Hybrid material systems are a promising approach for extending the capabilities of silicon photonics. Given the weak electro‐optic and thermo‐optic effects in silicon, there is intense interest in integrating an ultrafast‐switching phase‐change material with a large refractive index contrast into the waveguide, such as vanadium dioxide (VO2). It is well established that the phase transition in VO2thin films can be triggered by ultrafast, 800 nm laser pulses, and that pump‐laser fluence is a critical determinant of the recovery time of thin films irradiated by femtosecond pulses. However, thin‐film experiments are not reliable guides to a VO2:Si system for all‐optical, on‐chip switching because of the differences in VO2optical constants in the telecommunication band, and the complex sample geometry and alignment issues in a waveguide geometry. This paper reports the first demonstration that the reversible, ultrafast photoinduced phase transition in VO2can achieve sub‐picosecond response when small VO2volumes are integrated into a silicon waveguide as the active element. The result suggests that VO2can be pursued as a strong candidate for waveguide switching with sub‐picosecond on‐off times.
We study the ultrafast time resolved response of 30 nm films of VO2on a TiO2substrate when 3.1 eV (400 nm wavelength) pump pulses were used to excite the insulator to metal transition (IMT). We found that the IMT threshold for these samples (≤30µJ/cm2) is more than 3 orders of magnitude lower than that generally reported for a more traditional 1.55 eV (800 nm wavelength) excitation. The samples also exhibited unusual reflectivity dynamics at near-threshold values of pump fluence where their fractional relative reflectivity ΔR/R initially increased before becoming negative after several hundreds of picoseconds, in stark contrast with uniformly negative ΔR/R observed for both higher 400 nm pump fluences and for 800 nm pump pulses. We explain the observed behavior by the interference of the reflected probe beam from the inhomogeneous layers formed inside the film by different phases of VO2and use a simple diffusion model of the VO2phase transition to support qualitatively this hypothesis. We also compare the characteristics of the VO2films grown on undoped TiO2and on doped TiO2:Nb substrates and observe more pronounced reflectivity variation during IMT and faster relaxation to the insulating state for the VO2/TiO2:Nb sample.
more » « less- Award ID(s):
- 1827536
- NSF-PAR ID:
- 10154704
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optical Materials Express
- Volume:
- 10
- Issue:
- 6
- ISSN:
- 2159-3930
- Page Range / eLocation ID:
- Article No. 1393
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract The M1 form of vanadium dioxide, which exhibits a reversible insulator–metal transition above room temperature, has been incorporated into nanoscale heterostructures through solution‐phase epitaxial growth on the tips of rutile TiO2nanorods. Four distinct classes of VO2‐TiO2‐VO2nanorod heterostructures are accessible by modulating the growth conditions. Each type of VO2‐TiO2‐VO2nanostructure has a different insulator–metal transition temperature that depends on the VO2domain sizes and the TiO2‐VO2interfacial strain characteristics.
-
more » « less
Abstract The M1 form of vanadium dioxide, which exhibits a reversible insulator–metal transition above room temperature, has been incorporated into nanoscale heterostructures through solution‐phase epitaxial growth on the tips of rutile TiO2nanorods. Four distinct classes of VO2‐TiO2‐VO2nanorod heterostructures are accessible by modulating the growth conditions. Each type of VO2‐TiO2‐VO2nanostructure has a different insulator–metal transition temperature that depends on the VO2domain sizes and the TiO2‐VO2interfacial strain characteristics.
-
more » « less
Abstract Devices designed to dynamically control the transmission, reflection, and absorption of terahertz (THz) radiation are essential for the development of a broad range of THz technologies. A viable approach utilizes materials which undergo an insulator‐to‐metal transition (IMT), switching from transmissive to reflective upon becoming metallic. However, for many applications, it is undesirable to have spurious reflections that can scatter incident light and induce noise to the system. We present an electrically actuated, broadband THz switch which transitions from a transparent state with low reflectivity, to an absorptive state for which both the reflectivity and transmission are strongly suppressed. Our device consists of a patterned high‐resistivity silicon metamaterial layer that provides broadband reflection suppression by matching the impedance of free space. This is integrated with a VO2ground plane, which undergoes an IMT by means of a DC bias applied to an interdigitated electrode. THz time domain spectroscopy measurements reveal an active bandwidth of 700 GHz with suppressed reflection and more than 90% transmission amplitude modulation with a low insertion loss. We utilize finite‐difference time domain (FDTD) simulations in order to examine the loss mechanisms of the device, as well as the sensitivity to polarization and incident angle. This device validates a general approach toward suppressing unwanted reflections in THz modulators and switches which can be easily adapted to a broad array of applications through straightforward modifications of the structural parameters.
-
more » « less
Abstract Straining the vanadium dimers along the rutile
c ‐axis can be used to tune the metal‐to‐insulator transition (MIT) of VO2but has thus far been limited to TiO2substrates. In this work VO2/MgF2epitaxial films are grown via molecular beam epitaxy (MBE) to strain engineer the transition temperature (T MIT). First, growth parameters are optimized by varying the synthesis temperature of the MgF2(001) substrate (T S) using a combination of X‐ray diffraction techniques, temperature dependent transport, and soft X‐ray photoelectron spectroscopy. It is determined thatT Svalues greater than 350 °C induce Mg and F interdiffusion and ultimately the relaxation of the VO2layer. Using the optimized growth temperature, VO2/MgF2(101) and (110) films are then synthesized. The three film orientations display MITs with transition temperatures in the range of 15–60 °C through precise strain engineering.
