Structural, spectroscopic, and tunable laser properties of ${\mathrm{Yb}}^{3+}$-doped $\mathrm{Na}\mathrm{Gd}{\left(\mathrm{W}{\mathrm{O}}_4\right)}_2$

Single crystals of ${\mathrm{Yb}}^{3+}$-doped $\mathrm{Na}\mathrm{Gd}{\left(\mathrm{W}{\mathrm{O}}_4\right)}_2$ with up to $20\mathrm{mol}\%$ ytterbium content have been grown by the Czochralski technique in air or in ${\mathrm{N}}_2+{\mathrm{O}}_2$ atmosphere and cooled to room temperature at different rates $(4–250°\mathrm{C}∕\mathrm{h})$. Only the noncentrosymmetric tetragonal space group $I\overline{4}$ accounts for all reflections observed in the single crystal x-ray diffraction analysis. The distortion of this symmetry with respect to the centrosymmetric tetragonal space group $I{4}_1∕a$ is much lower for crystals cooled at a fast rate. ${\mathrm{Na}}^{+}$, ${\mathrm{Gd}}^{3+}$, and ${\mathrm{Yb}}^{3+}$ ions share the two nonequivalent $2b$ and $2d$ sites of the $I\overline{4}$ structure, but ${\mathrm{Yb}}^{3+}$ (and ${\mathrm{Gd}}^{3+}$) ions are found preferentially in the $2b$ site. Optical spectroscopy at low $\left(5\mathrm{K}\right)$ temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the ${}^{2}F_{7∕2}\left(n\right)$ and ${}^{2}F_{5∕2}\left(n^{\prime }\right)$ Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the $2b$ and $2d$ sites. As a novel uniaxial laser host for ${\mathrm{Yb}}^{3+}$, $\mathrm{Na}\mathrm{Gd}{\left(\mathrm{W}{\mathrm{O}}_4\right)}_2$ is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave ${\mathrm{Yb}}^{3+}$-laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the $\pi$ polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the $1014–1079\mathrm{nm}$ spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces $120\mathrm{fs}$ long pulses at $1037.5\mathrm{nm}$ with an average power of $360\mathrm{mW}$ at $\approx 97\mathrm{MHz}$ repetition rate. Using uncoated samples of $\mathrm{Yb}:\mathrm{Na}\mathrm{Gd}{\left(\mathrm{W}{\mathrm{O}}_4\right)}_2$ at normal incidence in simple two-mirror cavities, output powers as high as $1.45\mathrm{W}$ and slope efficiencies as high as 51% are achieved with different diode laser pump sources.

Figure 1

Evolution of the Yb:NaGdW crystal unit cell parameters versus Yb content in the crystal: ◇ this work (single crystal XRD), ◻ Rode et al. (powder XRD) (Ref. 15), ▵ Zharikov et al. (powder XRD of ground crystal) (Ref. 17), 엯 Mokhosoev et al. (powder XRD) (Ref. 14), ◀ JCPDS 25-0829 (powder XRD) (Ref. 16) The lines connect the values of Rode et al. (Ref. 15).

Figure 2

Temperature dependence of the absorption coefficient $\alpha$ of NaGdW near the UV band edge for the two polarizations $\sigma$ (a) and $\pi$ (b). The unpolarized IR spectrum recorded at room temperature is included in (a).

Figure 3

Refractive indices of NaGdW measured at $300\mathrm{K}$, $\mathbf{E}\perp \mathbf{c}$ ($n_o$, 엯), $\mathbf{E}\Vert \mathbf{c}$ ($n_e$, ◻), with black curves representing our Sellmeier fits. For Yb:NaGdW, the behavior of $n_o$ was very similar; the gray curve is the calculated Sellmeier fit obtained for $n_e$ in this case (the experimental data are not shown for clarity).

Figure 4

Polarized Raman spectra recorded with single crystals of Yb:NaGdW with $1\mathrm{mol}\%$ Yb doping grown under Cz2 conditions. The standard Porto notations designate the propagation/polarization configurations used for excitation and detection.

Figure 5

Low temperature $\left(5\mathrm{K}\right)$ spectroscopy of Yb:NaGdW for the $\pi$ and $\sigma$ polarizations. The absorption coefficient (left scale) is measured with a sample of $0.8\mathrm{mol}\%$ Yb in the crystal and the photoluminescence (right scale) with a sample of $3.7\mathrm{mol}\%$ Yb, excited at $961\mathrm{nm}$.

Figure 6

Unpolarized PL spectra of Yb:NaGdW with $0.8\mathrm{mol}\%$ Yb in the crystal recorded at $5\mathrm{K}$: (a) Comparison of the excitation spectra at several emission wavelengths, ${\lambda }_{\mathit{EMI}}$, with the optical absorption (OA) where the absorption coefficient $\alpha$ is rescaled for clarity. (b) Emission spectra for selected excitation wavelengths, ${\lambda }_{\mathit{EXC}}$. $2b$ and $2d$ indicate the two crystallographic sites characteristic for the $I\overline{4}$ SG.

Figure 7

Energy level scheme of ${\mathrm{Yb}}^{3+}$ in the $2b$ and $2d$ sites of the NaGdW host.

Figure 8

Absorption cross section ${\sigma }_{\mathit{ABS}}$ of Yb:NaGdW measured at room temperature (gray curves) and calculated emission cross sections ${\sigma }_{\mathit{EMI}}$ (black curves) for the $\pi$ and $\sigma$ polarizations. A $0.452\mathrm{mm}$ thick sample with $20\mathrm{mol}\%$ Yb in the crystal grown under Cz2 conditions was used for the absorption measurements.

Figure 9

Calculated gain cross section ${\sigma }_{\mathit{\text{GAIN}}}$ of Yb:NaGdW for different inversion ratios $\beta$.

Figure 10

Astigmatically compensated three-mirror cavity with Brewster geometry and an intracavity tuning element (a), and its extension for passive mode locking (b). In (a): $M_1$ and $M_2$: total reflectors for the laser radiation with radius of curvature (RC) equal to $-50\mathrm{mm}$ and $-100\mathrm{mm}$, respectively, $M_3$: plane output coupler. In (b): $M_1$, $M_2$, and $M_3$: total reflectors for the laser radiation with $\mathrm{RC}=-150\mathrm{mm}$, $-100\mathrm{mm}$, and $-100\mathrm{mm}$, respectively, $M_4$ and $M_5$: plane output couplers, $P_1$ and $P_2$: Brewster prisms, SAM: saturable absorber mirror.

Figure 11

Measured single pass absorption of the Yb:NaGdW sample A with lasing (full symbols) and with lasing interrupted in the arm containing the output coupler (open symbols), versus incident pump power $P_{\mathit{INC}}$ for several output couplers. The nonlasing data should be independent of $T_{\mathit{OC}}$ and the four data sets are indicative of the precision of this measurement.

Figure 12

Output power $P_{\mathit{OUT}}$ versus absorbed pump power $P_{\mathit{ABS}}$ (symbols) for the three-mirror Yb:NaGdW laser with Ti:sapphire laser pumping and linear fits (lines) for calculation of the slope efficiency $\eta$ for four different output couplers $\left(T_{\mathit{OC}}\right)$.

Figure 13

Output power $P_{\mathit{OUT}}$ versus absorbed pump power $P_{\mathit{ABS}}$ (symbols) of the CW Yb:NaGdW laser obtained with TDL pumping using a three-mirror cavity, and linear fits (lines) for calculation of the slope efficiency $\eta$ for two different output couplers $\left(T_{\mathit{OC}}\right)$.

Figure 14

Output power $P_{\mathit{OUT}}$ versus laser wavelength ${\lambda }_L$ for an incident pump power of $P_{\mathit{INC}}=2.0\mathrm{W}$ (Ti:sapphire laser pumping). The crystal orientation corresponds to $\pi$ polarization (a) with ${\lambda }_P=976\mathrm{nm}$ and $\sigma$ polarization (b) with ${\lambda }_P=975\mathrm{nm}$.

Figure 15

Autocorrelation trace (a) and spectrum (b) of the femtosecond Yb:NaGdW laser. The symbols in (a) are the experimental points while the fit is shown by the curve.

Figure 16

Output power $P_{\mathit{OUT}}$ and crystal absorption versus absorbed pump power $P_{\mathit{ABS}}$ of the CW Yb:NaGdW laser pumped by the unpolarized diode in a single pass. The linear fits (lines) were used for calculation of the slope efficiency $\eta$ for different output couplers $\left(T_{\mathit{OC}}\right)$.

Figure 17

Output power $P_{\mathit{OUT}}$ versus absorbed pump power $P_{\mathit{ABS}}$ of the CW Yb:NaGdW laser pumped by the polarized diode in a double pass with an output coupler having $\mathrm{RC}=-100\mathrm{mm}$ (a) and $-50\mathrm{mm}$ (b). The linear fits (lines) were used for calculation of the slope efficiency $\eta$ for different output couplers $\left(T_{\mathit{OC}}\right)$.