Spectra and energy levels of a layered Yb3+:CsGd(MoO4)

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Spectra and energy levels of a layered Yb3+:CsGd(MoO4)2 crystal with perfect cleavage: a candidate for microchip lasers Wang Zhao,a Yi-sheng Huang,b Zhou-bin Lin,b Bo Wei,c Feng-wu Wang,a Mai Xu,a Xing Zhao,a Qing-hua Zhenga and Wei-wei Zhou*a 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

Received 18th February 2015 Accepted 9th April 2015

Yb3+:LiLu(WO4)2. The emission cross-sections have been determined by the combinatorial reciprocity method and the Fu ¨ chtbauer–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

DOI: 10.1039/c5ra03125g

cleavage behavior, the crystal is particularly suitable for microchip lasers, in which thin platelets of gain

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media can be prepared by a simple cleavage technique.

1. Introduction Double molybdate (DMo) and double tungstate (DW) crystals with the general formula M+T3+(X6+O4)2 (X ¼ 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 shiing.1–4 The majority of the DMo and DW crystals with M ¼ Li and Na, have disordered tetragonal structure. Once doped with lasing ions (Yb3+, Tm3+, etc.), they exhibit low peak optical crosssections 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 ¼ K are characterized by the narrow bandwidths and the pronounced spectral anisotropy, resulting in high peak optical cross-sections in specic crystal orientations.4,8,9 In order to improve the laser performance especially for tunable and ultrafast ( 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.

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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 at 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 2F7/2 and 2F5/2 manifolds of Yb3+, and the detailed assignment will be discussed on Section 3.3. The maximum absorption cross-section sabs amounts to 3.9 1020 cm2 at 976 nm, which far exceeds than those of Yb3+:NaGd(MoO4)2 (2.2 1020 cm2 for the p polarization at 975 nm)4 and Yb3+:LiGd(MoO4)2 (1.6 1020 cm2 for the p polarization at 975 nm).21 The full-widths at halfmaximum (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 qualied to improve the spectral overlap with the pump beam, accommodate the thermal dri of the pump wavelength and hence make the title crystal adequate for commercial InGaAs laser diode (LD) pumping. The absorption spectrum enables us to estimate the radiative lifetime sr of the 2F5/2 upper laser level by the following formula:14 ð gl 8pcn2 sabs ðlÞdl sr 1 ¼ Ar ¼ (1) 4 gu labs where c, labs and g denote the velocity of light, the mean wavelength of absorption band. Since the Yb3+ ion (4f13 conguration) 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., gu ¼ 3 for 2F5/2 (J ¼ 5/2) and gl ¼ 4 for 2F7/2 (J ¼ 7/2). The refractive index n is roughly taken as 2.0. Thus the sr is determined to be 235 ms in this work. The uorescence 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

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

Fig. 3

internal reection will elongate remarkably the measured uorescence lifetime sf, especially for Yb3+ characterized with a large degree of overlap between absorption and emission. Not surprisingly, the sf (581 ms) for the bulk sample is signicantly longer than the sr (235 ms). 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 2F5/2 manifold.7,8,22,23 The bulk sample is ground to ne powder and dispersed in the monochlorobenzene uid (n ¼ 1.52). Detailed experimental procedure has been provided elsewhere.8 In principle, a uid with a refractive index close to that of the crystal (n z 2) is desirable in order to eliminate the total internal reection. But the uids with high n are toxic. So the monochlorobenzene (n ¼ 1.52) has been chosen for refractiveindex matching in this work. As expected, the powder method yields much shorter sf (274 ms) than that for the bulk sample (581 ms). The quantum yield, dened as h ¼ sf/sr, is slightly greater than 1. One possible explanation is that the powder method cannot eliminate the reabsorption and total internal reection completely, resulting in the overestimated sf.22,23 Anyway, the result indicates high h for the Yb3+:CGM crystal, which implies that nonradiative relaxation is rather inefficient. 3.3

Excitation and emission spectra

The electronic transitions only involve two manifolds due to the simple 4f13 electronic conguration of Yb3+, i.e., the 2F7/2 ground state and 2F5/2 excited state separated by about 10 000 cm1. 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 2F7/2 manifold and from 5 to 7 for the 2F5/2 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 cm1) are readily assigned to be resonant zero-phonon line transition between the lowest Stark levels of the 2F7/2 and

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

Fig. 4

F5/2 manifolds (1 4 5). The other two well-resolved excitation lines at 962 nm (10 395 cm1) and 929 nm (10 764 cm1) 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 reect 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 4 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. Eventually, the sequence of Yb3+ energy levels is established as follows: 2F7/2 ¼ 0, 255, 306, 612 cm1, and 2F5/2 ¼ 10 246, 10 395 and 10 764 cm1, as sketched in the inset of Fig. 6. To our knowledge, the overall splitting for the 2F7/2 manifold (DE ¼ 612 cm1) is the largest among the DMo and DW ever reported except Yb3+:LiLu(WO4)2 (DE ¼ 749 cm1).25 The high DE stems from a strong crystal eld characterized by a low point symmetry (C2) and a large distribution of Gd(Yb)–O distances, which is benecial to limit thermal population of 2


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Fig. 5

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

the terminal laser level and reduce the threshold pumping power. Our assignment is veried by the ‘barycenter plot’ method proposed by E. Antic-Fidancev.26 In principle, the energy separation between the 2F7/2 and 2F5/2 manifolds should be constant whatever the matrix. In other words, the 2F5/2 barycenter as a function of the 2F7/2 barycenter should obey a linear relationship with a slope of unity.22–24 Fig. 7 presents the evolution of the 2 F5/2 barycenter with the 2F7/2 barycenter for various Yb3+-doped crystals. The representative point for the title crystal is wellaligned with the theoretical line, conrming our interpretation. Knowledge of the Stark levels makes it possible to estimate the emission cross-section sem from the absorption spectrum in accordance with the reciprocity method (RM):27 sRM em (l) ¼ sabs(l)(Zl/Zu)exp[(Ezl hc/l)/kBT]

(2)

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.

Fig. 6

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The combination of the RM and FL methods can circumvent their shortcomings and describe precisely the wavelength dependence of the sem in the whole spectral region, as shown in FL Fig. 3. The sRM em is more reliable at l < 1007 nm, while sem is more accurate at l > 1007 nm. The sem are equal to 3.6 1020 at 1007 nm and 1.0 1020 cm2 at 1038 nm, respectively. The corrected sem is subsequently used to calculate the gain crosssection sg(l), which is dened as: sg(l) ¼ bsem(l) (1 b)sabs(l)

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

where Ezl denotes the zero-phone line energy, kB is the Boltzmann constant, Zl and Zu denote the partition functions of the lower 2F7/2 and the upper 2F5/2 manifolds, which are given by: Zk ¼

P dk exp(Ek/kBT)

(3)

where dk and Ek are the degeneracy and the energy of the Stark level k. The ratio Zl/Zu is 1.34 and Ezl is 10 246 cm1 in the present work. The RM is valid only in the vicinity of the fundamental transition where there is signicant absorption.14 The absorption is so low at long-wavelength wing of the absorption spectrum that even a noise will be magnied exponentially in terms of eqn (2). The RM is no longer reliable at longer wavelengths, and this is the reason why the sRM em increases abnormally at l > 1025 nm in Fig. 3. The sRM em in the long-wavelength range can be corrected by the F¨ uchtbauer–Ladenburg (FL) formula:28 sFL em ðlÞ ¼

l5 IðlÞ ð 8pcn2 sr lIðlÞdl

(4)

(7)

where b 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 b ¼ 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 inuenced 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 electronphonon coupling, are the most probable explanation for such a broad FWHM. The minimum theoretical duration Dtmin is derived as 27 fs according to the Fourier inequality DnDt $ 0.315 (sech2-shape pulse), indicating that the crystal permits the generation of sub-100 fs laser pulses.

3.4

Evaluation of the laser potential

The laser performance of the Yb3+:CGM crystal, can be estimated by the laser parameters: bmin, Isat and Imin.28 bmin 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 lext. The pump saturation intensity Isat describes the capability to get the usual bleaching phenomena in laser physics. Imin stands for the

where I(l) denotes the uorescence intensity as a function of the wavelength l. 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 uorescence intensity, resulting in an underestimate of the integral in eqn (4). For this reason, the eqn (4) is rewritten as:29 5 sFL em (l) ¼ Cl I(l)

(5)

where C represents all factors independent of wavelength and can be quantied by:29 C¼

sRM em ðlmax Þ lmax 5 Iðlmax Þ

(6)

where lmax ¼ 1007 nm in this work corresponds to the wavelength at which both RM and FL are applicable.

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


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minimum pump intensity required for transparency at the extraction wavelength lext. They can be calculated by the following equations:28


bmin ðlext Þ ¼

sabs ðlext Þ sabs ðlext Þ þ sem ðlext Þ

Isat lpump ¼

(8)

hc lpump sabs lpump sf

(9)

Imin ¼ bmin(lext)Isat(lpump)

(10)

If lpump ¼ 976 nm and sf ¼ 274 ms are selected, the Isat are calculated to be 19.0 kW cm2. Then the Imin are determined to be 2.9 kW cm2 for lext ¼ 1007 nm and 0.8 kW cm2 for lext ¼ 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, sub100 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 crosssections and hence a low gain. A tradeoff between optical bandwidths and emission cross-sections is achieved in the case

Table 1 Structural and spectroscopic features of several promising Yb3+-doped laser crystalsa

Crystals

NGM

NGW

KGW

CGM

Growth method Space group Multisite or disorder Symmetry (Gd) DE (2F7/2) (cm1) labs (nm) Dlabs (nm)

Cz. I41/a O S4