Charles Thiel's Research:

Rare-earth ions, also referred to as the Lanthanides, play an important role in much of modern optical technology as the active constituents of materials. There are an amazing number of applications for these rare-earth-activated materials and much of today's cutting-edge optical technology and future innovations rely on their unique properties.

• Perhaps the most well-known application is in rare-earth solid-state laser materials such as the archetypal Nd:YAG laser and the high-power Yb:YAG lasers. Rare-earth-doped materials, particularly garnets, vanadates, glasses, and fibers, have enabled the development of efficient, high-power, and long-lived laser sources from the near infrared to the ultraviolet regions of the spectrum. These lasers are used in countless applications including surgical instrumentation, industrial equipment, precision interferometry and holography, monitoring atmospheric composition and pollution, ultra-fast imaging, and fundamental research. For more information about rare earth lasers, click here, here, here, here, here, and here.

• Rare-earth ions also play a critical role in energy-efficient luminescent materials such as phosphors for fluorescent lamps, cathode ray tubes (CRT's), and plasma displays as both the active emitters as well as "sensitizing" agents that increase efficiency. Terbium based materials are standard green lamp phosphors (often with cerium as a sensitizer) such as the Tb:(Ln,Ce)PO₄ that is used as the efficient green phosphor in fluorescent lamps. Divalent europium is the active center of many commercially available blue phosphors such as the Eu²⁺:BaMgAl₁₀O₁₇ used in plasma display tubes, and trivalent europium in Eu³⁺:Y₂O₂S provides the "perfect" red color for television sets, which is considered to have been a critical factor in the commercial success of the color television. These materials are also of interest for solid-state lighting applications that replace conventional light sources, such as incandescent lamps, with energy-efficient and long-lived light emitting diodes, where the diode emission is converted into white light by a mixture of rare-earth phosphors. There is a strong motivation to replace mercury containing fluorescent lamps with more environmentally safe alternatives such as xenon lamps, and current research indicates that these new lamps will rely on new rare-earth phosphor materials to be energy efficient. For more information about rare earth phosphors, click here, here, here, here, here, here, and here.

• Rare-earth ions are also an important component of high-energy radiation detectors for applications such as digital x-ray imaging, computerized axial tomography (CAT scanning), positron emission tomography (PET scanning), high-energy particle physics, and even oil exploration. These scintillator materials absorb the energy from high-energy particles, gamma rays, or x-rays and convert it into visible or ultraviolet light that may be detected by conventional means, such as photomultiplier tubes or photodiodes. Lutetium materials provide the high density required for efficient absorption of the incident radiation, and cerium ions are the active centers in some of the fastest and most efficient scintillator materials available such as CeF₃, Ce³⁺:YAlO₃, and particularly Ce³⁺:Lu₂SiO₅ (LSO), which is used extensively in medical applications. For more information about rare earth scintillators, click here, here, and here.

• Semiconductors such as GaN or ZnS doped with rare-earth ions may exhibit electroluminescence: the emission of light when an electrical current is passed through the material. These materials extend the capabilities of traditional semiconductor light emitting diodes (LED's) and may enable new technologies for efficient and high-contrast emissive flat panel displays. These materials may also be used to improve many electro-optical applications that rely on the direct generation of either narrow or broad spectral emission from direct electrical pumping. For more information about rare earth electroluminescence, click here and here.

• Erbium-doped fiber amplifiers are essential to the expansion and development of the worldwide telecommunications network since they provide a means of amplifying signals in optical fibers at the 1.5 µm telecommunications band. In addition, the coherence properties of erbium materials may enable all-optical signal routing, correlating, and processing at 1.5 µm to provide new capabilities and enhanced performance for the telecommunications industry. For more information about Erbium-doped fiber amplifiers, click here, here, here, and here.

• The unique properties of rare-earth ions also make them excellent candidates for the next generation of optical data storage and processing, potentially allowing dramatic increases in both storage density and processing speed that may result in staggering advances in computationally intensive applications such as biological modeling. For more information about rare earth materials for optical data storage and processing, click here, here, here, here, and here.

• Recent work on laser frequency locking to rare-earth transitions has also shown tremendous possibilities for new laser sources that are ultra-stable. These stabilized lasers can be used as ultra-high precision frequency references or clocks, new sources for laser spectroscopy, or for demanding interferometric distance measurements. For more information about rare earth materials for laser stabilization, click here and here.

The number of applications for rare-earth-activated optical materials is constantly increasing and further studies and better understanding of these materials will undoubtedly lead to even greater advances in optical technology.

Among the elements, only the transition metals, rare earths, and actinides (sometimes referred to together as the transitional elements) form stable compounds with partially filled electronic shells. These partially filled shells of d or f electrons give rise to spectrally narrow localized electronic transitions that occur at wavelengths ranging from the far-infrared to the vacuum-ultraviolet and provide the basis for optical technology in which light may dynamically interact with a material that contains these ions.

The property of the rare-earth ions that sets them apart from the other transitional elements is that their 4f electrons remain highly localized to the ion and their optical transitions maintain much of an atomic-like character even when the ion is an element of a crystalline solid. This atomic-like behavior of the rare-earth ions' 4f electrons is in sharp contrast to the transition metals' d electrons, whose behavior is strongly affected by the presence of the host lattice and may show significant delocalization and mixing with the electronic states of other ions in the lattice. The actinides' 5f electrons provide an intermediate case whose properties may vary between these extremes depending on the nature of the material and ion. This characteristic of the rare-earth ions arises from the unique situation in which the lowest-energy electrons are not spatially the outermost electrons of the ion, and thus have a limited direct interaction with the ion's environment. The "shielding" of the 4f electrons from the environment by the outer filled shells of 5p and 5s electrons prevents the 4f electrons from directly participating in bonding and allows them to maintain much of the character exhibited by a free-ion. This non-bonding characteristic of the 4f electrons is responsible for the well-known chemical similarity of the different rare-earth ions. Since transitions between the electronic states of the "shielded" 4f electrons give rise to the ion's optical transitions, materials that contain rare-earth ions exhibit unique optical properties.

• Excited states of the 4f electrons can be incredibly long lived with lifetimes in excess of 10 ms. This is more than a factor of 10⁶ longer than the typical excited state lifetimes of other types of electronic states.

• Rare-earth ions can have highly efficient luminescence with large quantum efficiencies and very little energy wasted by phonon emission.

• The coherence lifetimes of rare-earth transitions can approach the fundamental lifetime limit with coherence times of up to several ms when the material is cooled to temperatures of a few Kelvin. This corresponds to the narrowest optical transitions ever observed in solids with lines as narrow as 75 Hz (a width that is narrower than one part in 10¹² of the transition energy).

• Transition energies are only weakly perturbed by the nature of the host material, allowing a universal "map" of the energy level structure to be constructed such as the classic "Dieke Diagram".

• The 4f^N to 4f^N transitions exhibit very weak electron-lattice coupling effects with Huang-Rhys parameters near zero (Debye-Waller factors near unity): most of the intensity is associated with the sharp zero-phonon line and phonon-sidebands are often not observed.

For more information about the properties of rare earth ions, click here, here, here, here, here, here, and here.

These properties not only enable exciting new technologies, they provide the means to study fundamental physical phenomena with a level of precision that would otherwise be inaccessible. The narrow lines and long coherence lifetimes provide sensitive probes for weak effects in the lattice and can be used to study long-range ion-ion interactions. Slight shifts in line positions caused by lattice defects would be entirely obscured for broader lines, but can be easily resolved using the narrow rare-earth resonances. Weak magnetic effects due to electron or even nuclear spins can be thoroughly explored by observing their effects on the rare earth's coherence lifetime. Together, these techniques provide new insights into solid-state materials and enhance our understanding of the interactions that occur within these materials.

Professor Rufus Cone's solid-state physics research group is the world's leader in ultra-high resolution spectroscopy of rare-earth-activated crystalline materials, measuring the narrowest electronic transitions ever observed in solids using optical coherent transient techniques. Professor Cone's group studies many optical, dynamic, and magnetic properties of rare-earth materials including the spectroscopic, luminescence, and coherence properties of the electronic 4f^N to 4f^N transitions for temperatures ranging from 1 K to 350 K and magnetic fields of up to 8 T. In addition, these studies examine relaxation effects, energy exchange, collective ion excitations, nuclear and electronic spin coupling, spectral diffusion due to ion-ion and host-ion interactions, lattice perturbations, and excitation induced dephasing.

This work is carried out using traditional spectroscopic methods as well as advanced nonlinear methods such as optical coherent transients, saturation spectroscopy, and persistent spectral hole burning. These techniques include optically detected nuclear magnetic resonance, photon echoes and stimulated photon echoes, field and orientation dependent Zeeman spectroscopy, optical nutation, site-selective photoluminescence excitation spectroscopy, nonlinear frequency-mixing (Raman spectroscopy and general four-wave mixing), excited-state absorption, direct two-photon absorption, frequency modulation spectroscopy, time-resolved photoluminescence, and electron photoemission spectroscopy.

Professor Cone's group maintain many active collaborations with researchers at institutions from around the world including Scientific Materials Corporation (USA), IBM Almaden Research Center (USA), University of Georgia (USA), Université Claude Bernard Lyon (France), A. F. Ioffe Physico-Technical Institute (Russian Federation), Australian National University (Australia), University of Lund (Sweden), Université de Caen (France), NASA Langley (USA), University of Oxford (UK), and Università degli Studi di Verona (Italy).

My research in Professor Cone's group involves exploring many aspects of the electronic energy level structure of rare-earth-activated optical materials and requires a wide variety of experimental techniques. The purpose of this work is to gain a complete picture of the electronic structure of these materials that includes understanding the relationships between the 4f^N levels, 4f^N–15d levels, and the host crystal's valence and conduction bands. This research is motivated by the need to improve our fundamental understanding of these materials as well as gain practical knowledge of immediate use in developing new rare-earth-activated optical materials for laser, phosphor, scintillator, and optical computing applications.

One current focus of this work is to develop new materials with optimal properties for use in optical computing applications. As our technological world continues to demand greater data-handling capabilities, improved techniques must be developed that increase our abilities to store and process data. One of the most promising of these new techniques is persistent spectral hole burning (PSHB) and its time-domain optical coherent transient counterparts. This method offers many advantages over present data processing and storage techniques since data may be stored at different "colors" as well as different spatial positions within the storage medium. The implementation of this new method of data storage could result in more than a thousand-fold increase over current storage densities and computing speeds.

The developing field of PSHB data storage relies on the optical properties of crystalline solids that have had ions of rare-earth elements inserted into their atomic lattice structures. Although individual ions have well-defined frequencies at which they may absorb light, interactions with the crystal's lattice cause these frequencies to be spread over a continuous range once the ions are inserted into the crystal. PSHB is an optical technique that uses lasers to excite the ions that absorb light at a selected frequency in the crystal. Once these ions have been excited, they can no longer absorb light at that frequency—a "hole" has been "burned" into the absorption spectrum. Using this process, manipulation of a crystal's ability to either absorb or transmit light at a particular frequency can be used to store binary data bits in the crystal's absorption spectrum and to perform optical data processing. This basic process may be used to store millions of bits in a micron sized spatial location within the crystal and to optically process data at tremendous rates.

Materials for PSHB applications require the excellent coherence properties exhibited by the 4f^N to 4f^N transitions of the rare-earth ions as well as a non-volatile mechanism for selectively addressing and manipulating subsets of atoms within a solid-state matrix. One obstacle to the practical implementation of this method of optical storage is its transient nature. Current techniques of PSHB require a metastable electronic or nuclear hyperfine "bottleneck" state in which the excited ions become trapped rather than completely relaxing back to their ground state. To prevent relaxation from this trap state, and the corresponding loss of stored data, the crystals must be kept at temperatures of only a few Kelvin and manipulated with low laser powers. Even with such precautions, the data lifetime is generally milliseconds to weeks, and each time the data is read, it is partially erased from the interaction with the probe laser. A technique that promises to overcome these problems is "photon-gated" photoionization PSHB. Photon-gated PSHB involves the use of a second laser to selectively photoionize the excited rare-earth ions. Photoionization permanently alters the excited ions by changing their valence state; therefore, the memory becomes permanent even at higher temperatures. Thus, when the second laser is present, information may be "written" into the rare-earth ions' population; when the second laser is not present, the information may be read back without disturbing the information previously written (non-destructive readout). This technique would overcome many of the problems inherent in present methods as well as introduce an additional element of control into the system through selective application of the second laser.

Another important technological application of spectral hole burning is to create ultra-stable frequency references, clocks, and laser sources. For these applications, a narrow spectral hole burned into the rare-earth absorption can provide a well-defined frequency reference that can be used to stabilize a laser's frequency. By using this technique, coupled with optical materials chosen for their optimal properties, professor Rufus Cone's group has shown that lasers may be stabilized to the remarkable level of 2 parts in 10¹³ of the optical frequency using a simple tabletop system without any elaborate environmental control. These stable lasers have many applications in both fundamental research, where they enable measurements over long time scales with unprecedented resolution, and technological applications, such as optical computing and measuring precise distances through interferometry. Since the stable lasers project shares many of the same requirements as optical computing applications, my work on developing PSHB materials is also important for developing materials for laser stabilization. Outstanding performance has been achieved in professor Cone's group using transient spectral holes as frequency references, however some applications could benefit from the ability to create permanent spectral holes to act as an absolute frequency reference and reduce long-term laser frequency drift. Thus, new materials for photon-gated PSHB would provide new opportunities to include both sharp short-term and stable long-term frequency references within a single crystal.

Recently, I have also become involved in the laser stabilization modeling project of Dr. Geoff Pryde in professor Cone's group. This project seeks to understand how the basic material and laser properties affect the maximum attainable stability through modeling the complete laser locking process. Within this computer model, an optically coherent treatment of the laser-material system is used to model the dynamic interactions that affect the ability of the system to reduce the noise on the laser source. This model includes the complete system of laser, active ions, host material, optical detectors, feedback electronics, as well as the stochastic and deterministic noise introduced by each of these components. By coupling experimental results with sophisticated computer modeling, improvements in laser performance can be dramatically accelerated and the limitations imposed by each component of the system can be evaluated and overcome.

A thorough understanding of both the static and dynamic properties of the electronic structure of rare-earth-activated optical materials is needed to guide the search for new materials that satisfy all of the requirements for optical computing, stable lasers, phosphors, scintillators, and the many other rare-earth enabled optical technologies. To improve our understanding of these materials, a variety of experimental techniques are employed and coupled with theoretical modeling of the observed properties. Thus, high-resolution linear and nonlinear spectroscopy of the intraconfigurational 4f^N to 4f^N transitions, ultraviolet and excited-state absorption spectroscopy of the interconfigurational 4f^N to 4f^N–15d transitions, and photoionization and electron photoemission spectroscopy of the 4f electrons and host states are all employed to map out the energy levels and interactions that are crucial for technological applications as well as for a fundamental physical understanding of rare-earth-activated optical materials.

Many optical applications for rare-earth ions rely on the unique properties of the localized 4f^N to 4f^N transitions (with scintillators and some phosphors as notable exceptions). By studying these transitions, we gain important information for practical applications and new insights into the fundamental physical processes in the crystalline environment. The exceptionally narrow transitions of the rare-earth ions provide unique opportunities for probing weak interactions within solid-state materials that cannot be studied directly. There are a vast variety of techniques used to study these interactions and the 4f^N to 4f^N transitions involved. My work on the 4f^N to 4f^N transitions is primarily concerned with the effect of the crystalline host lattice on the transition energies, intensities, coherence, and chemical stability of the rare earth ions as well as how these optical properties can manipulated by modifying the material's chemical composition.

The advanced techniques available in Professor Cone's laboratory are used to not only characterize the optically-active impurity ion and determine the wavefunctions of its electronic states, but to also explore the nature of the ion's local environment. Site-selective absorption studies, using a combination of transmission and photoluminescence excitation measurements, are used to distinguish groups of crystallographically inequivalent sites and even defects in the lattice. The symmetry and nature of the local environment of each spectroscopically unique group of ions are studied by observing the orientation and polarization dependent properties of the associated optical transitions. For example, by studying the angle-dependent Zeeman splittings and shifts of transitions under an applied magnetic field, ions that experience different local crystal symmetries can be unambiguously identified and characterized. Concentration dependent absorption and time-resolved photoluminescence measurements can be used to identify ion pairs and clusters as well as to study the energy-exchange interactions between neighboring ions. Together, these measurements give important insights into the solid-state chemistry of these materials and the structural defects that affect the optical properties.

One of the central points of my work, as well as for our group in general, is the coherence and spectral hole burning properties of rare-earth-doped materials. These properties are dominated by dynamic interactions such as phonon scattering and emission, spin-flips, radiative emission, and excitation effects. The coherence properties are studied by observing the material's time-dependent response to coherent radiation. For example, two-pulse photon echo decays can be used to determine the coherence lifetime—the average length of time that an ion can "store" phase information. Optical nutation can be used to directly determine the electric dipole moment of the transition results from observing the rate at which ions are optically driven between the upper and lower states involved in the optical transition. Three-pulse (stimulated) photon echo decays are used to probe the time scale and magnitude ion-ion and ion-lattice interactions in the material by observing how the coherence lifetime evolves in time. These measurements may be used to specifically identify the interactions within the lattice that contribute to the ion's dephasing (loss of coherence) and which physical mechanisms are responsible for driving each interaction (dipole-dipole coupling, direct phonon transitions, phonon scattering, etc.).

Spectral hole burning measures changes in a material's transmission during and after excitation with a spectrally narrow source. Analysis of hole "burn" rates and relaxation times gives information about relaxation mechanisms, branching ratios for different relaxation pathways, spin-flip processes, and the nuclear hyperfine or quadrupole structure of the ion and its neighbors. For example, patterns of holes (reduced absorption) and anti-holes (enhanced absorption) that appear in the hole burning spectrum due to changes in the ion's nuclear spin can be used to determine the energy splittings of nuclear spin states that result from the coupling to the ion's electronic levels, a mechanism known as the "hyperfine" interaction. In addition, the electronic levels may couple to the nuclear states of nearby ions through a "superhyperfine" interaction (also known as the transferred hyperfine interaction), which results in hole burning features due to the nuclear states of adjacent ions. Hole burning also provides a method for observing photoionization of the rare-earth ion since any ionized rare earths will be entirely removed from the transition's absorption profile, resulting in a corresponding hole in the spectrum.

The process of spectral diffusion is of particular interest to our group since it affects the coherence and hole burning properties of the rare-earth ions. The phenomenon of spectral diffusion corresponds to a shortening of coherence lifetimes, and a related broadening of linewidths, resulting from dynamic processes in the crystal lattice. The classic example of a spectral diffusion process is magnetic spin-flips and their effect on the ion's transition frequency. A random spin-flip will slightly shift the transition frequency of nearby ions and cause a change in each ion's phase development. When this process is averaged over an ensemble of ions, the resulting effect appears as a time-dependent broadening of the linewidth and a shortened coherence lifetime. By studying the different processes that lead to spectral diffusion, materials and ions may be chosen to dramatically reduce spectral diffusion and therefore provide improved performance in many optical applications.

The interconfigurational 4f^N to 4f^N–15d transitions of the rare-earth ions have become increasingly important in recent years because of their applications in fast scintillators and ultraviolet laser sources. The 4f^N to 4f^N–15d transitions involve the transfer of a single electron from the 4f shell to the empty 5d shell of the ion, and are therefore sometimes simply referred to as 4f to 5d transitions. These transitions are parity allowed (Laporte's selection rule) with oscillator strengths ranging from 10^-6 to values approaching 0.1 in some cases; this is up to 10,000 times stronger than the strongest 4f^N to 4f^N transitions, which are parity forbidden. Unlike the shielded intraconfigurational 4f^N to 4f^N transitions, the 4f to 5d transitions are strongly affected by the host lattice. This arises since the 5d electron is the outermost electron of the ion and mixes significantly with the electronic states of its neighboring ions so that the environment is the dominant effect on the 5d energy level structure. The 5d electrons also strongly couple to lattice vibrations, resulting in intense phonon sidebands and Debye-Waller factors of 0.01 or much smaller, with some transitions potentially having no observable zero-phonon line.

The 4f^N–15d levels exhibit electronic structure corresponding to the large crystal field splitting of the 5d levels coupled to the splittings of the 4f^N–1 electronic core. For the ions with more than a half-filled 4f^N shell, splittings are also observed that arise from the large difference in exchange interaction energy for states with the 5d electron spin aligned parallel or anti-parallel to the spin of the 4f^N–1 core. In these cases, the lowest energy 4f to 5d transition is spin-forbidden and corresponds to a 4f electron spin—initially antiparallel to the total 4f^N–1 spin—flipping during the 4f to 5d transition so that the total spin S increases by one. The fact that this transition is spin-forbidden reduces the oscillator strength by a factor of 1000 or more, increases the radiative lifetime by a similar factor, and significantly inhibits relaxation into this state from the higher energy 4f^N–15d states.

The energies of these levels are important for many optical applications and are of particular importance for spectral hole burning through photoionization. Due to the extended nature of the 5d state and its significant mixing with the host electronic states, it can act as an intermediate level for photoionization of the rare-earth ion. This would be a three-step hole burning process in which the first step would be a 4f to 4f transition providing the high-resolution spectral selectivity, the second step would be a strong excited-state absorption transition to a 5d state, and the third step would be an efficient autoionization of the 5d electron to the host conduction band. In my research, traditional methods of ultraviolet and vacuum ultraviolet absorption spectroscopy are initially used to locate the energies and study structure of the lowest 4f^N–15d levels. The second step of this process can also be studied in more direct manner through excited-state absorption and photoluminescence excitation experiments. These experiments are essential in order to choose the appropriate photon energy for the second step of the photoionization process and ensure the maximum efficiency is achieved.

Because of the large oscillator strengths and short lifetimes, the 4f^N–15d levels have important applications in scintillator materials for medical applications. In a scintillator material, the high-energy gamma-rays or x-rays are absorbed by the host crystal, eventually resulting in the excitation of electrons from the valence band into the conduction band. These electron-hole pairs may then transfer their energy to an optically active ion that emits a visible or ultraviolet photon that can be easily detected by traditional means (photomultiplier or diode). The high-density of rare-earth materials and efficient emission of the 4f^N–15d states of ions such as Ce³⁺ can produce a high-light yield, while the fast emission provides for improved time-resolution; both of these properties are essential for many modern scintillator applications. Since the energy transfer mechanism involves the exchange of charge between the lattice and the rare-earth ion, it is critical to know the position of the rare-earth levels relative to the host band states for important materials, such as Ce:LSO. In particular, scintillation can be completely quenched if the emitting 4f^N–15d level lies within the host conduction band, an effect that can also occur for potential phosphor materials. Thus, knowledge of the energy of the 4f^N–15d levels relative to the host band states is important to understand which materials are unlikely to be useful for scintillator or phosphor applications that rely on 4f^N–15d emission.

The energy of the 4f^N–15d states is also important for laser applications where they can potentially interfere with the lasing process. In a laser material, if the laser wavelength corresponds to a transition from the upper laser level to an even higher state, the absorption from the excited state into the higher level can reduce the laser power and scanning range, and may even result in crystal heating and optical damage. This process is particularly important for lasers with shorter wavelengths, where excited-state absorption may occur into a spectrally broad and intense 4f to 5d transition, possibly entirely quenching any laser action. Thus, measuring the energy of the 4f^N–15d levels is also important for predicting which materials will make poor laser materials because of excited-state absorption.

Interactions between the localized 4f electronic states of rare earth ions and the de-localized band states of the crystal lattice can strongly affect the optical properties of technologically important rare-earth-doped materials. In contrast to the well-developed understanding of the electronic structure of the 4f^N states, relatively little is known about the relationships between these states and the electronic states of the crystal. In recent years, the body of knowledge on this topic has steadily grown due to the strong interest in developing ultraviolet lasers and more efficient phosphor and scintillator materials. The performance of rare-earth-activated optical materials in these applications can be enhanced, reduced, or even entirely inhibited by energy exchange and charge transfer processes between the rare earth ions and the host crystal. This has made it increasingly important that the systematic trends and behavior of rare earth energies relative to crystal band states be explored and characterized.

For many of the modern applications for rare-earth-activated optical materials, knowledge of the position of the localized 4f electronic states relative to the band states of the crystalline host lattice is important for understanding the performance of an optical material. For example, in solid-state laser materials, ionization from the excited states of rare earth ions to the conduction band of the host lattice often cause a parasitic absorption that overlaps lasing wavelengths, resulting in crystal heating, reduction of both gain and tuning range, and may completely inhibit laser action, as for Ce³⁺ and Pr³⁺ in yttrium aluminum garnet (YAG). Excited state absorption can also create color centers and optical damage and is the dominant reason for the failure of otherwise promising tunable blue and ultraviolet laser materials. In contrast, ionization can be beneficial for applications such as proposed optical memories, optical processors, and frequency standards based on photorefractive effects or photon-gated photoionization hole burning, which may employ controlled ionization of the rare earth ions for non-volatile data storage and processing. The one- and two-photon photoionization process can potentially cause undesirable photodarkening of rare-earth-doped optical fibers and is a mechanism for the generation of optical gratings in these fibers. The radiation hardness of optical materials, which is essential for space based applications, is strongly influenced by the energy of the rare earth ions relative to the host bands. In the particular case of YAG, some rare earth ions resist radiation damage, while others suffer damage through oxidation or reduction. Recent studies suggest that the efficiency of scintillator and phosphor materials is influenced by the position of the 4f levels relative to the band states through both ionization of excited rare earth ions and energy exchange between band states and 4f states. In new luminescent materials for flat panel displays, the performance limitations of many potential electroluminescent materials may arise from field-induced or thermal ionization of the rare earth ions.

Several experimental techniques have been used to explore the relationships between the 4f states and the host electronic states, including vacuum-ultraviolet and excited-state absorption, photoconductivity, and photoelectron spectroscopy. Absorption spectroscopy provides one of the most direct methods for measuring interactions between the rare earth ions and the host crystal and also allows the study of exciton and other localized charge transfer processes that do not directly involve the host band states. However, interpretation is often difficult because of uncertainty in the assignment of initial and final states for observed transitions. In addition, due to the small overlap between the 4f electronic states and the host conduction band and the broad nature of the transitions, absorption due to photoionization is often difficult to observe and may be obscured by other features such as 4f to 5d or charge transfer absorption.

Photoconductivity experiments can be used to unambiguously assign the final state of a transition as involving either an electron in the conduction band or a hole in the valence band by measuring changes in the electrical resistance of an insulating material when light is applied. As the applied light is tuned into resonance with a transition that terminates in the host conduction band, the observed resistance of the material will be reduced. For the pure material, the photoconductivity spectrum will consist of a series of broad bands corresponding to transitions from occupied band states to unoccupied band states. The lowest energy feature that may occur in the spectrum occurs at the energy difference between the valence band maximum and the conduction band minimum (the band gap). Now, when an impurity such as a rare earth ion is introduced into the material, ionization or charge transfer transitions may become possible that involve the impurity levels and the host band states and new features may appear in the spectrum at energies lower than the band gap. The energy at which the impurity peaks occur gives the energy difference between the ground state of the active ion and either the conduction band or valence band, depending on whether the final state involves electrons in the conduction band or holes in the valence band. Photoconductivity peaks due to the presence of electron acceptor or donor defects in the lattice can give information about lattice defects, but may also create ambiguity regarding the origin of observed signals. Photoconductivity may also be used to determine the band gap of the host material.

Photoelectron spectroscopy, or photoemission, measures the energies of occupied electronic states relative to a common energy reference, providing information that can complement optical methods. Combined with the methods of inverse photoemission or bremsstrahlung isochromat spectroscopy, which measure the energies of unoccupied electronic states, photoemission can be used to provide a clear picture of the electronic states in a material. The primary difficulties in applying photoemission to the study of optical materials are the requirement of ion concentrations of at least a few atomic percent and complications arising from electrical charging due to the highly insulating nature of the materials being studied. However, in the garnets we have shown that energies determined from measurements on highly concentrated samples accurately reflect the energies in low-concentration samples and that sample charging problems can be overcome by combinations of experimental techniques and data analysis.

Recently we have used resonant photoemission to study the energies of the 4f electrons relative to the crystal valence band. We found that a two-parameter empirical model successfully describes the relative energies of the 4f^N ground states for rare earth ions in these materials. The success of this empirical model across the entire rare earth series indicates that measurements on as few as two different rare earth ions in a host are sufficient to predict the energies of all rare earth ions in that host. Analysis also shows that the energies of the 4f electrons relative to the valence band can be estimated from the photoemission spectrum of the undoped host, providing a simple method for surveying related host crystals. Additional data with more accurate measurements of electron binding energies will permit a detailed analysis of the empirical two-parameter model and test the range of materials for which it is applicable. Development of improved methods for charge compensation and reducing differential charging while insuring maximum uniformity in the photon beam intensity will improve the accuracy and sensitivity of the photoemission measurements. By comparing measurements in different materials with the empirical model, improved estimates for the free-ion ionization potentials may be obtained and used in the model to increase the accuracy of its predictions. Study of new materials will undoubtedly reveal lattice dependent trends that will enhance the predictive power of the empirical model.

Much work is still needed to advance the understanding of relationships between localized 4f electrons and the crystal band states. Measurements over the entire rare earth series in many different host materials are required to build a more complete picture of rare-earth-doped optical materials. Comparison of experimental results with theoretical predictions will provide insight into the electronic structure of the crystal lattice and provide guidance for calculations of lattice dependent properties. By supplementing photoemission with complementary techniques such as photoconductivity, excited state absorption, bremsstrahlung isochromat spectroscopy, and inverse photoemission, the relationships and interactions between rare earth ions and the occupied and unoccupied electronic states of the host crystal may be thoroughly explored. With sufficient data to guide the theoretical treatment of these processes, it may become possible to better understand the properties of current optical materials as well as direct the development of new materials for specific applications.

Selected Publications and Pre-prints Available for Download (Adobe pdf files)

"Progress in relating rare-earth ion 4f and 5d energy levels to host bands in optical materials for hole burning, quantum information, and phosphors," C. W. Thiel, Y. Sun, and R. L. Cone, J. Mod. Optics 49, 2399 (2002).

"Recent progress in developing new rare earth materials for hole burning and coherent transient applications," Y. Sun, C. W. Thiel, R. L. Cone, R. W. Equall, and R. L. Hutcheson, J. Lumin. 98, 281 (2002).

"Relating localized electronic states to host band structure in rare-earth-activated optical materials," C. W. Thiel, H. Cruguel, H. Wu, Y. Sun, G. J. Lapeyre, R. L. Cone, R. W. Equall, and R. M. Macfarlane, Opt. & Phot. News 12 (12), 64 (2001).

"Systematics of 4f electron energies relative to host bands by resonant photoemission of rare earth doped optical materials," C. W. Thiel, H. Cruguel, Y. Sun, G. J. Lapeyre, R. M. Macfarlane, R. W. Equall, and R. L. Cone, J. Lumin. 94-95, 1 (2001).

"Systematics of 4f electron energies relative to host bands by resonant photoemission of rare earth ions in aluminum garnets," C. W. Thiel, H. Cruguel, H. Wu, Y. Sun, G. J. Lapeyre, R. L. Cone, R. W. Equall, and R. M. Macfarlane, Phys. Rev. B 64, 085107 (2001).

Selected Presentations Available Online (HTML slides)

INVITED TALK, "Systematics of 4f Electron Energies Relative to Host Bands by Resonant Photoemission of Rare Earth Doped Optical Materials," C. W. Thiel, H. Cruguel, Y. Sun, G. J. Lapeyre, R. L. Cone, R. W. Equall, and R. M. Macfarlane, International Conference on Dynamical Processes in Excited States of Solids, Lyon, France, July 1-4, 2001.

"Terbium Compounds for Photon-Gated Persistent Spectral Hole Burning," C. W. Thiel, Y. Sun, R. L. Cone, R. W. Equall, R. L. Hutcheson, and R. M. Macfarlane, Optical Science and Laser Technology Conference, Bozeman, Montana, August 17-18, 1998.

Other Presentations and Documents Available Online

Click Here to View More of My Physics Documents

Click Here to View More of My Physics Presentations