II-VI Lasers Produce Powerful Green Emission
An international research team has constructed a very powerful, highly efficient green source that employs a violet nitride laser to optically pump a II-VI heterostructure. The 150 mW, pulse-driven 543 nm laser that results is a very promising candidate for the green source for low-cost, high-resolution picoprojectors that can be incorporated into smartphones, digital cameras and laptops.
The green laser created by researchers from Ioffe Physical-Technical Institute, Russia, the King Abdulaziz City for Science and Technology, Saudi Arabia, and the Institute of Physics of NAS Belarus, produces a conversion efficiency in excess of 25 percent and a wall-plug efficiency (WPE) of 6 percent. This is comparable to the WPE of second harmonic generation (SHG) lasers, which have values of 6-12 percent, and nearly three times that of green InGaN lasers emitting at 523-525 nm.
Sergei Sorokin from Ioffe Physical- Technical Institute says that the design of the II-VI converters is probably simpler than those employed in SHG laser diodes. What’s more, he claims that the output power of these IIVI lasers promises to exceed that of their InGaN cousins, which have relatively high threshold current densities when operating in the green spectral range. “Additionally, the lasing wavelength of the III-N/II-VI converters can be varied in the wide green range from 520 to 560 nm," adds Sorokin.
Historically, II-VI lasers have been plagued with short lifetimes. However, Sorokin says that with this wavelength-converting design, device lifetime should be governed by that of the violet nitride laser. “The undoped II-VI optically-pumped laser structures possess long enough lifetime."
The team have been developing lasers incorporating II-VI structures for several years, and in 2007 they reported a source with five electronically coupled CdSe/ZnSe quantum dot planes that delivered a pulsed output of 65 mW. The latest design produces nearly three times this power, thanks to the introduction of a superlattice graded-index waveguide. This introduces an electric field, which sweeps carriers that are optically generated in this waveguide into the quantum dots.
Fabricating this waveguide from II-VI materials is very challenging because minor adjustments to the composition of ZnMgSSe produce large changes in lattice constant. “The superlattice graded-index waveguide allows digital variation of composition profile, keeping all the growth parameters unchanged," says Sorokin. What’s more, strain balancing is possible in this structure.
The team forms its Cd(Zn)Se/ZnMgSSe quantum dot structure by MBE on a GaAs buffer layer. Migration enhanced epitaxy, which is similar to atomic layer epitaxy, is employed to deposit the initial ZnSe layer on GaAs. “We deposit only a part of monolayer during one deposition cycle, due to surface reconstruction peculiarities," says Sorokin. “The interruption between zinc and selenium fluxes is necessary to allow efficient redistribution of adatoms at the proper places on the surface."
The team’s green source, which employs a ToPGaN 416 nm laser with a 1W output in pulsed mode (pulse duration of 50 ns and frequency of 1 kHz), has a peak roomtemperature output of 154 mW.
Sorokin says that pumping must be performed with ultra-violet LEDs, rather than laser diodes, before it will be possible to make cheap, compact devices based on the marriage of II-Vs and III-Vs. “This, in turn, requires further improvements of the threshold and output characteristics of II-VI optically-pumped laser heterostructures, as well as in the converter design as a whole."
The team is addressing this issue, and in addition, it has set itself the goals of: Cutting the density of extended defects in the II-VI laser heterostructure to below 104 cm-2; reducing the threshold power density of II-VI laser structures below 1kW/cm2; and studying device reliability.
S.V. Sorokin et al. Electron. Lett. 48 189 (2012)