MBE Growth of Rare-Earth Doped Fluoride Insulators on Semiconductors for Laser Applications
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MBE GROWTH OF RARE-EARTH DOPED FLUORIDE INSULATORS ON SEMICONDUCTORS FOR LASER APPLICATIONS
M. LUI, R.A. MCFARLANE, AND D. YAP* *Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265.
ABSTRACT With the recent success of using rare-earth doped fluoride crystals as high power visible upconversion lasers, we have explored the use of MBE grown fluoride layers for a possible waveguide laser. By confining the pumped light in a waveguide with dimensions on the order of a few micron, the pump power density will increase promoting higher efficiencies at room temperatures. Initially, we have grown planar waveguides of erbium doped ZnF2 on MgF2 substrates using molecular beam epitaxy and have formed channel waveguides by ion milling. By exciting individual channels with an 800 nm pump, we have generated strong upconversion fluorescence at 410 nm, 550 nm and 670 nm and at numerous weaker peaks. The fabrication techniques can be adapted to semiconductor substrates for making compact diode-pumped visible and infrared lasers. A number of fluoride materials that are useful as laser host crystals are lattice matched to GaAs (100) and GaAs (111) offering the possibility of integrating the channel waveguide laser with the semiconductor diode laser pump source. For example SrF2 may be grown on GaAs (100) as a cladding layer followed by PbF2 doped with a rare-earth ion. Also LaF3 may be grown on GaAs (111) followed by CeF3 doped with a rare-earth ion. Both PbF 2 and CeF3 have low phonon energies and a higher index of refraction than their respective lattice matched cladding layers and should be capable of provide an attractive upconversion laser waveguide system. Our initial upconversion luminescence results on erbium doped PbF 2 on GaAs (100) using a intervening SrF 2 cladding layer are also reported. INTRODUCTION Laser operation at wavelenghts shorter than the pumping wavelength is accomplished in a new class of laser called an "Upconversion Laser." The laser operates by combining the energy of two or more infrared pump photons to produce an excited state of an impurity ion in an appropriate laser host material that is greater than the energy of one pump photon alone. This can occur by several excitation pathways. The most obvious route is a stepwise absorption of several pump photons in a system that has adequately long lifetimes at intermediate energy level positions and favorable level spacings and decay characteristics. This sequential absorption enables a stepwise excitation process to higher ionic energy levels, and is typically realized in fiber systems where doping levels are low (0.1% or less). In bulk cyrstals having rare earth doping levels of 1 to 10%, energy contained in two or more ions, each having absorbed one pump photon, can be accumulated by a single ion through energy exchange processes that are able to efficiently excite the ion to an energy level well above the single pump photon excitation level. Initial upconversion lasers operated at cryogenic temperatures, typically 10-100K. 1-7 The neces
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