Boqin Ma

Broadband efficient second harmonic generation in media with a short-range order

The authors demonstrate broadband efficient quasiphase matched second harmonic generation (SHG) in nonlinear LiNbO3 crystals. The distribution of ferroelectric domains in a short-range order provides the possibility for broadband second harmonic generation in the visible range. This process is similar to the broadband SHG in random media, but it results in much higher conversion efficiency. The unique distribution of reciprocal vectors provided by the structural short-range order plays an important role in the enhancement. The achieved broadband SHG with high conversion efficiency will aid in the development of optics integration and multiple-channel devices.

Second-harmonic generation in two-dimensional periodically poled lithium niobate using second-order quasiphase matching

Two-dimensional nonlinear photonic crystals of lithium niobate with square lattices were fabricated. In these crystals, efficient quasiphase-matched second-harmonic generation was demonstrated by using second-order quasiphase matching. As a mode-locked Nd:yttritium–aluminum–garnet laser at 1064 nm with 35 ps pulse was used, we obtained an average power of 0.86 mW at 532 nm with 2.73 mW input, which corresponds to 42% internal conversion efficiency. The variations of second-harmonic output with crystal temperature and incident angle were measured. In addition, the relation between the second-harmonic output and reversed duty cycle was studied. All the experimental results are well consistent with simulations.

Photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots under pressure

The photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots was investigated at 15 K under hydrostatic pressure up to 9 GPa. Photoemission from both the ground and the first excited states in large InAs dots was observed. The pressure coefficients of the two emissions were 69 and 72 meV/GPa, respectively. A nonlinear elasticity theory was used to interpret the significantly small pressure coefficients of the large dots. The sequential quenching of the ground and the excited state emissions with increasing pressure suggests that the excited state emissions originate from the optical transitions between the first excited electron states and the first excited hole states.

Quasiphase matched harmonic generation in a two-dimensional octagonal photonic superlattice

We have fabricated LiNbO3 nonlinear photonic crystals with a two-dimensional octagonal quasilattice. The effective collinear quasiphase matched second-harmonic orange and green lights, with conversion efficiencies of ∼38% and 23%, were realized. The noncollinear harmonic processes at multiple wavelengths were also observed with the conversion efficiency of around 5%. All the-same results can be obtained when the crystal is rotated by the integral multiples of π∕4.

Anisotropy of domain broadening in periodically poled lithium niobate crystals

We experimentally study the dependence of reversed domain broadening of lithium niobate (LiNbO3) wafers on their poling electrode configurations. Experiments show that the reversed domain area is anisotropic and has 60° symmetry. The slightest domain broadening takes place as the poling stripe electrode parallels to the diagonals of hexagonal domain walls. We suppose the anisotropy of domain expansions is directly related to the symmetry of LiNbO3 crystal lattices, and it can be understood by considering the anisotropoic distributions of tangential component of electric field and the spontaneous polarization on domain walls.

Temperature and pressure behavior of the emission bands from Mn-, Cu-, and Eu-doped ZnS nanocrystals

The temperature and pressure dependence of the photoluminescence from ZnS:Mn2+, ZnS:Cu2+, and ZnS:Eu2+ nanocrystals were investigated in the temperature range from 10 to 300 K and under hydrostatic pressure up to 6 GPa at room temperature. The orange emission (590 nm) from the 4T1-6A1 transition of Mn2+ ions, the green emission (518 nm) from the 4f65d1-4f7 transition of Eu2+ ions and the blue emission (460 nm) related to the transition from the conduction band of ZnS to the t2 level of Cu2+ ions were observed in the Mn-, Eu-, and Cu-doped samples, respectively. It was found that all of these emission bands decrease in intensity with increasing temperature. Among them the intensity of the Mn-orange emission dropped faster. The activation energies were estimated to be 58, 16, and 42 meV for the Mn-orange, Eu-green, and Cu-blue emissions, respectively. A negative pressure coefficient of −26 meV/GPa was obtained for the Mn-orange emission, which agrees with the value calculated from the crystal field theory. ...

Temperature behaviour of the orange and blue emissions in ZnS: Mn nanoparticles

The temperature dependences of the orange and blue emissions in 10, 4.5, and 3 nm ZnS:Mn nanoparticles were investigated. The orange emission is from the T-4(1)-(6)A(1) transition of Mn2+ ions and the blue emission is related to the donor-acceptor recombination in the ZnS host. With increasing temperature, the blue emission has a red-shift. On the other hand, the peak energy of the orange emission is only weakly dependent on temperature. The luminescence intensity of the orange emission decreases rapidly from 110 to 300 K for the 10 nm sample but increases obviously for the 3 nm sample, whereas the emission intensity is nearly, independent of temperature for the 4.5 nm sample. A thermally activated carrier-transfer model has been proposed to explain the observed abnormal temperature behaviour of the orange emission in ZnS:Mn nanoparticles.

Collinear second harmonic generations in a nonlinear photonic quasicrystal

The financial support of Chinese Academy of Science, Max Plank Society, and Deutscher Akademischer Austauschdienst DAAD is gratefully acknowledged.

Pressure behavior of Te isoelectronic centers in ZnS:Te

ZnS:Te epilayers with Te concentration from 0.5% to 3.1% were studied by photoluminescence under hydrostatic pressure at 15 K. Two emission bands related to the isolated Te-1 and Te-2 pair isoelectronic centers were observed in the samples with Te concentrations of 0.5% and 0.65%. For the samples with Te concentrations of 1.4% and 3.1%, only the Te-2-related peak was observed. The pressure coefficients of all the Te-1-related bands were found to be unexpectedly much larger than that of the ZnS band gap. The pressure coefficients for all the Te-2-related bands are, however, rather smaller than that of ZnS band gap as usually observed. Analysis based on a Koster-Slater model indicates that an increase of the valence bandwidth with pressure is the main reason for the faster pressure shift of the Te-1 centers, and the huge difference in the pressure behavior of the Te-1 and Te-2 centers is due mainly to the difference in the pressure-induced enhancement of the impurity potential on the Te-1 and Te-2 centers

Multi-color coherent radiation in a two-dimensional nonlinear photonic crystal with rectangular lattices

The temperature dependence of the magnetic field-induced strain (MFIS) and the field-controlled shape memory effect in Ni52Mn16.4Fe8Ga23.6 single crystals were investigated by measuring the MFIS and measuring the magnetic field-enhanced transformation strain with a field bias applied in the [001] and [010] directions of the parent phase, respectively. The results show that such material combined with the martensitic transformation can product large field-enhanced transformation strain and large MFIS. The strain accompanying the martensitic transformation is -1.61% in zero field and can be enhanced to -3.30% by a field of 960 kA/m. A MFIS of 1.04% has been induced along [001] in unstressed crystals with saturated magnetic field of 600 Win applied along the same direction at near martensitic transformation temperature. It was found that the MFIS is almost temperature independent; the maximum decrease of the saturated MFIS is less than 10%, from 265 K to 100 K. This well-behaved temperature response makes this alloy particularly valuable for industrial and military smart actuators and transducers. Furthermore, it was found that the direction in which the MFIS has the largest value is always the [001], namely, the growth direction of the crystals.