The most popular research group of Professor qiuji

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The research group of Professor qiujianrong of Zhejiang University found that the optical properties of any material can be derived from its dielectric constant. We know that the dielectric constant of a dielectric is greater than 0, while that of a metal is less than 0. When the dielectric constant of a material approaches 0, many wonderful changes will take place in its optical properties. For example, the phase velocity of light propagating in it will tend to be infinite, while the recent research of huanglianxi, chairman of the company, said that the nonlinear optical properties of materials also increase significantly when the dielectric constant approaches 0

at present, there are two main types of materials with a dielectric constant of 0 in the optical frequency band. One is metal/dielectric composites prepared by manual design; The other is some conductive oxides with less carrier concentration than common metals, such as ITO (indium tin oxide)

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figure o linear and nonlinear optical properties of sol nanocrystals: a) normalized absorption spectra of ITO nanocrystalline films with different doping concentrations; b) The dependence of carrier concentration on doping concentration; c) The dielectric constant (real part) of ITO nanocrystalline films with different doping concentrations; D the fuel efficiency can be increased by 6% to 8%) depending on the wavelength of the dielectric constant near zero (enz) and the carrier concentration; e) Z-scan curve of ITO nanocrystalline films doped with 5%, f) 12%

recently, associate professor liuxiaofeng of the research group of Professor qiujianrong of Zhejiang University found that ITO, a conductive oxide whose dielectric constant near zero point is in the near-infrared band, can be used to realize ultrafast optical switching in the optical communication band. Relevant achievements were published on advancedmaterials[29, (2017)]

in the experiment, guoqiangbing, a doctoral student of the research group, prepared ITO nanocrystals with different doping concentrations with a size of about 10nm by wet chemical method, and made them into thin films. It was found that the near zero point of their dielectric constant varied in the range of 1300 – 1600nm due to different carrier concentrations, and the nonlinear absorption test showed that these ITO nanocrystalline films showed negative nonlinear absorption coefficients in the corresponding bands, that is, saturated absorption. The ultrafast spectrum measurement shows that the response rate from the free carrier is in the order of 100fs, so it can be used to realize ultrafast optical switch

figure 2 Ultrafast laser based on ITO nanocrystalline film: a) schematic diagram of Er doped fiber pulse laser (EDF, Er doped fiber; PC, polarization controlled precision instrument controller; OC, output; pi-tiwdm, polarization independent wavelength division multiplexer); b) Laser output spectrum; c) Mode-locked pulse output sequence; d) Autocorrelation spectrum

in order to demonstrate the ultrafast optical switch based on the saturable absorption effect, the research group made the ITO nanocrystals into a composite film, coupled to an Er doped fiber laser (Fig. 2), and successfully realized the mode-locked pulse laser output at 1550nm optical communication band, with a minimum pulse width of 593fs and a signal-to-noise ratio of 56dB. By modulating the doping concentration, the material can realize optical switching in a wide band from near infrared to mid infrared

compared with various optical switches based on two-dimensional materials at present, this kind of optical switch based on oxide has better stability, and it can be prepared in large quantities through various commercial technologies. Therefore, this research result will provide a more reliable and economic solution for the optical switch of ultrafast mode-locked lasers, and is expected to break the current monopoly of SESAM in the commercial saturable absorber market

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