Toward the realization of erbium-doped GaN bulk crystals as a gain medium for high energy lasers

Abstract

Er-doped GaN (Er:GaN) is a promising candidate as a gain medium for solid-state high energy lasers (HELs) at the technologically important and eye-safe 1.54 μm wavelength window, as GaN has superior thermal properties over traditional laser gain materials such as Nd:YAG. However, the attainment of wafer-scale Er:GaN bulk or quasi-bulk crystals is a prerequisite to realize the full potential of Er:GaN as a gain medium for HELs. We report the realization of freestanding Er:GaN wafers of 2-in. in diameter with a thickness on the millimeter scale. These freestanding wafers were obtained via growth by hydride vapor phase epitaxy in conjunction with a laser-lift-off process. An Er doping level of 1.4 × 1020 atoms/cm3 has been confirmed by secondary ion mass spectrometry measurements. The freestanding Er:GaN wafers exhibit strong photoluminescent emission at 1.54 μm with its emission intensity increasing dramatically with wafer thickness under 980 nm resonant excitation. A low thermal quenching of 10% was measured for the 1.54 μm emission intensity between 10 K and 300 K. This work represents a significant step in providing a practical approach for producing Er:GaN materials with sufficient thicknesses and dimensions to enable the design of gain media in various geometries, allowing for the production of HELs with improved lasing efficiency, atmosphere transmission, and eye-safety.

Enhancement of 1.5 μm emission under 980 nm resonant excitation in Er and Yb co-doped GaN epilayers

“…[4][5][6][7] In particular, the erbium (Er) doped GaN (Er:GaN) has been extensively studied. [8][9][10][11][12][13][14] Er:GaN is a very promising gain material for optical amplifiers 15 and high energy lasers 16 due to its emission at the wavelength of 1.5 lm, which coincides with the wavelength of lowest attenuation in silica optical fibers, [17][18][19] and it has high transmission in the atmosphere, 20 and has a relatively high upper limit of eye-safe laser exposure. 21 Moreover, GaN possesses a high thermal conductivity (j ¼ 253 W/mÁK) and a low thermal expansion coefficient (a % 3.53 Â 10 À6 C À1 ), 22 making it highly attractive as a laser gain medium because the maximum achievable lasing power for a solid-state laser, to the first order, is characterized by the thermal shock parameter of j/a 2 .…”

Growth and fabrication of GaN/Er:GaN/GaN core-cladding planar waveguides

“…Furthermore, it was shown that the 1.5 lm emission in Er:GaN has an excellent thermal stability because GaN has a wide energy bandgap. [16][17][18] Compared with gain media in the bulk geometries such as disks, rods, and slabs, core-cladding planar waveguides (PWGs) have the advantages of enhanced optical gain and reduced lasing threshold as the optical energy is confined in the core layer (waveguide). Meanwhile, core-cladding PWGs possess sufficiently large surface areas to provide an excellent heat removal capability, thereby minimizing the thermal impact during high power operation.…”

Resonant excitation cross-sections of erbium in freestanding GaN bulk crystals

“…Moreover, prior results have demonstrated that 1.5 lm Er emission in GaN have a high temperature stability due to the wide bandgap nature of GaN. [15][16][17] To realize practical design of gain materials for HEL, thick layers of Er:GaN are needed to provide adequate dimensions to support a sufficient pumping light absorption and surface area for heat removal as well as mechanical strength. On the other hand, the excitation and emission mechanisms as well as the transition cross sections of the pump and laser wavelengths are of paramount importance for understanding the performance of lasers and amplifiers made of Er:GaN materials.…”

“…After the HVPE growth, the sapphire substrate was removed by a laser-lift-off process. 16 The obtained freestanding Er:GaN was then polished and cut into different geometries. The PL emission spectra were measured using a 980 nm laser diode as an excitation source, and a monochromator (SpectraPro-300i, Acton Research Corporation) in conjunction with an IR detector was used to disperse the PL signal.…”