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Terahertz quantum sensing

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Science Advances  13 Mar 2020:
Vol. 6, no. 11, eaaz8065
DOI: 10.1126/sciadv.aaz8065


  • Fig. 1 Scheme and nomenclature for the theoretical analysis.

    In addition to a laser pump (for simplification, not drawn here), the signal (s1) and idler (i1) input modes enter the nonlinear crystal (NL). The interaction in the crystal leads to the generation of signal and idler photons in the output modes s1 and i1, respectively. They are separated by an indium tin oxide (ITO)–coated glass. Afterward, the signal radiation and the pump beam are reflected back into the crystal by the mirror Ms. The input modes for the second passage are denoted by i2 and s2, which is, because of the alignment, equal to s1. The idler mode i1 passes through the object (O), is reflected by the mirror Mi, and propagates through the object again. This acts as a beam splitter (BS) with second input mode 3 and output modes i1 and 3. Aligning the idler beams, the mode i1 corresponds to i2. The output modes after the second passage are s2 and i2. Last, the signal radiation (in mode s2) is detected by the detector. The inset shows the simulated interference signal in the Stokes (red) and anti-Stokes (blue) regions based on the detailed model (see Materials and Methods).

  • Fig. 2 Schematic of the experimental setup.

    A continuous-wave laser with a wavelength of 659.58 nm is reflected by a VBG (VBG1) into the interferometer part of the setup through a zero-order half-wave plate (λ/2) controlling the polarization. It is then focused by a lens f1 into a periodically poled 1-mm-long MgO-doped LiNbO3 (PPLN) crystal generating signal and terahertz photons that are separated by an ITO. Signal and pump radiation are reflected at Ms directly into the crystal. The terahertz radiation passes the object twice, being reflected by a moveable mirror Mi. In the second traverse of the pump through the PPLN, additional signal and idler photons are generated. Afterward, the lens f1 collimates the pump and signal radiation for the detection starting with filtering the pump radiation by three VBGs and spatial filters (SF). To obtain the frequency-angular spectrum, the signal radiation is focused through a transmission grating (TG) by the lens f2 onto a sCMOS camera. The inset shows a frequency-angular spectrum for the used crystal (poling period Λ = 90 μm, pumped with 450 mW). The scattering angle corresponds to the angle after the transmission from the crystal to air.

  • Fig. 3 Terahertz quantum interference.

    In the collinear forward spot of the signal, interference is observed in the (A) Stokes and (B) anti-Stokes regions. (C and D) Corresponding FFTs peaks at about 1.26 THz. By placing an additional ITO glass in the idler path, no interference can be observed, and the peaks in the FFTs disappear.

  • Fig. 4 Terahertz quantum sensing.

    The envelope of the interference is shifted depending on the thickness of the PTFE plate in the (A) Stokes and (B) anti-Stokes parts. (C) Thickness of the PTFE plate measured by quantum interference over PTFE thickness measured by a micrometer caliper. The solid line is the angle bisector. The horizontal error bars (hidden by the data points) take into account the uneven thicknesses of the PTFE plates and the inaccuracy of the reference measurement. The vertical error bars result from the precision of determining the shift of the envelope center of the interference.

  • Fig. 5 Investigation of induced emission by idler radiation generated in the first passage.

    The count rates of the collinear Stokes (red, cross) and anti-Stokes (blue, plus) regions are shown for the cases of the idler beam being blocked (crosses) and unblocked (colored dots) for various pump powers. The triangles are the ratios between the unblocked and blocked cases. All values are close to a ratio of 1.00 (dashed line), which lies in the range of all error bars.

  • Fig. 6 Idler angular distribution.

    The idler angular density Γ(θi) corresponding to collinear signal angles is plotted as a function of the idler angle inside the PPLN crystal θi for three different pump radii.

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