Fig. 1. Configuration of the EOT metamaterials for the THz region. (a) Schematic of the all-optical, reconfigurable, non-volatile phase-change metamaterials induced EOT switch: single nanosecond pulsed laser transits a 100 nm thick GST225 film reversibly between the amorphous and crystalline states. (b) The FEM simulated transmission spectra of the chalcogenide metamaterials with the amorphous state at the various diameter of d = 40, 60, and 80 μm (top panel) and various heights of h_Au = 50, 80, 100, 200, and 300 nm (bottom panel). (c) Optical microscope (top panel) and FIB cross-sections (bottom panel) images of the EOT metamaterials. The geometrical parameters of the subwavelength holes array are p = 100 μm, d = 60 μm, respectively; the thicknesses of the Au and GST225 layers are h_Au = 0.2 μm and h_GST= 0.1 μm, respectively. (d) The temperature-dependent optical conductivity (σ) of the 100 nm thick GST225 film. (e) The behavior of resonant transition in the metamaterials: numerical simulated (top panel) and experimental measured (bottom panel) transmission spectra at the various temperatures ranging from 25°C to 300°C. The reduction of the peak intensity can be experimentally and theoretically observed by increasing the temperature. (f) The modulation efficiency against the annealing temperature varied from 25 °C to 300 °C.

Fig. 2. In sequence processing for the reversible state change. (a) Schematic of the reversible state change of the GST225 layer hybridised with an EOT metamaterials: the AD-AM GST225 is initially heated above T_C=250 °C to switch to the CR-GST225 via a hot plate. A single ns pulsed laser is transited to thermally anneal the CR-GST225 layer above T_M=600 °C that reamorphises the CR-GST225. Consequent quenching leads to the MQ-AM GST225. A temperature above T_C=250 °C but below T_M=600 °C is needed to recrystallise the MQ-AM GST225, which is achieved by using a hot plate. (b) The σ of 100 nm thick GST225 film at the various structural states of the as-deposited amorphous (AD-AM, black line), crystalline (CR, red line), melt quenched amorphous (MQ-AM, grey line), and re-crystallised (R-CR, orange line) over a spectral range of 0.2-1.8 THz. Experimental realisation of reversibly tunable EOT effect: the THz-TDS measurement of the transmission spectra of the chalcogenide metamaterials with the various structural phases of (c) AD-AM, CR and (d) MQ-AM, and R-CR.

Fig. 3. Numerical simulation of total E- field distribution along the x-z plane of the metamaterials at the various temperatures of (a) 25 °C, (b) 150 °C, (c) 200 °C, and (d) 300 °C.

Fig. 4. (a) Measured transmission spectra of the EOT chalcogenide metamaterials for twenty switching times. (b) The values of transmission peaks for the amorphous (shown by black dots) and crystalline (indicated by red dots) states with twenty switching times. The morphology (top panel) and cross-sectional (bottom panel) images of the chalcogenide metamaterials (c) before and (d) after 20 transition times.