Terahertz Metamaterials

Terahertz radiation has a frequency of 0.1 to 10 THz and is located between microwaves and infrared in the electromagnetic spectrum, in the transition region from electronics to photonics. The photon energy of terahertz radiation is very low, which will not cause damage to the measured material and can be detected without damage.

Basic Concepts and Characteristics

Terahertz radiation, with a frequency of 0.1~10 THz, is located between microwave and infrared in the electromagnetic spectrum, in the transition region from electronics to photonics. The development of terahertz time-domain spectroscopy based on ultrafast lasers has promoted the rapid development of terahertz technology. The photon energy of terahertz radiation is very low, which will not cause damage to the measured material and can be detected without damage; it is transparent for most dielectric substances and can be used for transmission imaging; it can simultaneously measure the amplitude and phase of the terahertz electric field, which can further directly obtain the sample's complex refractive index, complex permittivity, and complex conductivity, and it can realize the kinetic analysis with femtosecond time resolution; many condensed matter systems phonon and other meta-excitations of many condensed matter systems, as well as the vibrational and rotational energy levels of many biological macromolecules are in the terahertz band, thus allowing for the detection and fingerprinting of matter through characteristic resonances.

However, there are relatively few functional devices in the terahertz band, limiting the further development of terahertz technology. Metamaterials can achieve flexible and diverse control over the amplitude, phase, polarization, and propagation of terahertz waves, thus providing an effective way to realize terahertz functional devices. On the other hand, terahertz time-domain spectroscopy can simultaneously detect the amplitude and phase of the electric field, which can more comprehensively measure the electromagnetic response properties of metamaterials, thus, the development of terahertz technology and metamaterials are complementary to each other.

Composition of terahertz metamaterials

-Basic Components and Sensors

Metamaterials generally consist of subwavelength metallic structures fabricated on a dielectric or semiconductor substrate. The conductivity of the metal affects the strength of the metamaterial's resonance; the better the conductivity, the stronger the resonance; in addition, an increase in the thickness of the metal over a certain range also strengthens the resonance, and the thickness dependence varies for different metals.

The substrate material can be rigid or flexible; the smaller the absorption of the substrate, the better, to obtain a strong transmission response. The presence of the substrate leads to a resonance redshift; the larger the dielectric constant of the substrate, the lower the metamaterial resonance frequency. When the thickness of the substrate is much smaller than the wavelength, the substrate thickness also affects the resonance frequency of the metamaterial, and an increase in thickness causes a resonance redshift. At this time, the substrate and air can be regarded as an equivalent substrate, and the increase in thickness increases the dielectric constant of the equivalent substrate and thus leads to the resonance redshift.

Selection of constituent materials and tunable metamaterials

Materials whose dielectric properties are tunable under applied excitation, such as semiconductors, superconducting materials, thermosensitive materials, phase change materials, ferroelectric materials, etc., can be realized as tunable metamaterials by joining with metals to form a subwavelength structure or by acting as a substrate for a metallic structure. Some materials can even be made into non-metallic metamaterials instead of metals.

Semiconductors can control the concentration of carriers by light and charge temperature excitation, thus changing the dielectric properties, and are widely used in making tunable metamaterials. Among them, the most studied is optical excitation. Optical excitation generates photogenerated carriers, and the concentration of photogenerated carriers can be controlled by the power of the excitation light or the relative delay time between the excitation light and the incident terahertz wave. A photosensitive semiconductor as a substrate can be used as an ultrafast optical switch by controlling the resonance intensity of the metamaterial through optical excitation to realize dynamic modulation of terahertz wave transmission. The photogenerated carriers simultaneously lead to the reduction of terahertz wave transmission in the non-resonant frequency domain, thus playing a dual role in the terahertz transmission in the resonant frequency domain, where the enhancement of the substrate reflection decreases the transmission and the weakening of the resonance strength increases the transmission.