Brief Description of the Drawings
FIG. 1 shows the emission spectrum of LiNd.sub.0.9 Yb.sub.0.1 P.sub.4 O.sub.12,
FIG. 2 shows the responsivity curve of a Si-PIN photodetector,
FIG. 3 is a diagram showing the relationship between the luminescence intensity and the Yb content of LiNd.sub.1-y Yb.sub.y P.sub.4 O.sub.12, and
FIG. 4 is a diagram showing the relationship between the luminescence intensity and the M' content of LiM'.sub.0.9-x Nd.sub.x Yb.sub.0.1 P.sub.4 O.sub.12.
Detailed Description
The infrared - infrared fluorescent material according to this invention contains ytterbium ions (Yb.sup.3+) together with neodymium ions (Nd.sup.3+) as an activator, and it is represented by the following general formula:
where M denotes at least one element selected from the group consisting of Li, Na, K, Rb and Cs, M' denotes at least one element selected from the group consisting of Sc, Y, La, Ce, Gd, Lu, Ga, In, Bi and Sb, 0.05 .ltoreq. x .ltoreq. 0.999, 0.001 .ltoreq. y .ltoreq. 0.950, and x + y .ltoreq. 1.0.
In the fluorescent material according to this invention, Nd.sup.3+ acts as an absorbing and emitting ion. That is, it absorbs external exciting light, and it emits light by itself owing to part of the absorbed energy. Simultaneously therewith, it stimulates Yb.sup.3+ and causes it to emit light owing to the remaining energy. Accordingly, the emission spectrum of the fluorescent material according to this invention has a peak near 980 nm caused by Yb.sup.3+, and a peak near 1,050 nm caused by Nd.sup.3+.
FIG. 1 shows the emission spectrum of LiNd.sub.0.9 Yb.sub.0.1 P.sub.4 O.sub.12 which is one of the fluorescent materials according to this invention. This emission spectrum was obtained in such a way that a GaAlAs light emitting diode having an emission peak in the vicinity of 800 nm was employed as an exciting light source and that the intensity of luminescence from the fluorescent material was measured by means of a photomultiplier tube having a photocathode of the S-1 type.
In FIG. 1, luminescence in a region of wavelengths shorter than 940 nm is principally the light emission by the GaAlAs light emitting diode of the exciting light source. Luminescence in the vicinity of 980 nm is the light emission of Yb.sup.3+ which has had energy transmitted from the excited Nd.sup.3+. Luminescence in the vicinity of 1,050 nm by Nd.sup.3+ is feeble.
In case of a prior-art fluorescent material employing only Nd.sup.3+ as an activator, the luminescence is principally composed of light emissions near 1,050 nm and 900 nm, both of which are light emissions from the 4F.sub.3/2 excited level of Nd.sup.3+.
The efficiency of the luminescence from this excited level is usually close to 100%. However, the luminescence near 1,050 nm is of only approximately 60% of the energy from the level.
As apparent from FIG. 1, the luminescence near 900 nm overlaps the light emission of the GaAlAs light emitting diode. In order to detect the luminescence from the fluorescent material, therefore, only the luminescence near 1,050 nm which overlaps the exciting light little must be detected by the use of a proper filter. Ultimately, only about 60% of the energy from the excited level is utilized.
On the other hand, in the case of this invention, although the efficiency of transmission of energy from the 4F.sub.3/2 excited level of Nd.sup.3+ to Yb.sup.3+ depends upon the concentration of Yb.sup.3+, the luminescence efficiency of the excited Yb.sup.3+ is substantially 100%.
More specifically, in the prior-art fluorescent material, the luminescence near 1,050 nm is done by only about 60% of the energy from the 4F.sub.3/2 excited level of Nd.sup.3+. In the fluorescent material according to this invention, Yb.sup.3+ is employed as the light emitting ion, whereby part of the energy from the 4F.sub.3/2 excited level is converted into the luminescence at 980 nm, and substantially 60% of the remaining energy is obtained in the form of the luminescence at 1,050 nm. Thus, luminescence at an efficiency higher than in the prior art can be achieved.
As a detector for detecting the luminescence from the fluorescent material, a solid-state detector device made of Si or the like is of course favorable. The spectral sensitivity of a Si-PIN photodector which is a kind of Si detector is indicated by a curve in FIG. 2, and it has the highest responsivity in the vicinity of 900 nm.
Accordingly, in case where the luminescence by Nd.sup.3+ (at 1,050 nm) and the luminescence by Yb.sup.3+ (at 980 nm) are measured with the Si-PIN photodetector, a photoelectric conversion output in which the luminescence by Yb.sup.3+ is much higher than that by Nd.sup.3+ is provided even if the luminescence intensities by Nd.sup.3+ and Yb.sup.3+ are equal.
The contents of Nd.sup.3+ and Yb.sup.3+ in the fluorescent material of this invention need to lie within predetermined ranges, respectively.
FIG. 3 represents the luminescence intensities of LiNd.sub.1-y Yb.sub.y P.sub.4 O.sub.12 (O .ltoreq. y .ltoreq. 0.95) upon condition that the luminescence intensity of a fluorescent material containing quite no Yb.sup.3+, i.e., LiNdP.sub.4 O.sub.12 is made 100. Thus, the association of the luminescence intensities with the Yb.sup.3+ concentrations (y) is illustrated.
As apparent from FIG. 3, the luminescence intensity of the fluorescent material according to this invention is higher than in the case of containing no Yb.sup.3+ when y falls within a range of 0.001 - 0.95. Especially favorable results are obtained when y lies in a range of 0.04 - 0.90. When y is smaller than 0.001 or greater than 0.95, the effect of the addition of Yb.sup.3+ is scarcely noted, and hence, the range of y needs to be from 0.001 to 0.95.
The characteristic diagram of FIG. 3 was prepared in such a way that a GaAlAs light emitting diode (emission peak wavelength: 800 nm) was employed as an exciting light source and that the luminescences from the fluorescent materials were measured by a Si photodetector through a filter made of polycrystalline InP.
Even when an ion other than the neodymium ion and the ytterbium ion as can assume trivalence and as exhibits no absorption over wavelengths from 800 nm to 1,000 nm, that is, the ion of at least one element selected from the group consisting of Sc, Y, La, Ce, Gd, Lu, Ga, In, Bi and Sb coexists in the fluorescent material as M' in the aforecited general formula, the fluorescent material produces a higher luminescence intensity as compared with the prior-art fluorescent material activated with only the neodymium ion. By way of examples, FIG. 4 illustrates the relations between the luminescence intensities of fluorescent materials (the outputs of a silicon photodetector) and the contents of M' in a general formula of LiM'.sub.0.9-x Nd.sub.x Yb.sub.0.1 P.sub.4 O.sub.12 (the ytterbium ion concentration being fixed to 0.1) upon condition that the luminescence intensity of the prior-art fluorescent material LiNdP.sub.4 O.sub.12 is made 100. In FIG. 4, a curve 1 corresponds to a case of using bismuth as M', a curve 2 corresponds to a case of using yttrium, and a curve 3 corresponds to a case of using cerium. In comparison with the fluorescent material activated with only the neodymium ion, the fluorescent materials according to this invention provide higher outputs in the favorable concentration ranges of neodymium and ytterbium, and a concentration range of 0 - 0.3 is the optimum for the concentrations of the elements.
It has been confirmed that similar effects are obtained when Sc, La, Gd, Lu, Ga, In and Sb other than Bi, Y and Ce are used as M'.
As set forth above, the fluorescent material according to this invention exploits the large absorption cross-section of the neodymium ion in the infrared region and transmits energy from the neodymium ion to the ytterbium ion. Thus, the luminescence of the ytterbium ion which is more adapted to the spectral sensitivity of the conventional silicon photodetector than the luminescence of the prior-art fluorescent material can be attained at high efficiency. As the result, the emission output in the case of employing the silicon photodetector is allowed to sharply increase.
In the above, only the case of exciting the fluorescent material with infrared rays has been described in detail in relation to the enhancement of the luminescence intensity in the fluorescent material of this invention. It is obvious, however, that a higher luminescence intensity than in the prior-art fluorescent material is attained also in case where the energy level of the neodymium ion lying in the visible or ultraviolet wavelength region is excited by an argon laser or the like.
Example 1
The powdery raw materials were mixed well, and were packed into a lidded crucible made of quartz. The crucible was put into an electric furnace, and was heated from the room temperature up to 700.degree. C at a constant temperature-raising rate for 2 hours. Thereafter, the mixture was baked at 700.degree. C for 2 hours. As soon as the baking was completed, the crucible was taken out from the electric furnace and was cooled in the air. A fluorescent material obtained was put into water and was boiled together with the crucible. After cooling, the fluorescent material was cleansed with 1 N-nitric acid, was washed with water and was dried. The fluorescent material synthesized had a composition of LiNd.sub.0.9 Yb.sub.0.1 P.sub.4 O.sub.12, and was fine powder having a permeability grain size of about 5 .mu.m. The luminescence intensity of this fluorescent material was approximately 6.5 times as high as that of a fluorescent material activated with only neodymium (LiNdP.sub.4 O.sub.12).
Example 2
The powdery raw materials were mixed well, and were packed into a lidded crucible made of alumina. The crucible was put into an electric furnace, and was heated from the room temperature up to 700.degree. C at a constant temperature-raising rate for 2 hours. Subsequently, the mixture was baked at 700.degree. C for 2 hours. As soon as the baking was finished, the crucible was taken out from the electric furnace and was quickly cooled in the air. A fluorescent material obtained was put into water and was boiled. After cooling, the fluorescent material was cleansed with 1 N-nitric acid, was washed with water and was dried. The fluorescent material synthesized was represented by a composition formula of LiBi.sub.0.2 Nd.sub.0.7 Yb.sub.0.1 P.sub.4 O.sub.12.
This fluorescent material exhibited substantially the same emission spectrum as that in Example 1. The luminescence intensity thereof was approximately 8 times as high as that of the fluorescent material activated with only neodymium (LiNdP.sub.4 O.sub.12).
Example 3
The powdery raw materials were mixed well, and were packed into a lidded crucible made of quartz. The crucible was put into an electric furnace, and was heated from the room temperature up to 740.degree. C at a constant temperature-raising rate (300.degree. C/hour. Thereafter, the mixture was baked at 740.degree. C for 2 hours. As soon as the baking was finished, the crucible was quickly cooled in the air. A fluorescent material obtained was boiled with water together with the crucible. After cooling, the fluorescent material was cleansed with 1 N-nitric acid, was washed with water and was dried.
The fluorescent material synthesized had a composition of NaNd.sub.0.9 Yb.sub.0.1 P.sub.4 O.sub.12, and its luminescence intensity was approximately 6.4 times as high as the luminescence intensity of the neodymium-activated fluorescent material (LiNdP.sub.4 O.sub.12).
While the foregoing examples employed Li and Na as M, similar effects were obtained when K, Rb and Cs were used. Whether these elements were used alone or in combination consisting of two or more, no difference was noted in the luminescence characteristics.