Zhu Shaolan

A full numerical calculation of the Franz--Keldysh effect on magnetoexcitons in a bulk semiconductor

We have performed a full numerical calculation of the Franz–Keldysh (FK) effect on magnetoexcitons in a bulk GaAs semiconductor. By employing an initial value method in combination with the application of a perfect matched layer, the numerical effort and storage size are dramatically reduced due to a significant reduction in both computed domain and number of base functions. In the absence of an electric field, the higher magnetoexcitonic peaks show distinct Fano lineshape due to the degeneracy with continuum states of the lower Landau levels. The magnetoexcitons that belong to the zeroth Landau level remain in bound states and lead to Lorentzian lineshape, because they are not degenerated with continuum states. In the presence of an electric field, the FK effect on each magnetoexcitonic resonance can be identified for high magnetic fields. However, for low magnetic fields, the FK oscillations dominate the spectrum structure in the vicinity of the bandgap edge and the magnetoexcitonic resonances dominate the spectrum structure of higher energies. In the moderate electric fields, the interplay of FK effect and magnetoexcitonic resonance leads to a complex and rich structure in the absorption spectrum.

Excitonic absorption of semiconductor nanorings under terahertz fields

The optical absorption of GaAs nanorings (NRs) under a dc electric field and a terahertz (THz) ac electric field applied in the plane containing the NRs is investigated theoretically. The NRs may enclose some magnetic flux in the presence of a magnetic field perpendicular to the NRs plane. Numerical calculation shows that the excitonic effects are essential to correctly describe the optical absorption in NRs. The applied lateral THz electric field, as well as the dc field leads to reduction, broadening and splitting of the exciton peak. In contrast to the presence of a dc field, significant optical absorption peak arises below the zero-field bandgap in the presence of a THz electric field at a certain frequency. The optical absorption spectrum depends evidently on the frequency and amplitude of the applied THz field and on the magnetic flux threading the NRs. This promises potential applications of NRs for magneto-optical and THz electro-optical sensing.

A Digital Pulse Drive Circuit for Continuously Modulated Semiconductor Laser

A digital designing way of laser drive circuit for continuously modulate the frequency, pulse width and amplitude of the output of semiconductor laser has been demonstrated. Complex programmable logic device (CPLD) was utilized to generate square waves with tunable frequency, the accurate delaying chip and logic circuit which are digital controlled were used to generate electrical pulses with adjustable pulse width. The pulses were amplified by the laser diode driver. The maximum output current of the driver can be up to 600 mA. When the signal was used to drive the semiconductor laser, the shape of the pulses generated by the laser is identical with the electrical pulses. The repetition rate and pulse width of the laser pulse can be continuously adjustable from 1 to 10 kHz and 3 to 20 ns, respectively. Furthermore, the time of rising and falling edges for the laser pulse were less than 2 ns.

Coherent Radiation of Picosecond Ultra-short Electromagnetic Pulse Radiated by Antenna Arrays

The experiments are setup to study the picosecond ultra-short pulses coherent radiation characteristic. The transmitting and receiving antenna employ the tie horn antennas.16 channels electrical pulses with 300 ps duration and jitter less than 50 ps are used. The different frame of 4x1, 4x2, 4x3, 4x4 antenna array radiate the pulses. The receiving antenna measures the electrical field in different distance to the transmitting antennas array. The results show that if the pulses are in coherent condition, the output pulse peak power of antennas array with N elements are N of the single element antenna output.

Compact single-stage femtosecond multipass Ti : sapphire amplifier at 1kHz with high beam quality

A compact femtosecond Ti:sapphire amplifier system is reported using single-stage multipass configuration with high beam quality. A high dispersion glass stretcher and a pair of double prisms for compression are introduced based on broadband femtosecond seed pulses. The non-grating-based pulse stretcher and compressor are advantageous to increase high beam quality and to reduce the high-order dispersion. A Gaussian filter is used to reduce the gain narrowing effect in amplification. The compact femtosecond Ti:sapphire nine-pass amplifier delvers pulses with a duration of 26 fs and an energy of 800 mu J at 7 mJ pumping pulses energy at 1 kHz. The 1-kHz femtosecond amplifier with a high beam quality and high stability is very suitable for ultrafast physics research applications, such as attosecond science, ultra-precision micromatching, and THz wave generation.