1. INTRODUCTION

Vectorial beams have wide application prospects in polarization imaging, anticounterfeiting technology, target recognition, and optical communication [1,2]. Traditional methods to generate vectorial beams are based on interferometry, spatial light modulation, and spatially variant metasurfaces [3,4]. Vortex beams have widespread applications in quantum information, optical microscopy, and optical tweezers due to the disparate energy distribution of a circular symmetric doughnut-shape [5,6]. The spiral phase distributions are generally generated by space light modulators, spiral phase plates, and $q$ plates [7,8]. Traditional optical components used to generate vectorial and vortex beams suffer from bulky size, narrow operation bandwidth, and large phase pixels, which impede the use in applications that require miniaturization, broadband, and high stability.

Benefiting from advantages such as ultrathin, arbitrary structure design, and flexible manipulation, metasurfaces [9–13] have been investigated for photonic spin-orbit interactions (SOIs) [14,15]. Although steps have been made in polarization and phase manipulation, most existing methods based on SOIs can only generate a specific beam, which means the meta-devices could not realize tunable beam manipulation [16,17]. The solutions of the above problems could push many potential applications, such as secure communication, anticounterfeiting, and so on.

Currently, with the interest in tunable metasurfaces controlling [18], scientists realize tunable control according to electrochemistry [19], the change of voltage [19], temperature variation [20], and the use of phase-change materials (PCMs) [21]. Among all the methods above, the use of PCMs is promising for nonvolatile optical devices. PCMs, whose refractive index can be artificially controlled, have been used for optical disk storage before [22]. In recent years, the combination of PCM and metasurfaces is used to design reconfigurable, stabilized, and ultrafast meta-devices [21,23,24]. The optical response produced by these meta-devices could be tuned selectively by exploiting the large difference of the refractive index between amorphous and crystalline states. Ways to modulate PCMs, including heating, lighting, and electronic control are discussed in [25–27]. Researchers have demonstrated that the phase transition (from amorphous to the crystalline state) of a GST refractive index change can be induced by adjusting the femtosecond pulse sequence [26]. Moreover, the reverse process (from crystalline to the amorphous state) can also be achieved with the same femtosecond laser by modifying the excitation conditions (less pulse duration but higher pulse peak power). And the phase can basically be changed in real time (nanosecond magnitude) [28]. Up to now, researchers have designed tunable absorbers, displays, and filters as well as beam steering meta-devices using PCMs [20,25,27–29]. Recently, our group has reported a plasmonic metasurface for switchable photonic SOIs based on PCMs [30].

In this paper, a polarization converter and a vortex beam switcher have been designed based on a Pancharatnam–Berry phase [31,32], using phase-change and low-loss properties of PCMs ${{\rm Ge}_2}{{\rm Sb}_2}{{\rm Te}_5}$ (GST) operating at the working wavelength of 10.6 µm. Compared with our previous work [30], the meta-devices have higher polarization conversion ratios (PCR) when the GST layer is in two different states. First, a convertible vectorial beam converter concentrating on manipulation of the circularly polarized light (CPL) could produce azimuthally polarized (AP) and radially polarized (RP) light when the GST layer is in the crystalline and amorphous states, respectively. The polarization conversion efficiency of the convertible vectorial beam converter is as high as 83% in the amorphous state and 79% in the crystalline state. Second, a switchable vortex generator is designed using a similar method but different geometries. When the GST layer is in the amorphous state, the PCR of the metasurface is 6%, corresponding to the “OFF” state of the wave plate. When the GST layer is in the crystalline state, the PCR of the metasurface is 93%, corresponding to the “ON” state of the wave plate. Simultaneously, the reflected light carries OAM with a topological charge of $ \pm {2}$ depending on the handedness of incidence. This switchable metasurface could offer a promising route to design high-efficiency reconfigurable devices and encrypted optical communications.

2. PRINCIPLE

Figure 1(a) shows the schematic of the unit cell. The metasurface consists of a gold (Au), GST, and ${{\rm MgF}_2}$ multilayer stack. The GST film acts essentially as a switchable dielectric medium that changes the phase response of the output. Here, we defined that the electric field of transverse electronic (TE) polarization light is parallel to the $y$ axis (Ey), and the electric field transverse magnetic (TM) polarization light is parallel to $x$ axis (Ex). The phase retardation between TE and TM polarization can be exactly adjusted by optimizing the geometric parameters (thicknesses and widths) of the unit cell. The ${{\rm MgF}_2}$ layer acts as an isolation layer on top of the GST layer to prevent GST from oxidation in the atmosphere [30].

 

Fig. 1. (a) Schematic of the metasurface. Phase distribution of TE and TM with linearly polarized beam of (b) quarter- or three-quarter wave plate and (c) mirror or half-wave plate for different states of GST. (d) Co-polarization and cross-polarization reflectance for quarter- or three-quarter-wave plate. (e) Co-polarization and cross-polarization reflectance for mirror or half-wave plate. RCP with respect to the frequency for (f) quarter- or three-quarter-wave plate and (g) mirror or half-wave plate.

Download Full Size | PDF

 

Fig. 2. (a) Three-dimensional (3D) schematic of the switchable vectorial converter. Inset is the top view of the super unit cell. (b) Top view of the tunable vectorial beam generator.

Download Full Size | PDF

 

Fig. 3. Diffraction pattern of the designed switchable vectorial generator at the transverse plane 12 µm away from the metasurface. (a)–(c) When the GST layer is in the amorphous state, the transmitted electric fields for $x$ and $y$ components, and the polarization distribution, respectively. (d)–(f) Electric field distributions when the GST layer is in the crystalline state.

Download Full Size | PDF

As a proof of concept, different geometric parameters are selected to implement switchable vectorial beams and vortex beam generation, respectively. Both tunable beam generators are based on the switchable SOIs of the phase-change metasurface, with  the  unit  cells  behaving  as  quarter-   or  three-quarter-wave plates for switchable vectorial beam generation, while a mirror or half-wave plate is used for switchable vortex beam generation. The unit cell is simulated and optimized using the frequency domain solver in CST Microwave Studio with unit cell boundary conditions. According to the measured results in our previous report [30], GST has a large permittivity difference between amorphous (${\varepsilon _1} = {12}$) and crystalline (${\varepsilon _2} = {25}$) states at the working frequency of 28.3 THz. By placing the metasurface on a hot plate, GST could achieve phase change from an amorphous to crystalline state.

The geometric parameters are optimized as follows: the period is $p = {4.2}\,\,\unicode{x00B5}{\rm m}$; the thicknesses of Au, GST, and ${{\rm MgF}_2}$ layers are ${t_1} = {1.1}\,\,\unicode{x00B5}{\rm m}$, ${t_2} = {0.6}\,\,\unicode{x00B5}{\rm m}$, and ${t_3} = {0.1}\,\,\unicode{x00B5}{\rm m}$, respectively. The widths of protuberant GST and gold layers are ${w_1} = {1.3}\,\,\unicode{x00B5}{\rm m}$ and ${w_2} = {0.7}\,\,\unicode{x00B5}{\rm m}$. As shown in Fig. 1(b), the reflected phase retardation between TE and TM polarizations is 91° with the GST layer in the amorphous (red line). When the GST layer is in the crystalline state (blue line), the phase retardation between TE and TM polarizations is approximate to $ - {270}^\circ $ (90°). Both TE and TM polarization varies from the amorphous to crystalline state. Therefore, the wave plate in the crystalline state is obtained by rotating 180° in the amorphous state. As shown in Fig. 1(d), the reflectance of cross-polarization and co-polarization is 83% and 0.2%, when GST in the amorphous state. When GST changes to the crystalline state, the reflectance of cross-polarization and co-polarization is 79% and 0.1%. The simulated PCR defined as PCR=Rcross/(Rcross+Rco) is depicted in Fig. 1(f). The PCR is 100% whether the GST layer is in the amorphous or crystalline state at 28.3 THz.

Moreover, we also develop a switchable unit cell function as a mirror or half-wave plate at two different states of GST. The geometric parameters of the unit cell are as follows: the period is $p = {3.9}\,\,\unicode{x00B5}{\rm m}$, and the thicknesses of Au, GST, and ${{\rm MgF}_2}$ layers are 1.7, 1, and 0.1 µm, respectively. The widths of protuberant GST and gold layers are ${w_1} = {2.95}\,\,\unicode{x00B5}{\rm m}$ and ${w_2} = {2.35}\,\,\unicode{x00B5}{\rm m}$. As shown in Fig. 1(c), the reflected phase retardation between TE and TM polarizations is nearly 0° (red line) with the GST layer in the amorphous. When the GST layer is in the crystalline state (blue line), the phase retardation between TE and TM polarizations approximate to 180°. As shown in Fig. 1(e), the reflectance of cross-polarization and co-polarization are 0.3% and 88%, when the GST layer is in the amorphous state. When GST changes to the crystalline state, the reflectance of cross-polarization and co-polarization is 85% and 0.6%. The simulated PCR is depicted in Fig. 1(g), which shows the PCR is 0% and 100%, when the GST layer is in amorphous and crystalline states at 28.3 THz.

3. RESULTS AND DISCUSSION

A. Convertible Vectorial Beam Converter

For LCP incidence, the output light property can be described by the Jones vector in Eq. (1):

Above all, a switchable vectorial beam converter is designed, as shown in Fig. 2(a), and it is a circular structure with a radius of 80 µm. As shown in Fig. 2(b), the switchable vectorial converter is realized by eight super-units. As shown in the inset in Fig. 2(a), each super cell is composed of a rotating unit cell with an orientation angle of 45° with the radial axis. The geometric parameters are as follows: the period is $p = {4.2}\,\unicode{x00B5}{\rm m}$; the thicknesses of Au, GST, and ${{\rm MgF}_2}$ layers are ${t_1} = {1.1}\,\unicode{x00B5}{\rm m}$, ${t_2} = {0.6}\,\unicode{x00B5}{\rm m}$, and ${t_3} = {0.1}\,\unicode{x00B5}{\rm m}$, respectively; the widths of protuberant GST and gold layers are ${w_1} = {1.3}\,\unicode{x00B5}{\rm m}$ and ${w_2} = {0.7}\,\unicode{x00B5}{\rm m}$. Figures 3(a) and 3(b) show the simulated $x$ and $y$ polarized electric field components. As shown in Fig. 3(c), the polarization distribution also verifies that, when the GST layer is in the amorphous state, the generator could realize RP light. Figures 3(d) and 3(e) show the simulated electric fields for $x$ and $y$ components, which constitute AP light. As shown in Fig. 3(f), the polarization distribution verifies that, when the GST layer is in the crystalline state, the generator could realize AP light. Because the AP and RP light have OAM phases, as shown in Eq. (1), in order to see the resulting beam clearly, we add an impact factor ${\exp}({j}\beta )$. In actual experiment and applications, the factor is easy to implement by adding a spiral sheet in front of the receiving screen. Similarly, when we set the incident source to RCP, we can obtain the RP light (amorphous state) and AP light (crystalline state).

 

Fig. 4. (a) 3D schematic of the switchable OAM generator. The illustration is part of the generator. In the actual simulation, it is composed of 10 rings. (b) Top view of the tunable vortex generator.

Download Full Size | PDF

B. Switchable Vortex Generator

For a circularly polarized incidence ${{E}_{in}}(r,\varphi ) = {({1},j)^T}$, the reflected output beam can be expressed as

 

Fig. 5. Diffraction pattern of designed switchable vortex generator at the transverse plane 12 µm away from the metasurface. (a) and (b) The incident light is LCP. The electric field intensity distributions when the GST layer is in different states. (c) Phase distribution. (d)–(f) When the incident light is RCP, the electric field intensity and phase distributions for different states.

Download Full Size | PDF

The simulated result of the wave plate is shown in Fig. 5, and the intensity distribution is the result after filtering. The simulated intensity distribution for LCP incidence and GST of the amorphous state is shown in Fig. 5(a). The topological charge is identified by interference with a spherical wave with a focal length of 10 µm. Figure 5(b) shows the intensity of electromagnetic for the crystalline state. We can clearly see the contrast of the intensity when the GST layer is in two different states. The phase distribution for the crystalline state is shown in Fig. 5(c). It is obviously seen that the output beam with topological charge $l = + {2}$. For RCP incidence and amorphous state GST, the intensity distribution of electromagnetic is shown in Fig. 5(d). Figure 5(e) shows the intensity of electromagnetic for the crystalline state of GST. The phase distribution for the crystalline state is shown in Fig. 5(f). It achieves our previous design well.

4. SUMMARY

In conclusion, we designed an ultrathin metasurface platform composed of rotating grating structures. Because the phases of TE and TM polarization light can be arbitrarily designed with a high PCR when the GST layer is in the amorphous or crystalline state, the designed metasurface could act as a different wave plate according to a change in the geometric parameters of the metasurface. Last, a switchable vortex generator and a convertible vectorial beam converter are proposed. The PCR of the vectorial generator is as high as 100% whenever the GST layer is in the amorphous or crystalline state. The switchable vortex generator could realize OAM-carrying beams in the crystalline state; in the amorphous state, however, this phenomenon disappears. The simulated PCR is 0% and 100% for the amorphous and crystalline states, respectively. This tunable metasurface proposed here may provide a promising route for high-efficiency reconfigurable devices, such as deflectors, tunable hologram generation, and encrypted optical communications and can be extended to other wavebands.