Songhao Liu

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...

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.

Strong piezoelectricity in nanosized silicon nitride prepared by laser‐induced chemical vapor deposition

It has been found for the first time that laser‐induced chemical vapor deposition (LICVD) processed nano‐Si3N4 possesses strong piezoelectric effects. The piezoelectricity of LICVD nano‐Si3N4 stems from the charge accumulation in the interfaces and the surfaces of microvoids.

Characterization of passive optical components with ultra-fast speed and high-resolution based on DD-OFDM.

The passive optical components with very fine structures in wavelength domain are very sensitive to the mechanical vibrations or thermal fluctuations. If the measurement speed is lower than the temperature and mechanical fluctuation, we cannot measure the dynamic characteristics of the optical components. In this paper, we propose and demonstrate a novel method with ultra-fast measurement speed and high-resolution based on optical channel estimation using direct-detected orthogonal frequency division multiplexing (DD-OFDM) signal, which can be used to measure the dynamic characteristics and fine structure of the passive optical components. In experiment, by using fast Fourier transform (FFT) and a low-cost electro-absorption modulated laser (EML), we can achieve the transfer function characteristics with 3.9 MHz resolution. Compared with the optical channel estimation using coherent OFDM signal reported before, the proposed measurement technique is cost-effective.

Active packing method for blue light-emitting diodes with photosensitive polymerization: formation of self-focusing encapsulates.

A novel light-emitting diode (LED) packaging method, named the active packaging (AP) method, is presented in this paper. In this method, during the LED packaging process, the light emitted from a GaN LED chip itself is employed to package the LED encapsulant, thereby eliminating the need to utilize a mold. Current injection into a bare LED chip, triggers a photosensitive epoxy to polymerize, leading to the formation of mushroom lamp cap on the LED chip. The emission properties of LEDs fabricated by this method, including their emission beam profiles and light outputs, were characterized. The results showed that a self-focusing effect happened with the addition of an epoxy on the chip. The simulation demonstrated that the geometry the encapsulant controlled the beam pattern of emission. Further, the self-focusing effect was believed to be caused by the combination of the threshold energy of epoxy polymerization, the beam pattern and the power output of the LED chip.

Encoding random hot spots of a volume gold nanorod assembly for ultralow energy memory

Data storage with ultrahigh density, ultralow energy, high security, and long lifetime is highly desirable in the 21st century and optical data storage is considered as the most promising way to meet the challenge of storing big data. Plasmonic coupling in regularly arranged metallic nanoparticles has demonstrated its superior properties in various applications due to the generation of hot spots. Here, the discovery of the polarization and spectrum sensitivity of random hot spots generated in a volume gold nanorod assembly is reported. It is demonstrated that the two-photon-induced absorption and two-photon-induced luminescence of the gold nanorods adjacent to such hot spots are enhanced significantly because of plasmonic coupling. The polarization, wavelength, and spatial multiplexing of the hot spots can be realized by using an ultralow energy of only a few picojoule per pulse, which is two orders of magnitude lower than the value in the state-of-the-art technology that utilizes isolated gold nanorods. The ultralow recording energy reduces the cross-talk between different recording channels and makes it possible to realize rewriting function, improving significantly both the quality and capacity of optical data storage. It is anticipated that the demonstrated technology can facilitate the development of multidimensional optical data storage for a greener future.

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.

Optical gears in a nanophotonic directional coupler

Gears are rotating machines, meshing with each other by teeth to transmit torque. Interestingly, the rotating directions of two meshing gears are opposite, clockwise and counterclockwise. Although this opposite handedness motion has been widely investigated in machinery science, the analogue behavior of light remains undiscovered. Here, we present a simple nanophotonic directional coupler structure which can generate two light beams with opposite handedness of polarization states-optical gears. Due to the abrupt phase shift effect and birefringence effect, the angular momentum (AM) states of photons vary with the propagation distance in two adjacent waveguides of the coupler. Thus, by the choice of coupling length, it is able to obtain two light beams with opposite handedness of polarization, confirming the appearance of optical gears. The full control in the handedness of output beams is achieved via tuning the relative phase between two orthogonal modes at the input port. Optical gears thus offer the possibility of exploring light-matter interactions in nanoscale, opening up new avenues in fields of integrated quantum computing and nanoscale bio-sensing of chiral molecules.

Novel wavelength division multiplex-radio over fiber-passive optical network architecture for multiple access points based on multitone generation and triple sextupling frequency

Abstract. An innovative wavelength division multiplex-radio over fiber-passive optical network architecture for multiple access points (AP) based on multitone generation and triple sextupling frequency is proposed and demonstrated. A dual-drive Mach–Zehnder modulator (DD-MZM) is utilized to realize the multitone generation. Even sidebands are suppressed to make the adjacent frequency separation twice the frequency of the local oscillator by adjusting the modulation voltage of the DD-MZM. Due to adopting three fiber Bragg gratings to reflect the unmodulated sidebands for uplink communications source free at optical network unit (ONU), is achieved. The system can support at least three APs at one ONU simultaneously with a 30 km single-mode fiber (SMF) transmission and 5  Gb/s data rate both for uplink and downlink communications. The theoretical analysis and simulation results show the architecture has an excellent performance and will be a promising candidate in future hybrid access networks.