Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2001-07-30
2003-12-02
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C423S021100, C423S023000, C423S069000, C423S084000, C423S089000, C423S099000, C423S111000, C423S115000, C423S155000, C423S179000, C428S403000, C428S323000
Reexamination Certificate
active
06656588
ABSTRACT:
TECHNICAL FIELD
The subject invention pertains to solid state powder lasers and nanophosphors, to processes for producing powders suitable for such use, and to the powders produced thereby.
BACKGROUND ART
Past attempts to utilize highly scattering solid materials as candidates for stimulated emission have required irradiation with high intensity energy sources, such as lasers, to demonstrate stimulated emission. “Laser paints”, surfaces coated with solid particulates, have required such a high threshold level of optical energy input, because of the excessive attenuation (and therefore loss) that normally accompanies scattering, that their suitability for most practical purposes is highly questionable.
Although electron beams are known to cause emission in solid, dielectric phosphors, these materials are not known to support continuous-wave laser action. Atoms in the solid lattice that absorb the electron energy subsequently release it spontaneously and randomly. A large portion of the energy emitted is reabsorbed by the particulate, by its neighbors, or is highly scattered. Phosphors used in CRT tubes, for example, absorb electron beam energy and display spontaneous but not stimulated luminescence, scattering, etc.
Since the discovery of solid state lasers in the early 1960s, lasers have become progressively more commonplace in society, finding applications ranging from compact disc systems and supermarket scanners to precision surgical, optimetric and cutting instruments. The available types of lasers are very diversified, and now comprise solid, liquid, gas and plasma media pumped by light, electrons, chemical reactions, or other means. Generally, lasers require an external cavity to operate.
Luminescence in the multiple-scattering regime has been reported by several researchers from powders containing rare earth and transition metal ions, and extensively investigated theoretically. B. M. Tissue, “Synthesis and luminescence of lanthanide ions in nanoscale insulating hosts,” Chem Mater. 10, 2837-45 (1988), and laser action, N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes and E. Sauvain, “Laser action in strongly scattering media,” Nature 368, 436-438 (1994); C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Migus, “Generation of spatially incoherent short pulses in laser-pumped neodymium stoichiometric crystals and powders,” J.O.S.A. B10, 2358-2363 (1993); D. Wiersma and A. Lagendijk, “Light diffusion with gain and random lasers,” Phys. Rev. E54, 4256-4265 (1996); V. S. Letokhov, “Generation of light by a scattering medium with negative resonance absorption,” Sov. Phys. JETP 26, 835 (1968); S. John and G. Pang, “Theory of lasing in a multiple-scattering medium,” Phys. Rev. A54, 3642-3652 (1996). Spectral narrowing and threshold behavior have been reported in the absence of external cavities, and measured transient behavior shown to be characteristic of inverted systems of impurity ions with optical feedback mediated entirely by scattering. Lossless powders in which gain is encountered despite very short scattering mean free path lengths are sometimes referred to as “laser paints.” D. Wiersma and A. Lagendijk, “Laser action in very white paint,” Physics World, 33-37, January 1997.
Laser paint media in which the mean transport length l* is actually less than the wavelength itself have interesting properties which may be useful for speckle-free lithography at sub-micron dimensions or applications in which bright, omni-directional output is desired for displays or light sources of arbitrary shape. However, highly-scattering powders are difficult to pump and study optically because the very scattering that provides the feedback for laser action causes pump light to be scattered backwards very efficiently as it enters the medium. Incident light does not penetrate the medium well and the overall efficiency of any pumping and lasing processes is destined to be low. This effect would be particularly true under conditions of “strong localization,” when light propagation undergoes an Anderson transition, P. W. Anderson, “Absence of diffusion in certain random lattices,” Phys. Rev. 109, 1492 (1958); P. W. Anderson, “The question of classical localization: a theory of white paint?,” Philos. Mag. B52, 505 (1985), to a regime of recurrent scattering which results in completely localized electromagnetic “transport.” D. S. Wiersma, M. P. van Albada, B. A. van Tiggelen, and A. Lagendijk “Experimental evidence for recurrent multiple scattering events of light in disordered media,” Phys. Rev. Lett. 74, 4193-4196 (1995). In this regime, scattering would be so strong that the direction of light would be randomized before it has propagated a distance of even one wavelength.
Previous experiments, and theory, on localization of light and stimulated emission in scattering media, as well as interest in the consequences of recurrent scattering events have all heightened current interest in electromagnetic phenomena in multiple-scattering media. See, e.g., M. P. Van Albada and A. Lagendijk, Phys. Rev. Lett. 55, 2692 (1985); V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, Sov. J. Qu. El. 16, 281 (1986); A. Z. Genack and N. Garcia, Phays. Rev. Lett. 66, 2064 (1991); J. X. Zhu, D. J. Pine, and D. A. Weitz, Phys. Rev. A44, 3948-3959 (1991); C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Migus, J.O.S.A. B10, 2358-2363 (1993); N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes and E. Sauvain, Nature 368 436-438 (1994); M. Siddique, R. R. Alfano, G. A. Berger, M. Kempe, and A. Z. Genack, Opt. Lett. 21, 450 (1996); M. A. Noginov, N. E. Noginov, H. J. Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov, J.O.S.A. B13, 2024 (1996); D. Wiersma and A. Lagendijk, Physics World, 33-37, January 1997; S. John, Phys. Rev. Lett. 53 2169 (1984); P. W. Anderson, Phil. Mag. B52, 505 (1985); V. S. Letokhov, Sov. Phys. JETP 26, 835. (1968); S. John and G. Pang, Phys. Rev. A54, 3642-3652 (1996); R. M. Balachandran, N. M. Lawandy, and J. A. Moon, Opt. Lett. 22, 319 (1997); D. Wiersma and A. Lagendijk, Phys. Rev., E54, 4256-4265 (1996); G. A. Berger, M. Kempe, and A. Z. Genack, Rev. E56, 6118 (1997); D. S. Wiersma, M. P. van Albada, B. A. van Tiggelen, and A. Lagendijk, Phys. Rev. Lett. 74, 4193-4196 (1995); 18. D. S. Wiersma, P. Bartolini, A. Lagendijk and R. Rhigini, Nature 390, 671 (1997). Traditionally, multiple scattering has been of interest to researchers studying statistical aspects of weakly localized light coherence or imaging. See e.g., M. Kaveh, M. Rosenbluh and I. Freund, Nature 326, 778 (1987), G. Gbur and E. Wolf, Opt. Lett. 24, 10 (1999); E. Wolf, T. Shirai, G. Agarwal, L. Mandel, Opt. Lett. 24, 367 (1999); E. Leith, C. Chen, H. Chen, Y. Chen, D. Dilworth, J. Lopez, J. Rudd, P.-C. Sun, J. Valdmanis, and G. Vossler, J.O.S.A. A9, 1148 (1992). Others have demonstrated powder lasers in the diffusive propagation regime. See, V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, Sov. J. Qu. El. 16, 281 (1986); C. Gouedard, D. Husson, C. Sauterei, F. Auzel, and A. Migus, J.O.S.A. B10, 2358-2363 (1993); M. A. Noginov, N. E. Noginov, H. J. Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov, J.O.S.A. B13, 2024 (1996). However, at the boundary between the diffusive and strong scattering regimes it is expected that significant changes will occur in the interaction of light with matter which are fundamentally new. Severe scattering is predicted to cause strong Anderson localization of light when absorption is negligible. Three-dimensional confinement of light within regions of sub-wavelength dimensions could have profound implications for the degree of coherence and back-scattered intensity of elastically-scattered light. The dielectric constant becomes difficult to define when propagation is restricted to sub-wavelength “transport” distances (l*<1) in random media, because fluctuations in the structure of the medium and non-uniformities in the field amplitude occur on the same distance scale. See, S. John, Phys. Rev. Lett. 53, 2169 (1984); 11. P. W. Anderson, Phil. Mac.
Hinklin Thomas
Laine Richard M.
Rand Stephen C.
Williams Guy R.
Brooks & Kushman P.C.
Le H. Thi
The Regents of the University of Michigan
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