Self-Q-switched, orthogonally polarized, dual-wavelength laser using long-lifetime Yb³âº crystal as both gain medium and saturable absorber.

A novel self-Q-switched and orthogonally polarized dual-wavelength laser has been investigated with Yb³âº-doped CGB disordered crystals. By slightly inclining output coupler to introduce the Fresnel loss, we realized simultaneously dual-wavelength laser operation at 1052.6 nm in E//b polarization and 1057.7 nm in E//c polarization with a frequency difference of 1.38 THz. Self-Q-switched pulse generation was observed in this free-running laser, originating from the nonlinear reabsorption effect of Yb:CGB as well as the strong storage of inversion population induced by the long excited-state lifetime (~1 ms). Pulse duration of 287 ns was obtained with an output average power of 416 mW and a repetition rate of 35 kHz. The self-Q-switching effect increased the peak power by 100 times the average power. This very simple laser, free from the complexity and high-cost of additional intracavity polarization modulator and Q-switcher, may be applied for constructing miniature, integrated and portable laser system.

Spectral analyses of Dy3+/Sr2+: LaF3 and Dy3+/Ca2+: LaF3 mixed crystals for laser applications.

Mixed crystals of Dy3+/Sr2+: LaF3 and Dy3+/Ca2+: LaF3 were grown by Bridgman technique and their spectral properties were investigated. Spectra broadening and peak shifts were observed, indicating the co-doping of Sr2+/Ca2+ brings about a more disordered local symmetry of Dy3+, which makes both crystals favorable for tunable lasing action. Low-temperature high resolution excitation and emission spectra were carried out for exploring the types of luminescent center of Dy3+ in crystals. Room-temperature absorption and emission spectra, together with the fluorescence decay curves were studied in both crystals for estimating their potentials for yellow and MIR lasers. Under 450 nm excitation, the largest emission cross-sections at 571 nm of 1.51 × 10-21 cm2 for Dy3+/Sr2+: LaF3 crystal and 1.56 × 10-21 cm2 for Dy3+/Ca2+: LaF3 crystal, along with the lifetimes of Dy3+: 4F9/2 level as 0.983 ms for Dy3+/Sr2+: LaF3 crystal, 1.143 ms for Dy3+/Ca2+: LaF3 crystal were obtained, respectively. Besides yellow emissions, MIR emissions approximately at 3 µm are more appealing. Under 1280 nm excitation, the largest emission cross-sections of 0.304 × 10-20 cm2 at 2885 nm in Dy3+/Sr2+: LaF3 crystal, and 0.319 × 10-20 cm2 at 2880 nm in Dy3+/Ca2+: LaF3 crystal, together with rather long lifetimes of Dy3+: 6H13/2 in the level of milliseconds were achieved, making them useful media for MIR lasers.

The formation of a hydrated homochiral helix from an achiral zwitterionic salt, spontaneous chiral symmetry breaking and redox chromism of crystals.

A unique supramolecular helix assisted by water tetramers is obtained by static evaporation of aqueous solution of a zwitterionic salt, 1,1'-bis(4-carboxylatebenzyl)-4,4'-bipyridinium; spontaneous chiral symmetry breaking occurs in crystallization; interestingly, redox chromism behavior of crystals upon exposure to alcohol atmosphere has been observed.

[Study on the growth and spectral characteristics of LiNbO3:Cr:ZnO crystal].

The LiNbO3:Cr:ZnO crystals with the size up to phi20 mm x 50 mm were grown by the Czochralski technique. The absorption spectrum shows that there are two strong absorption wideband peaks and one weak absorption peak of to Cr3+ ion in the crystal. The peak values of the two strong absorption wideband peaks are 480 and 660 nm, which correspond to 4A2-->4 T1 and 4A2-->4 T2 transitions, respectively. The weak absorption peak at 727 nm corresponds to 4A2-->2 E(R line) transition; The fluorescence spectrum shows that one emission wideband peak ranging from 802 to 988 nm coexists with one weak emission peak at 754 nm corresponding to 4T2-->2 E transition. The peak value of emission wideband is at 871 nm, which corresponds to 4T2-->4A2 transition. The crystal field and Racah parameters were calculated, and Dq/B = 2.72 indicates that it has an stronger crystal field. The result shows that the wideband tunable laser can be obtained from this crystal because it not only has the necessary spectral characteristics required for tunable laser crystal but also has good physical chemical properties. Furthermore, the UV laser at about 420 nm can also be obtained from the crystal by its self-frequency doubling because it has a higher frequency-doubling coefficient.

[Crystal growth and spectroscopy of Er/Yb:KGW crystal].

The Er3 /Yb3+:KGW crystal with the dimensions of 30 mm x 25 mm x 15 mm was grown from K2W2O7 solvent by modified Czochralski method. The absorption spectrum was measured at room temperature and its absorption peaks were assigned. The emission spectrum was obtained under the excitation of 980 nm. There are two broad emission bands at 1024 and 1535 nm, whose FHWM are 60 and 36 nm respectively. It shows that this crystal is suitable for InGaAs LD pumping, and may be a promising laser crystal tunable at near 1 024 and 1 535 nm.

[Study on the growth and spectral characteristics of Cr3+: Al2 (WO4)3 crystal].

Cr3+:Al2(WO4)3 Crystal was grown from the Na2WO4 solvent system by using the TSSG method. The absorption spectrum shows that there are two strong absorption wideband peaks and one we ak absorption peak belonging to Cr3+ ion in the crystal. The peak values of two strong absorption wideband peaks are at 422 and 595 nm, respectively. The values of two weak absorption peaks are at 684 and 663 nm, respectively. The fluorescence spectrum shows that one emission wideband peak ranging from 650 to 820 nm coexists with one stronger emission peak at 680 nm. The peak value of emission wideband is at 740 nm. The crystal field and Racah parameters were calculated, where Dq/B = 2.420 indicates that it has an intermediary crystal field. The result shows that Cr3+:Al2(WO4)3 crystal is promising for tunable laser because it not only has the spectral characteristics required for tunable laser crystal but also has the good physicochemical properties.

Ultrasensitive nonlinear absorption response of large-size topological insulator and application in low-threshold bulk pulsed lasers.

Dirac-like topological insulators have attracted strong interest in optoelectronic application because of their unusual and startling properties. Here we report for the first time that the pure topological insulator Bi2Te3 exhibited a naturally ultrasensitive nonlinear absorption response to photoexcitation. The Bi2Te3 sheets with lateral size up to a few micrometers showed extremely low saturation absorption intensities of only 1.1 W/cm(2) at 1.0 and 1.3 µm, respectively. Benefiting from this sensitive response, a Q-switching pulsed laser was achieved in a 1.0 µm Nd:YVO4 laser where the threshold absorbed pump power was only 31 mW. This is the lowest threshold in Q-switched solid-state bulk lasers to the best of our knowledge. A pulse duration of 97 ns was observed with an average power of 26.1 mW. A Q-switched laser at 1.3 µm was also realized with a pulse duration as short as 93 ns. Moreover, the mode locking operation was demonstrated. These results strongly exhibit that Bi2Te3 is a promising optical device for constructing broadband, miniature and integrated high-energy pulsed laser systems with low power consumption. Our work clearly points out a significantly potential avenue for the development of two-dimensional-material-based broadband ultrasensitive photodetector and other optoelectronic devices.

[The up-conversion of frequency in Yb3+ :KGd(WO4)2 material].

By using the method of solid phase synthesis, a series of samples of KYbxGd(1 - x) (WO4)2 with different Yb3+ mole fraction of x = 0.03, 0.08, 0.10, 0.12, 0.15, 0.18, 0.20, 0.25, 0.28, respectively were synthesized when they were heated at 1,000 degrees C. Their fluorescence spectra were measured by using LD with a wavelength of 980 nm as the pumping source and RF-540 fluorescence spectrum measurement device. The weak fluorescence at 1,020 nm and strong blue fluorescence at 476 nm were obtained. Also, the relationship between the Yb3+ doping mole fraction and the fluorescence intensity at 476 nm was measured. The result indicates that the intensity of blue fluorescence increases rapidly with the increase of Yb3+ doping mole fraction and reaches the highest point at the Yb3+ doping mole fraction of 25%, then drops quickly with the increase of Yb3+ doping mole fraction. The result also indicates that the fluorescence intensity at 1,020 nm reaches the highest point when the Yb3+ doping mole fraction is about 8%-10%.