Finite Element Modeling of Near Field Optics

In many THz microspectroscopy configurations, detection of a transmitted THz beam is performed in the far-field. This is often done to facilitate environmental control of the sample, such as the humidity needed for protein crystals or vacuum conditions needed to isolate properties of semiconductor materials. Sample sizes of only 100-300 μm are typically used to measure the structural dynamics in fingerprinting protein dynamics[1][2], for example. This presents a need to use a subwavelength aperture to filter out the components of the beam that do not pass through the sample is paramount. Post-measurement corrections of artifacts arising in the detected absorbance spectrum account for known mechanisms such as etalon or signal drift. However, a loss of spectral fidelity is observed in the far field whose mechanisms remain largely unknown.Frequency-dependent loss tangent and dielectric permittivity plot for c-cut single crystal sucrose along the a-axis and schematic of a sample mounted on aperture with an incident Gaussian terahertz beam focused at the aperture plane.

The electric field of a THz beam focused on an aperture-mounted sample with known absorption properties may be modeled by finite element methods (FEM) to examine physical mechanisms responsible for various artifacts arising in the spectral content detected in the far-field. Spectroscopic calibration of a THz microspectroscopy instrument may be facilitated with single crystal sucrose, an excellent material choice as an asborbance standard as it is readily manufactred and presents well-established resonances in the THz range.[3] We presently simulate the electric field of a focused Gaussian THz beam with Ansys HFSS using the absorbance spectrum of c-cut single crystal sucrose1

Through a complete understanding of mechanisms that give rise to absorbance artifacts in the detected absorbance spectrum of modeled samples, it is presently known that only near-field detection schemes sufficiently preserve the spectral fidelity of the material. Measurements made in the far-field are subject to absorbance artificats unrelated to the absorbance spectrum of the material.

 

  • 1. (Full citation pending) T. LaFave, A. Lee, T.-Y. Kao, A. Markelz, "THz Transmission Through Submillimeter Apertures," IRMMW , Nov 7-23, Buffalo, NY (2020).

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