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Optical and magnetic materials
175
Optical and magnetic materials Editorial overview Olivier Kahn* and Stephen
Paynet
Addresses ‘Institut de Chimie de la Mattere Condensea de Bordeaux (ICNCB), Laboratoire de Sciences Moleculaires, Avenue du Docteur Schweitzer, 33603 Peaaac, France tLaaer Program, Lawrence Livermore National Laboratory, University of California, PC Box 808, Livermore, CA 94551-9900, USA; e-mail: payne3@!llnl.gov Current Opinion in Solid State 6 Materials Science 1996, 1 :175-l 76 0 Current Science Ltd ISSN 1359-0286
Research on luminescence has been particularly active in recent years. This field is boosted by opto-electronic applications, such as lasers, displays and optical storage media. In their article instead of covering the whole field superficially, Hazenkamp and Giidel (pp 177-182), have preferred to choose a few subareas which have been particularly innovative over past years. These include the luminescence of transition metal ions either in doped materials or in inorganic complexes, upconversion phenomena in lanthanide(III)or U(JV)-doped materials, high-resolution spectroscopy (the ultimate limit being single-molecule spectroscopy) and scanning near-field optical spectroscopy, which is a brand new technique allowing microscopy and spectroscopy with a very high spatial resolution. Hazenkamp and Giidel also devote a short section to the use of silicon in opto-electronic applications. What they make particularly clear is the interplay between the applications of the luminescence studies to the design of new laser materials, and the use of new lasers to perform more sophisticated and accurate luminescence studies. For many years, magnetic materials have been in the forefront of materials science as a whole, in terms of both scientific progress and new applications. The use of magnetic materials is so widespread as to make them truly indispensable in everyday life. Two articles in this section are devoted to new aspects of magnetism. Some other very active subfields, such as permanent magnets, will be covered in a subsequent issue of Cu~trf Opinion in Solid State d hfatetidf Scimce. The review by Givord, Lacroix and Schmitt (pp 183-191) deals with the latest findings in magnetism of intermetallics. These authors focus on four facets, namely itinerant electron magnetism, the magnetism of rare earth-transition metal compounds, the interplay between magnetism and superconductivity as observed, for instance, in the lanthanide-transition metal borocarbides (the archetype being LuNizBzC), and finally magnetism
in quasicrystals with an icosahedral symmetry. The authors point out that recent progress in the field of intermetallics’ magnetism has been characterized by both a better understanding of. the phenomena (this is particularly true as far as the interplay between magnetism and superconductivity is concerned), and the discovery of new materials which could lead to new magnetic devices. The second review dealing with magnetism, written by Gatteschi (pp 192-198), is devoted to molecular magnetic materials. Molecular magnetism is a field of research which emerged about a decade ago. This field deals with the chemistry and physics of molecular assemblies involving open-shell units. The heart of the discipline concerns the design and study of the physical properties of supramolecular assemblies exhibiting bulk magnetic properties such as long-range magnetic ordering (molecular magnets) or bistability with a hysteresis effect, which confers a memory effect on the system. Gatteschi also discusses the design of large, but finite, magnetic clusters, which can be considered as small magnetic particles of nanoscale size. Some of these high-spin clusters, with integer ground state spin, could exhibit the quantum tunneling effect. Molecular magnetism provides totally novel classes of materials. Most of these materials, in contrast with the classical magnets, are transparent or slightly colored. One of the main issues for research might be the synergy between magnetic and optical (or photophysical) properties. Several groups (in Japan and in Western Europe) are presently working along this line, and some exciting new results have recently been reported. During the past year we have seen that the close association of materials science and optical physics has produced remarkable advances in laser technologies. Four reviews in this section offer concrete examples of this phenomenon, including progress in research into ultrashort pulse formation, UV and mid-infrared wavelengths, and specialized architecture - yielding systems that are more flexible, useful and reliable. The article by Jarman (pp 199-203) reviews progress in novel optical fiber lasers, where the 155 pm Er-doped SiOe fiber amplifier, or EDFA, has essentially revolutionized the telecommunications industry by way of its simplicity and optical transparency. As we write, an effort to develop a comparable 1.3 pm telecommunications amplifier is proceeding, focusing on the Pr-doped ZBLAN fluoride fiber. Alternative fiber materials such as sulfides are also being explored for 1.3pm amplification, as well as for use in upconversion visible lasers and for mid-infrared
176
Optical and magnetic materials
devices. Other specialized techniques, such as in-fiber Bragg gratings, have proved useful for Raman amplifiers and other fiber lasers. Developments among borate crystals, as discussed by Keszler (pp 204-Zll), are directed toward the promotion of new frequency conversion schemes. Progress in BBO (B-BaBZ04) and LB0 (LiB305) has permitted the operation of practical optical parametric oscillators and the generation of shorter ultraviolet wavelengths. On the other hand, the novel borates CLBO (CsLi(B30&), BCBF (BaCaB03F), and others have the potential for permitting growth of larger crystals with high quality. Efforts to employ borates for frequency conversion at < 190nm are continuing. Diode laser devices are electrical/optical systems for which materials issues are central. One crucial goal has been to develop devices that operate efficiently and reliably in the mid-infrared, as discussed by Choi (pp 212-217). Here, Sb alloys and use of strained-layer quantum wells have proved quite valuable. Other approaches are the consideration of Type II lasers, where the valence and conduction bands are spatially separated in the structure. Quantum cascade designs operate out to 8.4pn-r at low temperature, based
on careful tailoring of the quantum confined structures and the electron transitions between them. Use of MBE (molecular beam epitaxy) growth, bandgap engineering, strained layers, and control of quantum size effects should offer the opportunity to create efficient room temperature mid-infrared devices. Keller (pp 218-224) describes in her article how the anti-resonant Fabry-Perot saturable absorber (A-FPSA) can lead to stable performance of Kerr lens modelocked (KLM) ultrashort-pulse lasers. Manipulation of the AIAsGaAs Bragg reflectors, saturable absorbers, and variation in the finesse allow for flexible design of the saturation intensity and recovery time. Efforts with a variety of gain media have proved successful with the A-FPSA approach, including Ti:sapphire, Nd:glass, Cr:LiSAF, and other materials where femtosecond sources have been demonstrated. In summary, fiber lasers, borate frequency conversion crystals, diode lasers, and KLM lasers with the A-FPSA offer striking examples of how materials science and optical physics can come together and yield new technologies. This close partnership is likely to pave the future of laser science.
Optical and magnetic materials Editorial overview Olivier Kahn* and Stephen
Paynet
Addresses ‘Institut de Chimie de la Mattere Condensea de Bordeaux (ICNCB), Laboratoire de Sciences Moleculaires, Avenue du Docteur Schweitzer, 33603 Peaaac, France tLaaer Program, Lawrence Livermore National Laboratory, University of California, PC Box 808, Livermore, CA 94551-9900, USA; e-mail: payne3@!llnl.gov Current Opinion in Solid State 6 Materials Science 1996, 1 :175-l 76 0 Current Science Ltd ISSN 1359-0286
Research on luminescence has been particularly active in recent years. This field is boosted by opto-electronic applications, such as lasers, displays and optical storage media. In their article instead of covering the whole field superficially, Hazenkamp and Giidel (pp 177-182), have preferred to choose a few subareas which have been particularly innovative over past years. These include the luminescence of transition metal ions either in doped materials or in inorganic complexes, upconversion phenomena in lanthanide(III)or U(JV)-doped materials, high-resolution spectroscopy (the ultimate limit being single-molecule spectroscopy) and scanning near-field optical spectroscopy, which is a brand new technique allowing microscopy and spectroscopy with a very high spatial resolution. Hazenkamp and Giidel also devote a short section to the use of silicon in opto-electronic applications. What they make particularly clear is the interplay between the applications of the luminescence studies to the design of new laser materials, and the use of new lasers to perform more sophisticated and accurate luminescence studies. For many years, magnetic materials have been in the forefront of materials science as a whole, in terms of both scientific progress and new applications. The use of magnetic materials is so widespread as to make them truly indispensable in everyday life. Two articles in this section are devoted to new aspects of magnetism. Some other very active subfields, such as permanent magnets, will be covered in a subsequent issue of Cu~trf Opinion in Solid State d hfatetidf Scimce. The review by Givord, Lacroix and Schmitt (pp 183-191) deals with the latest findings in magnetism of intermetallics. These authors focus on four facets, namely itinerant electron magnetism, the magnetism of rare earth-transition metal compounds, the interplay between magnetism and superconductivity as observed, for instance, in the lanthanide-transition metal borocarbides (the archetype being LuNizBzC), and finally magnetism
in quasicrystals with an icosahedral symmetry. The authors point out that recent progress in the field of intermetallics’ magnetism has been characterized by both a better understanding of. the phenomena (this is particularly true as far as the interplay between magnetism and superconductivity is concerned), and the discovery of new materials which could lead to new magnetic devices. The second review dealing with magnetism, written by Gatteschi (pp 192-198), is devoted to molecular magnetic materials. Molecular magnetism is a field of research which emerged about a decade ago. This field deals with the chemistry and physics of molecular assemblies involving open-shell units. The heart of the discipline concerns the design and study of the physical properties of supramolecular assemblies exhibiting bulk magnetic properties such as long-range magnetic ordering (molecular magnets) or bistability with a hysteresis effect, which confers a memory effect on the system. Gatteschi also discusses the design of large, but finite, magnetic clusters, which can be considered as small magnetic particles of nanoscale size. Some of these high-spin clusters, with integer ground state spin, could exhibit the quantum tunneling effect. Molecular magnetism provides totally novel classes of materials. Most of these materials, in contrast with the classical magnets, are transparent or slightly colored. One of the main issues for research might be the synergy between magnetic and optical (or photophysical) properties. Several groups (in Japan and in Western Europe) are presently working along this line, and some exciting new results have recently been reported. During the past year we have seen that the close association of materials science and optical physics has produced remarkable advances in laser technologies. Four reviews in this section offer concrete examples of this phenomenon, including progress in research into ultrashort pulse formation, UV and mid-infrared wavelengths, and specialized architecture - yielding systems that are more flexible, useful and reliable. The article by Jarman (pp 199-203) reviews progress in novel optical fiber lasers, where the 155 pm Er-doped SiOe fiber amplifier, or EDFA, has essentially revolutionized the telecommunications industry by way of its simplicity and optical transparency. As we write, an effort to develop a comparable 1.3 pm telecommunications amplifier is proceeding, focusing on the Pr-doped ZBLAN fluoride fiber. Alternative fiber materials such as sulfides are also being explored for 1.3pm amplification, as well as for use in upconversion visible lasers and for mid-infrared
176
Optical and magnetic materials
devices. Other specialized techniques, such as in-fiber Bragg gratings, have proved useful for Raman amplifiers and other fiber lasers. Developments among borate crystals, as discussed by Keszler (pp 204-Zll), are directed toward the promotion of new frequency conversion schemes. Progress in BBO (B-BaBZ04) and LB0 (LiB305) has permitted the operation of practical optical parametric oscillators and the generation of shorter ultraviolet wavelengths. On the other hand, the novel borates CLBO (CsLi(B30&), BCBF (BaCaB03F), and others have the potential for permitting growth of larger crystals with high quality. Efforts to employ borates for frequency conversion at < 190nm are continuing. Diode laser devices are electrical/optical systems for which materials issues are central. One crucial goal has been to develop devices that operate efficiently and reliably in the mid-infrared, as discussed by Choi (pp 212-217). Here, Sb alloys and use of strained-layer quantum wells have proved quite valuable. Other approaches are the consideration of Type II lasers, where the valence and conduction bands are spatially separated in the structure. Quantum cascade designs operate out to 8.4pn-r at low temperature, based
on careful tailoring of the quantum confined structures and the electron transitions between them. Use of MBE (molecular beam epitaxy) growth, bandgap engineering, strained layers, and control of quantum size effects should offer the opportunity to create efficient room temperature mid-infrared devices. Keller (pp 218-224) describes in her article how the anti-resonant Fabry-Perot saturable absorber (A-FPSA) can lead to stable performance of Kerr lens modelocked (KLM) ultrashort-pulse lasers. Manipulation of the AIAsGaAs Bragg reflectors, saturable absorbers, and variation in the finesse allow for flexible design of the saturation intensity and recovery time. Efforts with a variety of gain media have proved successful with the A-FPSA approach, including Ti:sapphire, Nd:glass, Cr:LiSAF, and other materials where femtosecond sources have been demonstrated. In summary, fiber lasers, borate frequency conversion crystals, diode lasers, and KLM lasers with the A-FPSA offer striking examples of how materials science and optical physics can come together and yield new technologies. This close partnership is likely to pave the future of laser science.