Real-time terahertz near-field microscope

Abstract

Here, we report a real-time THz near-field microscope with a high dynamic range that can capture images of a 370 x 740 µm 2 area at 35 frames per second. We achieve high spatial resolution on a large area by combining two novel techniques: THz pulse generation by tilted-pulse-front excitation 7 and electrooptic (EO) balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a THz pulse.Our results are the first demonstration of a direct quantification of a 2-dimensional subwavelength THz electric field taken in real-time.The very first demonstration of THz imaging 8 clarified that microscopic resolution is required for applications ranging from semiconductors inspections 9 to biomedical imaging 3,4 . For a corresponding wavelength at 1 THz, however, the far-field spatial resolution of an image is restricted to the Rayleigh criterion that corresponds to ∼180 µm in vacuum. To overcome this limit, we must work in the near-field region of a sample. In this case, the THz wave has to be captured very close to the sample surface in a region before light diffraction occurs 10 . Many schemes have been proposed since the first demonstration of THz near-field imaging 11 , most of which involve a subwavelength probe or a aperture placed very near the sample 9,11-17 .

Near-field terahertz imaging of a discontinuity in split ring resonator array

“…We used a THz microscope, as previously reported [5]. A large probe beam at 800 nm illuminated a LiNbO 3 (LN) crystal used for the purpose of two-dimensional electro-optic (EO) imaging in the reflection scheme.…”

“…When dealing with THz images, a very thin EO crystal is needed in order to preserve the high spatial resolution of the optical elements. This constraint enables the THz field to be collected from a sample before diffraction [5]. The losses in sensitivity caused by a thin EO crystal were compensated by a tightly focused pulse ranging from 0.1 to 2.5 THz and with a peak field of 600 kV/cm [6].…”

Full vectorial mapping of the complex electric near-fields of THz resonators

“…[3][4][5][6][7][8] Attempts to measure and characterize near-fields were done first at optical frequencies, which paved the way for near-field detection techniques at THz frequencies. [9][10][11][12][13][14][15][16][17][18][19][20] Owing to the relatively long wavelengths of THz radiation (∼100's of microns), the creation of highly confined THz local fields in small volumes is critical for enhancing the interaction of THz waves with matter. The use of resonant structures for this purpose has been the topic of many interesting studies, especially for spectroscopy of deep-subwavelength structures.…”

Extremely large extinction efficiency and field enhancement in terahertz resonant dipole nanoantennas

“…In addition, the layer of gold, onto which the slots are engraved, compromises the overall transparency of the host medium. Very recently, linear wire [25,26] and bowtie [27] dipole antennas at THz frequencies have been proposed. However, their very large lateral dimension, of the order of tens of microns, have limited the antenna field enhancement factor to 2.6 [25], and the antenna extinction efficiency to ~5 [27].…”

“…Very recently, linear wire [25,26] and bowtie [27] dipole antennas at THz frequencies have been proposed. However, their very large lateral dimension, of the order of tens of microns, have limited the antenna field enhancement factor to 2.6 [25], and the antenna extinction efficiency to ~5 [27]. The half-wavelength THz nanoantennas proposed in this work show instead local field enhancement factors and extinction efficiencies of the order of hundreds.…”