Terahertz near-field microscopy based on an air-plasma dynamic aperture

Terahertz(THz)near-field microscopy retains the advantages of THz radiation and realizes sub-wavelength imaging,which enables applications in fundamental research and industrial fields.In most THz near-field microscopies,the sample surface must be approached by a THz detector or source,which restricts the sample choice.Here,a technique was developed based on an air-plasma dynamic aperture,where two mutually perpendicular air-plasmas overlapped to form a cross-filament above a sample surface that modulated an incident THz beam.THz imaging with quasi sub-wavelength resolution(approximately λ/2,where λ is the wavelength of the THz beam)was thus observed without approaching the sample with any devices.Damage to the sample by the air-plasmas was avoided.Near-field imaging of four different materials was achieved,including metallic,semiconductor,plastic,and greasy samples.The resolution characteristics of the near-field system were investigated with experiment and theory.The advantages of the technique are expected to accelerate the advancement of THz microscopy.