The vast majority of data-storage devices are based on ferroelectric or magnetic materials. [1,2] The magnetic-memory effect is derived from the spin order, while the ferroelectric memories are based on the spontaneous polarization of electric dipoles. In the quest for superior efficiencies, the operating speed of the devices based on these memories has become a major focus of research. The time needed for the memory parameter to respond to ultrafast electric, magnetic, or optical stimuli is vital to determine its operating speed. [3][4][5] The ultrafast dynamics of spin (magnetic) memory over a picosecond time scale upon femtosecond excitation is well established. [3][4][5][6][7][8][9][10][11] Upon femtosecond excitation, the spin reorientation or the ultrafast demagnetization in ferromagnetic and antiferromagnetic materials occurs in the time scale of 1-2 ps, which suggests that when subjected to ultrafast stimuli, the magnetic memory can be manipulated at an exceptionally high speeds. However, the analogous situation of the ultrafast functionality of spontaneous polarization in ferroelectric/multiferroic materials remains unknown.The use of terahertz (THz) radiation-based spectroscopic techniques [12][13][14][15][16][17][18] has uncovered phenomena in a wide range of materials, such as semiconductors, [15,16] strongly correlated electron systems, [11][12][13][14][15] magnetic films, [9][10][11] and biological molecules.[19] The emission of THz radiation provides a direct measure of transient change in electric/magnetic fields over a picosecond time scale. [9][10][11][12][13][14][15][16][17] In any medium, THz emission according to the Maxwell wave equation is governed bywhere E, eô, m, and t are the electric field, electric susceptibility of free space, magnetic permeability, and time, respectively. In this relationship, THz emission can occur through a time-varying current density (J) and/or polarization (P). The polarization can be decomposed into P ¼ P S þ P NL , which indicates that THz emission can be due to time-varying spontaneous polarization P S (such as partial/complete depolarization) and/or optical rectification in non-linear medium (P NL ). [12,13,16,17] While the latter is a well-established source of THz emission, [13,16] the effect of P S is not known. Terahertz emission due to P S is of great importance, as it is a direct means of probing the ultrafast polarization dynamics of ferroelectric memories. [20] It should also provide a better understanding of the time scale at which the electric dipoles in the spontaneous polarized state are accessible for read or write operations. This time scale is crucial in determining the operating speed of ferroelectric memory devices, thus paving the way to improve their efficiency and ultimately resulting in their widespread adoption in various technological applications. To realize this, it is essential to utilize a ferroelectric system with a large P S . A choice popular with many researchers is the room-temperature ferroelectric antiferromagnet, BiFeO 3 ....

We investigate the nonlinear effect of a planar superconducting metamaterial made from niobium nitride (NbN) at terahertz frequencies. As the variation of the incident intense terahertz field alters the intrinsic conductivity in the NbN, a consequent giant amplitude modulation is observed due to the strong nonlinearities. The high sensitivity of the chosen metamaterial even allows observing the nonlinear behaviors at various temperatures, but the resonance modulation induced by the nonlinear effect was distinct from that induced by the heating effect. The presented results illustrate a clever implementation of strongly enhanced nonlinearities and thus may bring nonlinear metamaterials into novel applications.

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