Highly efficient valence state switching of samarium in BaFCl:Sm nanocrystals in the deep UV for multilevel optical data storage

Hans Riesen; Kate Badek; Tanya M. Monro; Nicolas Riesen

doi:10.1364/OME.6.003097

1. Introduction

Although the capacity to store digital data using modern magnetic and solid-state based devices is very impressive, data storage capacities are currently outpaced by the exponential growth of data generation by at least a factor of three [1]. In this context, it is recognized that magnetic storage is close to its maximum possible surface density i.e., the traditional hard-disc drive (HDD) has almost reached its theoretical capacity imposed by the ~10 nm spatial resolution limit and hence new approaches are required to increase digital data storage capacity. It has been demonstrated that such limits can be overcome by optical data storage devices [2, 3]. There is also a need for optical data storage on a chip to overcome the von Neumann bottleneck between the processor and the memory [4]. Conventional optical data storage is restricted by the diffraction limit which is determined by the numerical aperture (NA) of the focussing lens and the laser wavelength λ, with the area A of the diffraction-limited spot given by [5],

A = {(\frac{λ}{2 N A})}^{2}

and the maximum aerial density D being proportional to the inverse of A,

D = \frac{b}{A}

where b is the number of bits per pixel. Equation (2) shows that the storage density can be increased by writing more than one bit per spot – known as multilevel encoding. Multilevel encoding becomes feasible if the signal-to-noise ratio (SNR) of the measured pixel (2D) or voxel (3D) is high and the physical property that undergoes a photoinduced change has a large dynamic range [6]. Multilevel storage also increases the data transfer significantly as several bits are read out simultaneously. Theoretically the maximum number of bits that can be encoded in one pixel or voxel depends on the number of levels M,

b = \log_{2} (M)

Multilevel encoding has been demonstrated, for example, in polymers but these undergo photobleaching and the number of levels (typically less than 10) and the linearity reported so far are limited [7–9]. In the present paper we report on our investigations into nanocrystalline BaFCl:Sm³⁺, an inorganic material that has significant potential for multilevel encoding. Alkaline-earth fluorohalides MFX (M = Ba, Sr, Ca; X = Cl, Br, I), such as BaFCl, with the P4/nmm matlockite structure (PbFCl) doped with divalent samarium have been the subject of a vast number of studies over recent decades [10–12], since such systems exhibit the rare feature of room temperature spectral hole-burning [13–15]. These systems therefore have the potential to be used in practical frequency or time domain optical data storage [16] as they allow for the writing of multiple bits into the same spatial location. We note also here that BaFBr(I):Eu²⁺ is the most successful X-ray storage phosphor [17, 18] for applications in computed radiography, and it is currently the modality of choice for intraoral dental imaging. In the context of dosimetry and computed radiography, we have recently reported on the properties of the photoluminescent nanocrystalline BaFCl:Sm³⁺ X-ray storage phosphor as prepared by a facile co-precipitation method [19–22]. The size of the nanocrystallites, averaged over all three dimensions, is typically ~200 nm, and their sensitivity as X-ray storage phosphors is increased by ∼500,000 times because of the higher defect density in comparison with microcrystalline samples prepared by sintering at high temperature [20]. The storage mechanism relies on the reduction of Sm³⁺ to Sm²⁺ by trapping electrons that are created upon exposure to ionizing radiation in the BaFCl host. The ⁵D_J-⁷F_J f-f luminescence lines of the Sm²⁺ can be very efficiently excited by employing the parity allowed 4f⁶→4f⁵5d transitions at around 420 nm. This wavelength is in the region of readily available blue-violet laser diodes, facilitating efficient excitation of the electric dipole allowed transition which is relatively intense (~400 l/(mol⋅cm)) [23]. For the case of BaFBr(I):Eu²⁺ the X-ray storage mechanism appears to involve oxide impurities and it has been demonstrated by optical [24–26], EPR [27], ODEPR [28, 29], ENDOR [27] and NMR [30] spectroscopy that such impurities are ubiquitous in BaFX (X = Cl, Br) at the 100 ppm level unless extreme caution is taken during the preparation i.e., it is very difficult to grow pure samples as the material scavenges oxygen from the surrounding environment. Crystals with negligible oxygen contamination were however succesfully grown by Radzhabov and Otroshok [25] by employing “vacuum-drying” of molten BaFX for up to 24 h and single crystals were subsequently grown with the addition of pieces of teflon.

For the case of BaFCl, oxygen can enter the lattice as oxide, O^2-, ions on the F or Cl sublattice, with oxygen-anion vacancy O_X^2- - ν_a⁺ centres being formed. It was found that the oxide ion preferentially enters the lattice by substitution of fluoride ions and the vacancy appears to be on the chloride lattice sites. From EPR and NMR spectroscopy it has been shown that oxygen can also enter the BaFCl lattice in the form of peroxide, (O₂)^2- [27]. After X-irradiation the superoxide ion was detected by EPR and on the basis of symmetry it was concluded that it sits in a chloride site [24]. However, NMR results demonstrated that the peroxide ion can also enter the lattice interstitially. In addition, NMR has shown that hydroxide impurities are present in the BaFCl system even when great care is taken during the preparation [30]. These findings strongly imply that nanocrystalline BaFCl, as prepared by co-precipitation followed by a drying and grinding step in air, may contain significant concentrations of oxygen and hydroxide impurities. The former can be incorporated as O^2- or (O₂)^2-.

As is reported for the first time in this paper, the oxygen impurities allow for highly efficient Sm³⁺→Sm²⁺ valence state switching upon excitation into their electronic transitions in the deep UV region around 200 nm. The conversion behaviour is also compared with results obtained by X-irradiation, and preliminary results are presented that demonstrate that the phosphor has significant potential to be used as a rewritable multilevel optical data storage material. It is noted here that in previous work, we have reported the Sm³⁺→Sm²⁺ valence state switching by multiphoton processes (at visible wavelengths) but relatively high conversion power densities on the order of 10 GW/cm² were required [31], compared with ~10 µW/cm² levels (at UV wavelengths) used in the present work. Most likely, the multiphoton reduction mechanism relies on the presence of oxygen impurities as well.

2. Experiment

Nanocrystalline BaFCl:Sm³⁺ was prepared as described previously [20]. In brief, a 0.4 M aqueous solution of BaCl₂ with 0.6 mg SmCl₃.6H₂O per 25 ml was mixed with a 0.2 M aqueous solution of NH₄F at 20 °C, resulting in a fine, white suspended precipitate that was separated by centrifugation at 4000 rpm for 12 min and then the supernatant solution was decanted. The precipitate was subsequently dried at 50 °C for 12 h and the powder was then gently ground with a mortar and pestle. The powder was pressed into countebores of 2.5 mm radius and 0.5 mm depth on black-anodised aluminium holders, i.e., the pressed powder samples were 0.5 mm thick discs with a volume of ~10 mm³. A total of >100 samples were prepared with high reproducibility. For the greyscale imaging experiments, samples were also prepared in the form of 50 μm thin films on a poly (vinyl acetate) substrate employing 5 wt-% Kraton as the binding medium. Samples were characterized by powder XRD (Rigaku MiniFlex600, Cu Kα, 40 kV, 15 mA), SEM (Hitachi S900), TEM (JEOL 1400) and ICP-OES (Perkin-Elmer, Optima 8000). From the latter analysis it follows that the samples contain 250 ± 20 ppm samarium. Luminescence spectra were measured with a Jobin Yvon Horiba Spex Fluoromax-3 fluorometer or with a 0.5 m Spex 500M monochromator equipped with a 150 grooves/mm grating blazed at 500 nm and an Andor iDus DV401A-BV CCD camera cooled to −60 °C. For excitation spectra, the Fluoromax-3 fluorometer was used. For measurements at low temperatures a Cryodyne CTI 22C closed-cycle refrigerator was employed. Samples were exposed to deep UV radiation either in the Fluoromax-3 or with a low power mercury discharge lamp. X-ray exposure was either conducted by a Sirona Heliodent dental X-ray source or with the Rigaku MiniFlex600 powder X-ray diffractometer.

3. Results and discussion

The measured powder X-ray diffraction pattern of the nanocrystalline BaFCl:Sm³⁺ sample prepared by co-precipitation, is compared in Fig. 1 with the calculated (Crystalmaker 8.7 software) pattern using the standard BaFCl data (ICCD PDF No. 34-064). All prominent diffraction peaks can be indexed to the PbFCl (matlockite) structure with space group P4/nmm. The slight differences between the calculated and measured intensities are due to the preferential orientations of the ~40 nm thick plate-like crystallites, as seen in the scanning electron micrograph shown in the inset of Fig. 1. The average crystal size (averaged over all three dimensions) is ~200 nm as follows from an analysis of the width of the diffraction lines and the SEM micrographs [20,21].

Fig. 1 Powder X-ray diffraction pattern (λ = 0.154 Å) of nanocrystalline BaFCl:Sm³⁺ prepared by co-precipitation. The upper and lower traces are calculated and measured, respectively, and the Miller indices are shown for the prominent peaks. The inset shows a typical SEM micrograph with the dimension marker indicating 1 μm.

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Figure 2 compares the 425 nm excited Sm²⁺ luminescence spectra as obtained upon low-dose X-ray and UV exposure of the nanocrystalline BaFCl:Sm³⁺. It follows that a fluence of 8 mJ/cm² at 185 nm in the deep UV results in approximately the same conversion of Sm³⁺→Sm²⁺ as 10 mGy X-irradiation at 60 kVp. For comparison, the 401 nm excited Sm³⁺ spectrum before exposure is also shown in panel a). Exposure to 425 nm and 401 nm excites the Sm²⁺ and Sm³⁺ ions with high selectivity in 4f⁶→4f⁵5d¹ and f-f transitions, respectively. The prominent ⁵D_J→⁷F_J and ⁴G_J→⁶H_J luminescence lines for the Sm²⁺ and Sm³⁺ ions, respectively, are indicated in Fig. 2.

Fig. 2 Luminescence spectra of nanocrystalline BaFCl:Sm³⁺ excited at 425 nm before (green dash-dot trace) and after (solid red trace) a) exposure to 8 mJ/cm² 185 nm UV light (unfocused, unpolarized) and b) 10 mGy 60 kVp X-irradiation. The blue dashed trace in panel a) shows the 401 nm excited luminescence spectrum of Sm³⁺ before UV/X-ray exposure (right-hand intensity scale). Spectra were measured on the Fluoromax-3 spectrometer.

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Figure 3 summarizes the dependence of the Sm²⁺ luminescence intensity (integrated luminescence intensity of the ⁵D₀→⁷F₀ line at 687.5 nm) on deep UV fluence. An enormous dynamic range is observed and the signal appears to increase in a near linear fashion up to 600 mJ/cm² with saturation starting to occur above ~1 J/cm². From this data it follows that to saturate 10 mm³ of material requires about 3 J. Hence to saturate a nanocrystal of ~(200 nm)³ size, a mere 24 pJ is required.

Fig. 3 Dependence of 687.5 nm ⁵D₀→⁷F₀ luminescence intensity (integrated) on deep UV exposure. The data in panels a) and b) was obtained by exposing the sample with unpolarized light at 200 nm (2 nm bandwidth) in the Fluoromax-3 spectrometer with a power density of ~0.3 mW/cm² (beam size ~0.1 cm²). The data in panel c) was obtained by exposure of the sample with a mercury lamp (185 nm line, unfocused and umpolarized) with a power density of ~70 mW/cm².

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The wavelength dependence of the formation spectrum of Sm²⁺ is shown in Fig. 4 in comparison with the absorption spectrum of oxygen impurities in BaFCl as reported in [25]. It is important to note here that oxygen-free samples are absent of absorption features (excitons and bandgap) down to <185 nm. The formation spectrum of F(Cl^-) for a single crystal of BaFCl (with oxygen impurities), as extracted from [25], is also shown for comparison and appears to be very similar to the formation spectrum of Sm²⁺. It follows that the formation of Sm²⁺ is much more effective when the second oxide-based band at around 200 nm is excited, although it appears that the 250-260 nm band also yields some Sm²⁺ formation in contrast to the reported formation spectrum of F(Cl^-) [25]. The wavelength dependence clearly indicates that the reduction of Sm³⁺ to Sm²⁺ is based on direct excitation of oxygen impurities.

Fig. 4 Formation spectrum of Sm²⁺ in the deep UV region for nanocrystalline BaFCl:Sm³⁺ (black solid circles; also shown as inset in a semi-logarithmic plot) in comparison with the absorption spectrum of oxide impurities in a single crystal (dotted blue line) as extracted from [25]. The formation spectrum was obtained by exposing the sample in the Fluoromax-3 spectrometer with a 2 nm bandwidth and is corrected for the wavelength dependence of the power density. The red square data points show the formation spectrum of F(Cl^-) centres as reported in [25].

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The simplest possible photochemical reaction can be expressed as,

O^{2 -} + {Sm}^{3 +} + h v \to O^{-} + {Sm}^{2 +}

i.e., this is a photoinduced electron-transfer reaction between an oxide impurity and a Sm³⁺ ion. We have also conducted photoreduction experiments at 10 K (not illustrated here). Importantly, the formation spectrum and yield appear to be temperature independent in contrast to what has been observed for the formation of F(Cl^-) centres [25], where a strong temperature dependence of the quantum yield was reported, indicating that the thermally activated motion of electron and hole centres is important in this latter system. Thus the photoactive configuration of the Sm³⁺-oxygen impurity defect pair must be relatively static in BaFCl and not subject to significant motion within the crystal lattice.

Figure 5 compares luminescence spectra before and after long exposure to deep UV light and X-irradiation. Whereas the Sm³⁺ luminescence clearly decreases with increasing X-irradiation dose (Sm³⁺ + X-ray→Sm²⁺) as has been reported previously [20], it appears that this luminescence stays almost constant even after prolonged UV exposures that yield comparable Sm²⁺ intensities. This behaviour indicates that the UV-generated Sm²⁺ luminescence stems from Sm³⁺-oxide impurity defects that are initially dark i.e., the particular configuration of the Sm³⁺-oxygen impurity defect pair must lead to the quenching of the Sm³⁺ luminescence.

Fig. 5 Luminescence spectra of nanocrystalline BaFCl:Sm³⁺ excited at 401 nm before (blue dashed trace) and after (solid black trace) a) long UV (~20 J/cm² 185 nm light, unfocussed and unpolarized) and b) X-ray (~40 Gy 40 kVp) exposure. The insets shows semi-logarithmic plots (log(Intensity)) in the region of the ⁴G_J - ⁶H_J Sm³⁺ transitions.

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In Fig. 6 the excitation spectrum of Sm²⁺ after UV exposure of BaFCl:Sm³⁺ is compared with the spectra obtained after X-irradiation and from a BaFCl:Sm²⁺ sample directly prepared by ball milling. The “baseline” for the unexposed sample is also shown as well as the excitation spectrum for Sm³⁺. From these results it clearly follows that the energies and shapes of the 4f⁶→4f⁵5d¹ transitions are essentially the same for all three samples with the exception that there seems to be some extra background signal generated in the 300-400 nm region after UV exposure that adds to the intensity of the excitation spectrum. Hence it is possible to conclude that the local environment for the UV and X-ray generated Sm²⁺ ions is essentially the same as for the sample where Sm²⁺ is directly built into the lattice by ball milling. Thus the oxide impurity that is responsible for the reduction of Sm³⁺ upon UV exposure cannot be a next nearest neighbour on the F- or Cl-sublattice but the separation must be at least a few interionic distances as otherwise the d-levels, and hence the 4f⁶→4f⁵5d¹ transitions, would be subject to significant shifts.

Fig. 6 Excitation spectra (687.5 nm observed with 0.5 nm bandwidth) in the region of the 4f⁶→4f⁵5d¹ transitions of Sm²⁺ in BaFCl:Sm³⁺ as generated by X-ray or UV exposure as compared with a BaFCl:Sm²⁺ sample directly prepared by ball milling. The spectrum (“baseline”) is also shown for the unexposed sample and the excitation spectrum of Sm³⁺ (638.5 nm observed with 2 nm bandwidth) is also included.

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The Sm²⁺ can be bleached back to the 3+ valence state by excitation into the 4f⁶→4f⁵5d¹ transitions. This is illustrated in Fig. 7 where the dependence on power density for this bleaching is shown (semi-logarithmic graph). It is clear that the bleaching curves do not follow a simple exponential behaviour but are subject to strong dispersion. This indicates that the ease with which photoionization happens is dependent on the samarium site and its environment, most likely depending on the separation between the samarium ion and the oxide impurity. The bleaching curves were fitted by functions based on dispersive first order kinetics as is discussed in the following paragraph.

Fig. 7 Power dependence of the reverse Sm²⁺→Sm³⁺ photoionization process. A 425 nm laser diode was used for the photoionization of the Sm²⁺. a) Integrated intensity of the ⁵D₀→⁷F₀ luminescence line at 687.5 nm with 425 nm excitation laser power densities of i) 36, j) 112 and k) 600 mW/cm² (unpolarized light, spot size ~0.025 cm²). b) Rate constant k₀ as a function of power density. The green line shows a fit to Eq. (10).

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Models based on dispersive first order kinetics have been heavily used for the description of non-photochemical spectral hole-burning within the model of distributed two-level systems [32–34]. In general, if a conversion rate is given by a function of,

R = ν_{0} \exp (- x)

where ν₀ is an attempt frequency, and x is subject to a probability distribution function D(x), then the decay can in general be described by,

N (t) = \int_{- \infty}^{\infty} D (x) \times \exp [- R_{0} \exp (- x) t] d x

with

R_{0} = ν_{0} \exp (- x_{0})

Equation (6) only relies on two parameters e.g. when a Gaussian distribution is assumed for D(x); R₀ determines the timescale of the decay and σ determines the dispersion of the decay [32].

For our particular case we invoke the following assumptions. First, it is assumed that each Sm²⁺ centre has a correlated oxide defect, such as O^-, in its neighbourhood separated by a few interionic distances. Then we assume that the separation R between the oxide impurity and samarium can be described by a standard gamma distribution and the electron transfer rate in the excited state, in which photoionization occurs, is given by an exponential profile i.e., we assume tunneling of the electron from the Sm²⁺ to the oxide centre when the Sm²⁺ ion is in a highly excited state. The effective electron transfer rate k is then given by,

k = k_{0} \exp (- R / a_{f})

where k₀ is the rate for R = R₀ and a_f is a scaling parameter. The bleaching curves, as a function of time t, can then be modelled by,

N (t) = \int_{R_{m}}^{\infty} \frac{{(R - R_{m})}^{γ - 1} \exp [- (R - R_{m})]}{Γ (γ)} \times \exp [- k_{0} \exp (- R) t] d R

where R_m is the radius of an excluded volume i.e., the distance to the nearest possible oxygen impurity within the crystal lattice and N(t) is the normalized Sm²⁺ concentration. As discussed above, the oxygen impurity cannot be on nearest neighbour F and Cl sites and hence we chose a radius for the excluded volume of R_m = 5 Å. In Eq. (9), k₀ determines the timescale of the bleaching curve and γ determines its deviation from single exponential behaviour i.e., it is a measure of dispersion. Naturally, k₀ is an effective rate constant that measures the photoionization for a particular excitation power and its power dependence indicates whether the photoionization is a one or multiphoton process. A global fit of the bleaching curves Eq. (9) was conducted with only k₀ as an individual fit parameter and some of these fits are shown in Fig. 7(a). The fit yields a_f = 1.3 ± 0.3 Å and γ = 5.4 ± 1.2, with the latter indicating that the oxide centre is separated from the Sm²⁺ ion by ~10 Å on average. However, these parameters should not be overanalysed as they are not fully independent. The main results are the relative values of k₀ that provide the power dependence, and the evidence that the oxide and Sm²⁺ centres are separated by a few interionic distances. It was also observed that it is very difficult to bleach the last 1-2% of Sm²⁺ ions. The residual Sm²⁺ may be due to the temperature induced motion of the defect pairs which may lead to larger separations which are inaccessible for the electron transfer.

A fit with a simple power law of the form,

k_{0} = A \times p^{x}

where p denotes the power density of the laser, yields a value of x = 1.58 ± 0.04. This shows unambiguously that the ionization mechanism is a two-photon process. The deviation from a simple quadratic dependence is due to saturation effects as is common in multiphoton processes. This is confirmed in Fig. 7(b) by the observation that the slope for the low power density points is close to 2.

Figure 8 is a schematic diagram that illustrates the two-step two-photon process. After excitation into the 4f⁶→4f⁵5d¹ transitions at 425 nm, the system non-radiatively relaxes to the ⁵D₀ (4f⁶) level. A second photon of the same wavelength then electronically excites the Sm²⁺ ion to a higher lying state in the deep-UV region where the electron transfer to O^- (or (O₂)^-) occurs. Importantly, the k₀ values as obtained by the global fit discussed above, contains the power dependence of the two-photon process that populates the Sm²⁺ level in the UV region.

Fig. 8 Schematic diagram for the bleaching mechanism of UV-C induced Sm²⁺ in nanocrystalline BaFCl:Sm³⁺.

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Figure 3 demonstrated that the present UV storage phosphor has an enormous dynamic range and hence lends itself to the application of multilevel (greyscale) encoding. A preliminary test of multilevel encoding for potential 2D memory applications is shown in Fig. 9, where a 50 μm thick BaFCl:Sm³⁺ film containing 4 weight-% Kraton as binder was exposed to different doses of UV-C light. The signals shown in this figure have to be compared to the saturation level of ~100,000,000 counts. The results in Fig. 9 were obtained with a simple CCD-camera imaging system with a very low numerical aperture and hence a low efficacy for photon collimation. With a better numerical aperture and using photomultiplier or photodiode based readouts, signals at least 10 × weaker can easily be read out. Thus in principle it is possible to write >100,000 levels (~17 bit) per pixel into this material. Naturally, the signal-to-noise ratio may impose limitations on the readout of data i.e., the discrimination between level n and n + 1 when n is large. This could be overcome to some extent by using logarithmic write-read schemes. Nanocrystalline BaFCl:Sm³⁺ therefore presents a very strong candidate for optical data storage as it allows a very large dynamic range for multilevel encoding. The readout may be conducted in photoluminescence excitation or reflectance measurement mode. The former is somewhat limited by the 2 ms excited state lifetime, but this limitation may be circumvented by near-field or confocal readout methods.

Fig. 9 Multilevel encoding. a) Ten levels as read out by a simple CCD based imaging apparatus and b) a profile plot of this data.

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4. Conclusions

We have demonstrated for the first time that nanocrystalline BaFCl:Sm³⁺ as prepared by co-precipitation is a highly efficient UV-storage phosphor. The mechanism is based on oxygen impurities such as O^2- and (O₂)^2-. To saturate the valence state switching in a nanocrystal of ~200 nm size, light energies in the low pJ range are merely required. This points to the potential of the material to be used for 2D and 3D optical data storage applications and also highly efficient non-volatile optical memory applications on photonic chips. Importantly, the UV-induced Sm²⁺ can be reversibly bleached back to Sm³⁺ by a two-photon ionization mechanism by applying blue-violet light. The 200 nm write wavelength would also enable higher storage densities a priori by the tighter focal spot as determined by the diffraction limit. In terms of 2D memory, the phosphor shows an enormous dynamic range that would allow for multilevel optical storage i.e., greyscale encoding. In order to allow for readouts with high signal-to-noise ratio over a huge dynamic range we envision that the coding may be done on a logarithmic scale. We also note that since the barium ion plays no part in the UV induced reduction of Sm³⁺ to Sm²⁺, SrFCl and CaFCl host lattices may also be employed for the UV-storage phosphor. Moreover, the MFX:Sm³⁺ samples could be prepared by ball-milling resulting in an average crystallite size of 30 nm [23, 35], and hence could yield an even higher data storage density.

Funding

Australian Research Council Linkage Project (LP 110100451); Georgina Sweet Laureate Fellowship (FL130100044).

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Highly efficient valence state switching of samarium in BaFCl:Sm nanocrystals in the deep UV for multilevel optical data storage

Abstract

1. Introduction

2. Experiment

3. Results and discussion

4. Conclusions

Funding

References and links

Cited By

Figures (9)

Equations (10)

Optical Materials Express