Daozhong Zhang

Tensile tests of ropes of very long aligned multiwall carbon nanotubes

We have directly measured the Young’s modulus and tensile strength of multiwall carbon nanotubes by pulling very long (∼2 mm) aligned nanotube ropes with a specially designed stress-strain puller. This puller can apply an axial force to the rope and simultaneously measure the corresponding rope elongation and the change in rope resistance. The average Young’s modulus and tensile strength obtained were 0.45±0.23 TPa and 1.72±0.64 GPa, respectively, which are lower than those calculated and measured previously. The factors that affect the mechanical strengths of nanotubes are discussed.

Band gap and wave guiding effect in a quasiperiodic photonic crystal

A two-dimensional octagonal quasiperiodic photonic crystal composed of alumina cylinders is prepared. The transmission spectra of the quasicrystal are measured in the microwave region for the TM wave. We find that the position and width of the band gap do not depend on the incident direction, while the band structure can appear for quite a small piece of the quasicrystal. Two types of waveguide, a straight guide and a bending guide with two sharp 90° corners, are fabricated by removing three rows of cylinders. The measured transmittances show that the guiding efficiency for both waveguides is high.

10 fs ultrafast all-optical switching in polystyrene nonlinear photonic crystals

An ultrafast all-optical switching with the response time on the order of 10 fs is demonstrated in a three-dimensional opal polystyrene nonlinear photonic crystal by means of precise femtosecond pump-probe technique. The switching is realized by the shift in the photonic band gap under external optical pumping of 8 fs Ti:sapphire pulse laser with a peak pump power of 20.6 GW/cm2. The good performance of optical switching is attributed to the very strong and fast Kerr nonlinear optical response of the polystyrene material.

Broadband efficient second harmonic generation in media with a short-range order

The authors demonstrate broadband efficient quasiphase matched second harmonic generation (SHG) in nonlinear LiNbO3 crystals. The distribution of ferroelectric domains in a short-range order provides the possibility for broadband second harmonic generation in the visible range. This process is similar to the broadband SHG in random media, but it results in much higher conversion efficiency. The unique distribution of reciprocal vectors provided by the structural short-range order plays an important role in the enhancement. The achieved broadband SHG with high conversion efficiency will aid in the development of optics integration and multiple-channel devices.

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.

Pressure Controlled Self-Assembly of High Quality Three-Dimensional Colloidal Photonic Crystals

A concise pressure controlled isothermal heating vertical deposition (PCIHVD) method is developed, which provides an optimal growing condition with better stability and reproducibility for fabricating photonic crystals (PCs) without the limitation of colloidal sphere materials and sizes. High quality PCs are fabricated with PCIHVD from polystyrene spheres with diameters ranging from 200 nm to 1 mu m. The deep photonic band gap and steep photonic band edge of the samples are most favorable for realizing ultrafast optical devices, photonic chips, and communications. This method makes a meaningful advance in the quality and diversity of PCs and greatly promotes their wide applications.

Second-harmonic generation in two-dimensional periodically poled lithium niobate using second-order quasiphase matching

Two-dimensional nonlinear photonic crystals of lithium niobate with square lattices were fabricated. In these crystals, efficient quasiphase-matched second-harmonic generation was demonstrated by using second-order quasiphase matching. As a mode-locked Nd:yttritium–aluminum–garnet laser at 1064 nm with 35 ps pulse was used, we obtained an average power of 0.86 mW at 532 nm with 2.73 mW input, which corresponds to 42% internal conversion efficiency. The variations of second-harmonic output with crystal temperature and incident angle were measured. In addition, the relation between the second-harmonic output and reversed duty cycle was studied. All the experimental results are well consistent with simulations.

Rapidly Infrared-Assisted Cooperatively Self-Assembled Highly Ordered Multiscale Porous Materials

In this paper, cooperative self-assembly (CSA) of colloidal spheres with different sizes was studied. It was found that a complicated jamming effect makes it difficult to achieve an optimal self-assembling condition for construction of a well-ordered stacking of colloidal spheres in a relatively short growth time by CSA. Through the use of a characteristic infrared (IR) technique to significantly accelerate local evaporation on the growing interface without changing the bulk growing environment, a concise three-parameter (temperature, pressure, and IR intensity) CSA method to effectively overcome the jamming effect has been developed. Mono- and multiscale inverse opals in a large range of lattice scales can be prepared within a growth time (15-30 min) that is remarkably shorter than the growth times of several hours for previous methods. Scanning electron microscopy images and transmittance spectra demonstrated the superior crystalline and optical qualities of the resulting materials. More importantly, the new method enables optimal conditions for CSA without limitations on sizes and materials of multiple colloids. This strategy not only makes a meaningful advance in the applicability and universality of colloidal crystals and ordered porous materials but also can be an inspiration to the self-assembly systems widely used in many other fields, such as nanotechnology and molecular bioengineering.

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.

More direct evidence of the fcc arrangement for artificial opal

We present a scanning electron microscopy (SEM) investigation of artificial opals prepared by the sedimentation of suspension with sub-micrometer silica spheres. An analysis of the SEM images confirms that the fcc stacking is the most stable phase for the artificial crystals. Optical diffraction and transmission spectrum indicate a good alignment of the top (111) crystal plane. We also show that defects and dislocations of opal crystals are caused mainly by the irregular spheres. Slow sedimentation and uniform size of silica spheres may be the key factors for growing high quality opal crystals.