Limitations in Rechargeability of Li-O2 Batteries and Possible Origins.

Quantitative differential electrochemical mass spectrometry (DEMS) is used to measure the Coulombic efficiency of discharge and charge [(e(-)/O2)dis and (e(-)/O2)chg] and chemical rechargeability (characterized by the O2 recovery efficiency, OER/ORR) for Li-O2 electrochemistry in a variety of nonaqueous electrolytes. We find that none of the electrolytes studied are truly rechargeable, with OER/ORR <90% for all. Our findings emphasize that neither the overpotential for recharge nor capacity fade during cycling are adequate to assess rechargeability. Coulometry has to be coupled to quantitative measurements of the chemistry to measure the rechargeability truly. We show that rechargeability in the various electrolytes is limited both by chemical reaction of Li2O2 with the solvent and by electrochemical oxidation reactions during charging at potentials below the onset of electrolyte oxidation on an inert electrode. Possible mechanisms are suggested for electrolyte decomposition, which taken together, impose stringent conditions on the liquid electrolyte in Li-O2 batteries.

Solvents' Critical Role in Nonaqueous Lithium-Oxygen Battery Electrochemistry.

Among the many important challenges facing the development of Li-air batteries, understanding the electrolyte's role in producing the appropriate reversible electrochemistry (i.e., 2Li(+) + O2 + 2e(-) ↔ Li2O2) is critical. Quantitative differential electrochemical mass spectrometry (DEMS), coupled with isotopic labeling of oxygen gas, was used to study Li-O2 electrochemistry in various solvents, including carbonates (typical Li ion battery solvents) and dimethoxyethane (DME). In conjunction with the gas-phase DEMS analysis, electrodeposits formed during discharge on Li-O2 cell cathodes were characterized using ex situ analytical techniques, such as X-ray diffraction and Raman spectroscopy. Carbonate-based solvents were found to irreversibly decompose upon cell discharge. DME-based cells, however, produced mainly lithium peroxide on discharge. Upon cell charge, the lithium peroxide both decomposed to evolve oxygen and oxidized DME at high potentials. Our results lead to two conclusions; (1) coulometry has to be coupled with quantitative gas consumption and evolution data to properly characterize the rechargeability of Li-air batteries, and (2) chemical and electrochemical electrolyte stability in the presence of lithium peroxide and its intermediates is essential to produce a truly reversible Li-O2 electrochemistry.

Highly spin-polarized room-temperature tunnel injector for semiconductor spintronics using MgO(100).

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.

Digital holographic data storage in a high-performance photorefractive polymer composite.

Binary information in the form of a 256 x 256 pixel array has been stored holographically within a polymeric photorefractive composite of 130-mum thickness. Devices used consisted of 55-wt. % organic chromophore 1-(2?-ethylhexyloxy)-2, 5-dimethyl-4-(4?nitrophenylazo) benzene and up to 11.2-wt. % 2, 4, 7-trinitro-9-fluorenone (TNF) dispersed within a poly(N-vinyl carbazole) matrix. In a degenerate four-wave mixing arrangement, the refractive-index modulation and speed were 10(-3) and 200 ms, respectively. The high TNF concentration leads to short digital hologram recording times of 2 s without detriment in optical quality. Although not yet optimized, a figure of merit, M#, for holographic storage in this composite has been measured to be 0.017, and this is estimated to reach 0.18 in a revised experimental geometry.

Two-Color Holographic Recording Scheme Allowing Nonvolatile Reading in Mn:YAlO(3).

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).

Volume holographic data storage at an areal density of 250 gigapixels/in.(2).

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) .

Distortions in pixel-matched holographic data storage due to lateral dimensional change of photopolymer storage media.

Dimensional changes in polymerizable storage materials during exposure are a source of pixel misregistration in digital holographic storage. Lateral shrinkage or expansion of photopolymer recording materials can be deduced from quantitative measurement of the shift and the magnification errors of reconstructed holograms.

Experimental study of the effects of a six-level phase mask on a digital holographic storage system.

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.

Effects of multilevel phase masks on interpixel cross talk in digital holographic storage.

We study the interpixel cross talk introduced to digital holographic data storage by use of a multilevel phase mask at the data-input plane. We evaluate numerically the intensity distribution at the output detector for Fourier plane hologram storage in a limited-aperture storage medium. Only the effect at an output pixel of interpixel cross talk from the four horizontal and vertical neighboring pixels is considered, permitting systematic evaluation of all possibilities. For random two-level and pseudorandom six-level phase masks, the influence of the pixel fill factor, as well as the aperture size of the storage medium, is studied. Our simulations show that, for a given aperture size, a random two-level mask is more susceptible to interpixel cross talk than is a pseudorandom six-level mask. Decreasing the pixel fill factor below 94% with a pseudorandom six-level phase mask makes it theoretically possible to have a system with no errors from interpixel cross talk if one particular 5-pixel pattern is forbidden through modulation coding. Reducing the input fill factor below 85% means that no patterns need to be excluded.

Pixel-matched holographic data storage with megabit pages.

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.

Holographic digital data storage in a photorefractive polymer.

We report high-contrast storage of 64-kbit digital data pages in a photorefractive polymer material. Singlepage writing, reading, and erasing operations were demonstrated with a dual-function-dopant polymeric material having a dark lifetime of several days. Data were reconstructed without error by use of several simple readout algorithms.

A precision tester for studies of holographic optical storage materials and recording physics.

The design and the realization of an advanced precision optical test stand for evaluating materials and developing tools and techniques for holographic digital data storage are described. This apparatus allows studies of holographic recording materials and recording physics to be performed in the context of practical data storage. The system concept, its implementation, and its performance are described, and examples of holographic storage in photorefractive materials are discussed.

Time dependence of quantum fluctuations in solitons.

We investigate quantum fluctuations in coherent solitons propagating in a dispersive nonlinear waveguide. Optimal squeezing or noise reduction occurs for a local oscillator width near the soliton width in time, i.e., the quantum fluctuations are localized.

Photon-gated hole burning: a new mechanism using two-step photoionization.

We have observed photon-gated spectral hole burning, i.e., hole burning that occurs only in the presence of an additional gating-light source. Gating enhancement factors of 10(4) were observed. In BaClF:Sm(2+) this involves two step photoionization of Sm(2+) and leads to persistent holes in the (4)F(0) --> (5)D(0) (687.9-nm) and (7)F(0) --> (5)D(1) (629.7-nm) absorption lines. The hole widths of 25 MHz at 2 K are much narrower than the inhomogeneous broadening of 16 GHz. The action spectrum of the gating shows a threshold behavior around 2.5 eV. Erasing studies show that Sm(3)+ acts as a trap for the released electrons. A remarkable and novel feature is that the holes can be recovered after temperature cycling to 300 K.

Squeezing of classical noise by nondegenerate four-wave mixing in an optical fiber.

Nondegenerate four-wave mixing in an optical fiber is shown to attenuate one quadrature of random sideband fluctuations created by external modulators. A theory of the nonlinear interaction that includes nonlinear dispersion fits the results. Analogous experiments on quantum noise inputs should prove successful.

Photoionization hole burning and nonlinear Zeeman effect in CaF(2):Sm(2+).

Persistent spectral hole burning has been observed in CaF(2):Sm(2+) and originates from photoionization of the Sm(2+) ion. This represents a new class of persistent hole-burning materials. We have used hole burning to measure the nonlinear Zeeman effect of the (7)F(0)A(1g) ? A(1u) transition at 695.8 nm with a resolution of ~100 MHz. We find a nonlinear coefficient of 0.93 Hz/G(2), which is close to the value of 1.0 Hz/G(2) calculated for the (7)F(0)-(7)F(1) interaction. This shows that the dominant contribution to the nonlinear Zeeman effect comes from the ground state.

Measurement of optical homogeneous linewidths in a glass with picosecond accumulated photon echoes.

Picosecond accumulated photon echoes have been used to study optical dephasing in the (4)I(9/2)?(2)G(7/2), (4)G(5/2) transition of Nd(3+) in silicate glass. The homogeneous width exhibited a T(2.2)temperature dependence similar to that of other rare-earth systems previously studied by fluorescence line narrowing. The wavelength dependence of the echo-decay curves shows that relaxation among crystal-field states takes place in a few picoseconds.

Measurement of the hyperfine structure of Pr(3:):YAG by quantum-beat free-induction decay, hole burning, and optically detected nuclear quadrupole resonance.

The hyperfine splitting of the ground state and the lowest electronic component of the (1)D(2) excited state have been measured for Pr(3+) in YAG. We show that a particularly simple and powerful technique is the production of sublevel coherence by short-pulse (impact) excitation, which is detected by coherent forward scattering of a weak probe beam and Fourier transformation of the resulting quantum-beat signal. This yielded excited-state splittings of 6.49 and 8.29 MHz. The ground state shows large pseudoquadrupole splittings of 33.4 and 41.6 MHz, which were obtained by optically detected nuclear quadrupole resonance.

Measurement of nuclear and electronic Zeeman effects using optical hole-burning spectroscopy.

The excited-state nuclear Zeeman effect and quadratic-electronic Zeeman effect were studied in Pr(3+):LaF(3) using laser-induced hole burning. Hole burning in an external magnetic field gave values for the excited-state enhanced nuclear g tensor of g(x)beta/h = 2.2, g(y)beta/h = 1.9, and g(z)beta/h = 3.6 kHz/G. By applying the field after hole burning, quadratic-electronic Zeeman coefficients as small as 0.1 Hz/G(2) were measured. The hyperfine coupling constant in the excited (1)D(2) level was found to be A(J) = 600 MHz.

Optical hole burning by superhyperfine interactions in CaF(2):Pr(3+).

Optical hole burning has been observed in the 5941-A transition of Pr(3+) in a charge-compensated tetragonal site of CaF(2) . The Pr(3+) ground state is doubly degenerate and shows a large first-order hyperfine splitting, which is clearly resolved because of the very narrow inhomogeneous linewidth of 650 MHz. The hole burning involves a new mechanism, in which optically induced spin flips of neighboring nuclei (here (19)F) shift the optical transition frequency outside its homogeneous linewidth. This mechanism was confirmed by optical rf double resonance.