Near-field THz micropolarimetry

Abstract

We introduce a method for rapid determination of anisotropic terahertz absorption with sub micron resolution and high spectral integrity in the terahertz range. The method is ideal for microscopic and environmentally sensitive materials such as 2-D materials and protein crystals where the anisotropic absorption is critical to understanding underlying physics. We introduce the idea of using an iso-response relationship between the THz polarization and electro optic probe polarization to enable stationary sample polarization measurements covering a full 2π polarization dependence measurement.

Fig. 1.

Schematic for proposed Ideal Anisotropic Terahertz Microscopy.

Fig. 2.

Electro optic response R(θ_THz,ϕ_NIR) surface plot for [110] cut ZnTe. For ATM, the sample is rotated and θ_THz, ϕ_NIR and R(θ_THz,ϕ_NIR) remain constant at a peak of the surface such as indicated by the light blue dot. For PV-ATM the sample is stationary, ϕ_NIR is constant and θ_THz is rotated, indicated by the solid white line. For IATM the sample is stationary and to ensure constant EO responsivity, θ_THz and ϕ_NIR are rotated synchronously following an iso-response curve, such as the solid black curve or orange dashed curve. See Data File 1 for iso response curve angle values.

Fig. 3.

Calculated Δabs for two anisotropic resonance for model samples. For model sample 1 (a) ATM method (b) IATM iso-response curve 1 and (c) IATM iso-response curve 2. For model sample 2 (d) ATM method (e) IATM iso-response curve 1 and (f) IATM iso-response curve 2.

Fig. 4.

ATM Δabs angular dependence at resonant frequency. (a) model sample 1 and (b) model sample 2. Green lines for 90 cm⁻¹ resonance, Black lines for 50 cm⁻¹ resonance and blue lines for 70 cm⁻¹ resonance. Dashed lines in (a) show fits to a narrow band polarizer model as discussed in text.

Fig. 5.

IATM Δabs angular dependence at resonant frequencies. (a) and (b) are for model sample 1 using the iso-response curve 1 and curve 2 respectively. (c) and (d) show the same as (a) and (b) respectively, but with Model Sample 2. The 50 cm⁻¹, 70 cm⁻¹, and 90 cm⁻¹ angle dependence spectra are shown by the black, green, and blue lines, respectively. The corresponding angle dependences of the ATM spectra are shown by the dotted lines and like colors. Additional peak directions and shifts in peak directions are found, leading to ambiguous or incorrect assignment of the dipole transition direction.

Fig. 6.

Slopes of the R(θ_THz,ϕ_NIR) along specific iso-response curves.

Fig. 7.

IATM using minimized slope iso-response curve for mode sample 1. (a) Δabs(ν,θ) surface plot curve for model sample 1. (b) Angular dependence of Δabs at 90 cm⁻¹ (green) and at 50 cm⁻¹ (black). The plot shows both ATM result (dashed) and IATM result (solid).