Xu-Dong Chen

Transient thermal effect, nonlinear refraction and nonlinear absorption properties of graphene oxide sheets in dispersion.

The nonlinear refraction (NLR) properties of graphene oxide (GO) in N, N-Dimethylformamide (DMF) was studied in nanosecond, picosecond and femtosecond time regimes by Z-scan technique. Results show that the dispersion of GO in DMF exhibits negative NLR properties in nanosecond time regime, which is mainly attributed to transient thermal effect in the dispersion. The dispersion also exhibits negative NLR in picosecond and femtosecond time regimes, which are arising from sp(2)- hybridized carbon domains and sp(3)- hybridized matrix in GO sheets. To illustrate the relations between NLR and nonlinear absorption (NLA), NLA properties of the dispersion were also studied in nanosecond, picosecond and femtosecond time regimes.

Polarization-dependent optical absorption of graphene under total internal reflection

It is shown that graphene exhibits strong polarization-dependent optical absorption under total internal reflection. Compared with universal absorbance of 2.3%, larger absorption was observed in monolayer, bilayer, and few-layer graphenes for transverse electric (TE) wave under total internal reflection. Our result indicates that reflectance ratio of transverse magnetic wave to TE waves can easily provide the information of number of graphene layers. Furthermore, the enhanced light-graphene coupling in a wide spectral range will be great potential in many applications such as photodetector, photovoltaics, and optical sensor.

Sensitive Real-Time Monitoring of Refractive Indexes Using a Novel Graphene-Based Optical Sensor

Based on the polarization-sensitive absorption of graphene under conditions of total internal reflection, a novel optical sensor combining graphene and a microfluidic structure was constructed to achieve the sensitive real-time monitoring of refractive indexes. The atomic thickness and strong broadband absorption of graphene cause it to exhibit very different reflectivity for transverse electric and transverse magnetic modes in the context of a total internal reflection structure, which is sensitive to the media in contact with the graphene. A graphene refractive index sensor can quickly and sensitively monitor changes in the local refractive index with a fast response time and broad dynamic range. These results indicate that graphene, used in a simple and efficient total internal reflection structure and combined with microfluidic techniques, is an ideal material for fabricating refractive index sensors and biosensor devices, which are in high demand.

The selective transfer of patterned graphene

We demonstrate a selective microcleaving graphene (MG) transfer technique for the transfer of graphene patterns and graphene devices onto chosen targets using a bilayer-polymer structure and femtosecond laser microfabrication. In the bilayer-polymer structure, the first layer is used to separate the target graphene from the other flakes, and the second layer transfers the patterned graphene to the chosen targets. This selective transfer technique, which exactly transfers the patterned graphene onto a chosen target, leaving the other flakes on the original substrate, provides an efficient route for the fabrication of MG for microdevices and flexible electronics and the optimization of graphenes performance. This method will facilitate the preparation of van der Waals heterostructures and enable the optimization of the performance of graphene hybrid devices.

High-Precision Twist-Controlled Bilayer and Trilayer Graphene

Twist-controlled bilayer graphene (tBLG) and double-twisted trilayer graphene (DTTG) with high precision are fabricated and their controllable optoelectronic properties are investigated for the first time. The successful fabrication of tBLG and DTTG with designated θ provides an attractive starting point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.

Enhanced reverse saturable absorption and optical limiting properties in a protonated water-soluble porphyrin

Nonlinear optical properties of water-soluble porphyrin 5,10,15,20-Tetrakis (1-methyl-4-pyridinio) porphyrin tetra (p-toluenesulfonate) (TMPyP), protonated TMPyP (), TMPyP film with the gelatin matrix (TMPyP/Gelatin) were investigated by the open-aperture Z-scan technique in the nanosecond regime. Results show that exhibits a larger ratio of excited state absorption cross-section to that of the ground state and enhanced reverse saturable absorption properties compared with TMPyP and TMPyP/Gelatin. also shows the superior optical limiting performance, even better than the benchmark material C60 and the multi-walled carbon nanotubes (MWNTs) dispersion.

Transparent and flexible multi-layer films with graphene recording layers for optical data storage

Based on the polarization-sensitive absorption of graphene under conditions of total internal reflection, we demonstrate the fabrication and reading of transparent and flexible multi-layer-film optical data storage media based on graphene recording layers. We report a realization of the process of data writing-transferring-reading by repeatedly transferring recorded graphene and its strong polarization effect. The reading results show a high signal-to-noise ratio and stability and low crosstalk interference between the layers. In addition, the graphene-based multi-layer-film optical data storage medium has a high transparency and flexibility. The high signal-to-noise ratio remains stable after the structure is bent 1000 times.

Large tunable optical absorption of CVD graphene under total internal reflection by strain engineering

We have developed a method to tune polarization-dependent optical absorption of large-scale chemical vapor deposition (CVD) graphene under total internal reflection (TIR) by strain engineering. Through control of the strain direction, the optical absorption of graphene for transverse magnetic or transverse electric waves can be separately tuned. Strain-induced modulation of the optical absorption has been theoretically expected when light is normally incident through graphene. Under TIR, however, we experimentally observed a significant increase in the strain-induced tunability of optical absorption for CVD graphene, with the modulation efficiency of optical absorption in monolayer graphene increasing by a factor of three times that for normal incidence. We conclude that the strain sensitivity of optical absorption of graphene under TIR offers significant potential for application in many areas such as ultra-thin optical devices and strain sensors.

Accurate layers determination of graphene on transparent substrate based on polarization-sensitive absorption effect

Based on the polarization-sensitive absorption effect, we have proposed a method to accurately count the number of carbon atomic layers for both exfoliated and chemical vapor deposition graphene sheets on transparent substrate. With spatial scanning, the three-dimensional imaging of graphene sample can be achieved to test the uniformity of the sample. In addition, our method serves for graphene test on transparent substrate, which is different from the commonly used SiO2/Si substrate. Moreover, this method is also applicable to layers counting of other two-dimensional materials. Therefore, it paves the way for applications of two-dimensional materials on transparent medium.

Polarization dependence of optical pump-induced change of graphene extinction coefficient

We used pump-probe spectroscopy to study the dependence of optical pump-induced change of graphene extinction coefficient on polarization. We find that the optical pump-induced change of graphene extinction coefficient is polarization-dependent for light with energy identical to that of the excitation light; this dependence decreases rapidly after optical excitation. However, the change of extinction coefficient is isotropic for light with energy much smaller than that of the excitation light. The spatial distribution of momenta of non-equilibrium carriers accounts for this polarization dependence. These results provide a potential way for manipulating graphene’s transient optical response characteristics for applications in ultrafast photonics.