The Growth and Properties of Rare Earth-Doped NaY(WO4)2 Large Size Crystals

Recently, strong attention has been focused on development of a new-advanced material for optoelectronics applications. MRe(WO4)2 [M=alkali metal, Re=rare earth] single crystals is noticed as an interesting self-frequency conversion solid-state laser host material because of stimulated Raman scattering[1]. NaY(WO4)2 crystal is classified among the disorder crystalline host for lasing rare-earth ions[2]. Because of the disorder structure, the optical features in the absorption and emission spectrum even at low temperature are broadened. The lattice parameters of NaY(WO4)2 crystal are a=b=5.205 Å and c=11.251 Å respectively with the space group of I41/a [3]. This crystal is a typical tetragonal scheelite-type crystal with a formula MT(WO4)2 , where M is a monovalent alkali cation and T a trivalent cation . In these materials the M and T cations are randomly distributed in the 2b and 2d sites [4], which can be replaced by rare earth ions, such as Nd3+, Yb3+, Tm3+, Ho3+ and Ce3+. As a consequence, the optical absorption and emission lines of rare earth doping ions become broadened, which allow some laser tunability as well as a better match with the available diode laser emissions used for pumping. As it melts congruently, large size single crystal can be easily obtained by the Czochralski (CZ) method. Furthermore, the higher concentration of rare earth ions can be accepted in the crystal because of the higher covalent characteristic results in the lower luminescent quenching efficiency. Compared to the other laser host crystals such as YAG and YVO4 crystal, NaY(WO4)2 crystal has lower melting point and its raw materials for crystal growth is in-nocuity. As a result, NaY(WO4)2 crystal can serve as an excellent laser host. In this chapter, the crystal growth, thermal characteristic, optical and spectrum and laser properties of rare earth dopedNaY(WO4)2 crystals are presented.


Introduction
Recently, strong attention has been focused on development of a new-advanced material for optoelectronics applications.MRe(WO 4 ) 2 [M=alkali metal, Re=rare earth] single crystals is noticed as an interesting self-frequency conversion solid-state laser host material because of stimulated Raman scattering [1] .NaY(WO 4 ) 2 crystal is classified among the disorder crystalline host for lasing rare-earth ions [2] .Because of the disorder structure, the optical features in the absorption and emission spectrum even at low temperature are broadened.The lattice parameters of NaY(WO 4 ) 2 crystal are a=b=5.205Å and c=11.251Å respectively with the space group of I4 1 /a [3] .This crystal is a typical tetragonal scheelite-type crystal with a formula MT(WO 4 ) 2 , where M is a monovalent alkali cation and T a trivalent cation .In these materials the M and T cations are randomly distributed in the 2b and 2d sites [4] , which can be replaced by rare earth ions, such as Nd 3+ , Yb 3+ , Tm 3+ , Ho 3+ and Ce 3+ .As a consequence, the optical absorption and emission lines of rare earth doping ions become broadened, which allow some laser tunability as well as a better match with the available diode laser emissions used for pumping.As it melts congruently, large size single crystal can be easily obtained by the Czochralski (CZ) method.Furthermore, the higher concentration of rare earth ions can be accepted in the crystal because of the higher covalent characteristic results in the lower luminescent quenching efficiency.Compared to the other laser host crystals such as YAG and YVO 4 crystal, NaY(WO 4 ) 2 crystal has lower melting point and its raw materials for crystal growth is in-nocuity.As a result, NaY(WO 4 ) 2 crystal can serve as an excellent laser host.In this chapter, the crystal growth, thermal characteristic, optical and spectrum and laser properties of rare earth doped-NaY(WO 4 ) 2 crystals are presented.

The growth of large size crystals
Rare earth-doped NaY(WO 4 ) 2 crystals were grown in air along <001> direction by using Czochralski method [1~3] .The chemicals used were analytical grade Na 2 CO 3 , WO 3 , Y 2 O 3 and spectral grade Re 2 O 3 (Re=Yb, Tm, Ho,Ce, Nd, Er).The starting materials were prepared by mixing Y 2 O 3 , Na 2 CO 3 , WO 3 and Re 2 O 3 powders according to reaction formula:

The thermal characteristic
The a and c axes were obtained by the YX-2 X-ray Crystal Oridentation Unit (produced by Dandong Radiative Instrument Co,Ltd).Two pieces of square samples with the size 5×5×5 mm 3 having polished faces perpendicular to the a and c crystallophysical directions were used to carry out the measurements.The thermal expansion of as-grown Yb 3+ :NaY(WO 4 ) 2 crystal was measured by using Diatometer 402 PC instrument from 300 K to 1273 K [1] .Because of the relatively lower reliability of the room temperature cell parameter arising out of presence of water in the sample chamber, only the data from 473 to 1273 K is considered 99 in calculating the expansion coefficients.The thermal expansion pattern was obtained (shown in the Fig. 3.1).The thermal expansion coefficients of the Yb 3+ :NaY(WO 4 ) 2 crystal were calculated over different temperature ranges.In this case, the linear thermal expansion coefficients for different crystallographic direction c-and a-axes are , 1.83×10 -5 K -1 , 0.85×10 -5 K -1 , respectively.The thermal-expansion coefficient ij      of a crystal is a symmetrical second-rank tensor and it can be described by the representation quadric.The NaY(WO 4 ) 2 crystal belongs to the tetragonal system and 4/m point group.The unique symmetry axis is a fourfold axis along the crystallographic c-axis; the axes of the crystallographic and crystallophysical coordinate systems in NaY(WO 4 ) 2 have the same direction.In this case the value of thermal expansion along a-and b-axis are comparable and the values of 1

 and 3
 can be obtained by measuring the thermal expansion of the a-and c-oriented crystal.
The expansion coefficient in the [001] is about two times larger than that of the [100] direction according to our experimental results, which means that the NaY(WO 4 ) 2 crystal has anisotropic thermal expansion.The reason for the themal expansion coefficient along the c-axis being larger than that along the a-or b-axis can be explained by the structure of the NaY(WO 4 ) 2 crystal.The NaY(WO 4 ) 2 crystal has a scheelite structure according to the XRPD experiment results.According to Fig. 3.1, it can be seen that there are five layers and three layers perpendicular to the c-and the a-or b-axis, respectively.According to the XRPD experiment results, the distance of the interlayer of five layers and three layers are c/4 and a/2 (or b/2),which is equal to 2.813×10 -10 and 2.603×10 -10 m, respectively.The larger the distance of the interlayer is, the weaker the chemical bonds of the interlayer will be according to the crystal lattice vibration dynamics.It can be seen that the interaction force along the c-axis is weaker than that along the a-or b-axis, and there are more layers in the c-direction than in the a-direction.Thus when the crystal is heated, the thermal expansion of the Yb 3+ :NaY(WO 4 ) 2 crystal along the c-axis is larger than that along the a-or b-axis.www.intechopen.com The Growth and Properties of Rare Earth-Doped NaY(WO 4 ) 2 Large Size Crystals 103 4. The spectroscopic characteristics 4.1 The spectroscopic characteristic of Nd 3+ : NaY(WO 4 ) 2 crystal Fig. 4.1 shows the RT absorption spectrum of Nd 3+ :NaY(WO 4 ) 2 Crystal.Owing to the disordered structure and the high Nd-doping concentration, the strong absorption intensity and broad FWHM of every band are shown, especially for the 806 nm [1] .Its FWHM is about 16 nm and the cross-section is about 2.8×10 -20 cm 2 at 806 nm, which is benefit to the pumping of commercial laser diode.Table 4.4 Comparison of the emission spectroscopic parameters of some Nd-doped laser crystal.

4.3
The spectroscopic properties of Tm 3+ , Ho 3+ : NaY(WO 4 ) 2 crystal Fig. 4.8 shows the Room temperature absorption spectra of Tm 3+ -, Ho 3+ -doped and Tm 3+ /Ho 3+ co-doped NaY(WO 4 ) 2 crystals (a) in the range 300-850 nm and (b) in the range 1100-2100 nm.The spectrum of Tm 3+ : NaY(WO 4 ) 2 crystal consists of six resolved bands associated with the transitions from the 3 H 6 ground state to the 3 F 4 , 3 H 5 , 3 H 4 , 3 F 2, 3 , 1 G 4 and 1 D 2 excited states.It can be seen that the absorption band of the  polarization is narrower and has a larger peak cross section than the π absorption band.The spectrum of Ho 3+ : NaY(WO 4 ) 2 crystal consists of ten resolved bands associated with the transitions from the 5 I 8 ground state to the 5 I 7 , 5 I 6 , 5 F 5 , 5 [19] .Some absorption bands of Tm 3+ and Ho 3+ ions overlap in the Tm 3+ /Ho 3+ :NaY(WO 4 ) 2 crystal.Compared to Ho 3+ ions concentration in Ho 3+ : NaY(WO 4 ) 2 and Tm 3+ ions concentration in Tm 3+ /Ho 3+ :NaY(WO 4 ) 2 crystal, the concentration of Ho 3+ ions in Tm 3+ /Ho 3+ :NaY(WO 4 ) 2 crystal is very low; the 5 I 8 5 I 7 (Ho 3+ ) transition of Tm 3+ /Ho 3+ :NaY(WO 4 ) 2 crystal is extremely weak.Fig. 4.9 shows the absorption cross sections and polarized stimulated emission cross sections associated with the (a) 3 F 4 → 3 H 6 transition for the Tm 3+ :NaY(WO 4 ) 2 and (b) 5 I 7 5 I 8 for Ho 3+ :NaY(WO 4 ) 2 crystal derived by the reciprocity method.The maximum values of em  are 1.399×10 -20 cm 2 for  polarization at 2044 nm and 1.426×10 -20 cm 2 for  polarization at 2047 nm.For comparison, the em  obtained for Tm 3+ in NLuW are 2.0(±0.1)×10 -2 cm 2 at 1798 nm and 1.9(±0.1)×10 -2 cm 2 at 1830nm, respectively [20] .The FWHMs of the emission bands for  and  polarizations are 161 and 130 nm, respectively.Fig. 4.10 presents the gain cross-section calculated for different values of P (P=0.1~0.5) for (a) the 3 F 4 → 3 H 6 transition of Tm 3+ in NaY(WO 4 ) 2 crystal and (b) the 5 I 7 5 I 8 transition of Ho 3+ in NaY(WO 4 ) 2 crystal.The gain curves at a wavelength longer than 1900 nm are obscure due to the low signal-to-noise ratio of the absorption spectrum.The positive gain cross-section can be obtained at about 2.0 µm when P exceeds 0.2.The positive gains for P=0.5 are in a range from 1758 to about 1954 nm for  polarization and from 1758 to about 1977 nm for  polarization, respectively.Fig. 4.11 presents the room temperature fluorescence spectra of Tm 3+ -, Ho 3+ -doped and Tm 3+ /Ho 3+ co-doped NaY(WO 4 ) 2 crystals.Fig. 4.12 gives the decay curves of 3 F 4 manifold in the samples of bulk and powder in the Tm 3+ doped NaY(WO 4 ) 2 crystals.Fig. 4.13 also gives the decay curves of Ho: 5 I 7 level in the (a) samples of bulk and powder in the Ho 3+ :NaY(WO 4 ) 2 and (b) Tm, Ho:NaY(WO 4 ) 2 crystals.The Nd 3+ :NaY(WO 4 ) 2 crystal was made into laser stick and the laser experiment was performed using a xenon flash lamp as a pump source [1] .Maximum pulse energy of 786 mJ with a repetition rate of 1 Hz has been obtained.A maximum output power of 87 mW at 532 nm has been obtained and the double-frequency conversion efficiency is more than 25% when a LBO optical crystal was used as the frequency-doubling crystal.Table 5.1 shows the data of input and output energy and Fig. 5.1 presents the relationship between the Iuput energy and output energy.

The laser characteristics of Tm, Ho: NaY(WO 4 ) 2 crystal
An infrared laser output at 2.07 μm with Tm, Ho:NaY(WO 4 ) 2 crystal end-pumped by 795 nm laser diode at room temperature is reached [2,3] .Fig. 5. 4 shows the experimental configuration of the LD-end-pumping Tm, Ho: NYW laser.The crystal used with the concentrations of 5 at% Tm 3+ and 1 at% Ho 3+ was grown by the Czochralski method.The highest output power was up to 2.7 W corresponding to the crystal temperature being controlled at 283 K. Fig. 5.5 presents the output power versus pumping power at different temperatures.The overall optical conversion efficiency was 5.4% and the slop efficiency was 26%.The output characteristics and the laser threshold affected by the pulse duration and temperature have been studied.It can be found that the stability of the output power was correlative with the crystal temperature heavily.In addition, the wider pulse duration of pump could promote the output power efficiently as shown in Fig. 5.6, which presents the output power versus pulse duration.With Ti:sapphire laser pumping at 795 nm, a slope efficiency and a maximum output power as high as 48% and 265 mW, respectively, have been obtained at 2050 nm from a Tm:Ho: NaY(WO 4 ) 2 crystal by Prof.C.Zaldo [4] .Tuning from 1830 nm to 2080 nm has also been achieved using an intracavity Lyot filter.Prfo.A.A.Lagatsky and C.Zaldo [5] also reported the femtosecond-pulse operation of a Tm:Ho:NaY(WO 4 ) 2 laser at around 2060 nm by using an ion-implated InGaAsSb quantumwell-based semiconductor saturable absorber mirror for passive mode-locking maintenance for the first time.Transform-limited 191fs pulses are produced with an average output power of 82 mW at a 144 MHz pulse repetition frequency.Maximum output power of up to 155 mW is generated with a corresponding pulse duration of 258 fs.Fig. 5.9 presents the Input-output characteristics of the mode-locked Tm:Ho:NaY(WO 4 ) 2 laser.Two different operation regimes, shorter-pulse and higher-power, are indicated by squares and circles, respectively.Q-switching and mode-locking regimes are represented by open and closed symbols, respectively.

5.3
The laser characteristics of Tm, Ho, Ce: NaY(WO 4 ) 2 crystal An infrared laser output at 2.07 μm with Tm,Ho,Ce:NaY(WO 4 ) 2 single crystal end-pumped by 795 nm laser diode at room temperature [3,6] .The crystal used with the concentrations of 5 at% Tm 3+ , 1 at% Ho 3+ and 30 at% Ce 3+ was grown by the Czochralski method.The highest output power was up to 0.2 W corresponding to the pumping power of 50 W and the threshold was about 40 W at 293 K. Figure 5.10 shows the output power versus the pump power.The introduction of Ce 3+ brought about a novel phenomenon.End-pumping with the 795 nm LD, it was found the up-conversion was repressed heavily and the green emission disappeared thoroughly in Tm,Ho,Ce:NaY(WO 4 ) 2 crystal, which was particularly different from the crystal Tm,Ho:NaY(WO 4 ) 2, where the green emission was obvious and weakened the sensitized transition energy.The original intention of selecting the Ce 3+ was to compensate the up-conversion loss.The compensation mechanisms of the Ce 3+ lie in its transition energy.As shown in Fig. 5.11, the transition energy of 2 F 7/2 2 F 5/2 (Ce 3+ ) is close to that of 3 H 5 3 F 4 (Tm 3+ ) and half of the 3 H 4 3 H 5 (Tm 3+ ).Pumped with 795 nm, the electrons will transit from 3 H 6 to 3 H 4 , and jump to 3 H 5, 3 F 4 depending on the radiationless transition.Because the energy level 3 F 4 (Tm 3+ ) was close to 5 I 7 (Ho 3+ ), the electrons will transit from 3 F 4 (Tm 3+ ) to 5 I 7 (Ho 3+ ), which is just the sensitized process.At last, the transition 5 I 7 5 I 8 (Ho 3+ ) generates the2.07 m laser.In the complex sensitized process, only few of the electrons will transit from the upper pumping energy level 3 H 4 (Tm 3+ ) into the 5 I 7 (Ho 3+ ), which is the reason of the lower laser efficiency.By virtue of the Ce 3+ , the electrons of the 3 H 4 (Tm 3+ ) can transit fast into the energy level 3 F 4 (Tm 3+ ).More important, the multiple transition energy can guide the electrons towards 3 F 4 (Tm 3+ ) instead of irregular radiationless transition.That is to say, in the shorter time, there are more electrons gathering into the energy level 5 I 7 (Ho 3+ ), which is just the demand of the high laser efficiency.Here, in our experiment, the disappeared green emission is the certification of the function of the Ce 3+ , which contributes to the improvement of the 2 μm laser.

Conclusion
In this review, the growth of rare earth (Tm 3+ ,Ho 3+ ,Nd 3+ ,Yb 3+ , Er 3+ /Yb 3+ )-doped NaY(WO 4 ) 2 large crystal with the dimensions of Ф25 mm×100 mm is reported.The thermal, optical and spectrum characteristics of these crystals are presented.The laser characteristics of Nd 3+ ;Tm 3+ /Ho 3+ :NaY(WO 4 ) 2 laser crystals are also covered.Maximum pulse energy of 786 mJ with a repetition rate of 1Hz has been obtained from Nd 3+ -doped NaY(WO 4 ) 2 crystal pumped by xenon flash lamp.It can be found that the Nd:NYW crystal has the higher laser efficiency than Nd:YAG crystal.An infrared laser output of 2.7 W at 2.07 μm with Tm,Ho:NaY(WO 4 ) 2 crystal end-pumped by 795 nm laser diode at room temperature is also reached.Furthermore, the femtosecond-pulse operation of a Tm:Ho:NaY(WO 4 ) 2 laser at around 2060 nm is obtained for the first time.Transform-limited 191fs pulses are produced with an average output power of 82 mW at a 144MHz pulse repetition frequency.Maximum output power of up to 155 mW is generated with a corresponding pulse duration of 258 fs.Also, it is found that the co-doped Ce 3+ can depress the green up-conversion emission of Tm 3+ and thus improves the 2 μm laser.All the above performances demonstrate that NaY(WO 4 ) 2 crystal can serve as an excellent laser host.

Fig. 4 .
2 shows the RT emission spectrum with the pumping perpendicular to (001) planes.There are six emission peaks at follows wavelength: 894, 917,1063,1087,1339 and 1389 nm.The value of emission cross-section at 1063 nm is about 4.6×10 -20 cm 2 .Fig.4.3 shows the fluorescence decay of 4 F 3/2 level of Nd 3+ in NYW crystal at RT and the lifetime of 4 F 3/2 level is about 85 μs and relative luminescent quantum efficiency is about 47% .Tab.4.1 presents the integrated absorbance, the line strengths, the experimental and calculated oscillator strengths.

Fig. 4 .
5 shows the RT Polarized emission spectrum of Yb 3+ :NaY(WO 4 ) 2 Crystal.The emission cross-sections of crystal calculated from the fluorescence spectra by the reciprocity method and the Füchtbauer-Ladengurg formula are shown in Fig.4.6[16~18]  .The radiative lifetime r  of the 2 F 3/2 manifold is measured to be 0.902 ms.The gain coefficient was calculated for several values of population inversion P ( P =0, 0.1, 0.2…) and is shown in Fig.4.7 (a) and Fig.4.7 (b).Positive gain coefficient for P values larger than 0.5, which are encountered in a freerunning laser operation, implies a tuning range from 990 to 1070 nm.

Fig. 4 . 6 Fig. 4 . 7
Fig. 4.6 The emission cross-sections of crystal calculated from the fluorescence spectra by the reciprocity method and the Füchtbauer-Ladengurg formula.

Fig. 5 . 5
Fig. 5.5 The output power versus pumping power at different temperatures.

Fig. 5 .
Fig. 5.10 The output power versus the pump power.

Table 4
.2 shows the calculated radiative probabilities, radiative branching ratios and radiative time for the emissions from the 4 F 3/2 level of Nd 3+ :NYW crystal.Table4.3-4give the comparison of spectrum parameters in Nd:NYW and other Nd-doped crystals.

Table 4 .
The Growth and Properties of Rare Earth-Doped NaY(WO 4 ) 2 Large Size Crystals 105 1 The integrated absorbance, the line strengths, the experimental and calculated oscillator strengths of Nd:NYW crystal. www.intechopen.com

Table 4 .
2 Calculated radiative probabilities, radiative branching ratios and radiative time for the emissions from the 4 F 3/2 level of Nd 3+ :NYW crystal.

Table 4 .
3 Comparison of spectral values in Nd:NYW and other Nd-doped crystals.

Table 5 .
2 presents the Comparison of laser properties of Nd:NYW crystal and Nd:YAG crystal.It can be found that the Nd:NYW crystal has the higher laser efficiency than Nd:YAG crystal.Table.5.3 shows the frequency-doubling laser output power and conversion efficiency and Fig.5.2 presents the relationship between the pump power and output power of SH generation.Fig.5.3 shows the laser facula of the SH generation.

Table 5 .
1 Data table of pumping energy and output energy.

Table 5 .
2 Comparison of laser properties of Nd:NYW and Nd:YAG.

Table 5 .
3 SH generation power and conversion efficiency.