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ABSTRACT: We measured the optical decoherence times T2, or, equivalently, the homogeneous line width, in an Er-doped optical fiber at low temperature as a function of external magnetic field and temperature using two-pulse photon echoes. The decoherence times were up to 230 ns at fields above 3 T. The magnitude of the line narrowing induced by a magnetic field of 3 T is 2.5 MHz, which is anomalously large compared to that typical for oxide crystals with similar Er3+ concentration. This is interpreted as evidence for dephasing by coupled spin-elastic tunneling modes where the normal glass tunneling modes acquire a magnetic character by coupling to the Er3+ spin.
Physical Review Letters 02/2006; 96(3):033602. · 7.37 Impact Factor
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ABSTRACT: The spin polarization of current injected into GaAs from a CoFe/MgO(100) tunnel injector is inferred from the electroluminescence polarization from GaAs/AlGaAs quantum well detectors. The polarization reaches 57% at 100 K and 47% at 290 K in a 5 T perpendicular magnetic field. Taking into account the field dependence of the luminescence polarization, the spin injection efficiency is at least 52% at 100 K, and 32% at 290 K. We find a nonmonotonic temperature dependence of the polarization which can be attributed to spin relaxation in the quantum well detectors.
Physical Review Letters 03/2005; 94(5):056601. · 7.37 Impact Factor
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ABSTRACT: Postgrowth thermal annealing of a CoFe/MgO(100) tunnel spin injector grown on aGaAs/AlGaAs quantum well structure results in a significantly increased spin injection efficiency as inferred from the polarization of heavy-hole electroluminescence from a quantum well optical detector. The as-deposited sample displayed an initial polarization at 100 K of 43%, which was increased to 52% after a 1 h anneal at 300 °C, and finally to 55% after a second 1 h anneal at 340 °C. The polarization remained unchanged upon further annealing to temperatures as high as 400 °C. These results show that tunnel spin injectors based on CoFe/MgO are robust with high thermal stability, making them useful for device applications.
Applied Physics Letters 01/2005; 86(5):052901-052901-3. · 3.84 Impact Factor
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ABSTRACT: Detailed site-selective spectroscopy has been performed as a function of temperature on the 7F0↔5D0 transition of Eu3+:Y2SiO5 for Eu3+ concentrations of 0.02%, 0.1%, 0.5%, and 1%. Time-domain optical dephasing, spectral hole lifetimes, anisotropic absorption coefficients, inhomogeneous linewidths, and fluorescence lifetimes for Eu3+ ions at both crystallographic sites were measured. The temperature dependence of the optical dephasing, transition energy, and linewidth of the 7F0⃗5D0 absorption was measured and interpreted in terms of Raman scattering of phonons. Photon echo measurements of optical dephasing gave T2 values as long as 2.6 ms, approaching the limit set by the fluorescence decay time. Spectral hole lifetimes were measured for temperatures from 2 K to 18 K, with observed lifetimes varying from 1 s at 18 K to an estimated value of greater than 20 days at 2 K. Anisotropic absorption coefficients were measured, and an increase in Eu3+ concentration from 0.02% to 7% produced an increase in the inhomogeneous linewidth Γinh from 0.5 GHz to ∼150 GHz, indicating that Eu3+ doping induces significant strain in the crystal. New determinations of many energy levels of 7FJ multiplets have been made for J=0 to 6.
Phys. Rev. B. 08/2003; 68(8).
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ABSTRACT: We propose and experimentally realize the recording of two-color holographic gratings in Mn:YAlO(3), a potential material for holographic data storage. This type of recording allows for nonvolatile retrieval of recorded information at the recording wavelength. We demonstrate two-color recording and readout of a 256 x 256 pixel page using red and green laser beams with a bit error rate of 6 x 10(-7).
Applied Optics 09/2001; 40(23):3915-21. · 1.41 Impact Factor
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ABSTRACT: The energies of trivalent rare-earth ions relative to the host valence band were measured for a series of rare-earth-doped yttrium aluminum garnets RxY3-xAl5O12 (R=Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and 0<~x<~3), using ultraviolet photoemission spectroscopy. The 4f photoemission spectra were acquired using synchrotron radiation, exploiting the 4d to 4f “giant resonance” in the 4f electron photoemission cross section to separate the 4f contribution. Theoretical valence band and 4f photoemission spectra were fit to experimental results to accurately determine electron energies. The measured 4fn ground-state energies of these ions range from 700 meV above the valence band maximum for Tb3+ to 4.7 eV below the valence band maximum for Lu3+, and all ground-state energies, except for Tb3+, are degenerate with valence band states. An empirical model is successful in describing the relative energies of the 4fn ground states for rare-earth ions in these materials. This model is used to estimate the positions of the lighter rare-earth ions, giving good agreement with published excited-state absorption and photoconductivity measurements on Ce3+ in yttrium aluminum garnet. It is shown 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 extending these results to related host crystals. The success of this model suggests that further studies of additional host compounds will rapidly lead to a broader picture of the effect of the host lattice on the 4f electron binding energies.
Phys. Rev. B. 08/2001; 64(8).
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ABSTRACT: One thousand volume holographic data pages, each containing 1x10(6)pixels , are stored in a common volume of LiNbO(3) :Fe by use of the 90 degrees geometry. An effective transverse aperture of 1.6 mm x 1.6mm , realized by repetition of this experiment at each of the eight surrounding locations, results in a demonstrated areal density of 394pixels/mum (2) (254 Gpixels/in. (2)) . Short-focal-length Fourier optics provide a tightly confined object beam at the crystal; the reference beam is angle multiplexed. Data pages retrieved with a 1024 x 1024 CCD camera are processed to remap bad spatial light modulator pixels and to compensate for global and local pixel misregistration and are then decoded with a strong 8-bits-from-12-pixels modulation code. The worst-case raw bit-error rate (BER) before error correction was 1.1x10(-3) , sufficient to deliver a user BER of 10(-12) at an overall code rate of 0.61 user bits per detector pixel. This result corresponds to 1.08% of the well-known theoretical volumetric density limit of 1/lambda(3) .
Optics Letters 05/2001; 26(7):444-6. · 3.40 Impact Factor
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ABSTRACT: The authors have measured the critical exponent beta in TbVO4 and DyVO4, using liner birefringence to measure the order parameter. It is found that TbVO4 shows classical mean-field behaviour with beta =0.50+or-0.02, whereas DyVO4 shows fluctuation-dominated Ising-like behaviour with beta =0.34+or-0.02. This result is consistent with other measurements of the range of the interactions coupling the rare-earth ions. In DyVO4, the departure from mean-field behaviour appears to be due to a fortuitous cancellation of long-range components in the inter-ion coupling.
Journal of Physics C Solid State Physics 02/2001; 8(21):L451.
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ABSTRACT: Measurements of optical linear birefringence have been used to study the cooperative Jahn-Teller phase transition in the mixed crystals TbCGd1-cVO4 for 0.34<or=c<or=1. For two concentrations (c=0.34 and c=0.36) a double phase transition was observed as a function of temperature, in which an orthorhombic phase exists between two tetragonal phases. This is in agreement with earlier Raman studies. The origin of the birefringence and its relationship to the order parameter is discussed. A mean-field theory of the variation of the order parameter (i.e. the orthorhombic strain) with temperature is found to give a satisfactory explanation of the observed birefringence. Measurements of the critical behaviour of the order parameter are discussed.
Journal of Physics C Solid State Physics 02/2001; 10(15):2937.
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ABSTRACT: For pt.I see ibid., vol.10, p.2937 (1977). The authors have measured changes in linear birefringence ( Delta n) associated with the cooperative Jahn-Teller phase transition of DyVO4 near 14K as a function of temperature and magnetic fields, B, between 0.024 and 0.095 T. Theoretical arguments show that Delta n is directly proportional to the order parameter of the transition and that B2 is equivalent to the conjugate ordering field. By extrapolation to zero field they obtain the temperature dependence of the order parameter and the susceptibility. The data are compared with calculations based on a mean-field 'compressible' Ising model. For a reasonable choice of adjustable parameters this classical description gives a good fit to the data close to TD consistent with general theoretical arguments and more detailed calculations, but it deviates progressively away from TD presumably because of the known importance of short-range interactions in the system.
Journal of Physics C Solid State Physics 11/2000; 13(17):3161.
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ABSTRACT: Laser-induced spectral hole burning in the presence of applied electric and magnetic fields has been used to determine the symmetry and electronic character of the colour centre in X-irradiated NaF whose zero-phonon line is at 5770 AA. The centre is found to have Cs symmetry, perturbed only slightly from trigonal. The authors speculate that it, and other centres in this special region, may arise from F3+ centres perturbed by structural or chemical defects.
Journal of Physics C Solid State Physics 11/2000; 16(12):L395.
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ABSTRACT: Four zero-phonon lines of defect centres in diamond (GR1 (741 nm), N-V (637 nm), H4 (496 nm) and N3 (415 nm)) have been shown to exhibit persistent spectral hole burning. The phenomenon appears to be a rather general one in diamond and should prove very powerful for elucidation of the nature of defect centres. As an example, preliminary results for the N-V centre suggest a previously undetected splitting of the excited state.
Journal of Physics C Solid State Physics 11/2000; 17(8):L233.
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ABSTRACT: Saturation hole burning has been used to determine the homogeneous linewidth in the liquid helium temperature range of the lowest-energy transition of excitons bound at isoelectronic nitrogen defect centres in GaP. The defects involved are pairs of nitrogen atoms in nearest-neighbour phosphorus sites. The homogeneous linewidths ( Gamma h>70 MHz) are approximately three orders of magnitude greater than the limit set by population decay. The temperature dependence of Gamma h is consistent with a dephasing process dominated by one-phonon-induced transitions to nearby higher bound exciton levels.
Journal of Physics C Solid State Physics 11/2000; 16(31):L1121.
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ABSTRACT: gigabit-per-second (Gbitys) range. Both spatial light modulators and CCD's are available with 1024 3 1024 arrays of pixels and 1-kHz frame rates, and these yield an attractive data rate of 1 Gbitys. If 1000 such pages could be stored in a volume with a cross section of roughly 5 mm, an areal density of 4 Gbitsycm 2 would be achieved, which is very competitive with current magnetic storage technology. The raw bit-error rate (BER) must be better than ,10 25 , a level correctable to 10 212 with an acceptable overhead for error-correcting code. Although they are often-cited goals, 6,8 -- 10 the storage and retrieval of megabit (Mbit) holograms at such an error rate have never been accomplished to our knowledge, nor has a demonstration of Gbitys readout. This is because the precise imaging of each inp
04/2000;
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ABSTRACT: Summary form only given. Holographic gratings with high diffraction efficiency (>50%) can be recorded in YAlO<sub>3</sub>:Mn with green or blue laser beams. The storage time extrapolated to room temperature is much greater than ten years. Gratings can be erased by heating the crystal to about 270°C. These properties in combination with the high optical quality of the crystal give promise to this material for use in holographic data storage
Lasers and Electro-Optics, 2000. (CLEO 2000). Conference on; 02/2000
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ABSTRACT: Summary form only given. Congruent lithium niobate has a
disordered, lithium-deficient structure in which Nb<sub>Li</sub>
antisites provide charge compensation for lithium vacancies. The
disorder is produced by departures from stoichiometry, which play an
important role in the photorefractive properties of lithium niobate,
particularly two-color photorefractivity at ambient temperatures.
Disorder also increases the inhomogeneous linewidth (Γ<sub>inhom
</sub>) of optical transitions of impurity centers, which gives a
desirable increase in bandwidth for applications involving low
temperature spatial-spectral holography where the frequency and
time-domain phenomena of spectral hole burning and photon echoes are
combined with that of spatial holography. However it also has the
potential to increase the homogeneous linewidth (Γ<sub>hom</sub>)
by inducing low frequency tunneling modes or “two-level
systems” (TLS), reducing the figure of merit for the
time-bandwidth product: F=Γ<sub>inhom</sub>/Γ<sub>hom</sub>.
We investigate the role of disorder in lithium niobate by measuring both
the inhomogeneous and homogeneous broadening of optical transitions in
Pr<sup>3+</sup> and Er<sup>3+</sup> doped lithium niobate where the
rare-earth ions are generally accepted to substitute for Li<sup>+</sup>
ions in sites of nominally C<sub>3</sub> symmetry
Lasers and Electro-Optics, 2000. (CLEO 2000). Conference on; 02/2000
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ABSTRACT: We report spectral hole burning in the deuteride (D-) modified Tm3+ centers in CaF2, where the hole burning mechanism is known to involve localized displacement of the D- ions. Two main families of Tm3+-D- centers are present; the Li centers yield spectral holes that have been measured to be fully persistent for 48 hours at liquid-helium temperatures, while spectral holes of the Mi centers have a hole-recovery time constant of approximately 20–30 s. Hole widths vary from 18–40 MHz (full width at half maximum) for the different centers. Burn-down curves are in agreement with a simple and general model that takes into account the finite homogeneous linewidth but makes no other assumptions about the nature of the hole burning mechanism. The area of the spectral holes is found to be conserved after the sample temperature has been cycled to up to 70 K, while the hole profile is broadened by this process.
Phys. Rev. B. 05/1999; 59(22).
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ABSTRACT: We have demonstrated real-time decoding of 20-bit biphase-coded address header pulses, using stimulated photon echoes in a phase-matched crossed-beam configuration. This decoding is one of the functions required for coherent transient optical data routing, packet switching, and processing. The active medium used was single-crystal Y(2)SiO(5) doped with Er(3+), which provides an operating wavelength of 1536 nm.
Optics Letters 05/1998; 23(8):636-8. · 3.40 Impact Factor
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ABSTRACT: We measure the M/# and the bit-error rate of a digital holographic storage system with a 4f optical arrangement for three configurations: recording at the Fourier plane with and without a phase mask and recording outside the Fourier plane without a phase mask. Unexpectedly, no significant change in the dynamic range was observed when a phase mask was used to record in thick crystals. However, we show that a phase mask is a key component in a 4f digital holographic storage system if high-fidelity holograms with optimum volumetric density are to be stored.
Applied Optics 05/1998; 37(11):2094-101. · 1.41 Impact Factor
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ABSTRACT: Digital data-page holograms consisting of 1024 x 1024 arrays of binary pixels have been stored and subsequently retrieved with an optical exposure consistent with a data rate 1 Gbit /s. Each input pixel was precisely registered with a single detector pixel, and a raw bit-error rate as low as 2.4 x 10(-6) was demonstrated with global-threshold detection. To our knowledge, this is the first demonstration of the often-cited goal of holographic data storage of megabit data pages and a gigabit-per-second data rate.
Optics Letters 11/1997; 22(19):1509-11. · 3.40 Impact Factor
