Jiutao Wu

Ultrafast optical Kerr effect of Ag–BaO composite thin films

We report the ultrafast optical Kerr effect of Ag–BaO composite thin films by the femtosecond time-resolved pump-probe technique. The Ag–BaO thin films with Ag nanoparticles embedded into the BaO semiconductor matrix were prepared using a vacuum evaporation-deposition multimetallic layer method. The third-order nonlinear optical susceptibility of the thin films with the thickness of approximately 300 nm and the volume fraction of Ag nanoparticles in the thin films of about 25% was estimated to be 4.8×10−10 esu at the incident laser wavelength of 820 nm. The response time, i.e., the full width at half maximum of the Kerr signal, was as fast as 210 fs. The intrinsic third-order optical nonlinearity, or the optical Kerr effect of the thin films, can be attributed to the change of refractive index due to the intraband transition of electrons from the occupied state near the Fermi level to the unoccupied state in the Ag nanoparticles. Such nonlinearity is further enhanced by the local field effect that is pres...

Demonstration of a stable erbium-fiber-laser-based frequency comb based on a single rubidium atomic resonator

We propose a compact stabilization system for erbium(Er)-fiber-laser-based frequency comb with wavelengths ranging from 1510 nm to 1610 nm, which is widely used in fiber-based optical communication. In this stabilization scheme, both the repetition rate and mode of the comb is directly stabilized to a Rb atomic resonator, based on the physical phenomenon of coherent population trapping (CPT) and saturated absorption resonance, respectively. The stabilized frequency comb provides the long-term instabilities of ∼3 × 10−12 for repetition rate and of ∼7 × 10−12 for each optical mode of comb and can meet the requirement of optical-comb-based communication.

Analysis of Long-Term Phase-Locking Technique for Mode-Locked Laser With PID Regulator

We analyze the environmental effect on the standard phase-locking of the mode-locked laser. The mechanism of the improved phase-locking loop (PLL) with a standard proportional-integral-derivative (PID) regulator is described, and its stabilization principle is analyzed in detail. We utilized the standard PLL and the PLL with a standard PID regulator to phase-lock a mode-locked erbium-doped fiber laser to a rubidium atomic clock at 100 MHz for days. The agreement between the experimental results and simulation has proved that our theoretical analysis for phase-locking of mode-locked laser with PID regulator is correct. The phase-locking experiment also shows that the PLL with PID regulator has a good performance of long-term phase and frequency stabilities.

A stable frequency comb directly referenced to rubidium electromagnetically induced transparency and two-photon transitions

We demonstrate an approach to create a stable erbium-fiber-based frequency comb at communication band by directly locking the combs to two rubidium atomic transitions resonances (electromagnetically induced transparency absorption and two-photon absorption), respectively. This approach directly transfers the precision and stability of the atomic transitions to the comb. With its distinguishing feature of compactness by removing the conventional octave-spanning spectrum and f-to-2f beating facilities and the ability to directly control the combs frequency at the atomic transition frequency, this stable optical comb can be widely used in optical communication, frequency standard, and optical spectroscopy and microscopy.

Observation of Rb two-photon absorption directly excited by an erbium-fiber-laser-based optical frequency comb via spectral control

We demonstrated the observation of Rb two-photon absorption directly excided by an optical frequency comb at fiber communication bands. A chain of comb spectral control is elaborately implemented to increase the power of the second harmonic optical frequency comb generation and the two-photon transition strength. A two-photon transition spectrum is obtained with clearly resolved transition lines. It provides a potential approach to realize the optical frequency comb or optical clock at ~1.5{\mu}m with high stability and accuracy.

Erbium fiber laser-based direct frequency comb spectroscopy of Rb two-photon transitions

We demonstrate the direct frequency comb spectroscopy of Rb 5S→5D two-photon transitions directly excited by an optical frequency comb (OFC) in 1.5 μm fiber communication bands. A chain of comb spectral manipulation and quantum coherence control is implemented to enhance the efficiency of second harmonic OFC generation and eliminate the Doppler-broadening background. The direct frequency comb spectroscopy with clearly resolved transition lines is obtained. Our scheme provides a potential approach to realize the OFC at ~1.5 μm with high stability and accuracy.

Long-Term Stabilization of Fiber Laser Using Phase-Locking Technique With Ultra-Low Phase Noise and Phase Drift

We investigated the phase noise performance of a conventional phase-locking technique in the long-term stabilization of a mode-locked fiber laser (MLFL). The investigation revealed that the electronic noise introduced by the electronic phase detector is a key contributor to the phase noise of the stabilization system. To eliminate this electronic noise, we propose an improved phase-locking technique with an optic-microwave phase detector and a pump-tuning-based technique. The mechanism and the theoretical model of the novel phase-locking technique are discussed. Long-term stabilization experiments demonstrated that the improved technique can achieve long-term stabilization of MLFLs with ultra-low phase noise and phase drift. The excellent locking performance of the improved phase-locking technique implies that this technique can be used to stabilize fiber lasers with a highly stable H-maser or an optical clock without stability loss.

Highly precise stabilization of intracavity prism-based Er:fiber frequency comb using optical-microwave phase detector

In this Letter, we demonstrate a fully stabilized Er:fiber frequency comb by using a fiber-based, high-precision optical-microwave phase detector. To achieve high-precision and long-term phase locking of the repetition rate to a microwave reference, frequency control techniques (tuning pump power and cavity length) are combined together as its feedback. Since the pump power has been used for stabilization of the repetition rate, we introduce a pair of intracavity prisms as a regulator for carrier-envelope offset frequency, thereby phase locking one mode of the comb to the rubidium saturated absorption transition line. The stabilized comb performs the same high stability as the reference for the repetition rate and provides a residual frequency instability of 3.6×10(-13) for each comb mode. The demonstrated stabilization scheme could provide a high-precision comb for optical communication, direct frequency comb spectroscopy.

Long-term miniaturized stabilization of ultrafast laser based on rubidium coherent population trapping atomic resonator

Many locking approaches have been exploited to stabilize the mode-locked laser (MLL), helping to generate high-stable microwave sources. We present a novel miniaturized stabilization scheme for MLL, based on a rubidium coherent population trapping (CPT) atomic resonator. By directly frequency-locking the repetition rate of the MLL to a 85Rb CPT resonance, we demonstrate a very small stabilization system (length of 5 cm, width of 5 cm and height of 2.2 cm) for MLL with a long-term instability of ~3×10-12.

Fabrication of ultrathin ZnO nanowires and their photoluminescence properties

Ultrathin ZnO nanowires have been fabricated by a vapor-phase transport process via the vapor-liquid-solid mechanism. The average diameter of the nanowires is about only 10 nm and the length is up to several micrometers. The experimental results show that the growth of the nanowires is very sensitive to the temperature, catalyzer materials, the content of oxygen in the carrier gas, and the gas pressure. The diameter of the nanowires is directly determined by the size of eutectic alloy droplets related to the thickness of Au catalyzer film and the gas pressure. Ultrathin ZnO nanowires can only be synthesized at a certain pressure of about 300 Torr and the temperature in the range from 450-500 /spl deg/C when a layer of 20 nm Au catalyzer film is employed. Photoluminescence spectrum of the ultrathin ZnO nanowires is also measured, and an intensive ultraviolet light emission centered at 389 nm has been observed.