Zhi-Bo Liu

Porphyrin and Fullerene Covalently Functionalized Graphene Hybrid Materials with Large Nonlinear Optical Properties

The nonlinear optical properties of two novel graphene nanohybrid materials covalently functionalized with porphyrin and fullerene were investigated by using the Z-scan technique at 532 nm in the nanosecond and picosecond time scale. Results show that covalently functionalizing graphene with the reverse saturable absorption chromospheres porphyrin and fullerene can enhance the nonlinear optical performance in the nanosecond regime. The covalently linked graphene nanohybrids offer performance superior to that of the individual graphene, porphyrin, and fullerene by combination of a nonlinear mechanism and the photoinduced electron or energy transfer between porphyrin or fullerene moiety and graphene.

Nonlinear optical properties of graphene oxide in nanosecond and picosecond regimes

The nonlinear optical properties of graphene oxide (GO) were investigated at 532 nm in nanosecond and picosecond regimes. Results show that two-photon absorption dominates nonlinear absorption process of GO in the case of picosecond pulses, while excited state nonlinearities play an important role in the case of nanosecond pulses. Additionally, we compared nonlinear optical properties of three different dimensional carbon-based materials (two-dimensional graphene, one-dimensional carbon nanotube, and zero-dimensional fullerene) in nanosecond and picosecond regimes, respectively. The nonlinear mechanism of GO is distinctly different from nonlinear scattering of carbon nanotube and excited state nonlinearity of fullerene.

Covalently porphyrin-functionalized single-walled carbon nanotubes: a novel photoactive and optical limiting donor–acceptor nanohybrid

Novel covalently porphyrin-functionalized single-walled carbon nanotubes (SWNTs) have been synthesized by the reaction of SWNTs with in situ generated porphyrin diazonium compounds. The resulting nanohybrid was characterized by spectroscopic (UV-Vis-NIR, FTIR and Raman) and microscopic (TEM and AFM) methods. The Raman and absorption spectroscopy data showed that the electronic properties of the modified tubes were mostly retained, without damaging their one-dimensional electronic properties. The fluorescence from the porphyrin moiety was almost completely quenched by SWNTs, indicating that the unique direct linkage mode facilitated the effective energy and electron transfer between the excited porphyrin moiety and the extended π-system of SWNTs. This novel nanohybrid material also exhibited excellent optical limiting properties.

Ultrafast carrier dynamics and saturable absorption of solution-processable few-layered graphene oxide

Ultrafast carrier dynamics and saturable absorption of few-layered graphene oxide, well-dispersed in organic solvent, are studied using femtosecond pump-probe and Z-scan techniques. The results demonstrate that few-layered graphene oxide has a fast energy relaxation of hot carriers and strong saturable absorption, which is comparable with that of reduced graphene oxide. Fast carrier relaxation combined with well solution processing capability arises from the large fraction of sp2 carbon atom inside the few-layered graphene oxide sheet together with oxidation mainly existing at the edge areas. This superiority of few-layered graphene oxide will facilitate potential applications of graphene for ultrafast photonics.

Photoconductivity of Bulk-Film-Based Graphene Sheets

Time-resolved photoconductivity measurements are carried out on graphene films prepared by using soluble graphene oxide. High photocurrent generation efficiency is observed for these graphene-based films, and the relationships between their photoconductivity and different preparation methods, incident light intensity, external electric field, and photon energies are investigated. Higher photoconductivity is observed with higher photon energy at same incident light intensity. By fitting the experimental data to the Onsager model, the primary quantum yields for charge separation to generate bound electron-hole pairs and the initial ion-pair thermalization separation distance are calculated.

Wavelength-tunable, passively mode-locked fiber laser based on graphene and chirped fiber Bragg grating

We demonstrate a wavelength-tunable, passively mode-locked erbium-doped fiber laser based on graphene and chirped fiber Bragg grating. The saturable absorber used to enable passive mode-locking in the fiber laser is a section of microfiber covered by graphene film, which allows light-graphene interaction via the evanescent field of the microfiber. The wavelength of the laser can be continuously tuned by adjusting the chirped fiber Bragg grating, while maintaining mode-locking stability. Such a system has high potential in tuning the mode-locked laser pulses across a wide wavelength range.

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.

Ultrasensitive flow sensing of a single cell using graphene-based optical sensors.

On the basis of the polarization-dependent absorption of graphene under total internal reflection, we designed a graphene-based optical refractive index sensor with high resolution of 1.7 × 10(-8) and sensitivity of 4.3 × 10(7) mV/RIU, as well as an extensive dynamic range. This highly sensitive graphene optical sensor enables label-free, live-cell, and highly accurate detection of a small quantity of cancer cells among normal cells at the single-cell level and the simultaneous detection and distinction of two cell lines without separation. It provides an accurate statistical distribution of normal and cancer cells with fewer cells. This facile and highly sensitive sensing refractive index may expand the practical applications of the biosensor.

Nonlinear optical and optical limiting properties of graphene oxide–Fe3O4 hybrid material

The nonlinear optical (NLO) and optical limiting properties of a graphene oxide hybrid material coordinated with Fe3O4 nanoparticles (GO–Fe3O4) were studied by using the Z-scan technique at 532 nm in the nanosecond and picosecond regimes. Results show that GO–Fe3O4 exhibits enhanced NLO and optical limiting properties in comparison with the pristine GO in the nanosecond regime. Compared with fullerene (C60) in toluene at different concentrations, GO–Fe3O4 exhibits a weaker optical limiting effect than C60 at high concentration, but shows a stronger optical limiting effect than C60 at low concentration in the high input fluence region.