La3lu2ga3o12:cr3+:nd3+ Lazer Kristalinde Sıcaklığın Cr3+ Nd3+ Enerji Transferine Etkisi

Abstract

Son yıllarda katıhal lazer malzemeleri üzerine yapılan araştırmalar büyük bir artış göstermiştir. Bu katıhal lazer malzemelerinden en iyi bilinenlerden biri de YAG(Y3Al50i2) kristalidir. Bu kristal Nd3+ ile katkılandığında l,06um'de ışıma yapar, bu iş: gelmektedir. yapar, bu ışıma 4F3/2 metastable seviyesinden \m alt seviyesine geçişe karşılık YAG kristali katıhal lazer kristali olarak yaygın bir şekilde kullanılmasına rağmen lazer diyot uyarmalı yeni katıhal lazerleri üzerinde çalışmalar devam etmektedir. Bunlarda aranılan özellikler; daha uzun ışıma süresi, daha geniş soğurma bandı, daha yüksek soğurma katsayısıdır. Biz çalışmamızda LLGG(La3Lu2Ga30i2) katıhal lazer kristalini kullandık. LLGG gamet yapısındadır, gamet yapısındaki kristaller lazer host için istenen özelliklere sahiptir. Bu özellikler; kimyasal olarak dengede, mekaniksel olarak sert, optiksel olarak izotropik olmaları, nadir toprak ve demir gruplarının optiksel olarak aktif iyonlarım kabul edebilmeleridir. Buna ek olarak yüksek optiksel kalitede ve düşük optiksel kayıpla geniş boyutlara büyütülebilirler. LLGG ilk olarak Kokta ve Grasso tarafından hazırlanmıştır. Nd3+ ile katkılanmış ve [La1.xLu2](Lu1.yGay)Ga3012 bileşiminde olan LLGG Allik ve arkadaşları tarafından patansiyel lazer diyot ile uyarılmış ve katıhal malzemesi olarak incelenmiştir. LLGG kristali Nd3+ ve Cr3+ ile katkılandığında Cr3+ La iyonu ile, Nd3+ Lu iyonu ile yer değiştirmektedir. Bu çalışmada Cr3+ ve Nd3+ arasında oluşan enerji transferi prosesinin yapışım araştırmak amacıyla optik bölgede soğurma, lüminesans, yaşam süresi ve uyarma spektrumlarının ölçümleri 77°K ile 300°K sıcaklık aralığında yapılmıştır. Oda sıcaklığında 0.015ns"1 olarak bulunan enerji transferi hızının bu sıcaklık aralığında yaklaşık olarak sabit olduğu gözlenmiştir.

As is well known, the word "laser" is an acronym for the most significant feature of laser action; light amplification by stimulated emission of radiation. Many different kinds of lasers have been developed since it was first developed in 1960. This was a solid state laser based on the single crystal of ruby(crystalline aluminum oxide activated by trivalent chromium ions). Some of them are gas, semiconductor, liquids, organic dye lasers. All of these lasers share a crucial element: each contains materials capable of amplifying radiation called gain medium. The study of the spectroscopic characteristics of laser emission by activated compounds furnishes an important supplement to conventional luminescence and absorption techniques. These spectroscopic investigations are very important for quantum electronics and solid state physics. As a result of this developments in quantum electronics, a new method of experimental investigation has been established: Laser spectroscopy(stimulated emission spectroscopy). From the viewpoint of quantum electronics, the gain medium such as an activated glass or crystal is an entity with definite properties. In contrast, stimulated emission spectroscopy penetrates the interior of the Crystal to investigate the relationship between the intrinsic (structural and other) properties of the crystal and the parameters that determine the operating characteristics of a laser. New materials with high laser efficiency are the main topic of the current solid state laser research. It has been found that crystal with garnet structure has many properties which are desirable in a laser host since it is chemically stable, mechanically hard and has good thermal and optical properties. The most promising candidates are Y3AI5O12 (YAG), Gd3Ga50i2 (GGG), Gd3Sc2Ga30i2 (GSGG), La3Lu2Ga30i2 (LLGG),Y3Sc2Ga30i3 Sensitizers have been used to enhance the overall laser efficiency by increasing the pumping efficiency. A "sensitizer" is a second ion that both is better coupled to the_x000B_excitation source and also is able to transfer the excitation energy efficiently to Nd3+ ions, populating the 4F3/2 or higher energy level. The use of Cr3* ion as a sensitizer is desirable because the ion has two broad absorption bands in the red and blue-green wavelength regions, resulting from vibronic transitions to the 4T2 and 4Tj energy levels. The broad, strong absorption bands of Cr3+ ions in the visible range overlap well with the emission spectra of lamps. Moreover, the Cr3+ emissions are in the red and near infrared regions and overlap strongly with the absorption of rare earth ions. _3+ Cr doped materials can exhibit a broad band 3 -level system, if a relatively weak crystal field strength at the Cr site yields a reduced 4T2-4A2 splitting due to the thermal population of the 4T2 level and the high transition probability of the spin-allowed 4T2-4A2 transition, most of the fluorescence can be channeled into the broad band 4T2-4A2 transition. The relative intensity of the 2E-4A2 transitions (R-lines) and broad band 4T2- 4A2 transition depends on the energy gap AE=E(4T2)-E(2E). For instance, AE is nearly zero in GSGG(50 cm"1), and negative in LLGG (-1000 cm"1). As an example, Fig.l shows the emission spectra of different garnet crystals. c 900 800 700 X[nm] Fig. 1. Emission Spectra of Cr3* in various Ga-garnets. From top to the bottom crystal field strength is decreasing. XI_x000B_From the top to the bottom, the lattice constant increases with a corresponding decreasing crystal field strength. Thus, the relative intensity of the R-lines near 700 nm decreases and the broad band fluorescence is shifting to longer wavelengths. Following this approach, such behavior has been realized in many host lattices for Cr3+. If the energy gap AE is very small, both levels 2E and 4T2 are mixed through spin-orbit coupling. The typical laser wavelengths of tunable Cr3+ systems are located between 750 nm and 1000 nm depending on the actual crystal field of the host crystal at the Cr- site. Energy transfer processes are very important in quantum electronic laser devices, because they can provide the enhancement of the fluorescence emission and the consequent reduction of the laser threshold. This is usually achieved by the introduction in a crystal of an additional ion called sensitizer which strongly absorbs the pumping energy and transfers it as excitation energy to the ion(activator) responsible for the main fluorescence for the main fluorescence emission. Neodymium is one of the most important activators in solid state laser systems. It has an ideal four level system lasing at 1.06 urn. from the upper metastable state 4F3/2 to the lower state 4lun- However, because the trivalent neodymium ions exhibit partly forbidden electric dipole transitions between the 4& electronic states, their absorption and emission spectrum consists of rather weak and narrow sharp lines because the incompletely filled 4f shell is an inner one and the 4f electrons, which are responsible for the optical transitions are shielded by the outer filled 5s25p6 shells from external influences. The narrow absorption lines of Nd3+ overlap poorly with the radiation spectra of commercially available high power lamps. This leads to a waste of a large part of the pump energy. The sensitization of the Neodymium laser emission by codoping Neodymium and Chromium is an efficient way to increase the laser efficiency. Especially in rare earth gallium garnets this coactivation was found to be successful.. Doping with Chromium causes broad absorption bands which couple well to the flash lamp spectra. It has also been shown that the codoping of Gd3Sc2Ga30i2 with Cr and Nd increases the overall efficiency of the Nd emission due to the spin allowed nature of the Cr transition and the good overlap between the Cr emission and the Nd absorption. The conditions for such an overlap have been established by exploiting the effect of the crystal field strength on the spectral position of the emitting level of a transition metal ion.(Cr3+) If such an ion acts as sensitizer, a proper host may provide a crystal field that positions the broad band emission of Cr3+ as to create a better spectral coincidence between it and the sharp absorption levels of the rare earth activator ion Nd3+. An energy transfer from Cr3+ to Nd3+ increases the quantum efficiency Nd3+ emission. In its simplest form energy transfer involve the transfer of energy from one ion to another in glasses, crystals and molecules imbedded in solid state lattices. XII_x000B_Energy transfer can be analyzed in two groups; 1) Radiative Energy Transfer: This process can be divided into two steps, first sensitizer ion decays from the excited level to the ground level by emitting radiation then activator ion absorbs the radiation emitted by the sensitizer ion and decays from the ground level to the excited level. In radiative transfer interaction between the sensitizer ion and the activator ion is not necessary. 2) Non-Radiative Transfer: The effects that affect the efficient of this transfer process are; spectral overlap between the luminescence of the sensitizer and the absorption spectrum of the activator ion, life time measurements of the sensitizer ion and absorption coefficient of the activator ion. All transitions between states (e.g., S A, S A ) due to weak perturbations can be formulated in terms of a "Golden Rule": Probability(S*A^-SA*)^27t/h(p)S+A*,VI', corresponds to ^(S^A) and ¥f corresponds to ^(S^CA*). Since there are two electronic mechanisms by which Ti can undergo transition to 4^ we can break H up into two types of interaction i.e., He the exchange interaction and Hc the Coulombic interaction. The phenomenon of energy transfer in solid state materials has been known for most of the twentieth century; it was best modeled and described in the pioneering work of Förster, Dexter, Inokuti and Hirayama. For explaining the energy transfer if we model the ions involved in energy transfer each as an electron and a nucleus, with charge -e and +e respectively, in a harmonic oscillator potential. Assuming that one has simple one dimensional harmonic oscillators each with Hamiltonians Hi of the form H,-±-I?+\cx* (1) 2m 2 Xlll_x000B_for the i* oscillator where p is the momentum, Xj is the one-dimensional displacement of the Ith electron, m is the mass of an electron and C is the coupling constant. The ions themselves are coupled through the Coulombic interaction whose Hamiltonian is e2 e2 e2 e2 Hc= - + - - - (2) R R + x{ -x2 R + x^ R-x2 R is the distance between nuclei. The first term represents the nucleus-nucleus interaction, the second is the electron-electron interaction, and the third and fourth term are the electron-nucleus interaction between the electron of the first/second ion and the nucleus of the second/first ion, respectively. By expanding for small xi and X2 relative to R, this Hamiltonian can be approximated as Hc* 2eV2 (3) R3 When the Hamiltonian shown in Eqs.(l) and (3) are coupled together the new frequencies of oscillation can be calculated; the overall change in energy for the system is proportional to series of terms, the first of which is proportional to 1/R6. This is the origin of the R"6 dependence for the dipole-dipole interaction for energy transfer. The Coulombic interaction causes the energy to slosh between the electron and nucleus; the greater the interaction, the faster the energy transfer. The overall energy of the system is preserved. There are three basic mechanisms which may produce energy transfer between rare earth ions: 1) If photons are emitted by an ion S and absorbed by an ion A, the transfer of energy is said to take place by a cascade type of mechanism. Ion S must be by itself a good emitter of fluorescence in a region in which the ion A absorbs strongly. In this case the fluorescence lifetime of the ion A absorbs strongly. In this case the fluorescence lifetime of the ion S is not affected by the presence of the ion A, and the emission of fluorescence by S is decreased only at those wavelengths at which A absorbs. 2) Energy process may also take place through a mechanism of the resonant type which produces the so called sensitized fluorescence. The fluorescent ion is called the activator and the additional dopant ion is called the sensitizer, and it produces an enhancement of the absorption features of the compound. The sensitizer may not present strong fluorescence, and the activator may not present strong absorption in the fluorescence region of the sensitizer. If it can be measured, the lifetime of the XIV_x000B_sensitizer is found to decrease in the presence of the activator; also all the fluorescence lines of the sensitizer originating in the state participating in the energy exchange are quenched in the presence of the activator. 3) Another possible mechanism for energy transfer is of the phonon assisted type. The intervention of phonons is determined by the requirement of the conservation of energy and is to be found in systems where there is no overlap between the energy levels of the sensitizer and those of the activator. Absorption measurements provide basic information about the position of the energy levels and the pumping capability of an ion. Fluorescence measurements give information about the frequency and shape of the fluorescence lines. Excitation experiments, in which one emission frequency is monitored while the frequency of the exciting light is varied, provide information about the pumping bands responsible for populating of the metastable level. Thermal vibrations play an important role in fluorescent systems; they strongly affect the kinetics of the excitation of the metastable state by providing fast radiationless processes between the absorption bands and these states. They may also affect through the modulation of the crystalline field the width and position of the sharp fluorescence lines which in several cases are feasible to produce laser oscillations. Finally they may affect the lifetime of the metastable level by either depleting the population of this level through radiationless processes or by producing phonon assisted emission which appears in the form of sidebands to the main no-phonon lines. Activated Crystals, however, best revealed their properties after Johnson and Nassau developed a CaWC«4-Nd3+ laser emitting near 1 urn at room temperature with an extremely low excitation threshold This behavior was attributed to the fact that the terminal level for an induced transition m the Nd ion lies about 2000cm above the ground level and is practically unpopulated at operating temperatures. Currently the Nd3+ ions is the most widely used laser crystal activator. In solid state lasers, stimulated emission is attributed to transitions between electronic levels of activator ions. In recent years, scientists have searched for new laser crystals and for ways of improving their efficiency. Thus in order to improve the generation characteristics, the principle of sensitization was employed. During the emergence of quantum electronics, spectroscopic studies of activated crystals suggested their possible use in laser systems. Later, experience accumulated in studies of stimulated emission parameters allowed, in many cases, a much deeper and more complete analysis of the characteristics of these materials, and helped uncover their properties, adding to the information from such conventional spectroscopic methods as luminescence and absorption analyses. Thus, a new trend of spectroscopy originated at XV_x000B_the junction of quantum electronics and classical spectroscopy, namely, the stimulated emission spectroscopy of activated crystals which is currently being rapidly developed. In Cr:Nd:YAG the Nd3+ fluorescence decay time is shorter than the transfer time from Cr3+ to Nd3+ Thus a poor energy transfer efficiency results in the pulsed mode. In Cr:Nd:GGG and Cr:Nd:GSGG the transfer rate is much higher because the transfer occurs from the 4T2 state of Cr3+ and not from the 2E state as in YAG In these crystals the energy gap between 2T» and 4T2 is smaller than in YAG due to a lower crystal field strength. Therefore the 4T2 is thermalized with the 2E state, which causes a broad band luminescence. This chromium emission, which has a good overlap with Nd3+ absorption bands, and the short transfer time from Cr3+ to Nd3+ enable a high energy transfer efficiency. Pruss and coworkers reported a transfer efficiency of 0.86 in Cr:Nd:GSGG G. Armaöan et al. reported that the temperature dependence of the energy transfer process from Cr to Nd in GSGG:Cr:Nd is due to the thermal variation of the radiative decay probability of Cr. Lundth and Weidner found that Cr3+ to Nd3+ energy transfer in Cr:Nd:GGG and GSGG:Cr:Nd occurs with the same efficiency. In both materials this transfer leads to an increase in laser efficiency compared to Nd: YAG In this study, the effect of sample temperature on the energy transfer efficiency was investigated. Here Cr3+ was chosen as sensitizer ion and Nd3+ as activator ion in LLGG. The energy transfer from Cr3+ to Nd3+ enhances the emission intensity of Nd3+ emission centered at 1.06um. And the transfer rate was found to be 0.01 5US"1 at room temperature.