# Diode-pumped orange Sm:LiYF4 lasers emitting at 605 nm

Jonathan Demaimay, Pavel Loiko, Esrom Kifle, Florence Pau, Gilles Recoque, Abdelmjid Benayad, Thierry Georges, Patrice Camy, Alain Braud

► To cite this version:

Jonathan Demaimay, Pavel Loiko, Esrom Kifle, Florence Pau, Gilles Recoque, et al. Diode-pumped orange Sm:LiYF4 lasers emitting at 605 nm. Optics Express, 2025, 33 (5), pp.9412. 10.1364/oe.553759. hal-05371838

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**Title:** Diode-pumped orange Sm:LiYF lasers emitting at 605 nm

**Authors:** jonathan demaimay,Pavel Loiko,Esrom KIFLE,Florence Pau,gilles recoque,Abdelmjid Benayad,Thierry Georges,Patrice Camy,Alain BRAUD

**Accepted:** 30 January 25

**Posted** 31 January 25

**DOI:** https://doi.org/10.1364/OE.553759

Published by Optica Publishing Group under the terms of the [Creative Commons Attribution 4.0 License](https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

OPTICA
PUBLISHING GROUP
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# Diode-pumped orange Sm:LiYF₄ lasers emitting at 605 nm

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JONATHAN DEMAIMAY,¹ PAVEL LOIKO,¹ ESROM KIFLE,¹,² FLORENCE PAU,²

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GILLES RECOQUE,² ABDELMJID BENAYAD,¹ THIERRY GEORGES,² PATRICE

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CAMY,¹ AND ALAIN BRAUD¹,*

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¹Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICAEN, Université de Caen Normandie, 6 Boulevard Maréchal Juin, 14050 Caen Cedex 4, France

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²Oxpius SA, 4 rue Louis de Broglie, 22300 Lannion, France

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*alain.braud@ensicaen.fr

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**Abstract:** We report on the first visible orange Samarium laser directly pumped by a blue 465-nm GaN semiconductor laser diode. The diode-pumped continuous-wave Sm:LiYF₄ laser delivers 23.9 mW at 605 nm (⁴G₅/₂ → ⁶H₇/₂ transition) with a slope efficiency of 9.4%, a laser threshold of 385 mW and a linear polarization (π). We investigate the impact of pump beam quality on laser performance and revisit the polarized spectroscopic properties of Sm³⁺ ions in the LiYF₄ crystal. The stimulated-emission cross-section σₔₐ amounts to 1.27×10⁻²⁰ cm² at 604.8 nm for π-polarized light, and the luminescence lifetime of the ⁴G₅/₂ level is 4.02 ms for 1.1 at.% Sm-doping. Our work represents a proof-of-principle for compact low-threshold diode-pumped orange and red Samarium lasers.

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© 2024 Optica Publishing Group under the terms of the Optica Publishing Group Open Access Publishing Agreement

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## 1. Introduction

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Coherent light sources emitting in the visible are of immense interest due to their cutting-edge applications in medicine, quantum optics, and high-resolution imaging. For instance, laser emission in the orange (590-625 nm) is of key importance in fields such as flow cytometry, where it enables the use of a wider range of fluorescent probes with enhanced efficiency [1], but also astronomy, in which laser guide stars at 589 nm are used to correct wavefront distortions [2] and ozone gas detection, as it spectrally matches with an absorption band of ozone at 603 nm also known as the Chappuis band [3].

Generally, accessing the orange spectral range poses significant challenges, especially when a combination of power-scalable continuous-wave emission, high beam quality, and linear polarization is required. Several approaches are currently being investigated to address this issue:

i) *Raman lasers.* Mildren et al. reported on a nanosecond Raman laser based on a KGd(WO₄)₂ crystal pumped in the green delivering up to 100 mW at 606 nm [4] and Mao et al. designed an actively Q-switched Nd:YAlO₃ / YVO₄ / LBO laser delivering 0.5 W at 597 nm (with the three above mentioned crystals serving as the infrared gain medium, Raman shifter, and frequency doubler, respectively) [5];

ii) *Frequency-doubled semiconductor disk lasers.* Lin et al. reported on a continuous-wave vertical-external-cavity surface-emitting laser (VECSEL) with an intracavity LBO crystal, generating 0.52 W at 593 nm; however, it exhibits a low optical conversion efficiency [6]. Kantola et al. demonstrated power scaling of a similar laser to the multi-Watt level [7];

iii) *Rare-earth-ion based lasers.* Rare-earth ions, such as trivalent praseodymium (Pr³⁺) and samarium (Sm³⁺), offer emission in the orange, slightly above 600 nm. The recent progress achieved in the development of blue GaN laser diodes has facilitated their use as pump sources and opened a new paradigm for compact, continuous-wave, and high efficiency laser sources without the necessity for nonlinear optical elements [8]. Direct laser emission in the orange has
47 already been obtained by exploiting the $ ^{3}P_{0} \rightarrow ^{3}H_{6} $ transition of $Pr^{3+}$ ions in fluoride crystals,
48 such as $LiYF_{4}$ [9–11], $KYF_{4}$ [12] and $BaY_{2}F_{8}$ [13,14], yielding output powers of several
49 hundred mW at wavelengths of 604 nm and 607 nm. However, the generation of efficient and
50 high-power orange lasers based on $Pr^{3+}$-doped materials encounters several difficulties, namely
51 narrow absorption bands, high probability of cross-relaxation processes limiting the optimum
doping levels, short luminescence lifetimes and a strong competition with the high-gain red
52 emission. On the contrary, $Sm^{3+}$ ions offer broad absorption bands in the blue spanning from
53 400 to 480 nm, and long luminescence lifetimes of several ms. They exhibit multi-color
54 emissions in the visible related to transitions from the $^{4}G_{5/2}$ metastable state to the lower-lying
55 $^{6}H_{1}$ ($J = 5/2 - 11/2$) manifolds falling in the green, orange, red and deep red, respectively
56 [15,16].

58 These properties make $Sm^{3+}$-doped materials ideal candidates for the production of
59 luminescent phosphors that find use in field-emission displays and white light-emitting diodes
60 [17]. The first visible (red) laser emission from $Sm^{3+}$ ions was obtained in 1979 using a
61 $Sm:TbF_{3}$ crystal as gain material under flashlamp pumping at cryogenic temperatures [18].
62 Orange laser emission at 605 nm was reported for the first time for $Sm^{3+}$ in $LiTbF_{4}$ [19]
63 benefiting from an energy transfer process involving $Tb^{3+}$ ions being pumped by a 488 nm $Ar^{+}$
64 ion laser. Recently, Marzahl et al. reported on lasing at 593 nm and 606 nm in $Sm:SrAl_{12}O_{19}$
65 and $Sm:LiLuF_{4}$ crystals, respectively, employing a frequency-doubled optically pumped
66 semiconductor laser ($2\omega$-OPSL) as pump source [20].

67 Tetragonal lithium yttrium fluoride ($LiYF_{4}$) crystal is a well-known laser host material for
68 rare-earth ion doping targeting laser applications. It features good thermo-mechanical
69 properties [21] (thermal conductivity, $\kappa = 5.3$ and $7.2$ [Wm$^{-1}$K$^{-1}$] along the **a** and **c**-axes,
70 respectively [22]), a broad band gap, easy rare-earth ion doping, has a low phonon energy
71 ($h\nu_{ph} = 439$ cm$^{-1}$), and its growth by the Czochralski method is well developed. As compared
72 to its $LiLuF_{4}$ counterpart studied previously [20], $LiYF_{4}$ benefits from better thermal properties,
73 higher optical quality and less costly production in terms of the availability of yttrium oxide / 
74 fluoride reagents. This compound is also well recognized for generating coherent visible
75 radiation based on $Pr^{3+}$ ions.

76 In the present work, we report on the polarized spectroscopy and first laser operation of a
77 $Sm^{3+}$-doped $LiYF_{4}$ crystal in the orange (at 605 nm) and red (at 648 nm), as well as the first
78 diode-pumped visible Samarium laser.

## 2. Crystal growth and spectroscopy

### 2.1 *Crystal growth*

81 Two single crystals of $LiYF_{4}$ doped with 1.1 and 3 at.% $Sm^{3+}$ (in the melt) were grown using
82 the Czochralski method. The precursors used for the growth were LiF (Alfa Aesar, optical grade
83 99.95%) and $Y_{2}O_{3}$ (Alfa Aesar, 4N-REO). The oxide compound $Y_{2}O_{3}$ was fluorinated to $YF_{3}$
84 using molten ammonium hydrogen bifluoride ($NH_{4}HF_{2}$) as a fluorinating agent under heating.
85 The reagents were introduced in a molar ratio of 52% LiF – 48% $YF_{3}$ according to the phase
86 diagram of the LiF – $YF_{3}$ binary system.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Photograph of an as-grown 1.1 at.% $Sm:LiYF_{4}$ crystal boule under (a) natural light and (b) UV lamp illumination.</figcaption></figure>

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The growth of both Sm:LiYF₄ crystals was made under Ar + CF₄ atmosphere using a [001] oriented undoped seed and a vitreous carbon crucible at a temperature of ~810°C. The pulling rate was fixed at 0.5-1 mm/h, the rotation speed was between 2-6 rpm, and the growth duration was about 1 week. After the growth was completed, the crystals were slowly removed from the melt and cooled to room temperature at a rate of 30°C/h. The as-grown crystal boules (diameter: Ø30 mm, length: 40 mm) were transparent, free of cracks and inclusions, Fig. 1(a). They exhibited bright orange luminescence under UV lamp (λ<sub>max</sub> = 365 nm) illumination due to Sm<sup>3+</sup> doping, Fig. 1(b). The crystals boules were oriented using a Laue diffractometer (Thermo Scientific, ARL Equinox 100). For spectroscopic and laser studies, rectangular samples were cut for light propagation along the **a**-axis (**a**-cut).

## 2.2 Polarized spectroscopy

The electronic configuration of Sm<sup>3+</sup> is [Xe]4f<sup>5</sup> and the energy-level scheme of this ion features a ground-state <sup>6</sup>H<sub>5/2</sub>, two groups of relatively closely located <sup>6</sup>H<sub>J</sub> and <sup>6</sup>F<sub>J</sub> excited manifolds, and a metastable level <sup>4</sup>G<sub>5/2</sub> separated from the lower-lying manifold by an energy gap of ~7400 cm<sup>-1</sup>, see Fig. 2(a). Visible (orange and red) Sm lasers operate on a 4-level scheme: first, pumping in the blue leads to excitations into a group of higher-lying excited-states (e.g., <sup>4</sup>I<sub>11/2,13/2</sub> + <sup>4</sup>M<sub>15/2</sub>), then, these levels are rapidly depopulated by multiphonon non-radiative relaxation and Sm<sup>3+</sup> ions end in the metastable <sup>4</sup>G<sub>5/2</sub> multiplet from which multi-color emission in the visible spectral range happens. The two main terminal laser levels, <sup>6</sup>H<sub>7/2</sub> and <sup>6</sup>H<sub>9/2</sub>, are also drained in the same way.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Spectroscopy of Sm<sup>3+</sup> ions in the LiYF<sub>4</sub> crystal: (a) energy-level scheme of trivalent Samarium; (b) polarized absorption cross-sections, σ<sub>abs</sub>, in the blue spectral range; (c) stimulated-emission (SE), σ<sub>SE</sub>, cross-sections for the <sup>4</sup>G<sub>5/2</sub> → <sup>6</sup>H<sub>7/2</sub> and <sup>4</sup>G<sub>5/2</sub> → <sup>6</sup>H<sub>9/2</sub> transitions in the orange and red. The light polarizations are π and σ; (d) luminescence decay curves from the <sup>4</sup>G<sub>5/2</sub> Sm<sup>3+</sup> manifold in 1.1 at.% and 3 at.% Sm<sup>3+</sup>-doped crystals: points - experimental data, curves - their fits using the Inokuti-Hirayama model, Eq. (2); λ<sub>exc</sub> = 400 nm and λ<sub>lum</sub> = 603 nm.</figcaption></figure>

We investigated the key spectroscopic properties of Sm<sup>3+</sup> ions in LiYF<sub>4</sub>, that are relevant for laser operation under polarized light. The absorption spectra were measured using a Varian Cary 5000 spectrophotometer and a Glan-Taylor polarizer. The absorption cross-sections, σ<sub>abs</sub>, of Sm<sup>3+</sup> ions in the blue spectral range, see Fig. 1(b), were then calculated using the Beer-Lambert law using the Sm<sup>3+</sup> ion density, N<sub>Sm</sub> = 1.47×10<sup>20</sup> at/cm<sup>3</sup>, for the 1.1 at.% Sm<sup>3+</sup> doped crystal. The maximum σ<sub>abs</sub> value corresponds to the spin-allowed (ΔS = 0) <sup>6</sup>H<sub>5/2</sub> → <sup>6</sup>P<sub>3/2</sub> + <sup>6</sup>P<sub>5/2</sub> transition and amounts to 2.15×10<sup>-20</sup> cm<sup>2</sup> at 400.8 nm for the π-polarization (full width at half
124 maximum, FWHM: 1 nm). The broad but much weaker absorption band spanning from 460 to 125 485 nm corresponds to the $ ^{6}\text{H}_{5/2} \rightarrow ^{4}\text{I}_{11/2,13/2} + ^{4}\text{M}_{15/2} $ transitions and it is suitable for pumping 126 with both blue GaN semiconductor laser diodes and 2ω-OPSL. However, the peak $ \sigma_{abs} $ values 127 in this spectral range are by an order of magnitude lower due to the spin-forbidden nature of 128 the corresponding transitions ($ \Delta S \neq 0 $): $ 0.34 \times 10^{-20} \text{ cm}^2 $ at 462.6 nm and $ 0.33 \times 10^{-20} \text{ cm}^2 $ at 481.2 129 nm (absorption bandwidth, FWHM = 4.0 nm) for light polarization $ \pi $. No absorption bands 130 related to the presence of $ \text{Sm}^{2+} $ are noticeable in the absorption spectra. 131

The polarized luminescence spectra were measured by a confocal Raman microscope (inVia 132 Qontor, Renishaw) equipped with an $ \text{Ar}^{+} $ ion laser ($ \lambda = 488 \text{ nm} $). The stimulated emission (SE) 133 cross-sections, $ \sigma_{SE} $, for the $ ^{4}\text{G}_{5/2} \rightarrow ^{6}\text{H}_{7/2} $ and $ ^{4}\text{G}_{5/2} \rightarrow ^{6}\text{H}_{9/2} $ laser transition falling in the orange 134 and red spectral ranges, respectively, were calculated via the Füchtbauer-Ladenburg formula 135 [23]:

$$ \sigma_{SE}^{t}(\lambda) = \frac{\lambda^{5}}{8\pi c \langle n \rangle^{2} \tau_{rad}} \times \frac{\beta(JJ')I(\lambda)}{\frac{1}{3} \int_{2\sigma+\pi}^{\pi} \lambda I(\lambda) d\lambda'} \quad (1) $$

137 where $ \lambda $ is the light wavelength, $ c $ is the speed of light, $ \langle n \rangle $ is the polarization averaged 138 refractive index of the crystal at the mean emission wavelength, $ \tau_{rad} $ is the radiative lifetime of 139 the emitting state, $ \beta(JJ') $ is the luminescence branching ratio for the considered $ J \rightarrow J' $ transition, 140 $ I(\lambda) $ is the spectral profile of the emission band, and $ i(\pi \text{ or } \sigma) $ labels the light polarization. The 141 values of the radiative lifetime of the $ ^{4}\text{G}_{5/2} $ state ($ \tau_{rad} = 6.9 \text{ ms} $) and the luminescence branching 142 ratios ($ \beta(JJ') = 45.84\% $ for the orange emission and 22.0% for the red one) calculated using the 143 Judd-Ofelt theory were taken from [15]. The $ \sigma_{SE} $ spectra are plotted in Fig. 1(c). 144

The emission spectra are strongly polarized, and the peak $ \sigma_{SE} $ values amount to $ 1.27 \times 10^{-20} \text{ cm}^2 $ at 604.8 nm (emission bandwidth, FWHM = 2.4 nm) and $ 0.59 \times 10^{-20} \text{ cm}^2 $ at 648.0 nm (FWHM = 2.6 nm), both for $ \pi $-polarized light. These values are comparable to those reported 145 previously for $ \text{Sm:LiLuF}_4 $ and $ \text{Sm:SrAl}_{12}\text{O}_{19} $ crystals [20]. 146

The luminescence decay curves from the $ ^{4}\text{G}_{5/2} $ manifold of $ \text{Sm}^{3+} $ ions for the two doping 148 levels of 1.1 at.% and 3 at.% Sm are displayed in Fig. 2(d). They were recorded using a 149 spectrofluorometer (PTI Quantamaster-8075-22, Horiba) equipped with a 75 W pulsed Xe lamp 150 as excitation source (OB-75X, Horiba) and a photomultiplier tube (R928 Hamamatsu). The 151 lamp frequency was fixed at 25 Hz and the excitation and detection wavelengths were 400 nm 152 and 603 nm, respectively. The mean luminescence lifetimes $ \tau_{lum} $ are 4.02 ms and 2.67 ms for 153 1.1 at.% and 3 at.% $ \text{Sm}^{3+} $ doping, respectively, and they are shorter than the radiative lifetimes 154 obtained via the Judd-Ofelt formalism ($ \tau_{rad} = 6.9 \text{ ms} $) [15]. 155

Sm$^{3+}$ ions exhibit a self-quenching behavior for their visible emissions due to multiple 156 cross-relaxation (CR) processes involving the $ ^{4}\text{G}_{5/2} $ manifold, namely $ ^{4}\text{G}_{5/2}, ^{6}\text{F}_{11/2} \rightarrow ^{6}\text{H}_{5/2}, 157 ^{6}\text{F}_{11/2} $, $ ^{4}\text{G}_{5/2}, ^{6}\text{F}_{9/2} \rightarrow ^{6}\text{H}_{5/2}, ^{6}\text{F}_{9/2} $ and $ ^{4}\text{G}_{5/2}, ^{6}\text{F}_{5/2} \rightarrow ^{6}\text{H}_{5/2}, ^{6}\text{F}_{11/2} $ [24]. The Inokuti-Hirayama 158 model for multipolar interactions [25] was used to fit the luminescence decay curves: 159

$$ I(t) = I_0 \cdot \exp \left( -\frac{t}{\tau_0} - \frac{4}{3} \pi R_0^3 N \cdot \Gamma \left( 1 - \frac{3}{s} \right) \left( \frac{t}{\tau_0} \right)^{\frac{3}{s}} \right), \quad (2) $$

160 where $ I $ is the luminescence intensity, $ t $ is time, $ \tau_0 $ is the intrinsic luminescence lifetime, $ R_0 $ is 161 the critical radius at which the probability of radiative and cross-relaxation processes become 162 equal, $ N $ is the ion density, $ \Gamma $ is the gamma-function, and $ s $ is the parameter indicating the 163 character of interactions between the ions. The fit well matches with the experimental data for 164 both studied $ \text{Sm}^{3+} $ doping levels yielding the following best-fit parameters: $ \tau_0 = 7.0 \pm 0.1 \text{ ms}, R_0 = 10.0 \pm 0.7 \text{ Å}, $ and $ s = 6 $. This value of $ s $ indicates the dipole-dipole (D-D) character of 165 interactions between $ \text{Sm}^{3+} $ ions in the crystal. One can compare the critical radius obtained via 166 the Inokuti-Hirayama model and the average distance between the donor and acceptor ions $ R $, 167 given by: $ R = [3/(4\pi N_{\text{Sm}})]^{1/3} $. For the 1.1 at.% and 3 at.% $ \text{Sm:LiYF}_4 $ crystals, one obtains $ R = 168 11.8 \text{ Å} $ and 8.4 Å, respectively, showing that the critical radius value $ R_0 $ is close to the average 169 170 distance between $ \text{Sm}^{3+} $ ions in the studied samples. 171
172 **3. Laser operation**

173 **3.1. Laser set-up**

174 The scheme of the laser set-up is depicted in Fig. 3. A rectangular laser element was cut from
175 the 1.1 at.% Sm:LiYF₄ crystal for light propagation along the **a** crystallographic axis (**a**-cut)
176 having a thickness of 8.0 mm and an aperture of 3.0(**c**)×3.0 mm². It was polished to laser quality
177 from both sides with good parallelism (a few arcmin accuracy) and a small surface roughness
178 in the nm-range and mounted in a passively cooled massive Cu-holder using a silver paint (SPI
179 Supplies, 5002-AB) for better thermal contact. The nearly hemispherical laser cavity was
180 formed by a plane pump mirror (PM) coated for high transmission (HT) at the pump
181 wavelengths (T > 85% at ~465 nm and T > 90% at 479 nm) and high reflection (HR) at the
182 laser one (R > 99.9% at 605 nm), and a set of concave output couplers (OCs) having a radius
183 of curvature (RoC) of -50 mm and a transmission T<sub>OC</sub> at 605 nm in the range of 0.1% to 1.2%.
184 The crystal was placed near the PM leaving a small airgap of less than 1 mm, and the
185 geometrical cavity length was ~49 mm.

<figure><img src="image_4.png" /><figcaption>186
187 **Fig. 3.** Layout of the Sm:LiYF₄ laser, the pump configurations #1 to #3 are displayed below, λ/2 – AR-coated half-wave plate, FL – focusing lens, CL – collimating lens, N.A. – numerical aperture.
188
189</figcaption></figure>

190 Three different blue pump sources were employed, cf. Fig. 3. The spatial characteristics of
191 their emission were characterized, Fig. 4. Specifically, the beam profiles at the waist position
192 were captured using a beam profiling camera (BladeCam-XHR, *Dataray*) and the beam quality
193 factors M² in the horizontal (x) and vertical (y) directions were measured using a spherical lens
194 (focal length: *f* = 50 mm) following the ISO 11146 standard method. i) First, we employed a
195 frequency-doubled Optically Pumped Semiconductor Laser (2ω-OPSL, Genesis CX STM,
196 *Coherent*) delivering 1.3 W of linearly polarized radiation (aligned to the *π* polarization in the
197 crystal) at 479.1 nm (laser linewidth: 0.1 nm) with a near diffraction limited beam, with a M²
198 ~ 1.3. This pump source addresses the ⁶H₅/₂ → ⁴M₁₅/₂+⁴I₁₃/₂ absorption transition of Sm³⁺ ions.
199 ii) Secondly, we used a fiber-coupled (fiber core diameter: 200 µm, numerical aperture, N.A.
200 = 0.22) GaN diode laser module (*Oxxius*, subsequently referred to as diode n°1) emitting up to
201 13 W at 464-467 nm (with an unpolarized output beam) and a nearly circular beam profile
202 strongly multimode illustrated by the measured M² of about 28. iii) The last pump source was
203 an open space (single emitter) GaN diode (*Nichia*, labelled diode n°2) whose output was
204 collimated using an antireflection (AR) coated cylindrical short focal lens (*LIMO*, *f* = 1.19 mm)
205 producing a maximum of 5 W output power at ~465 nm (unpolarized output, laser linewidth:
206 1.6 nm) exhibiting a reasonably good beam quality factor M² of 2.5 on its fast axis and a larger
207 M² value of 10.2 on the slow axis. The radiation of both laser diodes addresses the ⁶H₅/₂ → ⁴I₁₁/₂
208 Sm³⁺ transition. The pump radiation was focused into the crystal through the PM by an
209 antireflection (AR) coated achromatic lens (*Thorlabs*). A long-pass filter (FEL500, *Thorlabs*)
210 was used to filter out the residual pump. The size of the pump beam at the focus (diameter, at
211 the 1/e² level) was measured to be 50±5 µm, 230±10 µm and 120×400±10 µm for 2ω-OPSL,
212 diode n°1 and diode n°2, respectively. The radius of the laser mode in the crystal was calculated
213 using the ABCD method yielding 70±10 µm (accounting for the thermal lens).

<figure><img src="image_5.png" /><figcaption>Fig. 4. Spatial characteristics of the pump radiation for different blue light sources employed for pumping of the Sm:LiYF₄ laser: top line: measured spatial mode profiles at the beam waist (at full power); bottom line: evaluation of the corresponding beam quality parameters M².</figcaption></figure>

218 The emission spectra of the three blue pump sources were measured by an optical spectrum
219 analyzer (OSA, Yokogawa, AQ6373B) with a spectral resolution of 0.05 nm, Fig. 5(a). The
220 blue pump radiation well overlaps with the absorption bands of Sm³⁺ ions. The single-pass
221 pump absorption ηₐbs in the 8 mm-thick 1.1 at.% Sm:LiYF₄ crystal was measured under non-
222 lasing conditions as a function of the incident pump powers Pₗnc, Fig. 5(b). The pump absorption
223 slightly decreases with Pₗnc due to a combined effect of ground-state bleaching and (for GaN
224 laser diodes) the temperature-induced red shift of the diode wavelength upon increasing the
225 driving current. At the threshold pump power, ηₐbs amounted to 20.3% (2ω-OPSL), 46.0%
226 (diode n°1) and 26.2% (diode n°2). As the absorption of Sm³⁺ ions in the blue spectral range is
227 spectrally broad and structured, the relatively broad emission bandwidth of the GaN diodes has
228 a positive effect on enhancing the pump absorption efficiency.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Pump absorption for different blue light sources employed for pumping the 1.1 at.% Sm:LiYF₄ laser crystal: (a) measured spectra of blue pump radiation (at full available power), the polarized absorption cross-section spectra of Sm³⁺ ions in LiYF₄ are shown for comparison; (b-d) measured single-pass pump absorption versus incident pump power for (b) 2ω-OPSL; (c) fiber-coupled GaN laser diode; and (d) open-space GaN laser diode.</figcaption></figure>

## 3.2. Laser performance

236 Under high-brightness 2ω-OPSL pumping, the continuous-wave (CW) orange Sm:LiYF₄ laser
237 generated a maximum output power of 12.3 mW at 605 nm (the ⁴G₅/₂ → ⁶H₇/₂ transition) with
238 a slope efficiency η of 9.7% (vs. absorbed pump power) and a laser threshold of 51 mW for an
239 output coupling transmission T<sub>OC</sub> of 0.9%, see Fig 6(a). As the output coupling increased, the
240 laser threshold varied from 27 mW for 0.1% OC to 82 mW for 1.2% OC. The spectra of laser
emission were weakly dependent on the output coupling, and the typical laser linewidth was about 0.3 nm, Fig. 6(b). The input-output dependences were linear, and further power scaling was restrained by the available pump. The round-trip passive losses in the cavity were estimated using the Findlay-Clay analysis [26], yielding $L = 0.4\%$ at 605 nm (absorption coefficient in the laser crystal, $\alpha_{loss} = 0.005$ cm$^{-1}$).

By carefully aligning the output coupler, it was also possible to achieve laser operation in the red spectral range (on the $^4G_{5/2} \rightarrow {}^6H_{9/2}$ transition). The red Sm-laser delivered an output power of 2.7 mW at 648 nm (laser linewidth: 0.2 nm) with a slope efficiency of 4% and a laser threshold of 108 mW for an output coupling of 0.31%, see Fig. 6(c,d). Despite the lower gain in the red ($\sigma_{SE}$ of $0.59 \times 10^{-20}$ cm$^2$ at 648.0 nm vs. $1.27 \times 10^{-20}$ cm$^2$ at 604.8 nm), laser oscillations were supported by the lower output coupling (0.31% at 648 nm vs. 0.55% at 605 nm) and possibly by the difference in the optical power of the thermal lens.

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** 2ω-OPSL pumped Sm:LiYF₄ laser: (a,c) input-output dependences and (b,d) typical spectra of laser emission for (a,b) orange emission at 605 nm and (c,d) red emission at 648 nm, for different output coupling rates $T_{OC}$, η – slope efficiency, $P_{th}$ – laser threshold.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Colasing in the red and orange spectral ranges in the Sm:LiYF₄ laser: (a,b) typical oscilloscope traces of laser emission captured at different time scales, (c) typical spectra of laser emission. $T_{OC} = 0.55\%$, the laser polarization is $\pi$.</figcaption></figure>

Simultaneous operation in the orange and red could also be observed by translating the output coupler towards the edge of the cavity stability. Both emissions were linearly polarized ($\pi$). They were separated using a bandpass filter (Thorlabs, FL647-10) and simultaneously detected using two fast Si photodetectors (Thorlabs, DET210) and a 7 GHz digital oscilloscope (Tektronix, TDS7704B), see Fig. 7. Stable true continuous-wave emission in the orange could be observed during a few ms, separated by chaotic regions of colasing, Fig. 7(a). It is interesting to note that during colasing, pulsed laser behavior was observed being similar to Q-switching, with typical single pulse durations of 3-4 µs and mean pulse repetition rates about a few tens of kHz. The orange and red pulses were synchronized in time, but their relative intensity was not constant and changed in an irregular way. A typical laser spectrum measured during

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simultaneous red and orange emissions is shown in Fig. 7(c). Such self-pulsed laser behavior is typical for free-running visible Sm lasers [20,27].

We further tested 1.1 at.% Sm³⁺-doped crystals with different thicknesses (5 mm and 11 mm), however, the laser output was inferior to the above described results. No laser action was achieved with the 3 at.% Sm:LiYF₄ probably because of the strong effect of cross-relaxation on the upper laser level lifetime.

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** GaN diode-pumped Sm:LiYF₄ lasers: (a) input-output dependences and (b) typical spectra of laser emission obtained for different output coupling rates *T*<sub>OC</sub> with the fiber-coupled and open-space diodes, *η* – slope efficiency, *P*<sub>th</sub> – laser threshold.</figcaption></figure>

Under pumping by the fiber-coupled diode at 465 nm, the continuous-wave orange Sm-laser delivered 11.8 mW at 604.7 nm with η of only 1.2% and a high laser threshold of 2.82 W for an output coupling rate of 0.55%, Fig. 8(a,b). The laser performance notably deteriorated as compared to the 2ω-OPSL pumping mostly because of the poor matching between the pump and laser modes determined by the spatial beam quality of this diode. The use of the single emitter diode enabled us to scale the output power to 23.9 mW at 605 nm corresponding to an improved slope efficiency of 9.4% and a reduced laser threshold of 385 mW when using the 1.2% output coupler, see Fig. 8(a). For low-transmission OCs, the laser operation close to the threshold was unstable preventing a complete measurement of the power transfer curve. This explains the relatively high laser thresholds for 0.2% and 0.39% OCs indicated in Fig. 8(a). Under true CW pumping, the output power of the Sm-laser degraded with time due to excessive heating of the laser crystal. It could be recovered under quasi-CW pumping (duty cycle: 1:4) achieved with a mechanical chopper modulating the pump beam. Active cooling of the laser crystal is believed to resolve this issue. The dual-color operation was not observed under diode-pumping most probably because the laser threshold for emission in the red was not reached.

The typical spectra of laser emission are shown in Fig. 8(b). The diode-pumped Sm:LiYF₄ laser operated on the ⁴G₅/₂ → ⁶H₇/₂ transition. The laser spectra were weakly dependent on the output coupling: the emission was centered at 605.0 nm and the laser linewidth was ~0.3 nm. The blue-shift of the laser spectrum when using diode n°1 is assigned to stronger heating of the laser crystal. Sm³⁺ ion in LiYF₄ provide broadband emission in the orange extending towards the wavelengths below 600 nm, peaking at 593.6 nm and 597.0 nm (for π-polarized light). Such emission lines could be addressed by introducing a spectrally selective intracavity element, e.g., a Lyot filter or an etalon.

A photograph of the orange laser beam on a black screen in the far-field is depicted in Fig. 9(a). The beam quality factors of the laser beam were measured at the maximum output power following the ISO 11146 standard method, Fig. 9(b), yielding M²ₓ = 1.46 and M²ᵧ = 1.53±0.05 indicating operation on the fundamental transverse mode with a small admixture of higher-order modes. The output of the orange Sm:LiYF₄ laser was linearly polarized (π) due to the intrinsic anisotropy of the gain. The polarization state was characterized using a Glan-Taylor polarizer placed after the output coupler, Fig. 9(c). The formula P(θ) = P<sub>u</sub>/2 + P<sub>p</sub>cos²(θ - π/2) where θ is the angle between the polarizer (P) axis and the x-axis and P<sub>u</sub> and P<sub>p</sub> correspond to unpolarized and polarized power fractions, respectively, provided a good fit for
313 the experimental data. The polarization degree defined as $X = P_p/(P_p + P_u)$ was close to unity.

314 A very similar behavior was observed while pumping with the open-space laser diode.

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Characteristics of the orange laser beam under 2ω-OPSL pumping: (a) a photograph of the laser beam in the far-field; (b) evaluation of the beam quality factors $M^2_{xy}$; and (c) analysis of the polarization state of laser emission, $T_{OC} = 0.55\%$, $P_{abs} = 600$ mW.</figcaption></figure>

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The output power and efficiency of visible Sm:LiYF₄ lasers could be improved in several ways. First, the overlap of the pump and laser modes in the crystal seems to be among the most crucial parameters. Second, the concentration of Sm³⁺ ions could be further optimized to find a compromise between efficient pump absorption and detrimental self-quenching of the upper laser level lifetime. As the pump absorption efficiency remained low in the present study, one could improve the output power using longer crystals. Third, the crystal quality has an impact on laser efficiency. Finally, the noticeable low slope efficiency of this laser can be partially explained by the presence of excited-state absorption (ESA) at the laser wavelength originating from the upper laser level ⁴G₅/₂ due to the complex energy level scheme of the Sm³⁺ ion, especially at high energies.

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## 4. Conclusion

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To conclude, we demonstrate the potential of blue 465 nm GaN laser diodes for direct pumping of Sm³⁺-doped crystals for generation of visible (orange) emission. Using a Sm:LiYF₄ crystal, a continuous-wave output power of 23.9 mW is extracted at 605 nm corresponding to the ⁴G₅/₂ → ⁶H₇/₂ transition with a reasonably high slope efficiency of 9.4%, a laser threshold of 385 mW and a linear polarization (π) determined by the gain anisotropy of this uniaxial material. We show that the pump beam quality (brightness) and the resulting overlap of the pump and laser modes is a key parameter for optimizing the output performance of this laser. Additionally, we expect that the laser efficiency could be further improved via careful selection of the Sm³⁺ doping level to balance the pump absorption efficiency and the detrimental self-quenching of the upper laser level lifetime. Given the positive thermal lens in *a*-cut LiYF₄ crystals, compact (microchip-type) diode-pumped Sm:LiYF₄ lasers with reduced intracavity losses via bonding the cavity components are envisioned.

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**Funding.** Agence Nationale de la Recherche (ANR), France (ANR-22-CE08-0025-01, NOVELA; ANR-23-LCV2-0001, LOVINOV); Région Normandie, France (Contrat de plan État-Région (CPER)).

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**Disclosures.** The authors declare no conflicts of interest.

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**Data availability.** Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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    <td>33. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
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    <td>384</td>
    <td>34. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
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    <td>385</td>
    <td>35. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
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    <td>386</td>
    <td>36. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
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    <td>387</td>
    <td>37. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>388</td>
    <td>38. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>389</td>
    <td>39. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>390</td>
    <td>40. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>391</td>
    <td>41. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>392</td>
    <td>42. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>393</td>
    <td>43. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>394</td>
    <td>44. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>395</td>
    <td>45. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>396</td>
    <td>46. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
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    <td>47. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>398</td>
    <td>48. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>399</td>
    <td>49. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>400</td>
    <td>50. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>401</td>
    <td>51. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>402</td>
    <td>52. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>403</td>
    <td>53. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>404</td>
    <td>54. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>405</td>
    <td>55. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>406</td>
    <td>56. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>407</td>
    <td>57. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
  <tr>
    <td>408</td>
    <td>58. A. Baillard, P. Loiko, D. Rytz, S. Schwung, M. Fromager, A. Braud, and P. Camy, "Red Sm:KGd(WO<sub>4</sub>)<sub>2</sub> laser at 649 nm," Opt. Lett., OL 48(18), 4721–4724 (2023).</td>
  </tr>
</table>