Research ArticlePHYSICS

Photon superbunching from a generic tunnel junction

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Science Advances  10 May 2019:
Vol. 5, no. 5, eaav4986
DOI: 10.1126/sciadv.aav4986
  • Fig. 1 Schematic of a scanning tunneling microscope combined with a Hanbury Brown and Twiss interferometer.

    Light radiating from a junction formed between a gold tip and Ag(111) substrate travels along two optical paths (1, 2) through a series of lenses (L), viewports (V), and optical filters (F) to a pair of single-photon avalanche diodes (SPADs). The number of photon coincidence events as a function of time delay t between the SPADs, g(2)(t), is measured with a time-correlated single-photon counter (TCSPC). The voltage bias (U) is applied to the substrate. The tunnel current (I) is measured with a picoammeter (A). A third optical path to an optical spectrometer is not shown.

  • Fig. 2 Tunnel junction characterization with photon pair generation schematic.

    (A) Ag(111) surface topography with a monatomic step imaged at 3 V, 100 pA. X marks the position of bunching measurements. Scale bar, 5 nm. The gradation spans one 240-pm Ag terrace step height. (B) Total light intensity (orange), tip retraction (purple), and density of states (DOS; green) during a linear voltage sweep at constant current. The position of the first FER maximum is indicated with a black line. arb. units, arbitrary units. kcts, kilocounts. (C) An energy level diagram of an inelastic electron tunneling event leading to photon pair production. The junction is biased by a fixed Ubias voltage. An electron at the tip Fermi level (EF,tip) tunnels through a junction potential barrier U(z), where z is a position in the gap, arriving on the sample side with an energy E = e · Ubias above the sample Fermi level (EF,sample). E can be aligned or misaligned with FERs nearby. E is an upper bound for the total photon energy because metals provide a continuum of initial and final states.

  • Fig. 3 Photon correlation measurements of a tunnel junction and picosecond light source.

    (A) Typical g(2)(t) measurement for the tunnel junction light source. The total number of true coincidence events (2620) is determined by integrating between ±1 ns after subtracting the level of accidental coincidence events (18.6) that corresponds to limtg(2)(t)=1 (black line) and is equal to the product of the two SPAD count rates. Total data accumulation time of 29,400 s. (B) A comparison of g(2)(t) rescaled to have unity peak height for the tunnel junction source (red) and an autocorrelation of a commercial picosecond white light source with 6-ps fundamental pulse width (blue). The full widths at half maxima are indicated. Solid lines are guides for the eye.

  • Fig. 4 Photon correlation measurements as a function of current.

    (A to E) The measured function g(2)(t) − 1 is for the currents shown. The unity shift aligns the normalized accidental coincidence level to zero across all measurements for ease of comparison. The horizontal axis in each window spans ±1-ns time delay t. Respective voltages of 4.47, 4.60, 4.65, 4.70, and 4.74 V are applied in (A) to (E). (F) Log-log plot of the absolute number of true and accidental coincidence events versus current. Both have a power-law dependence on the current equal to the slope of the fitted lines. The ratio of each data point pair yields the respective g(2)(0) − 1 peak values of 9.2, 1.4, 0.22, 0.060, and 0.028 for each trace from (A) to (E). Total data accumulation time of 1200 s (A to C) and 600 s (D and E).

  • Fig. 5 Photon correlation measurements at fixed tunnel conditions with varied spectral filtering.

    (A) Measured optical spectrum (orange) and its shortpass 600-nm cutoff spectrum (green). (B) Bunching is observed in the unfiltered light. (C) Bunching is not observed when the low-energy photons are blocked for both detectors (F1 = F2 = shortpass filter in Fig. 1), and the total energy of a photon pair is required to exceed the electron energy of 4 eV. Total data accumulation time in seconds: (B) 600 and (C) 39,000. Accidental correlation level in events per bin: (B) 42.05 and (C) 107.7.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaav4986/DC1

    Supplementary Text

    Fig. S1. Schematic of a one-dimensional model of tunneling.

    Fig. S2. Photon correlation measurements at fixed tunnel conditions with varied spectral filtering for a gold tip on Au(111).

    Fig. S3. Survey measurements of bunching for a gold tip on Cu(111).

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Schematic of a one-dimensional model of tunneling.
    • Fig. S2. Photon correlation measurements at fixed tunnel conditions with varied spectral filtering for a gold tip on Au(111).
    • Fig. S3. Survey measurements of bunching for a gold tip on Cu(111).

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