1 . A controllable wave-absorbing metamaterial, comprising: a substrate; and a metamaterial unit array layer disposed on the substrate and comprising a plurality of conductive geometric units that are periodically arranged, each of the conductive geometric units comprising a first hollow structure, second hollow structures, and conductive geometric structures; wherein the second hollow structures are respectively extended from four vertices of the first hollow structure, and the conductive geometric structure is disposed between each two adjacent second hollow structures; and each of the second hollow structures comprises a first end facing toward the center of the conductive geometric unit and a second end facing away from the center of the conductive geometric unit; wherein the first end of the second hollow structure is provided with a varactor diode connected to the conductive geometric structures at both sides, the second end of the second hollow structure is provided with a fixed capacitor and a fixed resistor that are spaced apart from each other; the fixed capacitor is connected to the conductive geometric structure at one side, and the fixed resistor is connected to the conductive geometric structure at the other side.
2 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein a metal material is disposed between the fixed capacitor and the fixed resistor, and the metal material is in contact with the fixed capacitor and the fixed resistor.
3 . The controllable wave-absorbing metamaterial as claimed in claim 2 , wherein the fixed capacitor and the fixed resistor are connected in series through the metal material, and the varactor diode is arranged in parallel with the fixed capacitor and the fixed resistor that are connected in series.
4 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the shape of the conductive geometric structure is half of the regular octagon that is divided along the central axis of the regular octagon.
5 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the two opposite conductive geometric structures are symmetrical about a central axis passing through the center of the conductive geometric unit.
6 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the first hollow structure comprises: two metal wires; wherein two ends of each metal wire are respectively in contact with conductive geometric structures that are opposite to each other, one of the metal wires is provided with a bent portion that is disposed in the substrate, the other one of the metal wires is disposed above the bent portion, such that the two metal wires are not in contact.
7 . The controllable wave-absorbing metamaterial as claimed in claim 6 , wherein the projection of the two metal wires is a cross structure.
8 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein one side of the second hollow structure is connected to the first hollow structure, and the other side of the second hollow structure is connected to a third hollow structure.
9 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the substrate comprises: a reflective layer; and a honeycomb structure layer; wherein the honeycomb structure layer is disposed above the reflective layer.
10 . The controllable wave-absorbing metamaterial as claimed in claim 1 , further comprising: a wave-transparent protective layer, wherein the wave-transparent protective layer is disposed on the metamaterial unit array layer.
11 . The controllable wave-absorbing metamaterial as claimed in claim 9 , wherein the thickness of the honeycomb structure layer is 7 mm, and the thickness of the reflective layer is 0.1 mm.
12 . The controllable wave-absorbing metamaterial as claimed in claim 10 , wherein the thickness of the wave-transparent protective layer is 0.8 mm, the thickness of the metamaterial unit array layer is 0.04 mm.
13 . The controllable wave-absorbing metamaterial as claimed in claim 9 , wherein the honeycomb structure layer is replaced by PMI (polymethacrylimide) layer or PVC (polyvinyl chloride) layer.
14 . The controllable wave-absorbing metamaterial as claimed in claim 9 , wherein the material of the reflective layer is carbon fiber or metal film.
15 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the shape of the conductive geometric unit is square.
16 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the resistance of the fixed resistor is 5005, the capacitance of the fixed capacitor is 1 pF, and the capacitance of the varactor diode is 1-16 pF.
17 . The controllable wave-absorbing metamaterial as claimed in claim 1 , wherein the side length of the conductive geometric unit is 30 mm, the distance between two opposite sides of the conductive geometric structure is 18.2 mm, the side length of the second hollow structure is 7.5 mm, the width of the second hollow structure is 3 mm.
18 . The controllable wave-absorbing metamaterial as claimed in claim 6 , wherein the width of each metal wire is 1 mm.
19 . The controllable wave-absorbing metamaterial as claimed in claim 6 , wherein the conductive geometric unit comprises four conductive geometric structures, four fixed resistors, four fixed capacitors, four varactor diodes, and the two metal wires.
