Amorphous WO3 thin films designed as gigahertz/terahertz dielectric lenses

Abstract

Herein, tungsten oxide thin films comprising excess oxygen are treated as optical resonator suitable for gigahertz/terahertz applications. WO3 thin films which are prepared by the thermal evaporation technique under a vacuum pressure of 10 -5 mbar are structurally, compositionally and optically evaluated. The amorphous WO3.33 films which showed high transparency permit electronic transitions within an indirect allowed energy band gap of 3.05 eV. The band gap comprised energy band tails of width of 190 meV. Four dominant dielectric resonators centered in the infrared (IR), visible (VIS) and ultraviolet (UV) ranges of light are detected. Analysis of the optical conductivity in accordance with the Drude-Lorentz approaches have shown that the drift mobility of free holes in this amorphous layer can be as large as 5.61 cm 2 /Vs an as low as 1.59 cm 2 /Vs when exposed to IR and UV light signals, respectively. In addition, the gigahertz/terahertz cutoff frequency (𝑓 𝑐𝑜 ) spectra demonstrated 𝑓 𝑐𝑜 values in the gigahertz frequency domain when exposed to IR light. Excitations with light signals in the VIS and UV spectral ranges allow 𝑓 𝑐𝑜 values that extends from 0.7-40.0 THz. The wide range of tunability of the WO3 dielectric resonators nominates them as dielectric lenses suitable for optical communications.

Lead-tungsten oxide interfaces designed as gigahertz/terahertz filters

“…The absorption coefficient increases with decreasing incident photon energy as a result of free absorption. For thin films of thickness of 1.0 m m free carrier absorption in the infrared range (1.74-1.14 eV) of light was previously observed in amorphous 3 thin films [10]. In the currently studied films (d 300 = nm) this phenomenon appears in the spectral range of 3.02-1.32 eV.…”

“…For this purpose we deposited a 300 nm thick WO 3 films onto semitransparent lead films of thicknesses of 100 nm and study their optoelectronic performances as gigahertz/ terahertz filters usable in communication technology. In our previous investigations we have grown amorphous layers of WO 3 onto glass substrates and studied their properties as dielectric lenses [10]. WO 3 thin films of thicknesses of 1.0 m m were measured and found sensitive to light signals in the ultraviolet, visible and infrared ranges of light showing terahertz cutoff frequency values in the range of 0.70-40.0 THz.…”

“…For this reason the current work targets enhancing the performance of WO 3 film as light absorbers, terahertz band filters and as microwave resonators so that it fits well with 5 G requirements. Previously WO 3 films coated onto glass substrates showed continuously varying terahertz cutoff frequency spectra based on the energy of the incident light signal [10]. From practical point of view, such behavior forces selection of monochromatic light signals to fix the value of terahertz cutoff frequency values.…”

Growth and Characterization of Lanthanum Germanide Thin Films by the Thermal Evaporation Technique

“…[25] The so called Urbach tails or band tails existed due to presence of more localized states in forbidden band of the films. [25][26][27] Urbach energy provide information about the disorder or defects: Urbach values can give insights into the degree of disorder or the presence of defects in a material. Higher Urbach energies are associated with higher levels of disorder, defects, or impurities, which can affect the material's optical and electronic properties.…”

“…To gain information about the nature of the optical transitions in the films Tauc's equation ( $${( {\alpha E} )^n} = {B^n}\;( {{E} - {E_{\mathrm{g}}}} )$$, B is Tauc parameter) was employed. [ 25–27 ] The fitting of the equation is illustrated in Figure 2d. As can be seen from the figure the best fit which linearize most of the experimental absorption coefficient data in the sharp absorption coefficient region (4.17–3.49 eV) is related to the indirect allowed transitions type ($$n\; = \frac{1}{2}\;$$).…”