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The experimental schematic for a general THz-TDS setup using the polarimetry method as described in the text. Four off-axis paraboloid mirrors guide and focus the THz pulse from the emitter, through the sample (gray), and then to the detector. Polarizer positions are indicated in red with an adjacent circle that illustrates their orientation if viewed along the beam path. The first fixed polarizer is used to clean up the vertically polarized THz pulse. The rotating polarizer and the second fixed polarizer are placed after the sample and are used to measure its optical activity. The THz setup is purged with N2, and additional details of the setup are described in the text.
The rotation angle φ(ω) of each frequency component of a THz pulse may be determined by measuring the transmission amplitude as a function of the polarizer angle (θ). After measuring the transmission spectrum at a set of polarizer angles and transforming these spectra into the frequency domain, φ(ω) may be obtained using the expression in Eq. (2) as a fitting function. In part (a), the value of φ(ω) for a particular frequency (1.23 THz) is extracted from each of four sets of measurements: free space, silicon, left-handed spirals, and right-handed spirals (black circles, gray squares, blue diamonds, and red triangles, respectively). When this procedure is carried out for every frequency in the experimental bandwidth, the result is a spectrum of optical rotation angles, as shown in the dashed traces of part (b). The solid traces in part (b) of the figure illustrate the fact that the values of φ(ω) may also be obtained by using a fitting function (Eq. (5)) in the time domain and Fourier transforming the resulting pair of time-domain amplitudes, as described in the text.
A vertically polarized THz pulse has both vertical (Ax ) and horizontal (Ay ) amplitude components after transmission through an array of right-handed spirals. The inset displays the relative amplitudes of these components in the frequency domain. These data were collected with the continuously spinning polarizer technique described in the text. At the mid-left of the figure, the first 3 ps are shown at 20 times higher sensitivity to illustrate the improved signal to noise ratio obtained using a double-modulation variation of the data acquisition.
The rotation angle φ(ω) and signed ellipticity ɛ ± (ω) of each frequency component in the THz pulse are obtained in a single time-domain measurement using the spinning polarizer method. The values of φ(ω) (black line) and ɛ ± (ω) (blue and red line) are plotted above. The same information is illustrated below the plot, where the polarization state of the light at each particular frequency is depicted. In these figures, blue indicates left-handed ellipticity and red indicates right-handed. For example, the light at 1.10 THz is left-hand elliptical but not rotated, while the light at 1.25 THz is rotated approximately 25° but is linearly polarized.
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