[2020] [paper notes] phase change materials and Hypersurfaces——

Preface

type
$Terahertz+Supersurface$
Periodical
$Journal\; of\; Physics$
author
$Long\; Jie,Li\; Jiusheng$
Time
$2020$

---

         \;\\\;\\\;

research objective

In recent years ,THz In communication 、 Security check 、 imaging 、 spectrum 、 Life medicine 、 Nondestructive testing and other fields are more and more widely used , Various types THz Wave control devices are also emerging in endlessly

These devices mainly focus on THz The frequency of the wave 、 Range 、 Polarization control

And as a THz The phase, one of the important parameters of wave, has gradually become a hot research topic , Therefore, carry out THz The research of phase shifter in wave band is of great significance

         \;\\\;\\\;

THz Phase shifter structure

• Vanadium dioxide $V{O}_2$ Embedded super surface composite structure （ Upper metal layer ）
• The liquid crystal layer
• Vanadium dioxide $V{O}_2$ Embedded subsurface composite structure （ Lower metal layer ）
• silicon dioxide $S{O}_2$ Matrix

Conductivity of metallic copper ：${\sigma }_{copper}=5.8\times 10^{7}S/m$
Metallic copper （ Yellow label ） The thickness of the ：$0.2\mu m$
Vanadium dioxide （ Orange marking ） The thickness of the ：$0.2\mu m$
Relative dielectric constant of silica matrix ：$\epsilon =3.9$, The thickness of the ：$40\mu m$
Liquid crystal layer thickness ：$20\mu m$

   \;

Vanadium dioxide （ Embedded super surface composite structure ） Double rectangular structure （ Upper metal layer ）

The unit period is $P=110\mu m,L_1=70\mu m,L_2=25\mu m,h_1=60\mu m,h_2=15\mu m$

Vanadium dioxide is embedded in the subsurface composite structure （ Lower metal layer ）

Rectangular notch metal copper layer embedded U Type structure vanadium dioxide
$L_3=70\mu m,h_3=40\mu m,L_4=30\mu m$

   \;
Because the top vanadium dioxide is embedded in the upper super surface composite structure, it is directly exposed to the air , So you can go through Laser beam expanding radiation temperature control Method , Regulate the temperature of vanadium dioxide on the top

The lower vanadium dioxide embedded in the lower super surface composite structure can Radiate the expanding laser from the bottom To control the temperature

Finally, the whole phase change material is embedded into the super surface to form a composite structure THz Phase shift of phase shifter , The switching control can be carried out through the upper and lower beam expanding lasers
$$Beam\; expanding\; laser\Rightarrow phase\; shift$$
         \;\\\;\\\;

Research models

Before phase transition
When the temperature is lower than the phase transition temperature 68° when , Vanadium dioxide is High resistance state Relative permittivity ${\epsilon }_i=9$

   \;

After phase transition
When the temperature is higher than 68° when , Vanadium dioxide is High conductivity state

stay THz The frequency band realizes the transformation from dielectric to metal state

The relative dielectric constant is （Drude Model ）
$$\begin{gathered} {\epsilon }_m(\omega )={\epsilon }_{\infty }-\frac{{\omega }_p^2}{(\omega +i/\pi )\omega }i \\ {\omega }_p^2=\frac{N{e}^2}{m^{\ast }\cdot {\epsilon }_0} \end{gathered}$$
among ${\omega }_p$ Is the angular frequency of the plasma ,${\epsilon }_i={\epsilon }_{\infty }=9$, Effective mass $m^{\ast }=2m_e$,$m_e$ It's free electron mass
Carrier mobility $\mu =2c{m}^2/Vs$

   \;

Conductivity of vanadium dioxide before phase transition ：$\sigma =200S/m$
Conductivity of vanadium dioxide after phase transition ：$\sigma =2\times 10^{5}S/m$

The liquid crystal simulation parameters of this paper are ：$n_0=1.52,n_e=1.78$ The former is the usual refractive index of liquid crystal , The latter is the refractive index of liquid crystal after applied electric field

         \;\\\;\\\;

Research methods

By setting the phase transition state of upper and lower vanadium dioxide 、 The refractive index of liquid crystal is taken as the initial condition , stay CST Simulation in software , Refractive index of liquid crystal after applied electric field $n_e$ How much . It also simulates THz Phase shift of phase shifter 、 transmission （transmittance） curve

1. Initial condition upper high conductivity , Lower layer high resistance
2. Initial condition upper high resistance , Lower high conductivity
3. Initial condition upper high resistance , Lower layer high resistance
4. Initial condition upper high conductivity , Lower high conductivity

         \;\\\;\\\;

Conclusion

By changing the temperature of the phase change material, the incident THz The phase shift is dynamically regulated

The simulation shows ,THz The phase shifter is $0.731\sim 0.752THz$（ bandwidth 22GHz） Within the scope of , Can be realized exceed 350° Phase shift of

Change the temperature of phase change materials , The frequency band range of the maximum phase shift ？ from $0.73\sim 0.734THz$ Change to $0.73\sim 0.752THz$

When $f=0.736THz$ when , The maximum phase shift reaches 355.37°

         \;\\\;\\\;

problem

The magnetic field 、 Comprehensive factors of electric field and temperature , What is the effect on this phase-shifting material ？

         \;\\\;\\\;