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Journal of Applied Physics

IRQC parameters for Er3+ occupying sites of C2v and C3v symmetry in CdF2

Norman E. Byer, T. C. Ensign, William M. Mularie, Stanley E. Stokowski

Published Apr 1, 1973• 9 citations
Publisher

Abstract

Measurements have been made of the optical parameters relevant to the operation of the Er3+ infrared quantum counter (IRQC) proposed recently to detect 1.5-μm radiation. The CdF2:Er3+ crystals used for this purpose were prepared under specific conditions selected to provide different local site symmetries at the Er3+ ions. EPR measurements were employed to determine the site symmetries and the relative concentration of ions at the various sites. In particular, high concentrations of C2v sites were generated by double doping with NaF prior to crystalline growth [CdF2: (Er3+,Na+)], whereas C3v sites were generated preferentially by oxygen firing sodium-free crystals subsequent to growth [CdF2: (Er3+,O2−)]. Selective generation of these two sites permitted the unambiguous determination of their characteristic optical absorption, emission, and excitation spectra, as well as the lifetimes and radiative efficiencies of relevant excited states. In addition, double excitation using two ir sources simultaneously, was used to determine the cross section for transitions between the 4I13/2 and 4S3/2 excited states and the lifetime of the former state. The optical properties of the Er3+ ion were observed to be sensitive to the local environment associated with each site and, in particular, to the magnitude of the noncubic component of the crystalline field. Phase-lag measurements confirmed the successive-excitation model proposed to explain the conversion of 1.5-μm radiation to 540- or 660-nm radiation in the presence of an 840-nm pump. Cooperative-excitation processes, on the other hand, were observed to produce 660-nm emission when 790- or 970-nm pumps were employed. The dependence of the over-all IRQC quantum efficiency on pump flux and on the optical parameters of Er3+ is derived from solutions to the appropriate rate equations. The analysis indicates that the performance of the 1.5-μm Er:IRQC, utilizing an 840-nm pump, is nearly optimized for CdF2: (Er3+,O2−).

DOI
10.1063/1.1662440

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High-resolution spectroscopy and novel crystal-field methods for rare-earth based quantum information processing

Sebastian P. Horvath

Rare-earth doped insulators have been widely recognized as excellent candidate materials for optical quantum computing and information processing. This is principally due to a combination of optical coherence times of up to milliseconds and ground-state hyperfine coherence times of seconds exhibited by rare-earth 4𝑓 𝑁 states. A key contender among rare-earth doped materials is Er3+:Y2SiO5, for which homogeneous linewidths as narrow as 50 Hz have been reported, and for which transitions between the ground and first excited states fall within the 1.5 𝜇m telecommunications wavelength band. However, attempts to utilize this material for spin-wave storage have been hampered by the complexity of the excited-state hyperfine structure, which inhibits the necessary theoretical work required to predict the location of zero first-order zeeman transitions. To address this, a novel crystal-field fitting method was developed to allow an accurate analysis of the 𝐶1 point-group symmetry sites of Y2SiO5. This work was complemented with spectral hole-burning spectroscopy, which revealed ground-state hyperfine level storage with a lifetime of 1.6 ± 0.4 seconds for the 4I15/2𝑍1 → 4F9/2𝐷1 transition. This type of storage time is uncharacteristic for erbium without an external magnetic field, since spin-flips of the unpaired electron in nearby Er3+ ions are typically understood to lead to fast hole filling. The development process of the theoretical and experimental tools required to perform the above work enabled several additional investigations. In particular, the novel crystal-field fitting methods were applied to the 𝑆4 site of Ce3+:LiYF4 and to the 𝐶2𝑣 site of Sm3+:Na+/Li+:CaF2/SrF2. These studies consisted of excellent tests for the developed methods yet also amounted to interesting investigations in their own respect. Similarly, the experimental work afforded an opportunity to directly determine the hyperfine splittings of the 5I8𝑍1 → 5F5𝐷1 transition in Ho3+:KY3F10. This was achieved using high-resolution excitation spectroscopy. Furthermore, the experimentally determined hyperfine structure could be accurately reproduced using a crystal field model. Finally, a lifetime measurement of the 6H7/2 state of Sm3+:LiYF4 was conducted. By utilizing an infrared free-electron laser, pico-second pump-probe spectroscopy allowed for a direct measurements of one of the shortest lifetimes of an RE3+ doped insulator reported in the literature to date. This confirmed the previously suggested two-gradient modification required for the energy-gap law at very short time scales.

•Jan 1, 2016•7 citations
Measurements have been made of the optical parameters relevant to the operation of the Er3+ infrared quantum counter (IRQC) proposed recently to detect 1.5-μm radiation. The CdF2:Er3+ crystals used for this purpose were prepared under specific conditions selected to provide different local site symmetries at the Er3+ ions. EPR measurements were employed to determine the site symmetries and the relative concentration of ions at the various sites. In particular, high concentrations of C2v sites were generated by double doping with NaF prior to crystalline growth [CdF2: (Er3+,Na+)], whereas C3v sites were generated preferentially by oxygen firing sodium-free crystals subsequent to growth [CdF2: (Er3+,O2−)]. Selective generation of these two sites permitted the unambiguous determination of their characteristic optical absorption, emission, and excitation spectra, as well as the lifetimes and radiative efficiencies of relevant excited states. In addition, double excitation using two ir sources simultaneously, was used to determine the cross section for transitions between the 4I13/2 and 4S3/2 excited states and the lifetime of the former state. The optical properties of the Er3+ ion were observed to be sensitive to the local environment associated with each site and, in particular, to the magnitude of the noncubic component of the crystalline field. Phase-lag measurements confirmed the successive-excitation model proposed to explain the conversion of 1.5-μm radiation to 540- or 660-nm radiation in the presence of an 840-nm pump. Cooperative-excitation processes, on the other hand, were observed to produce 660-nm emission when 790- or 970-nm pumps were employed. The dependence of the over-all IRQC quantum efficiency on pump flux and on the optical parameters of Er3+ is derived from solutions to the appropriate rate equations. The analysis indicates that the performance of the 1.5-μm Er:IRQC, utilizing an 840-nm pump, is nearly optimized for CdF2: (Er3+,O2−).
Photoluminescence (PL) spectra of Er3+ in an Er-doped silicon film were recorded at 2 and 77 K over the wavelength range from 1.52 to 1.68 μm, which includes optical transitions between the Er3+ I13/24 and I15/24 manifolds. These spectra were then analyzed to determine the nature of the electrostatic field at the site of the Er ion that governs the ion’s optical behavior. The PL spectra were analyzed three different ways: by considering the Er3+ ions to be occupying sites of Td, C3, or D2d point group symmetry. Transition energies and magnetic dipole transition probabilities were calculated for each case by diagonalizing a Hamiltonian representing the free-ion and crystal-field interactions in a Russell–Saunders basis of states spanning the lowest 15 J multiplets of the 4f11 electronic configuration of Er3+. Crystal-field parameters were varied in each case to find the best agreement with the experiment. Our results showed that the site symmetry of the Er3+ ions most consistent with the data is D2d. For this case, the rms deviation between the experimental and calculated energy levels is 3.3 cm−1 with the calculated magnetic dipole transition probabilities in qualitative agreement with the experiment.
An analysis of an ideal five-level ir quantum counter (IRQC) is presented. The importance of the two additional levels is discussed and analyzed. The parameters for achieving optimum performance as an ir detector and/or an image converter are presented. In theory, the IRQC is the only quantum amplifier with photon gain that can approach a noise temperature of absolute zero. In practice, with the constriction of available materials and technology, the most feasible use for the IRQC in the near term would be in an active system. An ir laser would be used to scan the target while another laser would pump the excited state transition. The IRQC action has been observed in this laboratory in the tripositive rare earths of Pr, Nd, Eu, Tb, Dy, Er, and Tm. These ions were incorporated in single crystal lattices of CaF(2), BaF(2), SrF(2), CdF(2), LaF(3), CaWO(4), A1(2)O(3), YAG, LaCl(3), and in solutions of the various trichlorides (dissolved in D(2)O). A total of 165 schemes are reported. The rare earth impurity concentration varied from 0.05% to 20%. In our laboratory, the primary range of interest for detection and/or image conversion is between 1 micro and 2 micro. Therefore, almost all the systems studied involved the detection of an ir photon in this region.
Positive optical gain around 5 cm−1 (∼21 dB/cm) has been observed in Er3+-doped Ba2NaNb5O15 single crystal using a pump and probe experimental setup. High power laser pulses at 532 nm have been used as the pump source in order to strongly populate the S43/2(H211/2) levels of the Er3+ ions due to ground state absorption. Low signal beam cw laser radiation at 850 nm has been used as the probe beam to stimulate the emission associated with the S43/2(H211/2)→I413/2 electronic transition of the Er3+ ions. The process has been modelized as a four level system, and its population has been analyzed and simulated in order to study the gain dynamics. Optical amplification of the probe signal has been observed during the first 60 μs, which represents a good agreement between the measured lifetime of the S43/2(H211/2) levels and the reported simulation.
Erbium-Doped Fiber Amplifiers (EDFAs) can very efficiently convert pump to signal energy. Quantum efficiencies higher than 80 % with 1.48 pm pump [1], and 93% when pumped at 980 nm [2] are reported. However, even higher values should be theoretically possible for an ideal EDFA. Various physical effects may lead to quantum efficiency (QE) reduction. Not all of them are quite well understood yet. At Erbium concentrations above 100 ppm by weight the pump loss is partly explained by energy transfer between two excited ions resulting in upcon- version process [3]. At low concentrations Excited-State Absorption (ESA) could be a more significant effect reducing QE. ESA of photons from 1.5 |im band have been predicted from absorption measurements [4] and also observed [5]. However, to our knowledge, no quantitative analysis has been made which would distinguish between the contributions of ESA and two-particle upconversion when measurements of short wavelength luminescence were performed.
We report on a scheme for particle detection based on the infrared quantum counter concept. Its operation consists of a two-step excitation process of a four level system, which can be realized in rare earth-doped crystals when a cw pump laser is tuned to the transition from the second to the fourth level. The incident particle raises the atoms of the active material into a low lying, metastable energy state, triggering the absorption of the pump laser to a higher level. Following a rapid non-radiative decay to a fluorescent level, an optical signal is observed with a conventional detector. In order to demonstrate the feasibility of such a scheme, we have investigated the emission from the fluorescent level 4S3∕2 (540 nm band) in an Er3+-doped YAG crystal pumped by a tunable titanium sapphire laser when it is irradiated with 60 keV electrons delivered by an electron gun. We have obtained a clear signature that this excitation increases the 4I13∕2 metastable level population that can efficiently be exploited to generate a detectable optical signal.
Rare-earth doped insulators have been widely recognized as excellent candidate materials for optical quantum computing and information processing. This is principally due to a combination of optical coherence times of up to milliseconds and ground-state hyperfine coherence times of seconds exhibited by rare-earth 4𝑓 𝑁 states. A key contender among rare-earth doped materials is Er3+:Y2SiO5, for which homogeneous linewidths as narrow as 50 Hz have been reported, and for which transitions between the ground and first excited states fall within the 1.5 𝜇m telecommunications wavelength band. However, attempts to utilize this material for spin-wave storage have been hampered by the complexity of the excited-state hyperfine structure, which inhibits the necessary theoretical work required to predict the location of zero first-order zeeman transitions. To address this, a novel crystal-field fitting method was developed to allow an accurate analysis of the 𝐶1 point-group symmetry sites of Y2SiO5. This work was complemented with spectral hole-burning spectroscopy, which revealed ground-state hyperfine level storage with a lifetime of 1.6 ± 0.4 seconds for the 4I15/2𝑍1 → 4F9/2𝐷1 transition. This type of storage time is uncharacteristic for erbium without an external magnetic field, since spin-flips of the unpaired electron in nearby Er3+ ions are typically understood to lead to fast hole filling. The development process of the theoretical and experimental tools required to perform the above work enabled several additional investigations. In particular, the novel crystal-field fitting methods were applied to the 𝑆4 site of Ce3+:LiYF4 and to the 𝐶2𝑣 site of Sm3+:Na+/Li+:CaF2/SrF2. These studies consisted of excellent tests for the developed methods yet also amounted to interesting investigations in their own respect. Similarly, the experimental work afforded an opportunity to directly determine the hyperfine splittings of the 5I8𝑍1 → 5F5𝐷1 transition in Ho3+:KY3F10. This was achieved using high-resolution excitation spectroscopy. Furthermore, the experimentally determined hyperfine structure could be accurately reproduced using a crystal field model. Finally, a lifetime measurement of the 6H7/2 state of Sm3+:LiYF4 was conducted. By utilizing an infrared free-electron laser, pico-second pump-probe spectroscopy allowed for a direct measurements of one of the shortest lifetimes of an RE3+ doped insulator reported in the literature to date. This confirmed the previously suggested two-gradient modification required for the energy-gap law at very short time scales.