WO2004054938A1

WO2004054938A1 - Chromium doped oxide glass

Info

Publication number: WO2004054938A1
Application number: PCT/GB2003/005541
Authority: WO
Inventors: Animesh Jha; Caroline Batchelor; Woon Jin Chung; Shaoxiong Shen
Original assignee: The University Of Leeds
Priority date: 2002-12-18
Filing date: 2003-12-18
Publication date: 2004-07-01
Also published as: AU2003298444A1; GB0229527D0

Abstract

An aluminosilicate glass comprising: a) SiO2 in the range 45 to 55 mol%; b) A12O3 in the range of 20 to 30 mol%; c) Cr2O3 in the range 0.001 to 0.1 mol%; d) at least one of Li2O or Na2O and mixtures thereof e) a charge transfer facilitating oxide; and f) further ingredients wherein the amounts of a + b + c + d + e + f total 100 mol%. In an alternative aspect of the invention there is provided a gallogermanate glass comprising: a) GeO2 in the range 65 to 85 mol%; b) Ga2O3 in the range 5 to 20 mol%; c) Cr2O3 in the range 0.001 to 0.01 mol%; d) at least one of Li2O and Na2O and mixtures thereof, e) a charge transfer facilitating oxide, and f) further ingredients wherein the amounts of a, b, c, d, e and f total 100 mol%.

Description

C romi m Doped Oxide Glass

The present invention relates to a chromium doped oxide glass. More particular, but not exclusively, the present invention relates to a chromium doped oxide glass including a charge transfer facilitating oxide.

Chromium doped glasses and crystals have been studied for their spectroscopic properties for many years, in particular for their use as tunable vibronic solid-state lasers. Cr³⁺ - doped BeAl₂0₄ (alexandrite) that is tunable over the range 700-850nm- (depending on the pumping conditions and the optics applied) has a lifetime of 260μs, whilst Cr⁴⁺ -doped Mg₂Si0₄ (forsterite) tunable over the range l.l-1.4μm (again depending upon the pumping conditions and optics) has a lifetime of less than 4μs . These well-known crystal hosts have much larger quantum efficiencies for Cr⁴⁺ and ^Cr3+ ions than when they are present in a glass host. Crystal hosts however suffer from a major disadvantage of being bulky, which then necessitates a large pump power requirement for tunable lasers requiring an efficient cooling system for the device.

In contrast, glass fibre lasers may be pumped using a commercially available semiconductor diode laser and therefore can be produced as compact devices. Much research has been undertaken to produce both Cr³⁺ and Cr⁴⁺ containing glasses suitable for replacing the well-established laser crystals .

The broadly emitting Cr³⁺ -doped glasses produced^" thus far, such as fluoride, borate; silicate and aluminosilicate glasses have possessed short lifetimes (below 60μs) and low quantum efficiencies (below 25%) . Cr⁴⁺ -doped glasses are difficult to produce. Emission has been reported in glass-ceramics and crystals but not to our knowledge in glasses unless they are subjected to strong oxidising atmospheres .

Accordingly, in a first aspect there is provided an aluminosilicate glass comprising: a) Si0₂ in the range 45 to 55 mol%; b) A1₂0₃ in the range of 20 to 30 mol%; c) Cr₂0₃ in the range 0.001 to 0.1 mol%; d) at least one of Li₂0 or Na₂0 and mixtures thereof; e) a charge transfer facilitating oxide; and f) further ingredients wherein the amounts of a + b + c + d + e + f total 100 mol% .

The aluminosilicate glass according to the invention has the advantage as it produces strong emissions from Cr⁴⁺ ions when pumped in visible and near infrared wavelengths.

Preferably, the concentration of Li₂0 is in the range 10-25 mol%, more preferably in the range 10-20 mol%.

Preferably, the concentration of Na₂0 is in the range 10-25 mol%, more preferably in the range 10-20 mol%.

The concentration of the mixture of Li₂0 and Na₂0 can be in the range 10-25 mol% .

The aluminosilicate glass according to the invention can further comprise at least one of ZnO or Ti0₂ or mixtures ^{' "}thereof. The^" concentration of ZnO or Ti0₂ ^"can^"be in the range 0-10 mol%, preferably in the range 0.05-10 mol%, more preferably in the range 0.5-5 mol%. The concentration of the mixture of ZnO and Ti0₂ can be in the range 0-10 mol%, preferably 0.05-10 mol%, more preferably 0.5-5 mol% . The charge transfer facilitating oxide can be an oxide of at least one of Mn, P, As, Sb, V, Nb, Ta, Mo, W, Te, Sn, Pb and mixtures thereof. The charge transfer facilitating oxide can be Bi₂0₃. The concentration of charge facilitating oxide can be in the range 1-5 mol% .

In an alternative aspect of the invention there is provided a gallogermanate glass comprising: a) Ge0₂ in the range 65 to 85 mόl%; b) Ga₂0₃ in the range 5 to 20 mol%; c) Cr₂0₃ in the range 0.001 to 0.01 mol%; d) at least one of Li₂0 and Na₂0 and mixtures thereof; e) a charge transfer facilitating oxide; and f) further ingredients wherein the amounts of a, b, c, d, e and f total 100 mol%.

The concentration of Li₂0 can be in the range 0-35 mol%, preferably in the range 0.05-35 mol%, more preferably in the range 5-20 mol%.

The concentration of Na₂0 can be in the range 0-20 mol%, more preferably in the range 0.05-20 mol%, more preferably in the range 5-15 mol%.

The concentration of the mixture of Li₂0 and Na₂0 can be in the range 0-35 mol%, preferably in the range 0.01 to 35 mol% the concentration of Na₂0 not exceeding 20 mol% .

The charge transfer facilitating oxide can be Bi₂0₃. The charge transfer facilitating oxide can be an oxide of at

^■ least orre of^'Mn, P,^"As, Sb, V, Nb, Ta, Mo, W, Te, Sri, Pb and mixtures thereof. The concentration of the charge transfer facilitating' oxide can be in the range 0-15 mol%, more preferably in the range 0.05-15 mol%, more preferably in the range 0.5-5 mol%.

The galloger anate glass according to the invention can further comprise at least one of ZnO or Ti0₂ and mixtures thereof. The concentration of ZnO or Ti0₂ or mixtures thereof can be in the range 0-10 mol%, more preferably in the range 0.05-7 mol%, more preferably in the range 5-10 mol%.

In a further aspect of the invention there is provided a laser or amplifier including an aluminosilicate or gallogermanate glass according to the invention.

The present invention will now be described by way of example only and not in any limitative sense with the reference to the accompanying drawings in which:

Figure 1 shows a Cr³⁺ and Cr⁴⁺ and energy level diagram in aluminosilicate and gallogermanate glasses;

Figure 2 shows absorption spectra of aluminosilicate glasses;

Figure 3 shows emission intensity against .wavelength for an aluminosilicate glass according to the invention;

Figure 4 shows emission intensity versus wavelength for an aluminosilicate glass according to the invention;

Figures 5 and 6 show plots of emission intensity versus time for an aluminosilicate glass ^"according to the invention, pumped at 514 nm and 797 nm respectively; Figures 7 and 8 show emission intensity versus wavelength for a gallogermanate glass according to the invention pumped at 514 nm and various infrared wavelengths respectively;

Figures 9 and 10 shows plots of emission intensity versus time for a gallogermanate glass according to the invention pumped at 502 nm and 795 nm respectively.

As shown in figure 1, Cr³⁺ -ions when doped in glass produced broad emission, which is centred around 800 nm. This broad emission is due to the ions being located in distorted low- field octahedral sites of modifier ions, which are present in the aluminosilicate and gallogermanate compositions. Chromium (Cr³⁺) ions in these low-field sites, when pumped at visible wavelengths, undergo a transition: ⁴A₂→⁴T_1A which then decays nonradiatively to the ⁴T₂ level . From the T₂ level, the ions emit radiation as the ions relax to the vibronic ground state via the transition ⁴T₂→⁴A₂.

When Cr³⁺ -ions reside in high-field environments, such as those found in crystals, the radiative decay occurs from the zero-phonon level via the transition ²E-⁴A₂. The zero-phonon level is invariably observed around 690 to 730 nm in silicate glasses and crystal hosts.

The Cr⁴⁺ -ions such as those found in the forsterite crystals prefer to occupy tetrahedral sites and absorb pump power to attain the metastable state via the ³A₂→³T₁ transition. The nonradiative relaxation to the ³T₂ state lowers the energy which is then followed by the ³T₂→³A₂ NIR emission peaked -^■ around^" 1300 ^"nm. The ³T^" ₂ ^"-^f3A₂ ^"transition is^" a^' vibronic transition, which is why the emission is quite broad compared to the non-vibronic transitions in the rare-earth doped glass systems. The aluminosilicate and gallogermanate glasses according to the invention include charge transfer facilitating oxides which modify the glass network. The Cr⁺ ions doped in such glasses preferably occupy low-field octahedral sites similar to the sites of the network modifying ions. Light emission from such glasses is via a charge transfer process which leads to meta stable lifetimes of at least as long as 300 μs .

The aluminosilicate glasses according to the invention include both low-field and high-field sites in which the Cr³⁺ ions preferentially occupy. The composition of the aluminosilicate glass is such that it resembles the crystal eucryptite, in which the layers of alternating A10₄ and Si0 tetrahedra are separated by Li ions. Cr³⁺ -ions can occupy either the low-field site between the layers, or by substituting for Al or Si ions within the high-field tetrahedral co-ordination.

Figure 2 shows in curve (a) the absorption spectrum of an aluminosilicate glass comprising Si0₂, Al₂0₃, Li₂0, ZnO and Cr₂0₃. Curve (b) shows an absorption spectrum for the same glass including Bi₂0₃. The excitation of the Cr³⁺ absorption band gives rise to a small emission peak between 700 nm and 800 nm as can be seen in figure 3.

The larger emission peak in figure 3, between 1000 and 1400 nm is due to the Cr⁴⁺ emission, which is evident in glass containing Bi₂0₃ according to the invention. When Bi ions are incorporated in an aluminosilicate glass, the optical absorption and carrier properties similar to those seen in ^" sillenite ^"structures (Bi₁₂G^"eO₂₀ and Bi₁₂SiO₂₀) are observed,^" see^" figure 2. These properties include a shoulder appearing in the absorption spectra, at around 500nm and a colour change in the samples from green for Cr³⁺ -doped glasses to a brown/red for Cr⁺ -doped glass. The shoulder is reportedly due to antisite bismuth ions occupying on germanium sites. Chromium can replace bismuth in this tetrahedral germanium site to produce Cr⁺.

Figure 1 shows a representative energy level diagram indicating the charge transfer processes that are likely to occur between the levels .

Aluminosilicate Glasses

The compositional range of the aluminosilicate glasses according to the invention are shown in Table 1. Compositions outside of this range result in emission from Cr³⁺ ions only.

Table 1 Compositional range for the aluminosilicate glasses according to the invention

X₂ is a charge transfer facilitating oxide. In one embodiment of the invention this is Bi₂0₃. Other oxides which participate in the charge transfer process as described below are also possible.

The Na₂0 can replace Li₂0 in part or completely. The aluminosilicate glass for the invention can include Zno or

TiO, The Ti0₂ can replace ZnO in part or completely. Shown in figures 3 and 4 are emission spectra of an aluminosilicate glass according to the invention pumped of various pump wavelengths in the visible region of the electromagnetic spectrum and at near infrared wavelengths respectively. The glass was prepared from high purity starting materials (>99.99%) by melting in a platinum crucible. The melting and casting of the homogenised glass was carried out at 1600 "C for three hours in air, before being removed from the furnace and allowed to cool to room temperature. Annealing at temperatures below 500 'C yield more stable glasses as thermal stresses are removed.

The decay curves for Cr⁴⁺ in the aluminosilicate glass according to the invention are shown in figures 5 and 6, pumped at 514nm and 797nm respectively. The lifetime of the emission decreases when pumped at longer wavelengths due to the change in the population of the excited states. The ³T₂→³A₂ level of Cr⁺ is split into 3 further levels, each with a different lifetime profile. When the sample is pumped at longer wavelengths only the lower levels of this transition are active and participate in spontaneous emission.

Galloqermanate Glasses

Gallogermanate glasses are a lower temperature analogue to the aluminosilicate glasses described above.

Table 2 Compositional range for gallogermanate glasses according to the invention

X₂ is a charge transfer oxide as previously described.

Figures 7 and 8 show emission spectra of a gallogermanate glass according to the invention pumped in the visible region and at various near infrared wavelengths respectively. The glass comprises Ge0₂, Ga₂0_3f Li₂0, Bi₂0₃ and Cr₂0₃. The glass was melted at 1300 ^°C for one hour before being removed from the furnace and being allowed to cool to room temperature. In this case the annealing was performed at 350 'C to relieve thermal stress.

Lifetime decays of the same glass is shown in figures 9 and 10, pumped at 502nm and 795nm respectively. Again the lifetime decreases as the pump wavelengths is increased.

MECHANISM OF ROOM TEMPERATURE Cr⁺ STATE EMISSION The addition of Bi₂0₃ or other charge transfer facilitating oxide promotes a charge transfer process, e.g. Cr³⁺ +Bi³⁺ →Cr⁴⁺ +Bi²⁺ (step 1) during pump excitation leading to Cr⁴⁺ - emission in both aluminosilicate and gallogermanate glasses. The second charge transfer process, in which the hexavalent Cr⁶⁺ -state is involved (see figure 1 the absorption peak at 375nm) , helps in recovering the 3+ state of Bi-ions via Cr⁶⁺ +2Bi²⁺ →Cr⁺ +2Bi³⁺, which is the essential for sustaining the emission. In the glass during melting some Cr⁶⁺ -ions also get reduced to Cr⁴⁺ state during melting in the presence of any Bi²⁺ state in the structure. Both steps are prevalent during the dynamic (excited) state, whereas the equilibrium concentrations of Cr⁴⁺ ions is only possible via melting in aluminosilicate and gallogermanate glasses.

Claims

1. An aluminosilicate glass comprising: a) Si0₂ in the range 45 to 55 mol%; b) A1₂0₃ in the range of 20 to 30 mol%; c) Cr₂0₃ in the range 0.001 to 0.1 mol%; d) at least one of Li₂0 or Na₂0 and mixtures thereof e) a charge transfer facilitating oxide; and f) further ingredients wherein the amounts of a + b + c + d . e + f total 100 mol% .

2. An aluminosilicate glass as claimed in claim 1, wherein the concentration of Li₂0 is in the range 10 to 25 mol%, more preferably in the range 10 to 20 mol% .

3. An aluminosilicate glass as claimed in either of claims 1 or 2, wherein the concentration of Na₂0 is in the range 10 to 25 mol%, more preferably -in the range 10 to 20 mol%.

4. An aluminosilicate glass as claimed in any one of claims 1^' to 3, wherein the concentration of the mixture of Li₂0 and Na₂0 is in the range 10 to 25 mol% .

5. An aluminosilicate glass as claimed in any one of claims 1 to 4, further comprising at least one of ZnO or Ti0₂ or mixtures thereof.

6. An aluminosilicate glass as claimed in claim 5, wherein the concentration is of at least one of ZnO or T_.i0₂ is in the range 0 to 10 mol%, preferably 0.05 to 10 mol%, more preferably 0.5 tθ^"5 mol% .

7. An aluminosilicate glass claimed in either of claims 5 or 6, wherein the concentration of the mixture of ZnO or Ti0₂ is in the range 0 to 10 mol%, preferably 0.05 to 10 mol%, more preferably 0.5 to 5 mol%.

8. An aluminosilicate glass as claimed in any one of claims 1 to 7, wherein the charge transfer facilitating oxide is an oxide of at least one of Mn, P, As, Sb, V, Nb, Ta, Mo, W, Te, Sn, Pb and mixtures thereof.

9. An aluminosilicate glass claimed in any one of claims 1 to 7, wherein the charge transfer facilitating oxide is Bi₂0₃_

10. An aluminosilicate glass as claimed in either of claims 8 or 9 wherein the concentration of charge facilitating oxides in the range 1 to 5 mol%.

11. A gallogermanate glass comprising: a) Ge0₂ in the range 65 to 85 mol%; b) Ga₂0₃ in the range 5 to 20 mol%; c) Cr₂0₃ in the range 0.001 to 0.01 mol%; d) at least one of Li₂0 and Na₂0 and mixtures thereof; e) a charge transfer facilitating oxide; and f) further ingredients wherein the amounts of a, b, c, d, e and f total 100 mol%.

12. A gallogermanate glass as claimed in claim 11, wherein the concentration of Li₂0 is in the range 0 to 35 mol%, preferably in the range 0.05 to 35 mol%, more preferably in the range 5 to 20 mol%.

13. A gallogermanate glass as claimed in either of claims 11 or 12, wherein the concentration of Na₂0 is in the range 0 to 20 mol%, preferably in the range 0.05 to 20 mol%, more preferably in the range 0.05 to 20 mol%, more preferably in the range 5 to 15 mol%.

14. A gallogermanate glass as claimed in any one of claims 11 to 13, wherein the concentration of the mixture of Li₂0 and Na₂0 is in the range 0 to 35 mol%, preferably in the range 0.01 to 35 mol% the concentration of Na₂0 not exceeding 20 mol%.

15. A gallogermanate glass as claimed in any one of claims 11 to 14, wherein the charge transfer facilitating oxide is Bi₂0₃ -

16. A gallogermanate glass as claimed in any one of claims 1 to 14, wherein the charge transfer facilitating oxide is an oxide of at least one of Mn, P, As, Sb, V, Nb, Ta, Mo, W, Te, Sn, Pb and mixtures thereof.

17. A gallogermanate glass as claimed in any one of claims 11 to 16 wherein the concentration of the charge transfer facilitating oxide is in the range 0 to 15 mol%, more preferably in the range 0.05 to 15 mol%, more preferably in the range 0.5 to 5 mol%.

18. A gallogermanate glass as claimed in any one of claims 11 to 17, further comprising at least one of ZnO or Ti0₂ and mixtures thereof.

19. A gallogermanate glass as claimed in claim 18 when the concentration of Zn or Ti0₂ or mixtures thereof is in the range 0 to 10 mol%, more preferably in the range 0.01 to 10 mol%, more preferably in the range 5 to 10 mol%.

20. A laser or amplifier including an aluminosilicate or gallogermanate glass as claimed in any one of claims 1 to 19