A study of background signals in terahertz apertureless near-field microscopy and their use for scattering-probe imaging

Abstract

Apertureless near-field microscopy is an imaging technique in which a small metal tip is held close to a surface, converting evanescent waves to propagating waves and permitting extreme subwavelength spatial resolution. This technique has recently been adapted for use in the terahertz region of the spectrum. Here, the interpretation of the measured signals and the suppression of background scattering can be complicated by the extremely broad bandwidth of the terahertz source and by the coherent (i.e., phase-sensitive) detection of the scattered radiation. We have analyzed the use of tip-sample distance modulation for the removal of background signals. We find that significant background signals, originating from scattering off the probe tip, can be observed even after modulation. These background signals result from path-length difference modulation, and thus are relevant when phase-sensitive detection is used. We use a dipole antenna model to explain the spatial variation in the scattered signal. Since this signal originates from the tip only, it can be used to characterize free-space terahertz wave fronts with subwavelength resolution.

Review of Near-Field Terahertz Measurement Methods and Their Applications

“…One can see that at a fixed frequency, such as 1 THz, a strong contrast is expected between doping concentrations in the range of 10 17 -10 19 cm −3 . The Mittleman group performed an interesting study on the background signal of these small metal tips when illuminated by a THz pulse [111]. The modulation of the probe/sample distance is responsible of this additional signal to the classical signal, expected from the general approach of ANSOM measurement.…”

The transition from a TEM-like mode to a plasmonic mode in parallel-plate waveguides

“…Our experimental approach is based on scattering probe imaging, 15,16 a technique for characterizing the spatial distribution of a THz field with subwavelength resolution. The concept is similar to apertureless near-field scanning optical microscopy ͑ANSOM͒.…”

Characterization of the terahertz near-field output of parallel-plate waveguides

“…This technique, illustrated in Fig. 1(c), is known as scattering-probe imaging [42]. Here, a sharp tungsten probe needle is mounted on a piezoelectric actuator, vibrating along the z direction with an amplitude of 20 μm and a frequency of 160 Hz.…”

“…This detection technique is similar to apertureless near-field microscopy, applied to the case of free-space propagating fields. The spatial resolution is limited by the size of the probe tip, and can therefore be much smaller than the free-space wavelength [27,42].…”