N. Fernelius’s research while affiliated with Wright State University and other places

Terahertz (THz) radiation generated by ultrafast laser pulses focused on each monoclinic semiconductor crystal, i.e., GaTe, exhibits unique features. By systemically measuring the dependence of the THz output on the ultrafast pump pulses, in terms of polarization, azimuth angle, incident angle, pump beam size, and pump intensity, we have observed the strong evidence of the unidirectional diffusion of photogenerated carriers within the surface layer of each crystal. Regardless of whether each crystal is pumped above or below its bandgap, the mechanism for THz generation is always attributed to diffusion of the photogenerated carriers. By analyzing data and introducing simplified models, it appears to us that the diffusion of the photogenerated carriers takes place along three different directions.

This paper presents the results of investigations on the influence of sulfur doping on properties of gallium selenide crystals, grown by the Bridgman method from melts with sulfur content of 0.01-3 mass %, using the Hall effect, optical absorption, photoconductivity and microhardness measurements on GaSe:S. The results obtained are explained by assuming the formation of solid solutions of GaS x Se 1-x and the presence of the competitive processes of point defects and structural disorder formation.

GaSe has a number of attractive properties for nonlinear optical applications including large birefringence for ease in phase matching. Its biggest drawback is its mechanical properties. GaSe has a strong tendency to cleave along the <100> plane which has made it difficult to grow and fabricate. We have developed a method to modify GaSe by structurally strengthening the material by doping. We have synthesized large boules of GaSe reacted mixtures and grown centimeter size single crystals by the Bridgman technique. Depending on the dopant and crystal quality, SHG measurements indicate a deff, of 51 to 76 pm/V. SHG power levels were theoretically calculated and appear to be in good agreement with the experimental data. The measured performance of crystals for the fourth harmonic generation and laser damage threshold are also reported in this paper. The damage threshold was greater than 2.8 J/cm2 and 85 KW/cm2 at the surface of the crystal.

We have investigated the characteristics of THz generation including the dependence of the output power and polarization on the incident angle and pump polarization from two series of InN films grown by plasma-assisted molecular beam epitaxy (PAMBE) and metal organic chemical vapor deposition (MOCVD), respectively. Following the analyses of our results, we have attributed the mechanism of the THz generation from these InN samples to the destructive interference between optical rectification and photocurrent surge. Under the average intensity of 176 W cm−2 for the subpicosecond laser pulses at 782 nm, the THz output powers were measured to be as high as 2.4 µW from the 220 nm InN film, with the output frequencies spanning the band from 300 GHz to 2.5 THz.

We comparatively studied laser performance in the mixed vanadate crystals Nd:YxGd1-xVO4 with direct and indirect pumping. The maximum CW output power at 1064 nm was measured to be 6.6 W with 12 W direct 880 nm laser diode pumping, and 4.4 W with 8.8 W indirect 808 nm pumping. The optical conversion efficiency depended on the mixture ratio (x), pumping wavelength (direct or indirect), and pumping power. The hightest optical efficiency of 59% and slope efficiency of 74% were achived.

THz average output power as high as 2.4 microwatts is generated from InN films, with the mechanism being the interference between optical rectification and photocurrent surge.

We observed that photoluminescence intensities clamped at certain values as the pump intensity was increased, due to the presence of nonlinear degenerate electron gas and saturation of photogenerated and localized holes in InN.

We present, for the first time, comparative studies of laser performance in the mixed vanadate crystals Nd:YxGd1-xVO4 with direct and indirect pumping. A series of mixed vanadate crystals Nd:Y xGd1-xVO4 with different yttrium and gadolinium composition ratios but the same Nd3+ dopant level is prepared under the same fabrication processes and their laser performance in the TEM 00 mode operation is studied using the same experimental configuration under direct pumping with laser diodes at 880nm as well as indirect pumping at 808nm. Remarkable improvement in laser efficiency and maximum achievable output power is achieved under direct 880-nm excitation over indirect 808-nm excitation. The maximum optical slope efficiency in the TEM 00 mode operation is 74% with direct 880-nm pumping, which also produces the maximum overall optical-to-optical conversion efficiency of 59%. The maximum attainable laser power is up to 50% higher under direct 880-nm pumping over the more conventional 808-nm pumping. These achieved results provide valuable insight into crystal composition selection and pump schemes for the mixed vanadate crystals, which have the potential to tailor laser parameters through crystal composition engineering.

We present, for the first time, comparative studies of laser performance in the mixed vanadate crystals Nd:YxGd1-xVO4 with direct and indirect pumping. The resulting highly efficient miniaturized laser has potential for laser communications.

A series of mixed vanadate crystals Nd:YxGd1-xVO4with the same Nd3+dopant level was investigated. The results revealed general characteristics and trends in laser performance at different Y/Gd ratios and at direct and indirect pumping.