Research ArticleAPPLIED OPTICS

Noninvasive, near-field terahertz imaging of hidden objects using a single-pixel detector

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Science Advances  03 Jun 2016:
Vol. 2, no. 6, e1600190
DOI: 10.1126/sciadv.1600190
  • Fig. 1 Schematic illustration of near-field, single-pixel THz imaging.

    The imaging scheme: An 800-nm pump pulse is spatially modulated and used to photoexcite a semiconducting wafer, which transfers the spatial encoding mask onto a coincident THz pulse. The subsequent THz pulse is then passed through an object onto a single-pixel detector.

  • Fig. 2 Pulsed THz imaging.

    (A) Electric field of our THz pulse recorded in the time domain using electro-optic sampling. The arrow shows the measurement point, at the peak of the THz field, for which images are recorded. (B) Normalized Fourier transform of our THz pulse. The central wavelength is approximately 375 μm, with a full width at half maximum of 840 μm. (C) Optical image of a resolution test target. Au marks the regions spanned by the gold film, whereas the regions that are marked “Si” show the exposed silicon wafer. (D) Image (128 × 128 THz) of the resolution test target in (C) obtained via a full set of Hadamard masks. The pixels are 20 μm in size. The arrows indicate the imaging resolution, evaluated as the maximal distance between the arms of the cartwheel for which the image contrast is diminished due to diffraction.

  • Fig. 3 Hadamard versus random versus raster imaging.

    (A) Circuit board design, where black indicates conducting, metallic regions. The individual wires are 50 μm in width, and 8-μm breaks have been introduced at points marked by the letters A and B. (B) Image acquired using raster scanning of a single opaque pixel. (C and D) Comparison of the same image acquired with a full set of masks derived from random and Hadamard matrices, respectively. (E and F) Compressed images obtained via random masks, where the number of measurements is 25% (E) and 75% (F) of the total number of pixels (we use a total variation minimization image recovery algorithm; see section S6 for more details). In all images, the THz electric field is polarized horizontally, and the number of pixels is 64 × 64 with 40-μm pixels. The signal acquisition time for a single measurement is 500 ms. Because of the considerably larger noise in the measurement, we have scaled the image in (B) by 0.25 to use the same color scale as in (C) and (D).

  • Fig. 4 Imaged polarization effects.

    (A) Images (64 × 64) of circuit board in Fig. 3A with vertical polarization. Pixels are 40 μm. We see that the contrast of each of the individual wires in the circuit depends on the THz polarization, with the highest contrast seen for polarization parallel to the wires. (B to E) Images (64 × 64) of the square regions in (A). Polarization is shown by the green arrow on the top left corner of each picture. Pixel size is 20 μm, and images have been denoised using the algorithm described in section S7. We see that the very subwavelength wiring breaks [marked by circles in (B) and (E)] give rise to transmissive regions in the THz image when the THz polarization is parallel to the wire. In (E), the diagonally orientated wire (indicated by the white arrow) also shows low contrast. Every image has been obtained via a full set of Hadamard masks. (F and G) Line plots through the 8-μm gaps in (B) and (E) with amplitude and space on the vertical and horizontal axis, respectively. The spatial coordinates of the plots are indicated by green rectangles in (B) and (E).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/6/e1600190/DC1

    section S1. Experimental schematics

    section S2. The silicon photomodulator

    section S3. Single-pixel detector imaging theory

    section S4. Scalar diffraction from two slits

    section S5. Signal with increasing number of pixels

    section S6. Total variation minimization reconstruction

    section S7. Image filtering

    fig. S1. Schematic of time-domain THz spectrometer.

    fig. S2. THz spectroscopy.

    fig. S3. [1, −1] versus [1, 0] masks.

    fig. S4. Diffraction from two slits.

    fig. S5. Increasing image size.

    fig. S6. Total variation minimized images.

    fig. S7. Unfiltered and filtered images.

    References (4555)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Experimental schematics
    • section S2. The silicon photomodulator
    • section S3. Single-pixel detector imaging theory
    • section S4. Scalar diffraction from two slits
    • section S5. Signal with increasing number of pixels
    • section S6. Total variation minimization reconstruction
    • section S7. Image filtering
    • fig. S1. Schematic of time-domain THz spectrometer.
    • fig. S2. THz spectroscopy.
    • fig. S3. 1, −1 versus 1, 0 masks.
    • fig. S4. Diffraction from two slits.
    • fig. S5. Increasing image size.
    • fig. S6. Total variation minimized images.
    • fig. S7. Unfiltered and filtered images.
    • References (45–55)

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