The yellow laser has promising applications, particularly in the field of medicine such as Freckle removal and the therapy of ophthalmic illness^[1-2]. Among the existing solutions for achieving yellow laser output include solid- state gain medium-based summing frequency mixing, frequency doubling techniques, and optically pumped semiconductor lasers^[3⇓-5]. But these approaches have low output efficiency, complex system design, and higher expenses, which in turn limit the application of yellow lasers. Trivalent dysprosium (Dy³⁺) ion possesses an abundant energy level structure that allows it to emit different colors of light in the visible region through 4f-4f energy level transitions^[6]. As one of the few rare- earth ions capable of directly producing yellow emission (Dy³⁺: ⁴F_9/2 → ⁶H_13/2), yellow lasers with Dy³⁺ doped crystals are promising for industrial, medical, and display applications, it has attracted much attentions. In 2012, the first InGaN LD pumped Dy: YAG crystal achieved yellow light laser operation with 150 mW output power^[7]. Later, a continuous yellow laser output of 574 nm was obtained with InGaN LD-pumped Dy, Tb: LiLuF₄ crystal. The slope efficiency is only 13.4%, and the output power is 55 mW^[8]. Furthermore, Dy: YAG crystal achieves laser operation at 582.7 nm with a single pulse energy of ~1.1 mJ at a repetition rate of 50 Hz^[9]. The CW yellow laser output of Dy: YAG crystal has also progressed, achieving a maximum output power of 166.8 μW at a laser center wavelength of 582.5 nm^[10]. Some studies of Dy co-doping with other ions have been carried out, such as Dy³⁺/Y³⁺: CaF₂^[11], Dy³⁺/(Tb³⁺, Eu³⁺): Sr₃Gd(BO₃)₃^[12], Dy³⁺/Tm³⁺: LiNbO₃^[13]. More importantly, to enhance the efficiency of the Dy³⁺ laser, it is indispensable to exploit suitable Dy³⁺-doped matrix crystals.

SrGdGa₃O₇ crystal is a member of the Melilite ABGa₃O₇ (A = Ca, Ba, Sr, B = La, Gd) crystals, which belongs to the P$\bar{4}$2₁m space group. The melting point is about 1600 ℃, and the congruent melting makes it suitable for crystal growth with the Czochralski method^[14⇓-16]. The ratio of the thermal expansion coefficient SrGdGa₃O₇ crystal in different polarization (α_a = 5.32×10^-6 /K, α_c = 5.365×10^-6 /K) is close to 1, which is beneficial for the growth of high-grade crystal and its applications in laser works^[17]. SrGdGa₃O₇ is a disordered laser crystal, and the activating ion is affected by the crystal field where it is located, causing an inhomogeneous broadening of the spectra, which is beneficial to the semiconductor laser pumping efficiency. The wide emission spectrum makes SrGdGa₃O₇ a good crystal material for new ultrashort pulse lasers. Furthermore, SrGdGa₃O₇ crystal with lower phonon energy (~680 cm^-1) can meet the requirements for matrix crystals in both visible and mid-infrared regions^[18]. At present, most of the studies on SrGdGa₃O₇ are focused on Nd³⁺, Er³⁺, and Tm³⁺-activated crystals^[15,17]. The visible and mid-infrared studies of Dy³⁺-activated SrGdGa₃O₇ crystal have not yet been reported.

In this work, Dy³⁺-doped SrGdGa₃O₇ crystal is successfully grown by the Czochralski method, and crystal structure, absorption, fluorescence, and fluorescence decay time of such crystal are investigated as a potential yellow laser gain medium.

3% (in mass) Dy³⁺-doped SrGdGa₃O₇ (Dy: SGGM for short) crystal was grown through the Czochralski method on (001) orientation. All chemical components were dried before weighing. The crystal was synthesized using SrCO₃ (AR), 99.999% purity Gd₂O₃, Ga₂O₃, and Dy₂O₃ raw materials according to the formula SrGd_0.97Dy_0.03Ga₃O₇. To compensate for the volatilization of Ga₂O₃ during the growth, additional 1% (in mass) Ga₂O₃ was added. The components were ground, mixed, and heated in a Corundum crucible at 1100 ℃ for 24 h. After cooling to ambient temperature, the mixtures were re-ground, pressed into bulks, and heated at 1200 ℃ for 48 h. An intermediate-frequency heater was used to melt the synthesized materials laid in the iridium crucible. During the crystal growth, the rotational rate was maintained at 8-12 r/min, and the pulling rate was 1-1.3 mm/h. When crystal growth completed, the crystal was cooled to ambient temperature at a rate of 15-30 K/h. As shown in Fig. 1, the as-grown Dy: SGGM crystal has dimensions of ϕ25 mm×40 mm. Effective concentration (C_top) for Dy³⁺ ions in Dy: SGGM crystal is 2.49% as measured by ICP-OES (Agilent 725 ES, USA), and segregation coefficient κ by calculation is 0.83.

The XRD (X-ray diffraction) pattern was obtained using a Rigaku MiniFlex-600 diffractometer and employing Cu Kα radiation (λ = 0.1540598 nm). The samples used for XRD measurements were powders obtained by grinding the crystals. A wafer with dimensions of 10 mm× 10 mm×1 mm is obtained by slicing and polishing for spectroscopic measurements. The rocking curve of the single crystal (002) plane was determined with an X-ray diffractometer (Germany-Bruker D8 Advance) with a scan step of 0.001°. Optical polarized absorption spectra were measured using a Perkin-Elmer UV-Vis-NIR spectrometer (Lambda-900) with a spectral resolution of 1 nm. Fluorescence spectra and fluorescence decay curves were measured with an Edinburgh FLS1000 photometer at continuous and pulsed Xe lamps, respectively, where the fluorescence spectra had a scanning slit width of 1 nm and the fluorescence decay curve had a slit width of 2 nm on the excitation side and 0.5 nm on the emission side. Experimental conditions were kept constant during the optical studies to obtain comparable results. All tests were carried out at ambient temperature.

Structure of the melilite SrGdGa₃O₇ crystal is shown in Fig. 2, the GaO₄ tetrahedron layer is in the a-b planes. There are two types of GaO₄ tetrahedra, one for Ga1 atoms and the other for Ga2 atoms, the symmetries are quartic symmetry and mirror symmetry. Gd³⁺ and Sr²⁺ are randomly distributed within the a-b layers in a ratio of 1 : 1. The Jahn-Teller effect results in the asymmetry of Sr²⁺ and Gd³⁺ coordination environment, resulting in the disordered structure of the crystal and spectral inhomogeneously broadening^[14]. In Dy: SGGM crystal, Dy³⁺ ions are equivalently substituted for Gd³⁺ ions, making rare-earth ion activation centers in the crystal with different coordination environments.

Fig. 3(a) displays XRD pattern of as-grown crystal. Diffraction peaks are quite matched to the standard JCPDF card of SrGdGa₃O₇ (50-1835). Rietveld refinement of XRD powder data from Dy: SGGM crystal was carried out by the GSAS-II program, and the result is illustrated in Fig. 3(b). The result is a reliable refinement with R-weighted profile residual (R_wp)=12.456% and Goodness-of-fit indicator (χ²)=1.35, where observed diffraction pattern agrees well with the calculated diffraction pattern, indicating that the Dy: SGGM crystal has a tetragonal phase and belongs to the space group of P$\bar{4}$1m. The doping of Dy³⁺ ions makes disorder of crystal structure increase and lattice constant becomes larger. The lattice parameters of Dy: SGGM crystal after refinement are shown in Table 1. Fig. 3(c) shows the rocking curve of the (002) plane of the Dy: SGGM crystal. 2θ of the (002) diffraction peak is 34.11°, and the peak shape is splitless and smoothly symmetric. The FWHM is 0.07°, reflecting the good crystalline quality of the grown crystal.

SrGdGa₃O₇ is a uniaxial crystal, and polarized absorption spectra of Dy: SGGM crystal in the wavelength range of 315-1900 nm are illustrated in Fig. 4. In Fig. 4, mainly 13 absorption peaks centered at 323, 347, 361, 385, 425, 452, 471, 770, 789, 896, 1076, 1248, and 1758 nm, corresponding to the transitions from ground state ⁶H_15/2 to ⁴G_9/2+⁶P_3/2+⁴M_17/2, ⁶P_7/2+⁴I_11/2, ⁶P_5/2+⁴D_3/2+⁴M_19/2, ⁴F_7/2+ ⁴I_13/2+⁴M_21/2+⁴K_17/2, ⁴G_11/2, ⁴I_15/2, ⁴F_9/2, ⁶F_3/2, ⁶F_5/2, ⁶F_7/2, ⁶F_9/2+⁶H_7/2, ⁶F_11/2+⁶H_9/2, ⁶H_11/2 level respectively are observed. In particular, the absorption peak corresponding to ⁶H_15/2→⁴I_15/2 transition is located at 452 nm, making it very suitable for laser diode (LD) pumping^[19]. Absorption cross-section (σ_abs) of Dy: SGGM crystal is given by the equation:

where N_c stands for lattice concentration, α represents the absorption coefficient. The lattice concentration of Dy³⁺ was calculated from ICP-OES measurements to be 1.498×10²⁰ cm^-3. The FWHM of the absorption peaks at 452 nm for π- and σ-polarization are 10.4 and 11.8 nm, and absorption cross-sections (σ_abs) are 0.594×10^-21 and 0.555×10^-21 cm², respectively. Here σ_abs of Dy: SGGM crystal around 452 nm representing the ⁴I_15/2 level is smaller than those of other Dy³⁺-doped crystals such as CeF₃^[20], CaYAlO₄^[21], CaGdAlO₄^[22], and YAG^[23]. However, Dy: SGGM crystal has a large FWHM compared with other crystals, which facilitates efficient diode pumping and is not easily affected by the temperature. The specific absorption spectral parameters are compared and listed in Table 2.

The Judd-Ofelt theory is an effective method to probe the optical properties of the 4f-4f transitions of trivalent rare-earth ions^[24-25]. Line strength S_calc(J, J′) of electric dipole transition and experimental line strength S_exp(J, J′) are calculated by following equations:

where ${{\left| \left\langle S,L,J\left\| {{U}^{(t)}} \right\|{S}',{L}',{J}' \right\rangle \right|}^{2}}$ represents the transition matrix element from J state to ${J}'$ state. $\left\langle ||{{U}^{(t)}}|| \right\rangle $ is the squared reduced matrix elements. $\text{OD(}\lambda \text{)}$ represents a function of wavelength λ about optical density. h is Planck constant (6.626×10^–27 erg∙s), c represents the speed of light (2.998×10¹⁰ cm∙s^–1), e is electron charge (4.803×10^–10 esu), $\bar{\lambda }$ represents the average wavelength of absorption bands, represents thickness of crystal, $n$ represents index of refraction of crystal taken from Ref. [17].

RMS (root-mean-square) between the calculated and experimental intensities is described by:

where N is the number of absorption bands. Calculated RMS is 0.074×10^-20 cm² (π-polarization) and 0.063×10^-20 cm² (σ-polarization) in Dy: SGGM crystal, which suggests that calculated line strengths are relatively close to experimental line strengths. It also proves that the results are of good authenticity and credibility. Calculated and experimental line strengths of Dy: SGGM crystal are given in Table 3.

Table 4 lists three J-O intensity parameters of the Dy: SGGM crystal and comparison of them with some Dy³⁺-doped materials. For polarization absorption, effective J-O intensity parameters are characterized by Ω_t,eff=(2Ω_t,σ + Ω_t,π)/3. The three effective J-O intensity parameters Ω_t,eff (t=2, 4, 6) are calculated to be 5.495×10^-20, 1.476×10^-20, 1.110×10^-20 cm², respectively. In particular, Ω₂ reflects symmetry of lattice environment around rare-earth ions, which is strongly influenced by the coordination environment^[29-30]. As shown in Table 4, the Ω₂ value in Dy: SGGM crystal is larger than that in Dy: CaYAlO₄^[21], Dy: Gd₃Ga₅O₁₂^[31], Dy: YAG^[23]. The higher value of Ω₂, the lower lattice symmetry of rare-earth ion. Ω_{4, 6} parameters reflect the rigidity and covalent of host material where rare-earth ions are located^[32].

Spontaneous radiative rates (A_ed) of electric dipole transition and spontaneous radiation rates (A_md) of magnetic dipole transition are calculated by using following equations:

As the variation of S_md is independent on host material, S_md values from Ref. [33] were used.

Radiative branching ratio β and radiative lifetime τ_r are described as:

The spontaneous transition rate A_total (A_total = A_md + A_ed), fluorescence branching ratio β, and radiation lifetime τ_r of Dy: ⁴F_9/2 level in Dy: SGGM crystal are listed in Table 5. Calculated β for transition ⁴F_9/2→⁶H_13/2 is 0.646, indicating that Dy: SGGM crystal has better yellow light emitting capability.

Emission spectra of Dy: SGGM crystal in 450-800 nm wavelength range are measured upon 452 nm excitation, and obtained spectra are presented in Fig. 5. Transitions from ⁴F_9/2 to ⁶H_15/2, ⁶H_13/2, ⁶H_11/2, and ⁶H_9/2 + ⁶F_11/2 are observed, corresponding to wavelengths of 476, 574, 660, and 754 nm, respectively. Yellow emission peak at 574 nm has the highest fluorescence intensity, which is in general consistent with calculated fluorescence branching ratio.

The excited level’s emission cross-section (σ_em) in the crystal is one of the most important factors, which affects the output power and light conversion efficiency of the lasers. Based on the absorption spectra, the emission cross- section was obtained by using the Füchtbauere-Ladenburg (F-L) equation^[37]:

where I(λ) is experimental fluorescence intensity as a function of wavelength λ, n means refractive index, c represents the speed of light, β is fluorescence branching ratio, τ_r stand for radiative decay time of an excited state.

For anisotropic uniaxial crystals, emission cross-section in the π- and σ- polarization needs to be calculated separately, and here the emission cross-section is expressed as:

Emission cross-section for the σ- and π- polarizations at 574 nm in the Dy: SGGM crystal are estimated to be 1.84×10^-21 and 2.49×10^-21 cm². As presented in Table 6, the values of σ_em is higher than those of Dy: CaGdAlO₄^[22], Dy: Sr₃Y(BO₃)₃^[28], and Dy: CNGS^[27], but smaller than those of Dy: CaYAlO₄^[21] and Dy: GGG^[31]. Furthermore, Dy: SGGM crystal has a broadband emission at around 574 nm for which the FWHM is 15.6 nm (π-polarisation) and 16.2 nm (σ-polarisation). It is beneficial to acquire tunable yellow laser and ultrashort pulsed laser outputs in the corresponding wavelength bands.

Chromaticity coordinates can be used to assess the color of overall emission in the visible range. For π- and σ-polarizations in Dy: SGGM crystal under 452 nm excitation, it can be calculated by CIE 1931 program as (x₁=0.4075, y₁=0.4527) and (x₂=0.3986, y₂=0.4431) respectively. The chromaticity coordinates belong to the yellow wavelength range, and correlated color temperature (CCT) can be computed according to the equation^[38]:

where $N=(x-{{x}_{\text{e}}})/(y-{{y}_{\text{e}}})$, chromaticity epicentre is (x_e=0.3320, y_e=0.1858). For π- and σ-polarizations, CCT value is calculated to be 3862 and 3982 K, respectively. Calculated chromaticity coordinates fall within the yellow light range, as illustrated in Fig. 6, which suggests the potential of Dy: SGGM crystal for yellow light applications.

Fig. 7 displays the fluorescence decay curve of Dy: ⁴F_9/2 level at 574 nm excited under 452 nm wavelength. The measured curve follows double-exponential functions, and the fluorescence lifetime (τ_f) of Dy³⁺:⁴F_9/2 level calculated is 0.531 ms. Besides, the radiative lifetime of Dy³⁺:⁴F_9/2 multiplet obtained from calculations of J-O theory was 0.768 ms. The cross-relaxation of Dy³⁺ ions as the main factor of the short fluorescence lifetime of ⁴F_9/2 multiplet can be improved by optimizing Dy³⁺ ion concentration^[39-40]. The quantum efficiency $(\eta ={{\tau }_{\text{f}}}/{{\tau }_{\text{r}}})$ of Dy³⁺:⁴F_9/2 multiplet is 69.1%, which is larger than that of Dy: CaYAlO₄ (54.0%)^[21], but smaller than those of Dy: LiNbO₃ (91.8%)^[35] and Dy: GGG (71.4%)^[31]. All these features indicate Dy: SGGM crystal as a promising candidate for yellow laser.

In summary, Dy: SGGM crystal with dimensions of ϕ25 mm×40 mm was successfully grown by the CZ method. Lattice parameters of Dy: SGGM crystal were optimized with Rietveld refinement based on XRD data. The polarized absorption spectra, polarized emission spectra, and fluorescence decay curves of the Dy: SGGM crystal were studied in detail. The absorption peak at the Dy: ⁴I_15/2 level is around 452 nm, and its absorption cross-section for π- and σ- polarization is 0.594×10^-21 and 0.555×10^-20 cm², respectively. The FWHM of σ_abs is 10.4 nm (π-polarization) and 11.8 nm (σ-polarization) accordingly, which is particularly favorable for blue LD pumping. Three effective J-O intensity parameters were calculated to be: Ω_2,eff=5.495×10^-20 cm², Ω_4,eff=1.476×10^-20 cm², and Ω_6,eff=1.110×10^-20 cm². Calculated emission cross-sections at 574 nm for π- and σ-polarization were 1.84×10^-21 and 2.49×10^-21 cm² by the FL method, and its FWHM was measured to be 15.6 nm (π-polarization) and 16.2 nm (σ-polarization). Fluorescence and radiation lifetime of Dy: ⁴F_9/2 level were 0.531 and 0.768 ms respectively with a quantum efficiency of 69.1%. All results show that Dy: SGGM crystal has a high potential to achieve a 574 nm yellow laser.