# Cr:ZnSe as a luminescent concentrator

Lisa Lopez, Hussein Taleb, Maxime Nourry-Martin, Frédéric Druon, Sophie Coumar, Clément Oriol, Patrick Georges, François Balembois

<figure><img src="image_1.png" /></figure>

To cite this version:

Lisa Lopez, Hussein Taleb, Maxime Nourry-Martin, Frédéric Druon, Sophie Coumar, et al. Cr:ZnSe as a luminescent concentrator. Optics Express, 2025, 33 (19), pp.41078. 10.1364/OE.573379. hal-05269530

HAL Id: hal-05269530

https://iogs.hal.science/hal-05269530v1

Submitted on 19 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

# Cr:ZnSe as a luminescent concentrator

LISA LOPEZ, HUSSEIN TALEB, MAXIME NOURRY-MARTIN, FRÉDÉRIC DRUON, SOPHIE COUMAR, CLÉMENT ORIOL, PATRICK GEORGES, AND FRANÇOIS BALEMBOIS*

Université Paris-Saclay, Institut d'Optique Graduate School, CNRS, Laboratoire Charles Fabry, 91127, Palaiseau, France

*francois.balembois@institutoptique.fr

**Abstract:** We demonstrate the first Cr:ZnSe luminescent concentrator. Enhanced by amplified spontaneous emission, it has the best efficiency ever reported for luminescent concentrators emitting in the SWIR band. It emits an output power of 1.6 W and a brightness of 5.4 W/cm²/sr over a broadband spectrum between 2.1 µm and 2.6 µm. It has a remarkably simple setup using a fibre-coupled laser diode at 1530 nm as a pump source.

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.

## 1. Introduction

Chromium-doped zinc selenide (Cr:ZnSe) is a well-known crystal, first reported in 1996 [1] as a tunable laser material in the shortwave infrared (SWIR), between 2 µm and 3 µm. Since then, many laser developments have been carried out leading to Cr:ZnSe single frequency oscillators, gain-switched oscillators, femtosecond oscillators and high gain amplifiers [2]. Moskalev et al. [3] even demonstrated a 140 W Cr:ZnSe laser system with remarkable optical-to-optical efficiency, confirming its potential for industrial and scientific applications. Cr:ZnSe is also used as a saturable absorber for Ho-doped, Tm-doped [4] and Er-doped lasers [5].

Cr:ZnSe is pumped by various lasers according to its broadband absorption located between 1.5 µm and 2 µm: laser diodes at 1.5 µm or 1.7 µm, Er-doped fibre lasers, Tm-doped fibre lasers, Tm:YAG lasers and Raman lasers [2].

Among the materials emitting in the SWIR (1-3 µm), Cr:ZnSe certainly has the widest emission bandwidth (860 nm full width at half maximum, FWHM [2]) with the highest quantum efficiency, close to 1 for low-doped samples [6]. Cr:ZnSe covers strong fundamental absorption bands - “molecular fingerprints” - located in the mid-IR spectral region (e.g. CH₄: ~3.3 µm; H₂S: ~2.7 µm; and NH₃: 2.3 µm) and can be used as a coherent laser source in many applications: remote sensing for environmental monitoring, medical diagnostics, surgical procedures, material processing of polymers and defence related applications. The Cr:ZnSe spectrum can be further broadened by supercontinuum generation [7].

In terms of its spectroscopic properties, Cr:ZnSe is also an excellent candidate for an ultra-broadband incoherent source in the SWIR. Such a source provides speckle-free lighting, which could be useful for imaging applications such as sorting plastics or quality control. To our knowledge, the only demonstration of a partially incoherent Cr:ZnSe source has been achieved with a Cr:ZnSe powder used as a random laser [8]. Another option to generate a broadband spectrum is to collect its spontaneous emission. To date, Cr:ZnSe samples have never been used in this configuration.

There are several architectures for spontaneous emission. The first is related to amplified spontaneous emission in fibres, which has been well demonstrated in Er-doped fibres [9] and Tm-doped fibres [10]. Although ZnSe can be deposited by high pressure chemical vapour
deposition (HPCVD) into a silica fibre template [11], this technique has not been demonstrated for Cr:ZnSe.

The second is related to “laser phosphors”, i.e. luminescent materials pumped by lasers. It is well developed in the visible for Ce-doped phosphors pumped by blue laser diodes [12]. In this case, the emitting zone is a “source point” made very small by focusing the pump diode on a surface covered with phosphor powders or short highly doped crystals. In the SWIR, laser-pumped phosphors are 2 to 3 orders of magnitude less powerful than in the visible [13]. This is related to the high pump power density, which imposes very significant thermal effects on the luminescent material and strongly reduces the emission efficiency. Considering the low absorption of Cr:ZnSe samples, it seems difficult to use this architecture for efficient spontaneous emission.

The third architecture is related to luminescent concentrators. This configuration uses an optically pumped parallelepiped with all its faces polished to trap the spontaneous emission and direct it towards the output face by total internal reflections. This geometry was inspired by luminescent solar concentrators [14,15]. It has also been studied to generate new light sources in the visible with Ce:YAG luminescent concentrators [16–18] and more recently in the SWIR with Er:Yb:glass [19], Er:Cr:YSGG [20] and CTH:YAG [21]. Luminescent concentrators are characterised by a large aspect ratio, with the output on the smallest area. This results in emitters with a much larger volume than laser phosphors. This architecture is suitable for low absorption materials such as Cr:ZnSe and can significantly reduce the pump power density compared to laser phosphors.

This paper is the first to report using a Cr:ZnSe as a luminescent concentrator. It discusses how its performance compares to other luminescent concentrators that emit in the SWIR.

## 2. Experimental setup

The experimental setup is shown in Fig. 1. The Cr:ZnSe luminescent concentrator is a rectangular parallelepiped with dimensions of 2.4 x 2.4 x 7 mm³. The Cr:ZnSe is cut and polished on all sides in our laboratory from a raw piece purchased from IPG Photonics. Its Cr concentration is $10^{19} \text{ cm}^{-3}$. We selected a source available in our laboratory to pump our Cr:ZnSe crystal. Although the pump wavelength is not optimal for the absorption spectrum of the Cr:ZnSe, we chose a 1530 nm fibre-coupled laser diode (QPC BrightLock laser). The fibre has a core diameter of 400 µm and a numerical aperture of 0.22. To avoid any thermal problems, the laser diode operates in quasi-continuous mode with a low duty cycle: 1 ms, 1 Hz and with a maximum peak power of 32 W. The crystal is longitudinally pumped in a close-coupled configuration, i.e. without any optics, resulting in a small air gap between the crystal surface and the fibre output port. We have measured the absorption of our Cr:ZnSe crystal at 1530 nm and found an absorption coefficient of $\alpha_p = 2.4 \text{ cm}^{-1}$.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Experimental setup of the Cr:ZnSe luminescent concentrator (CPC: compound parabolic concentrator).</figcaption></figure>

As the refractive index of Cr:ZnSe is high (2.44 at 2450 nm [22]), a large fraction of the spontaneous emission is confined within the crystal by total internal reflection. The escape cone of the interface air-Cr:ZnSe, corresponding to the rays that can pass through the interface has an
apex angle of $\theta = 24.2^\circ$. This means that only 4.4% of the rays emitted can be collected in air. In order to increase this ratio, a compound parabolic concentrator (CPC#17-709 from Edmund Optics) is bonded to the exit face using a UV-cured adhesive (Norland NOA 170, refractive index of 1.7), following a setup described in [23]. The CPC has a refractive index of $n = 1.52$ at 656 nm (glass Schott B270). We have no data on its refractive index in the SWIR, as it is designed for the visible. Assuming a value of $n = 1.5$ at 2450 nm, the equivalent output cone of the Cr:ZnSe has an apex angle of $\theta' = 37.9^\circ$ with almost 10.5% of the rays emitted. This is more than twice the amount emitted without a CPC.

### 3. Results

The first experiment concerns the emission spectrum. It is recorded with a Czerny–Turner monochromator using a 300 lines/mm grating and a photodiode (PDA 20H-EC from Thorlabs) positioned at the monochromator output. The spectral response of our monochromator is supposed to be flat over the spectral band 1.5-3.3 µm. The measured spectra are corrected from the spectral response of the photodiode. Figure 2 shows the emission spectrum of the Cr:ZnSe luminescent concentrator compared to the spontaneous emission of Cr:ZnSe deduced from [2]. Its spectral narrowing can be attributed to different effects. In the short wavelengths (typically below 2.2 µm), the emission and absorption bands overlap [1,2]. This effect reduces the spectral band. From the curve shapes, we estimate that the reabsorption process reduces the output power by 10%. We attribute to this process a global transmission denoted $T_{reabs} = 90\%$. In the long wavelengths, the CPC transmission strongly reduces the bandwidth. This effect can be seen with the comparison of the output spectrum with and without CPC. It is worth to note that we have reduced the length of the CPC from 10.5 mm to 8.3 mm in order to slightly reduce this absorption effect at long wavelengths. We estimate the spectral transmission of the CPC at 60%, averaged on the spectrum emitted by the luminescent concentrator. Finally, since the spectrometer is not purged from air, strong water absorption between 2.4 µm and 2.8 µm modifies the output spectrum.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Emission spectrum of the Cr:ZnSe luminescent concentrator with CPC (solid orange line) and without CPC (blue line) compared to the spontaneous emission of Cr:ZnSe deduced from the emission cross section given in [2] (dot line). Dark line: spectral transmission curve of the CPC (without Fresnel losses on the interfaces of the CPC) deduced from the data of Schott B270 glass and assuming an average propagation distance in the CPC of 10 mm.</figcaption></figure>

Figure 3(a) shows the pump and the signal measured by photodiodes (InGaAs Thorlabs DET05D2, risetime 17 ns). The fluorescence lifetime of Cr:ZnSe (5.5 µs) [2,6] slightly affects the risetime and the decay time of the fluorescence signal compared to the pump signal. Next, we measure the output energy with a joulemeter (ref. ES120C Thorlabs) positioned very close to the CPC (in order to avoid water absorption). We deduce the peak power from the pulse duration (1
ms for the pump and for the signal). Figure 3(b) presents the output power (peak values) versus the incident pump power (peak values). It shows a linear increase with a maximum output power at 1.6 W for an incident pump power of 32 W corresponding to an absorbed pump power of 23.6 W. This corresponds to an efficiency of 5% in incident pump power and 6.8% in absorbed pump power.

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** a Pump (dark line) and signal (orange line) versus time. Fig. 3b. Peak power measured at the output of the Cr:ZnSe versus the incident pump power.</figcaption></figure>

The radiation pattern, i.e. the intensity in a given direction, is recorded by a photodiode (PDA 20H-EC Thorlabs) mounted on a rotating arm (Fig. 4). When the fibre is close coupled to the Cr:ZnSe ($d=0$ mm), the full width at half maximum (FWHM) is 80°, slightly lower than the acceptance angle of the CPC (90° FWHM). This corresponds to a brightness of 5.4 W/cm²/sr.

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Radiation pattern of the Cr:ZnSe luminescent concentrator for different distances $d$ between the output of the pump fibre and the input face of the Cr:ZnSe. Dot curve is for $d=0$ mm. Plain curve is for $d=2.5$ mm.</figcaption></figure>

Table 1 compares the performance of this Cr:ZnSe source with other luminescent concentrators emitting in the SWIR. Cr:ZnSe offers similar output power and brightness. However, the remarkable parameter is here the optical efficiency, significantly higher than the other sources. The purpose of the next part is to understand the origin of such an efficiency.

## 4. Result analysis

First, we calculate the output power of the Cr:ZnSe concentrator as a function of the incident pump power, $P_p$. This output power is a combination of spontaneous emission directed towards the output, and emission directed backwards and reflected by the input surface ($z=0$) towards the output. Reflectivity depends strongly on the incidence angle. For an incidence angle below 24.2° (corresponding to the apex angle of the escape cone for the Cr:ZnSe-air interface), the reflectivity of the interface is given by Fresnel reflections; its value is $R = 21.7\%$ when averaged over all directions included in the escape cone. For higher incidence angles, total internal reflection
occurs. To account for this, we separate the output power into two parts: $P_{air}$, which is related to rays in the Cr:ZnSe-air escape cone, and $P_c$, which is related to rays in the corona between the Cr:ZnSe-air escape cone and the Cr:ZnSe-CPC escape cone (see Fig. 5).

<table><caption>**Table 1.** Comparison of the performance of different luminescent concentrators emitting in the SWIR. The optical efficiency is given in incident pump power.</caption><thead><tr><th>Luminescent<br>concentrator</th><th>Operating<br>regime</th><th>Wavelength range<br>(μm)</th><th>Output<br>(mW)</th><th>Brightness (W/cm²/sr)</th><th>Optical efficiency<br>(%)</th></tr></thead><tbody><tr><td>Er:Yb:glass [19]</td><td>cw</td><td>1.5 - 1.6</td><td>850</td><td>5.4</td><td>1.29</td></tr><tr><td>Er:Cr:YSGG [20]</td><td>Quasi cw<br>1.5 ms, 10 Hz</td><td>1.5 - 1.7</td><td>350</td><td>1.8</td><td>0.11</td></tr><tr><td>Cr:Tm:Ho:YAG [21]</td><td>Quasi cw<br>3.5 ms, 10 Hz</td><td>1.8 - 2.1</td><td>995</td><td>21.2</td><td>0.17</td></tr><tr><td>Cr:ZnSe<br>[this work]</td><td>Quasi cw<br>1 ms, 1 Hz</td><td>2.1 - 2.6</td><td>1600</td><td>5.4</td><td>5</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Representation of the escape cones for different interfaces: output interface (Cr:ZnSe-CPC) and input interface (air-Cr:ZnSe).</figcaption></figure>

$P_{air}$ is given by (1):

$$P_{air} = T_{reabs} T_{g} T_{cpc} \frac{1 - \cos\theta_{max}}{2} \eta_{r} \frac{\lambda_{p}}{\lambda_{em}} T_{p} P_{p} \frac{\alpha_{p}}{\alpha_{p} - \alpha} [e^{-\alpha l} - e^{-\alpha_{p} l}][1 + Re^{-\alpha l}] \quad (1)$$

where the different quantities are defined in Table 2.

The output power related to the rays in the corona is calculated as in [24], taking into account multiple reflections in the Cr:ZnSe between the input face (where total internal reflection occurs) and the output face (where the average reflectivity is 10% for the rays included in the corona). $P_c$ is given by (2):

$$P_c = T_{reabs} T_{cpc} \frac{\cos\theta_{max} - \cos\theta'_{max}}{2} \eta_r \frac{\lambda_p}{\lambda_{em}} T_p P_p \frac{\alpha_p}{\alpha_p - \kappa\alpha} [e^{-k\alpha l} - e^{-l\alpha_p l}] (1 + e^{-k\alpha l}) \frac{T_{gc}}{1 - e^{-2\alpha k l}(1 - T_{gc})^2} \quad (2)$$

where the different quantities are defined in Table 2.

At maximum pump power, we calculate:

$$P_{air} = 0.36 \text{ W}; P_{c} = 0.76 \text{ W and finally, } P_{out} = P_{air} + P_{c} = 1.12 \text{ W}$$

Clearly, the theoretical power is far lower than the experimental power (1.6 W). This underestimation could be due to additional power being collected in the output beam through the amplification of spontaneous emission (ASE). ASE can be difficult to calculate, particularly for bulk materials. This was recently demonstrated for Cr:ZnSe [25]. However, we can gain an initial understanding of the influence of ASE by calculating the small signal gain $G_0$ on the axis of the concentrator. $G_0$ can be expressed by (3):

$$G_0 = \exp \int_0^l \sigma n_2(z) dz \quad (3)$$

where $\sigma$ is the emission cross section (1.3 10⁻¹⁸ cm²), and $n_2(z)$ is the upper state population density at the point $z$.
<table><caption>Table 2. List of the quantities used for the estimation of the output power</caption><thead><tr><td>Parameter description</td><td>Value</td><td>Comments</td></tr></thead><tbody><tr><td>T<sub>reabs</sub>, overall transmission Cr:ZnSe after reabsorption losses</td><td>T<sub>reabs</sub> = 90%</td><td>Estimated from part.3</td></tr><tr><td>T<sub>g</sub>, transmission of the interfaces Cr:ZnSe-glue-CPC for rays in the Cr:ZnSe-air escape cone</td><td>T<sub>g</sub> = 97.2%</td><td></td></tr><tr><td>T<sub>gc</sub>, transmission of the interfaces Cr:ZnSe-glue-CPC averaged for rays in the corona</td><td>T<sub>gc</sub> = 90%</td><td></td></tr><tr><td>T<sub>cpc</sub>, transmission of the CPC taking into account its spectral transmission and the last interface CPC-air</td><td>T<sub>cpc</sub> = 57%</td><td>The spectral transmission is evaluated in part.3 (60%)</td></tr><tr><td>θ<sub>max</sub>, apex angle of the escape cone Cr:ZnSe-air</td><td>θ<sub>max</sub> = 24.2°</td><td>Refractive index of Cr:ZnSe n = 2.44</td></tr><tr><td>θ'<sub>max</sub>, apex angle of the escape cone Cr:ZnSe-CPC</td><td>θ'<sub>max</sub> = 37.9°</td><td>All the rays collected by the CPC are supposed to be transmitted through the last interface CPC-air.</td></tr><tr><td>η<sub>r</sub>, quantum efficiency of Cr:ZnSe</td><td>η<sub>r</sub> = 1</td><td></td></tr><tr><td>λ<sub>p</sub>, pump wavelength</td><td>λ<sub>p</sub> = 1530 nm</td><td></td></tr><tr><td>λ<sub>em</sub>, emitted wavelength</td><td>λ<sub>em</sub> = 2360 nm</td><td>Mean wavelength of the emitted spectrum</td></tr><tr><td>T<sub>p</sub>, transmission of the pump beam through the interface Cr:ZnSe-air</td><td>T<sub>p</sub> = 82.5%</td><td>Fresnel transmission at normal incidence for an interface n = 1 and n = 2.44. This transmission is valid here, since the maximum incidence angle is only 12.7° (NA = 0.22)</td></tr><tr><td>P<sub>p</sub>, peak pump power incident on the Cr:ZnSe</td><td>P<sub>p</sub> = 32 W</td><td>Maximum peak pump power</td></tr><tr><td>α<sub>p</sub>, pump absorption coefficient</td><td>α<sub>p</sub> = 2.4 cm<sup>-1</sup></td><td>Measured</td></tr><tr><td>l, length of the concentrator</td><td>l = 0.7 cm</td><td></td></tr><tr><td>α, loss coefficient of Cr:ZnSe at the signal wavelength</td><td>α = 5 10<sup>-2</sup> cm<sup>-1</sup></td><td>Estimated from typical data sheet of Cr:ZnSe crystals. This value does not include reabsorption losses.</td></tr><tr><td>k, coefficient to estimate the average propagation distance in the corona</td><td>k = 1.16</td><td>k = <sup>1</sup>&frasl;<sub>cos(θ<sub>max</sub>+θ'<sub>max</sub>)</sub></td></tr><tr><td>R, reflection coefficient of the Cr:ZnSe-air interface</td><td>R = 21.9%</td><td>Averaged on all the incidence angles in the escape cone Cr:ZnSe-air</td></tr></tbody></table>

At the stationary state, $n_2(z)$ can be written by (4):

$$n_2(z) = \tau \frac{\lambda_p \alpha_p T_p P_p e^{-\alpha_p z}}{hc \pi r(z)^2} \quad (4)$$

where $\tau$ is the fluorescent lifetime (5.5 µs [2,6]), and $r(z)$ is the pump radius at the point z in the Cr:ZnSe, $r(z) = r_0 + z.tan(\theta_p)$ with $r_0$ the beam radius at the input of the crystal ($r_0 = 200$ µm) and $\theta_p$ the divergence of the pump beam in the Cr:ZnSe ($\theta_p = 5.2°$) taking the numerical aperture of the pump beam in air (NA = 0.22).

At maximum pump power (32 W), we find a value $G_0 = 1.4$ showing that ASE could play a major role in the output power.

The next step is to detect the influence of the ASE. As a first step, we monitor the spectral emission of the concentrator versus the pump power. We don't see any modifications of the spectral shape as the pump power increases. This could be attributed to the very broad emission
band of Cr:ZnSe. Next, we monitor the effect of the pump power density (and therefore the small signal gain $G_0$) on the output power. To this end, we vary the distance $d$ between the output of the pump fibre and the input face of the Cr:ZnSe (see Fig. 6(a)). The maximum value of $d$ is 4.4 mm before clipping the pump-beam by the Cr:ZnSe aperture. Figure 6(b) shows that the output power decreases with $d$, while the pump power launched into the Cr:ZnSe remains constant. When $d = 4.4$ mm, we calculate that the small signal gain on the axis is reduced to $G_0 = 1.04$, which leads to a smaller contribution from the ASE. Therefore, this decrease could be seen as a consequence of the ASE's contribution to the output power. It should be noted that even at $d = 4.4$ mm, the output power is higher than the theoretical one. This suggests that ASE still have an effect.

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** a Position of the pump fibre output relative to the input face of the Cr:ZnSe. Fig. 6b. Evolution of the output power (normalized to the maximum output power) versus the distance $d$ between the output fibre and the input face of the Cr:ZnSe.</figcaption></figure>

ASE on the axis could also affect the divergence angle of the output beam. The increase in divergence of the output beam at $d = 2.5$ mm (88° FWHM) compared to at $d = 0$ mm (80° FWHM) could be seen as additional proof of the presence of ASE in the Cr:ZnSe luminescent concentrator. It is worth noting that the central peak observed in the radiation pattern at $d = 0$ mm occurs for all pump powers. Therefore, it is not a signature of ASE. We believe it is an effect of the CPC, analogous to spherical aberrations. The slight hole observed in the radiation pattern at $d = 2.5$ mm confirms this hypothesis.

## 5. Conclusion

As shown in Table 1, Cr:ZnSe provides an interesting opportunity to expand the coverage of the SWIR band, offering output power and brightness comparable to existing SWIR luminescent concentrators based on rare earth elements (e.g. Er, Tm, Ho). Notably, its optical efficiency is significantly higher. This can be explained by several factors: unlike the rare earths, the energy diagram of Cr$^{2+}$ in ZnSe is much simpler, with only two bands ($^5$T$_2$ and $^5$E). This limits the parasitic effects that can reduce the population of the upper state level. Consequently, Cr:ZnSe has a much higher quantum efficiency that is independent of pump power density. The second factor relates to reabsorption losses. The lower levels of the SWIR transitions in Er, Tm and Ho are close to the fundamental level, which leads to strong reabsorption losses. In the case of Cr:ZnSe, reabsorption also occurs, but only for shorter wavelengths. This results in losses of only 10% for our sample. The third factor is amplified spontaneous emission. This effect increases the Cr:ZnSe output power by 43%, according to our calculations. This effect has not been observed in rare earth concentrators. This is because the emission cross section is 100 times higher in Cr:ZnSe than for rare earth ions. Therefore, a given gain value would require an excited state population density that is 100 times higher in rare earths. However, many effects occur in rare earth dopants that limit the upper state level population, such as up-conversion, excited state
absorption and cross relaxation. Consequently, ASE is much more probable in Cr:ZnSe than in rare-earth-doped materials.

Stimulated emission in luminescent concentrators is a relatively new subject. Theoretical studies have recently been carried out on solar luminescent concentrators [26–28] with the aim of improving their performance. These studies propose using an additional laser source to trigger emission from the luminescent concentrator towards the photovoltaic cells. To the best of our knowledge, amplified spontaneous emission has never been reported experimentally in a luminescent concentrator. Our work reports it for the first time. It confirms experimentally that ASE can enhance luminescent concentrator performance.

Nevertheless, our Cr:ZnSe luminescent concentrator is far from optimal. Reducing its section could enhance the output power density and, consequently, the output brightness. An index matching between the pump fibre output and the Cr:ZnSe input could increase the pump transmission. Efficiency and brightness could also be enhanced by higher brightness laser diode at 1530 nm thanks to ASE. Additionally, using a more transparent CPC with glass optimised for the SWIR band could significantly improve the output power. Concerning the operating regime, pure continuous operation is preferred for lighting in quality control or plastic sorting. This would require efficient crystal cooling for thermal management, which appears feasible given the pump power (a few tens of watts) and the good thermal conductivity of Cr:ZnSe (14 W/m/K).

In conclusion, we present the first Cr:ZnSe luminescent concentrator. With an output power of 1.6 W and a brightness of 5.4 W/cm²/sr, it emits over a broadband spectrum between 2.1 µm and 2.6 µm. To our best knowledge, it is also the first luminescent concentrator pumped by laser diode. This pumping method favors amplified spontaneous emission, leading to the highest efficiency ever recorded for luminescent concentrators operating in the SWIR band. The system architecture is remarkably simple. In terms of power and brightness, this source lies between high-power, low-brightness halogen sources and high-brightness, fibre-based ASE sources. It could pave the way for a new class of sources particularly well-suited to industrial control applications, such as product quality monitoring and plastic sorting.

**Funding.** Agence Nationale de la Recherche (project NewLight ANR-21-CE08-0044); Agence de l'innovation de Défense (PhD funding Lisa Lopez).

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

**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.

**References**

1. L. D. DeLoach, R. H. Page, G. D. Wilke, et al., "Transition metal-doped zinc chalcogenides: spectroscopy and laser demonstration of a new class of gain media," *IEEE J. Quantum Electron.* **32**(6), 885-895 (1996).

2. S. B. Mirov, V. V. Fedorov, D. Martyshkin, et al., "Progress in Mid-IR Lasers Based on Cr and Fe-Doped II-VI Chalcogenides," *IEEE J. Select. Topics Quantum Electron.* **21**(1), 292-310 (2015).

3. I. Moskalev, S. Mirov, M. Mirov, et al., "140 W Cr:ZnSe laser system," *Opt. Express* **24**(18), 21090 (2016).

4. T.-Y. Tsai and M. Birnbaum, "Q-switched 2-µm lasers by use of a Cr2+:ZnSe saturable absorber," *Appl. Opt.* **40**(36), 6633 (2002).

5. A. Aubourg, J. Didierjean, N. Aubry, et al., "Passively Q-switched diode-pumped Er:YAG solid-state laser," *Opt. Lett.* **38**(6), 938 (2013).

6. A. Sennaroglu, U. Demirbas, A. Kurt, et al., "Concentration dependence of fluorescence and lasing efficiency in Cr2+:ZnSe lasers," *Opt. Mater.* **29**(6), 703-708 (2007).

7. S. Vasilyev, J. Gu, M. Mirov, et al., "Low-threshold supercontinuum generation in polycrystalline media," *J. Opt. Soc. Am. B* **38**(5), 1625 (2021).

8. C. Kim, D. V. Martyshkin, V. V. Fedorov, et al., "Mid-infrared Cr2+:ZnSe random powder lasers," *Opt. Express* **16**(7), 4952 (2008).

9. J. H. Lee, U.-C. Ryu, and N. Park, "Passive erbium-doped fiber seed photon generator for high-power Er3+-doped fiber fluorescent sources with an 80-nm bandwidth," *Opt. Lett.* **24**(5), 279 (1999).

10. Z. Y. Hu, P. Yan, Q. Liu, et al., "High-power single-stage thulium-doped superfluorescent fiber source," *Appl. Phys. B* **118**(1), 101-107 (2015).

11. M. G. Coco, S. C. Aro, S. A. McDaniel, et al., "Continuous wave Fe2+:ZnSe mid-IR optical fiber lasers," *Opt. Express* **28**(20), 30263 (2020).
12. N. Rondelez, A. Correia, W. Ryckaert, et al., "Efficient transmissive remote phosphor configuration for a laser-driven high-luminance white light source," *Opt. Express* **30**(4), 5107 (2022).

13. M. C. Tan, J. Connolly, and R. E. Riman, "Optical Efficiency of Short Wave Infrared Emitting Phosphors," *J. Phys. Chem. C* **115**(36), 17952-17957 (2011).

14. W. H. Weber and J. Lambe, "Luminescent greenhouse collector for solar radiation," *Appl. Opt.* **15**(10), 2299-2300 (1976).

15. M. G. Debije and P. P. C. Verbunt, "Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment," *Adv. Energy Mater.* **2**(1), 12-35 (2012).

16. S. Roelandt, Y. Meuret, D. K. G. de Boer, et al., "Incoupling and outcoupling of light from a luminescent rod using a compound parabolic concentrator," *Opt. Eng.* **54**(5), 055101 (2015).

17. A. Barbet, A. Paul, T. Gallinelli, et al., "Light-emitting diode pumped luminescent concentrators: a new opportunity for low-cost solid-state lasers," *Optica* **3**(5), 465 (2016).

18. J. Sathian, J. D. Breeze, and B. Richards, "Solid-state source of intense yellow light based on a Ce:YAG luminescent concentrator," *Opt. Express* **25**(12), 13714 (2017).

19. P. Pichon, J.-P. Blanchot, F. Balembois, et al., "New LED-based high-brightness incoherent light source in the SWIR," *Opt. Express* **26**(7), 9353 (2018).

20. L. Lopez, F. Druon, P. Georges, et al., "LED-pumped Er:YSGG light sources," *Opt. Express* **31**(17), 27604 (2023).

21. L. Lopez, P. Pichon, F. Druon, et al., "CTH:YAG: from laser medium to luminescent concentrator," *Opt. Express* **32**(8), 14321 (2024).

22. A. Sennaroglu and Y. Morova, "Divalent (Cr2+), trivalent (Cr3+), and tetravalent (Cr4+) chromium ion-doped tunable solid-state lasers operating in the near and mid-infrared spectral regions," *Appl. Phys. B* **128**(1), 9 (2022).

23. D. K. G. de Boer and L. Haenen, "Extraction optics for high lumen density sources," *J. Eur. Opt. Soc.-Rapid Publ.* **15**(1), 8 (2019).

24. P. Pichon, F. Balembois, F. Druon, et al., "3D luminescent concentrators," *Opt. Express* **29**(5), 6915 (2021).

25. J. Nilsson, "Modeling and simulation of amplified spontaneous emission in single-mode-pumped Cr2+:ZnSe bulk amplifiers with beam propagation and equivalent input-noise seeding," *Appl. Phys. B* **131**(3), 63 (2025).

26. M. R. Kaysir, S. Fleming, R. W. MacQueen, et al., "Luminescent solar concentrators utilizing stimulated emission," *Opt. Express* **24**(6), A497 (2016).

27. A. Flood and N. P. Kherani, "Influence of luminescent material properties on stimulated emission luminescent solar concentrators (SELSCs) using a 4-level system," *Opt. Express* **25**(24), A1023 (2017).

28. A. G. Flood and N. P. Kherani, "Large area stimulated emission luminescent solar concentrators modelled using detailed balance consistent rate equations," *Opt. Express* **30**(11), 18978 (2022).