Relationship measurement between ac-Stark shift of^40Ca^+clock transition and laser polarization direction

Ac-Stark shift of atom levels is caused by an ac-electromagnetic field. As an electromagnetic wave, laser light does induce ac-Stark shift. It is proved experimentally that if the light is linearly polarized, the dynamic polarizability changes with polarization direction. The polarization direction of the linearly-polarized laser is tuned by 720°, and the ac-Stark shifts of the 4S1/2,m=±1/2→3D5/2,m=±1/2 clock transitions in ^40Ca^＋ are measured in steps of 10°. The frequency shifts change with laser polarization in a periodical manner and have values opposite to each other.

A novel modulation and direct detection scheme of optical phase shift keying

This paper introduces a new modulation and direct detection scheme of optical phase shift keying (PSK) which is simple and practical in fiber optical communication. A phase modulator is used to modulate a continuous wave (CW) laser source and return-to-zero (RZ) signal that is changed from the initial transmitting information is used to control a phase modulator to form a optical PSK signal. In the receiver terminal, just add a signal delayed a half of one bit to itself so that the initial information can be restored.

Reducing the stray light of holographic gratings by shifting the substrate a short distance in the direction parallel or perpendicular to the exposure interference fringes

This research proposes a simple and practical method to make low-stray-light gratings, where the substrate shifts about a 1 mm distance in the direction parallel or perpendicular to the exposure interference fringes. When the substrate shifts, a reference grating next to the substrate is used to adjust in real time the phase of the exposure interference fringes relative to the substrate. Shifting eliminates the exposure defects and therefore decreases the stray light of gratings. Several gratings are successfully made by using this method, which have straighter grooves,smoother surfaces, and lower stray light than gratings made in conventional interference lithography.

Substituent Effects on Direct Arylation Polycondensation and Optical Properties of Alternating Fluorene-Thiophene Copolymers

Four thiophene derivatives were prepared by replacing the 3,4 positions of thiophene by―OCH3, ―CH3―COOCH3 and ―CN, respectively. The polycondensations via direct arylation took place between the four thiophene derivatives and 2,7-dibromo-9,9-dioctylfluorene under six various catalytic conditions to investigate the substituent effects. The substituent can affect the electron cloud density of the active C―H bond, which can be monitored by the NMR chemical shift. The experimental results show that the reactivity decreases with increasing the chemical shift of active C―H bonds in the four thiophene derivatives, and thus can promote the direct arylation polycondensation. This phenomenon is explained by the electrophilic aromatic substitution（SEAr） mechanism. UV-Visible absorption and photoluminescence were studied to investigate the substituent effect on optical properties of the four copolymers. The results show that these alternating fluorene-thiophene copolymers with different substituents are good fluorescent materials and promising in PLED applications.