Zhigang Cai

Intelligent integration of optical power splitter with optically switchable cross-connect based on multimode interference principle in SiGe∕Si

An intelligent and monolithic integration of optical power splitter with optically switchable cross-connect has been proposed. It is fabricated on the multimode interference principle in Si-based SiGe material system and configured for a 3×2 symmetrical structure of the three input waveguides and the two output waveguides. The central waveguide section is based on a multimode interference and incorporated with an activated carrier injection element. The operating wavelengths of the device are specially designed for 1545, 1550, and 1555nm conventional-band wavelengths. The measured crosstalk is at around −17dB and the average insertion loss is about 5.25dB. At switch-ON state, the measured injection current is 370mA corresponding to an injection current density of 950A∕cm2.

Picosecond transient photoluminescence spectra of ZnSe‐ZnS strained‐layer superlattices grown on GaAs(001) by molecular beam epitaxy

Picosecond transient photoluminescence spectra were carried out on ZnSe‐ZnS strained‐layer superlattices at 77 K. The typical exciton formation time and exciton lifetime are 40 and 100 ps, respectively. The relationship between exciton lifetime and the well width is also given.

A kind of two-dimensional electroconvection at ultralow electric frequency

We report a kind of two-dimensional (2D) electroconvection (EC) domain observed in a negative dielectric anisotropy nematic liquid crystal (LC) in a parallel-rubbed alignment cell. The domain is excited by an ac electric field with ultralow frequency, and can be transformed into a one-dimensional (1D) EC domain at low amplitude electric fields. We conclude that the 1D domain is a kind of Williams domain, while the 2D domain is induced by flexoelectric effect, the corresponding 2D texture coming from superposition of two sets of symmetrical 1D stripes oblique to the initial LC orientation.

Controlled light field concentration through turbid biological membrane for phototherapy.

Laser propagation through a turbid rat dura mater membrane is shown to be controllable with a wavefront modulation technique. The scattered light field can be refocused into a target area behind the rat dura mater membrane with a 110 times intensity enhancement using a spatial light modulator. The efficient laser intensity concentration system is demonstrated to imitate the phototherapy for human brain tumors. The power density in the target area is enhanced more than 200 times compared with the input power density on the dura mater membrane, thus allowing continued irradiation concentration to the deep lesion without damage to the dura mater. Multibeam inputs along different directions, or at different positions, can be guided to focus to the same spot behind the membrane, hence providing a similar gamma knife function in optical spectral range. Moreover, both the polarization and the phase of the input field can be recovered in the target area, allowing coherent field superposition in comparison with the linear intensity superposition for the gamma knife.

Voltage selectable dual-mode optically-induced grating in ZnTPP doped chiral nematic liquid crystals

Optically-induced grating in ZnTPP doped chiral nematic liquid crystals (CLC) is demonstrated by dual-mode operations, where the dynamic or storage functionality is selected by the amplitude of dc voltage. While the storage can be erased and rewritten by a high dc pulse, it can persist without an electric field, which is the result when the CLC undergoes a switching between a planar and a focal conic state. This tunable dual-functionality grating combines the advantages of bistable storage of CLC and high photosensitivity of dopant, and thus can be potentially exploited in photonic devices to implement different optical information processing in situ.

Generation of frequency-tunable ultrashort optical pulses with liquid-core fibers

Experimental generation of ultrashort, frequency-tunable optical pulses in liquid-core fibers is reported. Passage of high-energy optical pulses through a submillimeter liquid-core fiber reveals a quasi-linear frequency sweep. The subsequent use of a grating dispersion system and a spectral window shortens the Nd:YAG laser pulse duration from 45 ps to less than 2 ps. Based on spectral filtering after the grating pair, the wavelength of the generated short pulses is shown to be tunable.

Fourier Spectrometer based on a wide-range and nanometer stabilized Michelson Interferometer

A high signal-to-noise-ratio (SNR) and resolution Fourier spectrometer based on a wide-range and phase-locked Michelson interferometer is presented. A novel arrangement of a step motor, a nanometer resolution piezoelectric actuator and an active PID control loop ensures that the Michelson interferometer has a phase stabilized scanning range virtually without limit, resulting in the Fourier spectrometers high resolution and high SNR. Several experiments demonstrate that this spectrometer, using light sources such as write LED (WLED) and femtosecond laser, can accurately acquire the spectrum information of a given sample as well as the light source itself in a resolution of nanometer order.

3x2 integrated microphotonic switches

A microphotonic switch with three input waveguides and two output waveguides and integrated with an optical power splitter has been proposed. It is fabricated on the multimode interference principle in Si-based SiGe material system and configured for a 3x2 symmetrical structure of the three input waveguides and the two output waveguides of the device. The central waveguide section is based on a multimode interference and incorporated with an activated carrier injection element. The operating wavelengths of the device are specially designed for 1545, 1550, and 1555 nm conventional-band wavelengths. The measured crosstalk is at around -17 dB and the average insertion loss is about 2.3 dB. At switch-ON state, the measured injection current is 370 mA corresponding to an injection current density of 950 A/cm2.

Dynamics of localized exciton recombination and migration in GaAs multiple quantum wells

Abstract A theory on the dynamics of localized exciton recombination and their intralayer migration in GaAs quantum wells is developed. Non-exponential photoluminescence decay is successfully interpreted by exciton recombination and its influence on the quasi-equilibrium localized exciton population. The recombination rate and the effective downmigration rate can be obtained.

Experimental study of photoluminescence of trans-cisoid and cis-transoid polyphenylacetylene

The result of experimental studies of Photoluminescence (PL) in cis-transoid and trans-cisoid polyphenylacetylene were presented. The samples were prepared by a. newly-developed method using rare-earth metal catalyst. In contrast to polyacetylenea, the obtained PPA samples were soluble in common organic solvents and can be proceaaed into thin film. Comparing the results of diferent form of samples: powder, film and toluene solution samples, we concluded that, the observed fut PL decay is caused by the polaronic exciton radiative recombination, which will diminished when the inter-chain charge transfer took place, while the slow PL decay is related to the intra-chain bipolaron recombination.