Switchable 3D Photonic Crystals Based on the Insulator-to-Metal Transition in VO2

Abstract

Photonic crystals (PhCs) are optical structures characterized by the spatial modulation of the dielectric function, which results in the formation of a photonic band gap (PBG) in the frequency spectrum. This PBG blocks the propagation of light, enabling filtering, confinement, and manipulation of light. Most of the research in this field has concentrated on static PhCs, which have fixed structural and material parameters, leading to a constant PBG. However, the growing demand for adaptive photonic devices has led to an increased interest in switchable PhCs, where the PBG can be reversibly activated or shifted. Vanadium dioxide (VO₂) is particularly notable for its near-room-temperature insulator-to-metal transition (IMT), which is accompanied by significant changes in its optical properties. Here, we demonstrate a fabrication strategy for switchable three-dimensional (3D) PhCs, involving sacrificial templates and a VO₂ atomic layer deposition (ALD) process in combination with an accurately controlled annealing procedure. The resulting VO₂ inverse opal (IO) PhC achieves substantial control over PBG in the near-infrared (NIR) region. Specifically, the synthesized VO₂ IO PhC exhibits PBGs near 1.49 and 1.03 μm in the dielectric and metallic states of the VO₂ material, respectively, which can be reversibly switched by adjusting the external temperature. Furthermore, a temperature-dependent switch from a narrow-band NIR reflector to a broad-band absorber is revealed. This work highlights the potential of integrating VO₂ into 3D templates in the development of switchable photonics with complex 3D structures, offering a promising avenue for the advancement of photonic devices with adaptable functionalities.

Keywords: atomic layer deposition; insulator-to-metal transition; inverse opal; switchable photonics; vanadium dioxide.

Figure 1

ALD deposition of VO₂ thin film. (a) Mechanism of VO_x film deposition via ALD. This diagram illustrates the ligand-exchange process during the ALD cycles, showing how the surface becomes terminated with (i) −OH groups and (iii) −NMe₂ groups after (iv) the H₂O and (ii) the TDMAV pulses, respectively. (b) Thickness of the deposited films as a function of the number of ALD cycles, demonstrating a constant GPC of 0.56 Å/cycle. (c) GIXRD spectra for the as-deposited VO_x film and annealed VO₂ film. In the annealed VO₂ film, the peak near 2θ = 51° belongs to the substrate, while other peaks belong to VO₂. A more detailed labeling is shown in Figure S1. (d) Temperature-dependent resistance measurement of the annealed VO₂ film, revealing the IMT with over 3 orders of magnitude change in resistance.

Figure 2

IMT characterization of VO₂ thin film. (a) In situ temperature-dependent GIXRD measurement zoomed in near 2θ = 28° region. The peak shown belongs to the (011) plane of VO₂ (M) at low temperature while the (110) plane of VO₂ (R) at high temperature. (b) Raman spectra at low and high temperature. (c) Temperature-dependent Raman measurement. The peak at ∼520 cm^–1, which remains almost unaltered, is the signal from the SiO₂/Si substrate. Lorentz fitting analysis for (d) the peak position shift and (e) the peak area as peak intensity at ∼144.5 cm^–1 as a function of temperature.

Figure 3

Optical characterization of ALD-deposited VO₂ thin film. (a) The real permittivity ε′ and (b) the imaginary permittivity ε′ at the start (T_{start_E}), maximum (T_{max_E}), and end (T_{end_E}) temperatures during a continuous heating/cooling measurement obtained from spectroscopic ellipsometry as a function of wavelength λ. Note that the lines for T_{start_E} and T_{end_E}, plotted as a solid blue line and a dashed yellow line in (a) and (b), overlap. This match indicates a fully reversible change in the optical properties of the film. (c) Colormap of ε′ versus wavelength λ and temperature. (d) The film reflectance R_film converted from ε′ and ε″, taking the temperature at T_{start_E} as VO₂ (M), and at T_{max_E} as VO₂ (R). (e) Skin depth δ for the VO₂ (M) and VO₂ (R) phases plotted logarithmically. The skin depth increases significantly toward long wavelengths in the VO₂ (M) phase while remaining constant in the VO₂ (R) phase.

Figure 4

VO₂ IO PhC fabrication. SEM images of the VO₂ IO (a) from the side view at low magnification and (b) from the top view at higher magnification. (c) Statistics of center-to-center distance, D_{Center-to-center}, between adjacent PS spheres or macropores during the IO fabrication process: (I) Pristine PS opal template, (II) VO_x coated PS opal template (VO_x/PS opal), (III) VO_x IO, and (IV) VO₂ IO PhC. The D_{Center-to-center} is measured using ImageJ software on the corresponding SEM images. The distribution curves were normalized according to the area under the curve. (d) Raman spectrum for the VO₂ IO. Inset: optical image of the VO₂ IO. (e) Radially averaged profiles of the IO lamella’s electron diffraction ring using TEM before and after a heating/cooling cycle showing an unchanged VO₂ phase. Inset (i): TEM image for the VO₂ IO lamella. Inset (ii): corresponding electron diffraction pattern. Scale bars for (a), (b), (d-inset), (e-inset (i)), and (e-inset (ii)) are 2 μm,1 μm, 20 μm, 500 nm, and 5 nm^–1, respectively.

Figure 5

Switchable PBG of VO₂ IO PhC. (a) Reflection spectra R_IO during a heat-up and following cool-down measurement. (b) Experimental R_IO and simulated R_IO at high and low temperatures. The experimental results for T_{start_IO} and T_{end_IO} overlap. The small peak in the 900–1200 nm region of the curve marked as T_{max_IO} in (b) is highlighted in the roughly corresponding region with a black dashed box in (a). The oscillation in the simulated spectrum of the VO₂ (M) phase results from Fabry–Perot interference. (c) The Bragg peak position λ_c as a function of temperature, showing a switch function adapted to the external temperature. The Bragg peak position λ_c shifts from about 1.49 μm below the IMT transition temperature to about 1.03 μm above the IMT transition temperature.