The optical properties of any material can be derived from its dielectric constant. We know that the permittivity of dielectrics is greater than 0, while the permittivity of metal is less than zero. When the dielectric constant of a material approaches 0, there are many wonderful changes in its optical properties. For example, the phase velocity at which light propagates tends to infinity, and recent studies show that the nonlinear optical properties of materials increase significantly as the permittivity approaches zero.
At present, there are mainly two kinds of materials with a dielectric constant of 0 in the optical frequency band, one is a metal / dielectric composite material designed and manufactured by hand, and the other is a conductive oxide with a carrier concentration smaller than that of a common metal , Such as ITO (Indium Tin Oxide).
Fig.1 Linear and nonlinear optical properties of ITO sol nanocrystals: a) Normalized absorption spectra of ITO nanocrystalline films with different doping concentrations; b) Dependence of carrier concentration and doping concentration; c) D) the dependence of the wavelength of the near-zero dielectric constant (ENZ) and the carrier concentration; e) doping of 5%, f) 12% of ITO nanowires Z-scan curve of crystalline thin film.
Recently, Associate Professor Liu Xiaofeng from Zhejiang University's Qiu Jianrong Group recently found that ITO, a conductive oxide with near-zero dielectric constant in the near infrared band, can be used to realize the ultra-fast optical switch in the optical communication band. Relevant results published in Advanced Materials [29, 1700754 (2017)].
In the experiment, Guo Qiangbing, a doctoral candidate in the research group, prepared ITO nanocrystals with different doping concentrations of about 10 nm by wet chemical method and made them into thin films. The results show that the dielectric constant of near-zero due to current-carrying The results show that these ITO nanocrystalline films exhibit negative non-linear absorption coefficient, that is, saturated absorption in the corresponding band. Ultrafast spectroscopy tests show that the response rate of free carriers originates from the order of 100 fs and can therefore be used to achieve ultrafast optical switches.
Fig. 2 Ultrafast laser based on ITO nanocrystalline film: a) Schematic diagram of Er-doped fiber pulsed laser (EDF, Er doped fiber; PC, polarization controller; OC, output; PI-TIWDM, polarization- Demultiplexer); b) laser output spectrum; c) mode-locked pulse output sequence; d) autocorrelation spectrum.
In order to demonstrate the ultra-fast optical switch based on the saturable absorption effect, the research group made a composite thin film of ITO nanocrystal into an Er-doped fiber laser (Figure 2) and successfully achieved mode locking in the 1550 nm optical communication band Pulsed laser output with a minimum pulse width of 593 fs and a signal-to-noise ratio of 56 dB. By modulating the dopant concentration, this material enables optical switching from a wide range of wavelengths, from near-infrared to mid-infrared.
This oxide-based optical switch has better stability than any other current types of optical switches based on two-dimensional materials, and it can be mass-produced by a variety of commercial technologies, so this research result will be super Fast-mode-locked optical switches provide a more reliable and cost-effective solution and are expected to break the current monopoly of SESAM on the market for commercially available saturable absorbers.
Flat Face Mask Making Machine
Flat Face Mask Making Machine,Automatic Mask Machine,Robot Surgical Machines,Mask Earloop Welding Machine
Dongguan Pailade Machinery Equipment Co.,Ltd , https://www.pailades.com