THz. However, mirror and shutters will block or reflect all frequencies of terahertz waves because they are made of metals. Therefore, mirror and shutters cannot be used to develop frequency isolators and terahertz filters.
6. Conclusions
This work successfully deposits the terahertz metamaterial on the LCE film using the thermal evaporation via the programmed electronic shutter and high-efficiency cooling system. The metamaterial has a large switching contrast of 277 at the transmittance modulation not only because the separated silver SRRs have strong electromagnetic resonance at zero pump intensity but also because all the SRRs and the bent LCE film are not exposed to the terahertz beam at a pump intensity of 22.5 mW/cm2. In addition, the resonance transmittance of the metamaterial can be continuously tuned by the pump beam due to its constant resonance frequency at the transmittance modulation. The metamaterial-deposited LCE film is an optically tunable intensity modulator with a large switching contrast, and can be used in terahertz imaging and terahertz communication. The sensitivity of the resonance transmittance of the metamaterial to the bend angle of the LCE film can be tuned by moving its bend axis, and this result is verified by the related simulation. Therefore, the metamaterial-deposited LCE film has potential in developing mass flow meters with various measuring ranges for the detection of the pressure, density, viscosity and flow rates of fluids.
Funding
Ministry of Science and Technology, Taiwan (MOST 107-2112-M-029-005-MY3, MOST 109-2112-M-029-006).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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