Abstract
The sub-wavelength THz emission point on a nonlinear electrooptical crystal, used in broadband THz near-field emission microscopy, is computationally modeled as a radiating aperture of Gaussian intensity profile. This paper comprehensively studies the Gaussian aperture model in the THz near-field regime and validates the findings with dual-axis knife-edge experiments. Based on realistic parameter values, the model allows for THz beam characterisation in the near-field region for potential microscopy applications. An application example is demonstrated by scanning over a cyclic-olefin copolymer sample containing grooves placed sub-wavelengths apart. The nature of THz microscopy in the near-field is highly complex and traditionally based on experiments. The proposed validated numerical model therefore aids in the quantitative understanding of the performance parameters. Whilst in this paper we demonstrate the model on broadband electro-optical THz near-field emission microscopy, the model may apply without a loss of generality to other types of THz near-field focused beam techniques.
Highlights
Terahertz imaging offers many attractive advantages over existing imaging modalities especially in its ability to obtain spectroscopic information [1, 2]
The approach is comparatively simpler and does not heavily rely on micro-fabrication technologies. These sub-wavelength THz sources have been investigated in the far-field regime as a radiating aperture of Gaussian profile [26], using semi-analytical techniques commonly applied at microwave frequencies
The Gaussian aperture source is placed inside the crystal surface and the near-field wave propagation is simulated with a electromagnetic simulation tool based on the Finite-Volume Time-Domain (FVTD) method [32, 33]
Summary
Terahertz imaging offers many attractive advantages over existing imaging modalities especially in its ability to obtain spectroscopic information [1, 2]. Focused-beam techniques exploit the micron sized farinfrared pump or probe beam spot for generating or detecting THz radiation respectively to achieve sub-wavelength resolution [18,19,20,21,22,23,24,25]. The approach is comparatively simpler and does not heavily rely on micro-fabrication technologies These sub-wavelength THz sources have been investigated in the far-field regime as a radiating aperture of Gaussian profile [26], using semi-analytical techniques commonly applied at microwave frequencies. The numerical results based on a radiating Gaussian aperture source are validated experimentally, and a practical application of the model to extrapolate the THz beam spot to infer system resolution is demonstrated
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