Yuanhao Liu

Nonvolatile resistive switching in graphene oxide thin films

Reliable and reproducible resistive switching behaviors were observed in graphene oxide (GO) thin films prepared by the vacuum filtration method. The Cu/GO/Pt structure showed an on/off ratio of about 20, a retention time of more than 104 s, and switching threshold voltages of less than 1 V. The switching effect could be understood by considering the desorption/absorption of oxygen-related groups on the GO sheets as well as the diffusion of the top electrodes. Our experiments indicate that GO is potentially useful for future nonvolatile memory applications.

Ultrafast three-dimensional tunable photonic crystal

We demonstrate a continuously tunable photonic crystal with several picoseconds response, which is fabricated through a self-assembly formation method of polystyrene spheres. The large optical nonlinearity originates from the delocalization of the conjugated π-electron along polymer chains. The pump and probe scheme is adopted to measure the transmission changes based on the optical Kerr effect. The dynamic shift of the photonic band gap is studied. It is found that the band gap shifts about 13 nm when the input peak intensity is 40.4 GW/cm2, which is in agreement with the theoretical predictions.

Ultrafast all-optical switching in two-dimensional organic photonic crystal

An all-optical switching with high switch efficiency is realized based on two-dimensional nonlinear photonic crystal made of polystyrene. The prism-film coupling method is used to couple energy of probe light into photonic crystal waveguide. High transmittance contrast of more than 60% is realized for the probe light. Time response of the optical switching is around l0 ps. The dynamical shifts of photonic gap induced by pump light are measured and analyzed in detail. The photonic gap shifts l0 nm under the excitation of 16.7GW∕cm2 pump intensity, which is in agreement with the theoretical predictions.

Subpicosecond optical switching in polystyrene opal

An ultrafast all-optical switching is demonstrated in an organic three-dimensional nonlinear photonic crystal with a response time as short as 120 fs. The operating wavelength of the optical switching is in the visible spectrum. The switching function is realized by a shift of the photonic band gap under optical pumping. It is found that the photonic band gap shifts about 10 nm with the excitation of 27.5GW∕cm2 pump laser.

All-optical switching of defect mode in two-dimensional nonlinear organic photonic crystals

An all-optical switching is demonstrated by use of defect mode in two-dimensional nonlinear photonic crystals made of polystyrene. Transmittance contrast of about 70% is achieved for the probe light. The dynamical shifts of the defect mode induced by pump intensity are measured and analyzed in detail. It is found that the defect mode shifts 5 nm under the excitation of 18.7GW∕cm2 pump intensity, which is in agreement with the theoretical predictions. Time response of the optical switching is smaller than the laser pulse of about 10 ps.

Ultrafast tunable filter in two-dimensional organic photonic crystal

A tunable filter made from a two-dimensional polystyrene photonic crystal is fabricated by focused ion-beam etching. The pump and probe scheme is adopted to measure tunability based on the picosecond optical Kerr effect. The response time of the tunable filter, approximately 10 ps, is within the measurement resolution. The maximum shift of optical channels is estimated to be 6 nm under excitation of 15.9 GW/cm2 pump intensity, which is in agreement with the theoretical prediction.

All-optical switching with 20fs response

All-optical ultrafast switching is demonstrated in a two-dimensional polystyrene photonic crystal, which is based on the shift of photonic gap under optical pumping. Both the response and recover time constants are measured to be less than 20fs.

Multi-color coherent radiation in a two-dimensional nonlinear photonic crystal with rectangular lattices

The temperature dependence of the magnetic field-induced strain (MFIS) and the field-controlled shape memory effect in Ni52Mn16.4Fe8Ga23.6 single crystals were investigated by measuring the MFIS and measuring the magnetic field-enhanced transformation strain with a field bias applied in the [001] and [010] directions of the parent phase, respectively. The results show that such material combined with the martensitic transformation can product large field-enhanced transformation strain and large MFIS. The strain accompanying the martensitic transformation is -1.61% in zero field and can be enhanced to -3.30% by a field of 960 kA/m. A MFIS of 1.04% has been induced along [001] in unstressed crystals with saturated magnetic field of 600 Win applied along the same direction at near martensitic transformation temperature. It was found that the MFIS is almost temperature independent; the maximum decrease of the saturated MFIS is less than 10%, from 265 K to 100 K. This well-behaved temperature response makes this alloy particularly valuable for industrial and military smart actuators and transducers. Furthermore, it was found that the direction in which the MFIS has the largest value is always the [001], namely, the growth direction of the crystals.

Modulation of magnetization direction in flexible multiferroic heterostructures towards flexible spintronics

The discovery of giant magnetoresistance (GMR) opens the door of spintronics. Recently, GMR and tunneling magnetoresistance (TMR) spintronic devices prepared on flexible substrates, so called flexible spintronics, have attracted a lot of interest due to the mechanical flexibility, light weight, and low cost compared with rigid materials. Controlling the magnetization direction by tuning the magnetic anisotropy is the essential issue for the application of the flexible spintronics. In this talk, flexible FeGa/PVDF multiferroic heterostructures were prepared by magnetron sputtering under different strain. PVDF can supply in-plane uniaxial strain by external strain, electric field, and the change of temperature, which is transferred to the FeGa layer to tune the magnetic anisotropy. Under assistance of small magnetic field, we can well control the magnetization direction in flexible FeGa/PVDF multiferroic heterostructures.

Femtosecond optical switching effect In two-dimensional organic photonic crystal

All-optical switching with ultrafast time response of 20 fs is realized in a two-dimensional polystyrene photonic crystal. The high switching contrast of more than 45% is achieved. The pump and probe method is used to study the dynamical shift of the photonic band gap based on the nonlinear optical Kerr effect. It is found that the maximum shift of short wavelength band edge is about 6 nm under excitation of 13 GW/cm pump light.