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Nanoscale optical pulse limiter enabled by refractory metallic quantum wells

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Science Advances  15 May 2020:
Vol. 6, no. 20, eaay3456
DOI: 10.1126/sciadv.aay3456
  • Fig. 1 Comparison of the traditional bulk transmission-mode and the proposed nanoscale reflection-mode pulse limiters.

    (A and B) Conventional configurations (not to scale) widely used for optical limitation based on Kerr-induced self-defocusing (A) and Kerr-type nonlinear absorption (such as TPA) (B). The former is achieved by inserting a bulk Kerr medium behind the focal plane to accelerate the divergence of an incident Gaussian beam with a high intensity so that only a fraction of the beam is allowed to pass through a preassigned aperture. The latter is performed by placing a bulk Kerr medium ahead of the focal plane to absorb the incident beam’s high-intensity portion. Note that an inhomogeneously distributed bulk Kerr medium, as shown in (B), is desired to maximize the nonlinear absorption. (C) Recently emerging reflective optical limiter (not to scale). To limit the high-intensity transmission, instead of increasing the absorption (B), the reflection of the reflective pulse limiter will be enhanced because of off-resonance above the threshold intensity. (D) Schematic representation of the nanoscale reflective optical limiter (not to scale). The deeply subwavelength optical limiter film can be integrated onto the surface of an existing optical component.

  • Fig. 2 Multiple electronic subbands in the quantum-sized TiN films enabling extraordinarily high Kerr coefficients.

    (A) Conduction band diagram of a TiN MQW (left) and the corresponding electronic dispersion of subbands (right). The Fermi level EF (~4.6 eV) is shown as the dashed line. The red arrows indicate the single-photon intersubband transitions between subbands ∣2⟩ and ∣3⟩. (B) Wavelength dependence of the nonlinear optical constant n2 of a 2-nm-thick TiN film, measured by the z-scan technique using 45°-incident p-polarized laser pulses (100-fs pulse width, 1-kHz repetition rate; Astrella, Coherent) with the intensity of ~70 GW/cm2. Note that a minus “−” is used in the imaginary party of the n2. The red arrow corresponds to the calculated transition wavelength shown in (A), while the solid lines are the spline-fitted curves. The fluctuations in multiple measurements at various locations are indicated by the error bars (SD). Inset shows a typical transmission electron microscopy (TEM) cross-section of a TiN MQW thin film.

  • Fig. 3 Experimental demonstration of the reflection-mode nanoscale femtosecond pulse limiter using TiN-based MQWs.

    (A) Experimental configuration of the reflection-mode pulse limiter (not to scale). The attenuator is used to vary the incident powers for obtaining pulse-limiting curves. (B) Typical TEM cross section of a 7-unit MQW thin film. The layer on top of the MQWs is a protective layer used only for TEM cross section preparation during the focused ion beam cutting process. (C) Intensity dependence of the measured reflected power for samples with a single unit and 7 units of MQW at the wavelength of 1997 nm (100-fs pulse width, 1-kHz repetition rate, 130-μm beam radius, 45° incidence, and p polarization). The dashed lines show the corresponding linear reflection curves. The onset-of-limiting intensity Ion is defined in the main text. Insets show a zoomed-in TEM cross section of the 7-unit MQW thin film (left) and a dark-field high-resolution TEM image (right) showing the high quality of the grown multilayer.

  • Fig. 4 Physics of optical Kerr nonlinearities of the MQWs.

    (A and B) Intensity-dependent refractive index nI extracted from the experimentally measured reflectivity and transmissivity [“exp” in (A)] and fitted by the single-photon absorption (1PA) and two-photon absorption (TPA) saturation models [“fit” in (B)]. Inset of (B) shows diagrams representing the Kerr, 1PA, and TPA processes, respectively. The sample used has 7 units of MQW, and the data are taken at the wavelength of 1997 nm.

  • Table 1 Wavelength dependence of performance parameters for the nanoscale thin-film pulse limiter with 7 units of MQW.

    λ (nm)RlinIon (GW/cm2)DRRSLI

Supplementary Materials

  • Supplementary Materials

    Nanoscale optical pulse limiter enabled by refractory metallic quantum wells

    Haoliang Qian, Shilong Li, Yingmin Li, Ching-Fu Chen, Wenfan Chen, Steven Edward Bopp, Yeon-Ui Lee, Wei Xiong, Zhaowei Liu

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