Research ArticleAPPLIED PHYSICS

Excitons in 2D perovskites for ultrafast terahertz photonic devices

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Science Advances  21 Feb 2020:
Vol. 6, no. 8, eaax8821
DOI: 10.1126/sciadv.aax8821
  • Fig. 1 Ultrafast measurement of 2D perovskite.

    (A) Free carrier excitation and relaxation dynamics of 3D perovskite (black solid curve), mixed 3D-2D perovskite (red solid curve), and pure 2D perovskite (blue solid curve) thin films. All three perovskite thin films are photoexcited using a 400-nm pump beam at 750 μJ/cm2. (B) Schematic of the 2D perovskite crystal structure that form QWs. (C) Schematic of the quantum confinement of free carriers within the QW structure of the pure 2D perovskite. (D) Artistic illustration of the perovskite-coated hybrid metadevice, where the 2D perovskite is spin-coated (thickness 60 nm) on the top of TASR and is photoexcited using a 400-nm optical pump beam with THz as the probe. Inset shows the dimensions of the unit cell, where l = 60 μm, x = 15 μm, w = 6 μm, g = 3 μm, and square periodicity p = 75 μm. (E) Transmission of THz electric field (blue solid line) and photomodulation of the THz pulse ΔE(t) measured at τpump = 1.5 ps (red solid curve) and 20 ps (green solid line) in the 2D perovskite thin film. The inset shows the THz electric field zoomed at the zero-crossing point. a.u., arbitrary units.

  • Fig. 2 Transient real and imaginary THz photoconductivity at different pump fluences.

    (A) Transient real photoconductivity at different pump fluences recorded by scanning the peak THz amplitude by varying the pump delay time in picoseconds. (B) Transient imaginary photoconductivity recorded by monitoring the change in THz amplitude at zero crossing. For clarity, different pump fluence data are vertically offset. Black solid line (A and B) represents the biexponential fit (details are given in section S2). The individual spectra of real and imaginary part of photoconductivity at different pump fluences are shown in fig. S4.

  • Fig. 3 Photoswitchable response of the 2D perovskite-based hybrid metadevice.

    (A) Measured amplitude transmission spectra for the metamaterial at various optical pump fluences. (B) Numerically simulated transmission spectra of the metamaterial coated with 2D perovskites for different photoconductivity values. (C) Numerically calculated E-field distribution for the hybrid metadevice for different photoconductivity values of the 2D perovskite thin film.

  • Fig. 4 Ultrafast switching of Fano resonance.

    (A) Measured transmission spectra from the 2D perovskite-coated metadevice at different pump and probe delay times. Inset shows the charge carrier relaxation in the 2D perovskite after exciting with a 400-nm pump beam at 250 μJ/cm2. Black, blue, cyan, and green dots represent 10-, 14-, 20-, and 26-ps time delay between OPTP, respectively. (B) Amplitude modulation of Fano resonance at different optical pump fluences. The contour plot represents the amplitude modulation of the Fano resonance at different pump fluences (shown in the y axis).

  • Fig. 5 Active flexible hybrid metadevice.

    (A) Optical image of the 2D perovskite-based hybrid metadevice fabricated on the polyimide flexible substrate. (B) Real image of the flexible metadevice showing flexibility. (C) Experimentally measured THz transmission through the 2D perovskite-based hybrid metadevice at different pump-probe delay times (τp). Inset shows the charge carrier dynamics in the pristine 2D perovskite thin film deposited on the flexible polyimide substrate. (D) Experimentally measured THz transmission through the metadevice at different pump fluences. (E to G) THz transmission for different curvatures of devices where curvature 1 and 2 are 1.03 and 0.84 cm−1 respectively. Photo credit: Manukumara Manjappa, Nanyang Technological University, Singapore.

  • Table 1 Time constants of the free carrier dynamics extracted by fitting the THz transient measured (Fig. 2A) at peak position of THz pulse using biexponential decay function.

    Pump fluence (μJ/cm2)τ1 (ps)τ2 (ps)
    3000.23 ± 0.042.9 ± 0.2
    4000.18 ± 0.013.5 ± 0.4
    4500.26 ± 0.015.0 ± 0.1
    5000.17 ± 0.017.2 ± 0.9
    5500.20 ± 0.019.0 ± 2.0
  • Table 2 Time constants of the exciton dynamics extracted by fitting the THz transient measured (Fig. 2B) at zero crossing of THz pulse using biexponential decay function.

    Pump fluence (μJ/cm2)τ1 (ps)τ2 (ps)
    3003.2 ± 0.470 ± 30
    4002.5 ± 0.260 ± 10
    4501.6 ± 0.331 ± 6
    5001.2 ± 0.328 ± 5
    5501.1 ± 0.224 ± 5

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/8/eaax8821/DC1

    Section S1. Measurement of change in THz transmission at different THz pulse positions

    Section S2. THz transient

    Section S3. Transient photoluminescence

    Section S4. THz transmission from the perovskite thin film

    Section S5. THz transmission from bare quartz at different pump fluences

    Section S6. Free carrier density excited at various pump fluences

    Section S7. Photoconductivity extraction

    Fig. S1. AFM images of the 2D, mixed 3D-2D, and 3D perovskites.

    Fig. S2. Optical characterization of different perovskite (2D, mixed 3D-2D, and 3D) thin films.

    Fig. S3. Effect of phase change induced by exciton in THz amplitude (ΔT).

    Fig. S4. Free carrier and exciton dynamics in 2D perovskite.

    Fig. S5. Transient photoluminescence.

    Fig. S6. THz transmission through the perovskite thin film.

    Fig. S7. THz transmission through the bare z-cut quartz.

    Table S1. Extracted rate constant by fitting the transient photoluminescence spectra.

    Table S2. Free carrier number density at different pump fluences.

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Measurement of change in THz transmission at different THz pulse positions
    • Section S2. THz transient
    • Section S3. Transient photoluminescence
    • Section S4. THz transmission from the perovskite thin film
    • Section S5. THz transmission from bare quartz at different pump fluences
    • Section S6. Free carrier density excited at various pump fluences
    • Section S7. Photoconductivity extraction
    • Fig. S1. AFM images of the 2D, mixed 3D-2D, and 3D perovskites.
    • Fig. S2. Optical characterization of different perovskite (2D, mixed 3D-2D, and 3D) thin films.
    • Fig. S3. Effect of phase change induced by exciton in THz amplitude (ΔT).
    • Fig. S4. Free carrier and exciton dynamics in 2D perovskite.
    • Fig. S5. Transient photoluminescence.
    • Fig. S6. THz transmission through the perovskite thin film.
    • Fig. S7. THz transmission through the bare z-cut quartz.
    • Table S1. Extracted rate constant by fitting the transient photoluminescence spectra.
    • Table S2. Free carrier number density at different pump fluences.

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