Hai-Ying Liu

Three-Dimensional Orientation Sensors by Defocused Imaging of Gold Nanorods through an Ordinary Wide-Field Microscope

Gold (Au) nanoparticles, particularly nanorods, are actively employed as imaging probes because of their special nonblinking and nonbleaching absorption, scattering, and emitting properties that arise from the excitation of surface plasmons. Herein, we report a novel sensing method that detects feature orientation at the nanoscale via the defocused imaging of individual Au nanorods (AuNRs) with an ordinary wide-field optical microscope. By simultaneously recording defocused images and two-photon luminescence intensities for a large number of individual AuNRs, we correlate their defocused images with their three-dimensional spatial orientations. The spatial orientation of many individual AuNRs can be monitored in situ and in real-time within a single frame, enabling its use as a technique for high-throughput sensing. The probe size can be as small as several nanometers, which is highly desirable for minimization of any potential interference from the probe itself. Furthermore, the sensing property is insensitive to the excitation polarization and the distribution of the probe aspect ratio, which allows AuNRs of any length within a proper regime to be used as orientation sensors without changing the laser frequency and polarization. These unique features make the orientation probes proposed here outstanding candidates for optical imaging and sensing in materials science and biological applications.

High spatial frequency periodic structures induced on metal surface by femtosecond laser pulses

The high spatial frequency periodic structures induced on metal surface by femtosecond laser pulses was investigated experimentally and numerically. It is suggested that the redistribution of the electric field on metal surface caused by the initially formed low spatial frequency periodic structures plays a crucial role in the creation of high spatial frequency periodic structures. The field intensity which is initially localized in the grooves becomes concentrated on the ridges in between the grooves when the depth of the grooves exceeds a critical value, leading to the ablation of the ridges in between the grooves and the formation of high spatial frequency periodic structures. The proposed formation process is supported by both the numerical simulations based on the finite-difference time-domain technique and the experimental results obtained on some metals such as stainless steel and nickel.

Colorizing silicon surface with regular nanohole arrays induced by femtosecond laser pulses

We report on the formation of one- and two-dimensional (1D and 2D) nanohole arrays on the surface of a silicon wafer by scanning with a femtosecond laser with appropriate power and speed. The underlying physical mechanism is revealed by numerical simulation based on the finite-difference time-domain technique. It is found that the length and depth of the initially formed gratings (or ripples) plays a crucial role in the generation of 1D or 2D nanohole arrays. The silicon surface decorated with such nanohole arrays can exhibit vivid structural colors through efficiently diffracting white light.

Effects of optical forces on the transmission of magnetic fluids investigated by Z-scan technique

The dependence of the transmission behavior of magnetic fluids on the incident power density of a laser beam is investigated and the conventional Z-scan technique is employed to continuously vary the optical forces induced by the laser beam. We calculate the optical forces exerted on magnetic nanoparticles and compare them with those for gold and silica nanoparticles. It is found that the optical forces for magnetic nanoparticles are comparable to those for gold nanoparticles. In addition, the calculation results show that the absorption force is dominant at low incident power densities while the gradient and scattering forces become significant at high incident power densities when the clustering of magnetic nanoparticles occurs. In Z-scan experiments, it is observed that the evolution of the Z-scan trace of a magnetic fluid with increasing incident power density cannot be explained only by the nonlinear absorption of the magnetic fluid induced by the thermal diffusion of magnetic nanoparticles. Instead,...

Ultrafast carrier dynamics in undoped and p-doped InAs∕GaAs quantum dots characterized by pump-probe reflection measurements

We investigate the dependence of the differential reflection on the structure parameters of quantum dot (QD) heterostructures in pump-probe reflection measurements by both numerical simulations based on the finite-difference time-domain technique and theoretical calculations based on the theory of dielectric films. It is revealed that the value and sign of the differential reflection strongly depend on the thickness of the cap layer and the QD layer. In addition, a comparison between the carrier dynamics in undoped and p-doped InAs∕GaAs QDs is carried out by pump-probe reflection measurements. The carrier capture time from the GaAs barrier into the InAs wetting layer and that from the InAs wetting layer into the InAs QDs are extracted by appropriately fitting differential reflection spectra. Moreover, the dependence of the carrier dynamics on the injected carrier density is identified. A detailed analysis of the carrier dynamics in the undoped and p-doped QDs based on the differential reflection spectra i...

Assembling of three-dimensional crystals by optical depletion force induced by a single focused laser beam

We propose and demonstrate a method to achieve large effective Soret coefficient in colloids by suitably mixing two different particles, e.g., silica beads and Fe3O4 nanoparticles. It is shown that the thermophoretic motion of Fe3O4 nanoparticles out of the heating region results in a large nonequlibrium depletion force for silica beads. Consequently, silica beads are driven quickly to the heating region, forming a three-dimensional crystal with few defects and dislocations. The binding of silica beads is so tight that a colloidal photonic crystal can be achieved after the complete evaporation of solvent, water. Thus, for fabrication of defect free colloidal PCs, periodic structures for molecular sieves, among others, the proposed technique could be a low cost alternative. In addition as we use biocompatible materials, this technique could be a tool for biophysics studies where the potential of large effective Soret coefficient could be useful.We proposed a method to assemble microspheres into a three-dimensional crystal by utilizing the giant nonequilibrium depletion force produced by nanoparticles. Such assembling was demonstrated in a colloid formed by suitably mixing silica microspheres and magnetic nanoparticles. The giant nonequilibrium depletion force was generated by quickly driving magnetic nanoparticles out of the focusing region of a laser light through both optical force and thermophoresis. The thermophoretic binding of silica beads is so tight that a colloidal photonic crystal can be achieved after complete evaporation of solvent. This technique could be employed for fabrication of colloidal photonic crystals and molecular sieves.

Self-induced Anderson localization and optical limiting in photonic crystal coupled cavity waveguides with Kerr nonlinearity

The transmission behavior of photonic crystal coupled cavity waveguides (CCWs) with Kerr nonlinearity is investigated by numerical simulations based on the finite-difference time-domain technique. The authors find that a nearly ideal optical limiter can be realized by use of a nonlinear CCW. In addition, it is revealed that Anderson localization [Phys. Rev. 109, 1492 (1958)] of the extended states in the impurity band instead of the shift of the impurity band is responsible for the observed optical limiting. Therefore, nonlinear CCWs offer a convenient platform for studying Anderson localization of electromagnetic waves in a controlled fashion and will find potential applications in optical limiting and switching.

Self-induced transparency in colloidal liquids by Z-scan-based optical trapping

We demonstrated a transition from a disordered to an ordered state in a colloidal liquid by utilizing Z-scan-based optical trapping. The Z-scan process plays a role of gradually and continuously narrowing and deepening the optical potential well. When the trapping power was increased above a certain level, a self-induced transparency occurs, leading to a significant enhancement in transmission. The dynamic transition was confirmed by monitoring the diffraction pattern of the trapping region.

Magnetic Fano resonance of heterodimer nanostructure by azimuthally polarized excitation

The optical properties of a Si-Au heterodimer nanostructure, which is composed of an Au split nanoring surrounded by a Si nanoring with a larger diameter, are investigated both theoretically and numerically. It is found that a pure magnetic plasmon Fano resonance can be achieved in the Si-Au heterodimer nanostructure when it is excited by an azimuthally polarized beam. It is revealed that the pure magnetic Fano resonance is generated by the destructive interference between the magnetic dipole resonance of the Si nanoring and the magnetic dipole resonance of the Au split nanoring. A coupled oscillator model is employed to analyze the Fano resonance of the Si-Au heterodimer nanostructure. The pure magnetic response of the Si-Au heterodimer nanostructure is verified by the current density distributions and the scattering powers of the electric and magnetic multipoles. The Fano resonance in the Si-Au heterodimer nanostructure exhibits potential applications of low-loss magnetic plasmon resonance in the construction of artificial magnetic metamaterials.

Dependence of nonlinearity enhancement on power density in photonic crystals characterized by numerical Z -scan experiments based on the finite-difference time-domain technique

We investigate the enhancement of nonlinearity in one-dimensional (1D) photonic crystals (PCs) with Kerr nonlinearity by numerical Z-scan experiments based on the finite-difference time-domain technique. Focused Gaussian beams with well-defined waists and Rayleigh lengths necessary for Z-scan experiments are generated through a conjugated manipulation of the Gaussian beams propagating in free space. The Z-scan measurements used for bulk materials are naturally extended to 1D PCs after incorporating the frequency- and power-density-dependent reflections into their linear and nonlinear absorptions. The closed- and open-aperture Z-scan traces for the 1D PCs are obtained and a symmetric method is employed to modify the asymmetric closed-aperture traces. The nonlinearity enhancement factors at different frequencies in the first and second bands are derived numerically and analytically. A good agreement is found between the numerical and analytical results in the case of weak nonlinearity. Moreover, the dependences of the enhancement factor on the incident power density for different frequencies in the 1D PCs are extracted and they are found to be much different from those in bulk materials. It is revealed that the variation of the group velocity with increasing power density is responsible for the power-density dependence of the enhancement factor. It indicates that in practice one must deliberately choose the working frequency and power density of PC-based devices in order to achieve a maximum enhancement of nonlinearity.