Cubic sesquioxides, RE 2 O 3 , where RE = Y, Lu or Sc, are attractive host crystals for thulium (Tm 3+ ) doping. A comparison of the spectroscopic properties of Tm 3+ :RE 2 O 3 crystals in terms of ...transition cross sections and cross-relaxation (CR) efficiency required for efficient upconversion pumping is presented. Thermo-optic properties of Tm 3+ :RE 2 O 3 crystals (thermal lensing, fractional heat loading, and thermo-optic coefficients) are described. The positive thermal lens, broadband emission, and efficient CR of the Tm:RE 2 O 3 crystals enable the development of compact, highly efficient and power-scalable lasers operating above 2 μm, based on thermal guiding. Nowadays, Tm:Lu 2 O 3 microchip lasers are capable of generating nearly 5 W of output power at ~2.06 μm with a slope efficiency η of 67% and in a rod geometry-up to 47.5 W with η of 59%. For multiwatt output at even longer wavelengths around 2.15 μm, Tm:Sc 2 O 3 is an interesting candidate.
Microrods of calcium sodium bismuth fluorapatite doped with neodymium, denoted as Ca
8
NaBi
1-x
Nd
x
(PO
4
)
6
F
2
(with x ranging from 0 to 0.5), were synthetized using a modified Pechini sol–gel ...method. The crystalline structure of these phosphors was refined using the Rietveld method and exhibited hexagonal symmetry with space group P6
3
/m—C
2
6h
, where the lattice parameters were
a
= 9.3855(5) Å and
c
= 6.8998(6) Å for
x
= 0.2. The particles morphology was analyzed through SEM, revealing an average length of approximately 1.5 µm. When excited at 808 nm, the Ca
8
NaBi(PO
4
)
6
F
2
:0.2Nd
3+
microrods emitted strongly at 872, 957 and 1055 nm, falling within the near-infrared region. These emissions correspond to the
4
F
3/2
→
4
I
9/2
(P2),
4
F
5/2
→
4
I
11/2
(P3),
4
F
3/2
→
4
I
11/2
(P1) transitions of Nd
3+
ions, respectively, and are situated within the first and second biological windows. The luminescence lifetime of the
4
F
3/2
state of Nd
3+
was measured to be 294.4 µs for the sample with the lowest Nd
3+
concentration of
x
= 0.05. In addition, the luminescence intensity ratios P2/P1 and P3/P1 were found to be temperature dependent, potentially making it suitable for luminescent ratiometric thermal sensing. These findings suggest that the synthesized Ca
8
NaBi(PO
4
)
6
F
2
:Nd
3+
microrod thermometers exhibit favorable characteristics in terms of relative sensitivity, temperature uncertainty, and repeatability within the temperature range of 303–403 K.
Thermal lensing is studied in monoclinic Yb:Ca 4 YO(BO 3 ) 3 (Yb: YCOB) oxoborate crystals cut along the optical indicatrix axes. For all orientations, the thermal lens is positive. In the Z-cut ...crystal, the sensitivity factors of the thermal lens are M Z-X = 2.4 and M Z-Y = 2.8 m -1 /W (for E∥X), and the astigmatism degree is as low as S/M = 14%. The positive thermal lens in Yb:YCOB is related to the large thermal expansion and strong photoelastic effect. Microchip lasers are realized with 3-mm-long X, Y, and Z-cut 15 at.% Yb:YCOB crystals. With a Z-cut crystal, maximum output power of 8.35 W is achieved at ~1040 nm with slope efficiency of 70%. Using various crystal cuts and output coupler transmissions, multiwatt emission in the spectral range of 1033-1091 nm is demonstrated.
We demonstrate the suitability of monoclinic double tungstates (MDTs), KRE(WO 4 ) 2 , where RE = Gd, Y or Lu, doped with Nd 3+ , Yb 3+ , Tm 3+ , or Ho 3+ ions and co-doped with Yb 3+ -Tm 3+ , or Tm ...3+ -Ho 3+ ion couples, for highly efficient micro-lasers at ~1 μm and at ~2 μm. This is facilitated by the use of high rare-earth doping levels (up to 15 at.% for Tm, 10 at.% for Nd, and 25 at.% for Yb) and a special crystal cut along the Ng-axis providing the thermal guiding. Record slope efficiencies for bulk MDT lasers are achieved for each studied ion. A 15 at.% Tm:KLu(WO 4 ) 2 laser generated 785 mW at 1957...1965 nm with a slope efficiency η = 77%. The quantum efficiency for Tm 3+ ions amounted to η q = 1.98±0.02. With a 0.9 mm-thick 25 at.% Yb:KLuW micro-laser, η = 91% is achieved, approaching the theoretical limit set by the Stokes shift.
Novel transparent Ho-doped sesquioxide ceramics are synthesized by high-temperature vacuum sintering and hot isostatic pressing. The spectroscopic properties of 1.4 at% and 3 at% Ho:(Lu,Sc)2O3 ...ceramics are studied within Judd-Ofelt theory, yielding the intensity parameters Ω2 = 7.863, Ω4 = 1.843 and Ω6 = 0.531 10−20 cm2. The radiative lifetime of the 5I7 state is 10.63 ms. Absorption, stimulated-emission and gain cross-sections for the 5I7 → 5I8 Ho3+ transition are calculated. At > 2 µm, the maximum σSE = 3.41× 10−21 cm2 at 2.101 µm. Under GaSb diode-pumping at 1.929 µm, the 1.4 at% Ho:(Lu,Sc)2O3 ceramic laser generated 187 mW of quasi-continuous wave (CW) output at 2.114–2.135 µm with a slope efficiency η of 7.6%. When pumped at 1.946 µm with a Tm laser, η reached 25% while the CW output power was 20.5 mW. The Ho:(Lu,Sc)2O3 ceramics are promising for broadband tunable and ultrashort pulse laser operation at ~ 2.1 µm.
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•A series of Eu3+ -doped Li3Ba2Gd3(WO4)8 quaternary tungstates phosphors were synthesized.•A structural characterization of the prepared phosphors is performed by XRD, Raman and DTA ...analysis.•Temperature- and concentration-dependent photoluminescence properties are studied.•The symmetry of the local environment of Eu3+ activators has been described in the frame of Judd–Ofelt theory.•The developed quaternary tungstates are promising as red-emitting phosphors.
Red-emitting Li3Ba2Gd3(WO4)8: x at.% Eu3+ (0.5 ≤ x ≤ 10) phosphors were effectively produced using a conventional solid-state reaction method at high temperature in air atmosphere. The phosphors exhibited monoclinic crystalline structure with the C2/c space group and 2/m point group. The unit cell parameters determined from the Rietveld refinement for 3 at.% doped sample are a = 5.2149(4) Å, b = 12.7447(1) Å, c = 19.2058(3) Å, α = γ = 90°, β = a^c = 91.9315(1)° and Vcalc = 1275.748(2) Å3. Photoluminescence spectra, lifetimes, colour coordinates and purity, and temperature-dependent emission spectra have been systematically studied. Upon excitation at 395 nm, the phosphors displayed red emission with a peak centered at 614 nm, which was associated with the 5D0→7F2 electrical dipolar transition of Eu3+. At the same time, the influence of different Eu3+ ion concentration on the photoluminescence intensity is also discussed. The optimal concentration of Eu3+ ions in Li3Ba2Gd3(WO4)8 was found to be 3 at.%. The mechanism behind concentration quenching was ascribed to dipole–dipole interactions, and the critical distance for energy transfer among Eu3+ ions was determined to be 7.409 Å. The Judd–Ofelt theory was utilized to assess the intensity parameters and radiative properties. The fluorescence decay times were also determined for the different concentration levels of Eu3+ cations. The chromatic coordinates of Li3Ba2Gd3(WO4)8: Eu3+ were found to be (0.653, 0.345), closely resembling the standard red light (0.670, 0.330) and demonstrated high colour purity (98.42 %) under excitation at 395 nm. Moreover, the phosphors Li3Ba2Gd3(WO4)8: Eu3+ exhibited favorable thermal stability, characterized by an activation energy of 0.219 eV. The aforementioned findings indicate that the phosphors Li3Ba2Gd3(WO4)8 doped with Eu3+ have the potential to function as a red-emitting phosphor in near-ultraviolet-driven white light-emitting diodes.
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