Antimony thin films demonstrate programmable optical nonlinearity

Abstract

The use of metals of nanometer dimensions to enhance and manipulate light-matter interactions for emerging plasmonics-enabled nanophotonic and optoelectronic applications is an interesting yet not highly explored area of research beyond plasmonics. Even more importantly, the concept of an active metal that can undergo an optical nonvolatile transition has not been explored. Here, we demonstrate that antimony (Sb), a pure metal, is optically distinguishable between two programmable states as nanoscale thin films. We show that these states, corresponding to the crystalline and amorphous phases of the metal, are stable at room temperature. Crucially from an application standpoint, we demonstrate both its optoelectronic modulation capabilities and switching speed using single subpicosecond pulses. The simplicity of depositing a single metal portends its potential for use in any optoelectronic application where metallic conductors with an actively tunable state are important.

Fig. 1. Thickness dependence on optical and structural properties of Sb.

(A and B) The spectra (from ultraviolet to near infrared) of the refractive index n_a (A) and extinction ratio k_a (B) of thin-film Sb with different thicknesses (t_Sb) as deposited on silicon wafers measured by spectroscopic ellipsometry. (C and D) The spectra of n_c (C) and k_c (D) of the same Sb samples in (A) and (B) after annealing on a hot plate (270°C for 10 min). (E and F) The absolute change of refractive index |Δn| (|n_c − n_a|) (E) and extinction ratio |Δk| (|k_c − k_a|) (F) of Sb upon annealing, calculated from (A) and (C) and (B) and (D), respectively. (G) Raman spectra of annealed Sb films with different t_Sb. E_g and A_1g vibration modes are denoted by the dashed lines. Typical vibration modes F_2g and A_g of Sb₂O₃ are illustrated by the circles. The Raman spectrum intensity of 3-nm Sb (purple) has been enlarged by two times for clarification. a.u., arbitrary units.

Fig. 2. TEM characterization of Sb.

(A) TEM image of a 5-nm-thick Sb layer as deposited has an amorphous structure. (B) Selected-area (344 nm diameter) electron diffraction (SAED) of the as-deposited Sb, corresponding to the sample in (A). (C) TEM image of the same Sb sample in (A) after thermal annealing. (D) SAED (200-nm-diameter region) of the annealed Sb sample in (B) has formed diffraction spots from crystalline planes. Two patterns, corresponding to the zone axis of [001] (white) and $[\stackrel{\mathrm{–}}{1}00]$ (cyan), are overlapped. Miller indices of crystal planes with high symmetry are labeled. The arrows show merged diffraction spots coming from different planes. (E) TEM image of the annealed Sb with visible crystalline planes along the $[1\stackrel{\mathrm{–}}{2}0]$ direction. Inset: Fourier transform of the whole image showing reflections $(1\stackrel{\mathrm{–}}{2}0)$ with an interplane spacing (d) of 2.2 Å. (F) The hexagonal unit cell of the rhombohedral crystalline structure of Sb. The red lines show the primitive rhombohedral unit cell. a and c are the measured crystal constants in the hexagonal unit cell.

Fig. 3. Switchable reflective stacks using ultrathin-film Sb.

(A) Structure of reflective display based on phase-change Sb sandwiched between two ITO layers (ITO/Sb/ITO) on top of a Pt mirror. The phase-change Sb layer can be switched from amorphous to crystalline through thermal annealing. (B) Optical images show typical display samples with different thicknesses (t_ITO) of the bottom ITO layer, while Sb and the top ITO are fixed at 5 and 15 nm, respectively. The samples have an amorphous (top row) and a crystalline (bottom row) Sb layer. (C and D) Measured (C) and simulated (D) reflection spectra of samples corresponding to (B). Each measured spectrum curve was normalized to the peak of the corresponding simulated curve.

Fig. 4. Electro-optical switching of Sb using conductive AFM.

(A) Schematic of the electrical switching of Sb using CAFM. The Sb film is sandwiched between two ITO layers above a Pt mirror. The conductive probe of the CAFM is biased using a DC voltage (V_B) while contacting or scanning the Sb sample. The Pt substrate is grounded via a resistor R_S (3 kilohms) to limit the current (I_S) passing through the probe and the sample. (B) Static measurement of I_S while sweeping V_B on the probe that is in contact with different locations of the sample. Inset: Histogram distribution of the threshold voltage V_th of the switching during voltage sweeping. (C to E) Original grayscale (8-bit, 256 × 256) images used to modulate V_B. (C) is from Marco Schmidt’s standard test images database, (D) is reproduced from the logo of the University of Oxford, and (E) is from the University of Southern California-Signal and Image Processing Institute (USC-SIPI) Image Database. (F) The optical image of the Sb sample switched by CAFM, corresponding to the image in (C). The Sb sample structure is 15-nm ITO/5-nm Sb/100-nm ITO/Pt (from top to bottom layer). (G and H) The optical image of the Sb sample switched by CAFM, corresponding to the images in (D) and (E), respectively. The Sb sample structure is 15-nm ITO/3-nm Sb/50-nm ITO/Pt. (I to L), Optical images show zoomed-in regions of the switched areas: blue (I), orange (J), green (K), and red (L) boxes in (H).

Fig. 5. Ultrafast optical switching of Sb.

(A) Schematic of optical switching of Sb using femtosecond laser. LED, light-emitting diode; CCD, charge-coupled device; NA, numerical aperture. (B) Optical image shows amorphized regions (blue disks) using single laser pulse (200 fs) with increasing energy E_p (from bottom to top). (C) Large area switching of the c-Sb sample (c-Sb background) through single femtosecond pulse (200 fs, E_p = 0.56 nJ) while raster scanning the sample (moving speed, 500 μm/s). a-Sb1 and a-Sb2 are switched areas with different sizes. (D) Recrystallization (c-Sb1) of the amorphized region a-Sb1 in (C) with multiple femtosecond pulses (200 fs, E_p = 29 pJ, 80 MHz) while translating the sample at 200 μm/s. (E) Reflection spectra of different locations of the sample in (D). (F) The stability of the amorphized region by femtosecond laser switching. Optical images show the amorphized region after different aging times in ambient conditions at RT. The Sb sample used is 15-nm ITO/3-nm Sb/50-nm ITO/Pt.