An oral treatment unit for confining and modulating an electromagnetic field having a frequency in the range of 1MHz to 300 GHz and adapted to be inserted into the oral cavity of a user, the oral treatment unit comprising: a first cleaning element coupled to or integrated in the oral treatment unit; and an electromagnetic field focusing element coupled to or integrated in the oral treatment unit and adapted to receive an electromagnetic field having an initial frequency and output a focused electromagnetic field, wherein the electromagnetic field focusing element comprises a first metamaterial structure, wherein the first metamaterial structure comprises a plurality of unit cells and has a negative or near-zero dielectric permittivity and/or near-zero magnetic permittivity and/or negative or near-zero refractive index thereby to provide a first passive electromagnetic field focusing function, which confines the received electromagnetic field at a given distance from the first metamaterial structure by performing one or more of: an attenuation rate control function in the near-field region of the first metamaterial structure; or a propagation direction modification function on the electromagnetic field; and wherein the electromagnetic field focusing element has a modulation function that modulates the confined electromagnetic field, thereby generating a modulated electromagnetic field, wherein the electromagnetic field focusing element comprises a mechanical actuator for performing the modulation function, wherein the mechanical actuator is adapted to impart a mechanical oscillation to the first metamaterial structure. a first cleaning element coupled to or integrated in the oral treatment unit; and an electromagnetic field focusing element coupled to or integrated in the oral treatment unit and adapted to receive an electromagnetic field having an initial frequency and output a focused electromagnetic field, wherein the electromagnetic field focusing element comprises a first metamaterial structure, wherein the first metamaterial structure comprises a plurality of unit cells and has a negative or near-zero dielectric permittivity and/or near-zero magnetic permittivity and/or negative or near-zero refractive index thereby to provide a first passive electromagnetic field focusing function, which confines the received electromagnetic field at a given distance from the first metamaterial structure by performing one or more of: an attenuation rate control function in the near-field region of the first metamaterial structure; or a propagation direction modification function on the electromagnetic field; and an attenuation rate control function in the near-field region of the first metamaterial structure; or a propagation direction modification function on the electromagnetic field; and wherein the electromagnetic field focusing element has a modulation function that modulates the confined electromagnetic field, thereby generating a modulated electromagnetic field, wherein the electromagnetic field focusing element comprises a mechanical actuator for performing the modulation function, wherein the mechanical actuator is adapted to impart a mechanical oscillation to the first metamaterial structure.