Rotation of terahertz radiation due to phonon-mediated magnetoelectric coupling in chiral selenium

Anirban Pal, Sharmila N. Shirodkar, Prathamesh Deshmukh, Harshad Surdi, Bagvanth R. Sangala, S. S. Prabhu, Kailash C. Rustagi, Umesh V. Waghmare, and Pushan Ayyub

Phys. Rev. B 98, 235141 – Published 20 December 2018

Abstract

The trigonal form of selenium (t-Se) has an unusual, quasi-one-dimensional, chiral crystal structure. First-principles calculations have helped us uncover a polar, optic phonon in t-Se that exhibits a diagonal magnetoelectric coupling with the electric field of the incident THz radiation, and induces a parallel magnetic field due to its inherent chirality. We show that this phonon-mediated, magnetoelectric mechanism is predicted to cause optical rotation in t-Se in the 1–3 THz range and is quite distinct from the high frequency (>80 THz) activity due to electronic excitations that has been reported previously. In the second part of the paper, we report our experimental results based on THz time-domain spectroscopy as well as direct measurements of optical rotation that confirm this prediction in an aligned, monocrystalline array of t-Se microrods. The Se microrod array not only exhibits a large birefringence ( $Δ n = 1.3$ ) but also rotates the polarization of THz radiation by ∼3°/mm. To our knowledge, this is the only elemental solid known to rotate THz polarization, because the currently available THz rotators are based mainly on liquid crystals or metamaterials. The identification of new THz-active materials and a better understanding of the underlying physics are both clearly essential to the development of better sources, detectors and components.

Received 23 January 2018
Revised 22 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.235141

Physics Subject Headings (PhySH)

Magnetoelectric effect Optical & microwave phenomena Polarization of light

Elemental materials Single crystal materials

Density functional theory Optical techniques

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Anirban Pal¹, Sharmila N. Shirodkar², Prathamesh Deshmukh¹, Harshad Surdi¹, Bagvanth R. Sangala¹, S. S. Prabhu¹, Kailash C. Rustagi¹, Umesh V. Waghmare², and Pushan Ayyub^1,*

¹Department of Condensed Matter Physics & Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India
²Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India

^*Author to whom correspondence should be addressed: pushan.ayyub@gmail.com

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Issue

Vol. 98, Iss. 23 — 15 December 2018

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Images

Figure 1
Phonon modes in trigonal Se that couple with electric field applied along (a) $x$ , (b) $y$ , and (c) $z$ directions. The third one involves rotation of Se helical chain about its axis. (d) The position vectors of the Se atoms with respect to the center of the helix are indicated by arrows. (e) Calculated frequency dependence of the angle of rotation per unit length, φ/ $l$ , with (solid line) and without (dashed line) correction for finite phonon line width ( $\approx 7 c m^{- 1}$ ). The electric field of the incident radiation is along the axis of the Se helix. (f) Flowchart depicting the mechanism of polarization rotation in a chiral solid such as trigonal Se.
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Figure 2
(a) Scanning electron micrograph of a typical, perfectly faceted, hexagonal microrod of vapor-deposited, monocrystalline Se (diameter ≈ 50 μm). (b) Powder x-ray diffractogram showing the pure trigonal/hexagonal phase of Se ( $SG = 152$ or $P 3_{1} 21$ ). The Al lines are from the sample holder. (c) Electron diffraction pattern confirming the monocrystalline trigonal structure.
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Figure 3
(a) Schematic diagram of the time-domain spectroscopy (TDS) setup, consisting of the THz emitter, sample assembly, electro-optic detector, and associated optics. Legends: $M = plane mirror$ , $PM = parabolic mirror$ , $BS = beam splitter$ , $PCA =$ photoconducting antenna, $EOS = electro - optic sampling$ , $BPD = balanced photodiodes$ . (b) THz source spectra before and after passing through the microrod array sample.
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Figure 4
THz time domain spectra, i.e., contour plots of the amplitudes of the transmitted intensity as a function of the relative time delay, recorded at all polarization angles from 0° to 360°, for (a) an amorphous Se plate, and (b) a Se micro-rod array (color scales shown beside the spectra). The corresponding line scans for the micro-rod array at a few selected angles are shown in (c).
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Figure 5
(a) Experimental setup for measuring the optical rotation due to Se microrod array (sample rotated) or polycrystalline Se (analyzer rotated). (b) The experimental geometry pertaining to this problem, showing the orientations of the Se microcrystal axes with respect to the polarization vectors of the THz radiation at the point of incidence into the Se microrod and just after emerging from it.
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Figure 6
(a) Angular dependence of the intensity transmitted through the microrod array. (b) Angular shift showing the polarization rotation produced by a polycrystalline Se sample. (c) Thickness dependence of the THz rotation produced by polycrystalline Se.
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