Spectra and energy levels of a layered Yb³⁺:CsGd(MoO₄)₂ crystal with perfect cleavage: a candidate for microchip lasers

Abstract

Thin mica-like Yb³⁺:CsGd(MoO₄)₂ crystals, with a perfect (100) cleavage plane, have been grown by a top seeded solution growth method from a flux of Cs₂Mo₃O₁₀. The cleavage habit has been found to be closely connected with the layered structure. The Stark-level positions of Yb³⁺ have been deduced from the excitation and emission spectra at 77 K with the aid of the Raman spectrum, indicating the largest ground-state splitting among the double molybdates and double tungstates ever reported except for Yb³⁺:LiLu(WO₄)₂. The emission cross-sections have been determined by the combinatorial reciprocity method and the Füchtbauer–Ladenburg formula. The wavelength dependence of the gain cross-section predicts a broad tuning range and potential sub-100 fs laser pulse generation. Taking into account the cleavage behavior, the crystal is particularly suitable for microchip lasers, in which thin platelets of gain media can be prepared by a simple cleavage technique.

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1. Introduction

Double molybdate (DMo) and double tungstate (DW) crystals with the general formula M⁺T³⁺(X⁶⁺O₄)₂ (X = Mo or W), have aroused considerable interest due to their outstanding performance as gain media for thin-disk, tunable, and ultrafast pulsed lasers or as nonlinear elements for laser Raman shifting.^1–4 The majority of the DMo and DW crystals with M = Li and Na, have disordered tetragonal structure. Once doped with lasing ions (Yb³⁺, Tm³⁺, etc.), they exhibit low peak optical cross-sections and large inhomogeneously-broadened spectral bands owing to the structural disorder and multisite character of hosts.^4–7 On the contrary, the ordered monoclinic DMo and DW crystals with M = K are characterized by the narrow bandwidths and the pronounced spectral anisotropy, resulting in high peak optical cross-sections in specific crystal orientations.^4,8,9 In order to improve the laser performance especially for tunable and ultrafast (<100 fs) pulsed lasers, it is of great importance to explore new laser-active crystalline materials with intermediate bandwidths and optical cross-sections.

To our knowledge, the literatures on the DMo and DW crystals with M = Rb and Cs are still limited. Most likely, the difficulties (e.g., incongruent melting, structural transformation and cleavage) in the growth of single crystals with centimeter dimensions hamper the spectroscopic analysis and laser applications.^10,11 Our group have presented the growth and spectroscopic characterization of Nd³⁺:CsLa(WO₄)₂ crystal,¹¹ an interesting crystal with the ordered tetragonal structure. Unfortunately, we failed to obtain the nominal Yb³⁺:CsLa(WO₄)₂ crystal, since Yb³⁺ could hardly incorporate into the crystal lattice.

CsGd(MoO₄)₂ (CGM) crystallizes in the monoclinic system with the space group P2/c and the cell parameters: a = 9.5289(2) Å, b = 5.0823(2) Å, c = 8.0563(7) Å, β = 91.246(6)°, and Z = 2.¹² The CGM features a highly anisotropic layered crystal structure (Fig. 1a), which gives rise to some interesting phenomena as a dipolar magnet.¹³ The Eu³⁺:CGM red-emitting phosphor is also reported to be attractive for potential applications in the white light-emitting diode devices.¹² From the structural point of view, CGM offers a highly distorted cationic site suitable for lasing ions, especially for Yb³⁺. Yb³⁺ lasers operate in a quasi-three-level scheme, since the fundamental and terminal laser levels belong to the same ²F_7/2 manifold. In order to limit the thermal population of the terminal level, the crystal-field splitting of the ²F_7/2 manifold ΔE as large as possible is desirable. Considering the ΔE is sensitive to the crystal field, one important strategy to extend the ΔE is to employ hosts that accommodate Yb³⁺ with low site symmetry and different Yb–O distances.¹⁴ Similar to the famous KGd(WO₄)₂,¹⁵ Gd³⁺ occupies an eight-coordinated site (Wyckoff position: 2e) of CGM with C₂ symmetry and Gd–O distances ranging from 2.269 Å to 2.704 Å (Fig. 1b). The distortion degree of the [GdO₈] polyhedron in CGM is even greater than that in KGd(WO₄)₂ with the minimum Gd–O distance of 2.271 Å and maximum of 2.650 Å. Such a highly distorted local environment is expected to produce an intensive crystal-field strengths and thus large ΔE of Yb³⁺. Moreover, the absence of the structural disorder or multisite character in CGM predicts high optical cross-sections and moderate bandwidths. Based on the above consideration, we investigated the crystal growth and the spectroscopic characteristics of the Yb³⁺:CGM crystal so as to evaluate its feasibility for laser applications.

Fig. 1 The photographs of (a) crystal structure, (b) coordination environment of Gd³⁺ and (c) a typical (100) cleavage slice of the Yb³⁺:CGM crystal.

2. Experimental

2.1 Crystal growth

CGM was found to decompose into Gd₂MoO₆ in a peritectic reaction at 1035 °C.¹⁶ The incongruent-melting Yb³⁺:CGM crystals were elaborated by the top seeded solution growth (TSSG) technique. The growth was performed in a vertical tubular furnace equipped with a Ni–Cr wire and a temperature controller. Cs₂Mo₃O₁₀ was chosen as a flux, since the self-flux do not introduce impurity. According to the solubility curve of the CGM-Cs₂Mo₃O₁₀ system,¹⁶ the molar ratio of Yb³⁺:CGM to Cs₂Mo₃O₁₀ was set as 1 : 2, thereby ensuring appropriate saturation temperature (∼930 °C) for crystal growth. The starting materials including Cs₂CO₃ (A.R.), Gd₂O₃ (4N), Yb₂O₃ (4N) and MoO₃ (A.R.), were weighed, mixed, transferred into a Pt crucible and heated up to 1000 °C for 10 h to homogenize the solution. At first the crystal were grown by spontaneous nucleation using a Pt wire as seed, and then a seed cut from the obtained crystal was used to grow large single crystal. After the saturation temperature was determined by repeated seeding, the seed was put into the solution, and the growth proceeded with the cooling rate of 0.5 °C day⁻¹ and the rotating rate of 15 rpm. After a period of 20 days, the crystal was drawn out of the solution and cooled down to room temperature at a rate of −20 °C h⁻¹.

2.2 Characterizations

The crystal was pulverized and used for the phase identification, elemental analysis and Raman spectrum measurement. The X-ray powder diffraction (XPRD) pattern was collected on a powder diffractometer (DMAX2500, Rigaku) operating at 40 kV and 100 mA using Cu-K_α (λ = 1.54056 Å) radiation. It can be observed from Fig. 2 that all diffraction peaks match well with those of standard pattern of CGM (JCPDS card no. 41-0419). The Yb³⁺ concentration C_crystal was determined to be 4.4 at% (atomic percent with respect to trivalent cation of the host) by means of the inductively coupled plasma optical emission spectrometry (Ultima 2, Jobin–Yvon), namely [Yb³⁺] = 2.25 × 10²⁰ ions cm⁻³. Given the Yb³⁺ concentration C_melt is 5.0 at% in the starting melting, the segregation coefficient K of Yb³⁺ in the crystal, defined as K = C_crystal/C_melt, is 0.88. The Raman spectrum was measured on a Raman spectrometer (LabRAM HR800, Jobin–Yvon) with a frequency-doubled Nd³⁺:YAG laser (532 nm) at room temperature.

Fig. 2 The XPRD patterns of (a) JCPDS card no. 41-0419 and (b) the Yb³⁺:CGM crystal.

A piece of 0.8 mm thick cleavage plate with its main faces parallel to the (100) planes (Fig. 1c), was exfoliated from the as-grown crystal by a knife and directly used for the spectral measurements without polishing. The absorption spectra were recorded by a UV/VIS/NIR spectrometer (Lambda900, Perkin–Elmer). The 77- and 300 K excitation and fluorescence spectra as well as the decay curves were carried out on a fluorescence spectrophotometer (FLS920, Edinburgh) equipped with a 450 W continuous xenon lamp and a microsecond pulsed flashlamp.

3. Results and discussion

3.1 Cleavage habit and crystal structure

The as-grown crystals exhibit distinct cleavage behavior along the (100) planes, which can be explained by the layered structure.^12,13 Fig. 1a reveals that the –Gd–(MoO₄)–Cs–(MoO₄)– alternative layers run along the a-axis. All these layers are parallel to the (100) planes and adjacent layers are linked by the Cs–O or Gd–O bonds. Obviously, the bond strengths are sorted in the order of Mo–O > Gd–O > Cs–O bonds, which indicates the Cs–O bonds are the most probable bonds to break. Therefore, the as-grown crystal has a tendency to cleavage along the (100) planes owing to the Cs–O bond rupture.

So far the typical thickness of the CGM crystal has been reported to be 1 mm at most.^13,17 Much effort about crystal growth method and technique is still required to obtain crystals with large size. It should be noticed that the cleavage yields an interesting cleavage slice with smooth and parallel surfaces, as shown in Fig. 1c. The unprocessed cleavage slice can be used directly as a gain medium for microchip lasers.^18,19 A microchip laser consists of a small piece of laser medium polished flat and parallel on two sides, and the cavity mirrors deposited directly onto the medium faces.²⁰ The conventional fabrication processing includes slicing (to submillimeter thick wafers), polishing, coating and at last cutting (into 1 mm-square pieces). Each piece serves as a complete laser cavity. The cleavage technique is simpler than the conventional machining processing, since the slicing and polishing are dispensable for cleavage microchips. Effective laser operations have been realized in the unprocessed cleavage microchips of Nd³⁺:LaB₃O₆ (ref. 18) and Tm³⁺:BaGd₂(MoO₄)₄.¹⁹

3.2 Absorption spectrum and lifetime

Fig. 3 exhibits the absorption spectrum of the Yb³⁺:CGM crystal at room temperature. The spectrum is composed of broad mutually overlapping bands. Three main absorption peaks centered at 962, 976 and 1006 nm arise from the electronic transitions between the Stark levels of the ²F_7/2 and ²F_5/2 manifolds of Yb³⁺, and the detailed assignment will be discussed on Section 3.3. The maximum absorption cross-section σ_abs amounts to 3.9 × 10⁻²⁰ cm² at 976 nm, which far exceeds than those of Yb³⁺:NaGd(MoO₄)₂ (2.2 × 10⁻²⁰ cm² for the π polarization at 975 nm)⁴ and Yb³⁺:LiGd(MoO₄)₂ (1.6 × 10⁻²⁰ cm² for the π polarization at 975 nm).²¹ The full-widths at half-maximum (FWHM), another crucial factor affecting the pumping efficiency, reaches up to 8.1 nm for the Yb³⁺:CGM crystal. The value is much smaller than that of Yb³⁺:NaGd(MoO₄)₂ (50.0 nm)⁷ but larger than that of Yb³⁺:KGd(WO₄)₂ (3.7 nm).⁹ Such a broad bandwidth is qualified to improve the spectral overlap with the pump beam, accommodate the thermal drift of the pump wavelength and hence make the title crystal adequate for commercial InGaAs laser diode (LD) pumping.

Fig. 3 Absorption and emission cross-sections calculated by the RM and FL method.

The absorption spectrum enables us to estimate the radiative lifetime τ_r of the ²F_5/2 upper laser level by the following formula:¹⁴

where c, [small lambda, Greek, macron]_abs and g denote the velocity of light, the mean wavelength of absorption band. Since the Yb³⁺ ion (4f¹³ configuration) has an odd number of electrons in the 4f shell, the Stark levels are doubly degenerate in terms of the Kramer's degeneracy theorem. So the degeneracy g turns out to be (2J + 1)/2, i.e., g_u = 3 for ²F_5/2 (J = 5/2) and g_l = 4 for ²F_7/2 (J = 7/2). The refractive index n is roughly taken as 2.0. Thus the τ_r is determined to be 235 μs in this work.

The fluorescence decay curves were recorded under pulsed excitation at 976 nm and emission at 1039 nm, as shown in Fig. 4. It is universally known that the reabsorption and total internal reflection will elongate remarkably the measured fluorescence lifetime τ_f, especially for Yb³⁺ characterized with a large degree of overlap between absorption and emission. Not surprisingly, the τ_f (581 μs) for the bulk sample is significantly longer than the τ_r (235 μs). In the past few years, the powder method has been proved to be a simple and efficient approach to more accurately measure the intrinsic lifetime of the ²F_5/2 manifold.^7,8,22,23 The bulk sample is ground to fine powder and dispersed in the monochlorobenzene fluid (n = 1.52). Detailed experimental procedure has been provided elsewhere.⁸ In principle, a fluid with a refractive index close to that of the crystal (n ≈ 2) is desirable in order to eliminate the total internal reflection. But the fluids with high n are toxic. So the monochlorobenzene (n = 1.52) has been chosen for refractive-index matching in this work. As expected, the powder method yields much shorter τ_f (274 μs) than that for the bulk sample (581 μs). The quantum yield, defined as η = τ_f/τ_r, is slightly greater than 1. One possible explanation is that the powder method cannot eliminate the reabsorption and total internal reflection completely, resulting in the overestimated τ_f.^22,23 Anyway, the result indicates high η for the Yb³⁺:CGM crystal, which implies that nonradiative relaxation is rather inefficient.

Fig. 4 Fluorescence decay curves for bulk and power samples of the Yb³⁺:CGM crystal.

3.3 Excitation and emission spectra

The electronic transitions only involve two manifolds due to the simple 4f¹³ electronic configuration of Yb³⁺, i.e., the ²F_7/2 ground state and ²F_5/2 excited state separated by about 10 000 cm⁻¹. The C₂ site symmetry of Yb³⁺ in the CGM crystal is expected to split the two manifolds into four and three doubly degenerate Stark levels, respectively. Here the Stark levels are labelled as numbers from 1 to 4 for the ²F_7/2 manifold and from 5 to 7 for the ²F_5/2 manifold in the order of the increasing energy.

The low-temperature excitation and emission spectra are recorded at 77 K to reduce the spectral broadening and further determine the energies of Stark levels, as illustrated in Fig. 5. The common excitation and emission lines at 976 nm (10 246 cm⁻¹) are readily assigned to be resonant zero-phonon line transition between the lowest Stark levels of the ²F_7/2 and ²F_5/2 manifolds (1 ↔ 5). The other two well-resolved excitation lines at 962 nm (10 395 cm⁻¹) and 929 nm (10 764 cm⁻¹) are attributed to the electronic transitions 1 → 6 and 1 → 7, respectively. Different from the excitation spectrum, the emission spectrum displays more transition lines than expected. Obviously, some spectral lines in the 980–1030 nm spectral region, should belong to the vibronic sidebands originating in the strong electron-phonon interaction of Yb³⁺ with the lattice vibrations.^23,24 Given that the phonon sidebands should appear at longer wavelength than the corresponding electron transition in the emission spectrum,²³ these lines can only be associated with the 5 → 1, 5 → 2 or 5 → 3 electronic transitions. Keeping in mind the hypothesis that the Raman spectrum should reflect vibronic structures accompanying each pure electronic transition, the Raman spectrum is introduced to aid the interpretation of the electronic levels.²³ Fig. 6 compares the excitation and emission spectra with the Raman spectrum, with an aim to distinguish the vibronic transitions from pure electronic transitions. These spectra are adjusted to the same energy scale. The energy origins of the excitation and emission spectra are taken at 5 ↔ 1 transition, while three energy onsets of Raman spectrum are selected to identify the possible vibronic sidebands from the 5 → 1, 5 → 2 and 5 → 3 transitions, respectively. The common spectral lines in the emission and Raman spectra are marked by asterisks in Fig. 6 and interpreted as the vibronic sidebands, while the remaining lines which are present in the emission spectrum but absent in the Raman spectrum, belong to the electronic transitions.

Fig. 5 Excitation and emission spectra of the Yb³⁺:CGM crystal at 77 K.

Fig. 6 Comparison of 77 K excitation and emission spectra with 300 K Raman spectrum. For Raman spectrum, three positions of the origin have been considered from the 5 → 1, 5 → 2 and 5 → 3 transition lines, respectively. The inset indicates the distribution of Stark levels of Yb³⁺ in the CGM crystal.

Eventually, the sequence of Yb³⁺ energy levels is established as follows: ²F_7/2 = 0, 255, 306, 612 cm⁻¹, and ²F_5/2 = 10 246, 10 395 and 10 764 cm⁻¹, as sketched in the inset of Fig. 6. To our knowledge, the overall splitting for the ²F_7/2 manifold (ΔE = 612 cm⁻¹) is the largest among the DMo and DW ever reported except Yb³⁺:LiLu(WO₄)₂ (ΔE = 749 cm⁻¹).²⁵ The high ΔE stems from a strong crystal field characterized by a low point symmetry (C₂) and a large distribution of Gd(Yb)–O distances, which is beneficial to limit thermal population of the terminal laser level and reduce the threshold pumping power.

Our assignment is verified by the ‘barycenter plot’ method proposed by E. Antic-Fidancev.²⁶ In principle, the energy separation between the ²F_7/2 and ²F_5/2 manifolds should be constant whatever the matrix. In other words, the ²F_5/2 barycenter as a function of the ²F_7/2 barycenter should obey a linear relationship with a slope of unity.^22–24 Fig. 7 presents the evolution of the ²F_5/2 barycenter with the ²F_7/2 barycenter for various Yb³⁺-doped crystals. The representative point for the title crystal is well-aligned with the theoretical line, confirming our interpretation.

Fig. 7 A barycenter plot for various Yb³⁺-doped crystalline materials.

Knowledge of the Stark levels makes it possible to estimate the emission cross-section σ_em from the absorption spectrum in accordance with the reciprocity method (RM):²⁷

σ^RM_em(λ) = σ_abs(λ)(Z_l/Z_u)exp[(E_zl − hc/λ)/k_BT] (2)

where E_zl denotes the zero-phone line energy, k_B is the Boltzmann constant, Z_l and Z_u denote the partition functions of the lower ²F_7/2 and the upper ²F_5/2 manifolds, which are given by:

Z_k = ∑d_k exp(−E_k/k_BT) (3)

where d_k and E_k are the degeneracy and the energy of the Stark level k. The ratio Z_l/Z_u is 1.34 and E_zl is 10 246 cm⁻¹ in the present work.

The RM is valid only in the vicinity of the fundamental transition where there is significant absorption.¹⁴ The absorption is so low at long-wavelength wing of the absorption spectrum that even a noise will be magnified exponentially in terms of eqn (2). The RM is no longer reliable at longer wavelengths, and this is the reason why the σ^RM_em increases abnormally at λ > 1025 nm in Fig. 3. The σ^RM_em in the long-wavelength range can be corrected by the Füchtbauer–Ladenburg (FL) formula:²⁸

where I(λ) denotes the fluorescence intensity as a function of the wavelength λ.

The FL relationship also has its limitations: it is valid only in the spectral region where there is no reabsorption.¹³ It should be noted that the reabsorption decreases the fluorescence intensity, resulting in an underestimate of the integral in eqn (4). For this reason, the eqn (4) is rewritten as:²⁹

σ^FL_em(λ) = Cλ⁵I(λ) (5)

where C represents all factors independent of wavelength and can be quantified by:²⁹where λ_max = 1007 nm in this work corresponds to the wavelength at which both RM and FL are applicable.

The combination of the RM and FL methods can circumvent their shortcomings and describe precisely the wavelength dependence of the σ_em in the whole spectral region, as shown in Fig. 3. The σ^RM_em is more reliable at λ < 1007 nm, while σ^FL_em is more accurate at λ > 1007 nm. The σ_em are equal to 3.6 × 10⁻²⁰ at 1007 nm and 1.0 × 10⁻²⁰ cm² at 1038 nm, respectively. The corrected σ_em is subsequently used to calculate the gain cross-section σ_g(λ), which is defined as:

σ_g(λ) = βσ_em(λ) − (1 − β)σ_abs(λ) (7)

where β denotes the fraction of ions in the excited state. The FWHM of gain band is a measure of the wavelength tunability of a material for tunable or ultrafast laser applications.³⁰ The FWHM of gain band at β = 0.5 is as high as 39 nm (Fig. 8), which is superior to those of Yb³⁺:YAG (9 nm) and Yb³⁺:KGd(WO₄)₂ (25 nm)³¹ and comparable to those of Yb³⁺:NaLa(MoO₄)₂ (41 nm)⁷ and Yb³⁺:NaGd(MoO₄)₂ (43 nm).⁷ The Yb³⁺ bandwidths is known to be particularly host dependent and influenced by the splitting of energy level, electron-phonon coupling, structural disorder or multisites character of the host.^14,31 The latter two are excluded on the basis of the crystal structure. The combination of large splitting of energy level and strong electron-phonon coupling, are the most probable explanation for such a broad FWHM. The minimum theoretical duration Δt_min is derived as 27 fs according to the Fourier inequality ΔνΔt ≥ 0.315 (sech²-shape pulse), indicating that the crystal permits the generation of sub-100 fs laser pulses.

Fig. 8 Gain cross-section spectrum of the Yb³⁺:CGM crystal for different inversion ratios β.

3.4 Evaluation of the laser potential

The laser performance of the Yb³⁺:CGM crystal, can be estimated by the laser parameters: β_min, I_sat and I_min.²⁸ β_min denotes the minimum inversion fraction of Yb³⁺ ions that must be excited to balance exactly the gain with the ground-state absorption at the extraction wavelength λ_ext. The pump saturation intensity I_sat describes the capability to get the usual bleaching phenomena in laser physics. I_min stands for the minimum pump intensity required for transparency at the extraction wavelength λ_ext. They can be calculated by the following equations:²⁸

I_min = β_min(λ_ext)I_sat(λ_pump) (10)

If λ_pump = 976 nm and τ_f = 274 μs are selected, the I_sat are calculated to be 19.0 kW cm⁻². Then the I_min are determined to be 2.9 kW cm⁻² for λ_ext = 1007 nm and 0.8 kW cm⁻² for λ_ext = 1038 nm, respectively.

The important structural and spectroscopic parameters of the Yb³⁺:CGM crystal are summarized in Table 1 and compared with those of other Yb³⁺-doped DMo and DW crystals. From the spectroscopic point of view, the Yb³⁺:CGM crystal is situated at the intermediate position between the disordered tetragonal and ordered monoclinic DMo and DW crystals. Until now, sub-100 fs laser pulses have already been demonstrated in Yb³⁺:KGd(WO₄)₂ (78 fs),³⁴ Yb³⁺:NaY(WO₄)₂ (53 fs)³⁵ and Yb³⁺:NaY(MoO₄)₂ (91 fs).³⁶ The production of ultrafast pulses is known to depend critically on the optical bandwidths of gain media.^31,37 From this prospective, the disordered tetragonal DMo and DW are more advantageous to deliver shorter pulse duration and higher peak power as compared with the ordered monoclinic ones. But the former suffers from low optical cross-sections and hence a low gain. A tradeoff between optical bandwidths and emission cross-sections is achieved in the case of the Yb³⁺:CGM crystal. In addition, the crystal is suitable for LD pumping at 976 nm due to broad absorption bandwidth (FWHM = 8.1 nm), and operates in a quasi-three-level scheme with large ground-state splitting (ΔE = 612 cm⁻¹). And lastly, an unprocessed cleavage slice can be used directly for microchip lasers, simplifying fabrication processes. The unique combination of above-listed advantages proposes the potential application of the Yb³⁺:CGM crystal in the LD-pumped microchip laser systems capable to produce the tunable and sub-100 fs pulsed laser.

Table 1 Structural and spectroscopic features of several promising Yb³⁺-doped laser crystals^a

Crystals	NGM	NGW	KGW	CGM
a Notes: Czochralski (Cz.), NaGd(MoO4)2 (NGM), NaGd(WO4)2 (NGW), KGd(WO4)2 (KGW).
Growth method	Cz.	Cz.	TSSG	TSSG
Space group	I41/a	I4̄	C2/c	P2/c
Multisite or disorder	√	√	×	×
Symmetry (Gd)	S4	S4	C2	C2
ΔE (^2F7/2) (cm^−1)	<500	492	535	612
λabs (nm)	975	975	981	976
Δλabs (nm)	50	50 (π)	3.7	8.1
		43 (σ)
σabs (10^−20 cm^2)	2.2 (π)	1.78 (π)	12.0	3.9
		1.36 (σ)
λem (nm)	1000	1000	1023	1007
				1038
Δλ^*em (nm)	43	40 (π)	25	39
σem (10^−20 cm^2)	2.7 (π)	1.89 (π)	2.8	3.6
	1.4 (σ)	0.75 (σ)		1.0
τf (μs)	280	320	317	274
Δtmin (fs)	24	26	44	27
Isat (kW cm^−2)	33.1 (π)	35.7 (π)	5.3	19.0
Reference	4, 7	32, 33	9, 31	This work

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4. Conclusions

The crystal growth, morphology and spectroscopic properties of a layered Yb³⁺:CGM crystal have been systemically investigated in order to assess its laser potential. The cleavage habit and spectroscopic properties have been found to be closely associated with the crystal structure: Firstly, the highly distorted coordination environment of Yb³⁺ contributes to the most largest ground-state splitting of Yb³⁺ among the DMo and DW crystals ever reported other than Yb³⁺:LiLu(WO₄)₂, which is favorable for low-threshold quasi-three-level laser operation; secondly, single site and structure order give rise to high transition cross-sections and modest absorption bandwidth. The emission/gain bandwidth is remarkably high due to the strong electron-phonon coupling and large energy-level splitting, which is promising for tunable and ultrafast (sub-100 fs) pulse laser applications. Thirdly, the anisotropic layered structure is responsible for the perfect cleavage along the (100) planes. A high-quality cleavage slice is too thin to be used as a laser rod in laser cavity, but is well suited for microchip laser configuration. In conclusion, the Yb³⁺:CGM crystal can be regarded as a potential gain medium for microchip lasers.

Acknowledgements

We gratefully acknowledge the financial support of the NSF of China (61205213, 21201071 and 21176099), the NSF of Anhui Province (1208085QB42), the Science and Technology Project of Huainan City (2013A4105), and the Program for Innovative Research Team in Huainan Normal University.

Notes and references

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