In-depth study of spectroscopic properties of new $Pr^{3+}$-ion doped low-phonon sesquisulfide $Lu_2S_3$ material for mid-IR laser sources

Technical Summary: Spectroscopic Properties of Pr³⁺-Doped Lu₂S₃ for Mid-IR Lasers

Problem Statement
The development of robust, solid-state mid-infrared (mid-IR) laser sources is hindered by the scarcity of suitable host materials. Efficient mid-IR emission requires host matrices with low maximum phonon energies (typically ≤300 cm⁻¹) to minimize non-radiative multi-phonon relaxation. While materials like LaCl₃ offer low phonon energies, they suffer from poor mechanical properties and hygroscopicity. Other candidates, such as alkali-lead halides (MPb₂X₅) and chalcogenide crystals (e.g., ZnS, ZnSe), face limitations including difficulty in achieving high rare-earth (RE) doping concentrations, hygroscopicity, or challenges in bulk crystal growth. Specifically, ternary alkali RE sulfides (ALnS₂) show promise but have historically been limited to thin hexagonal plates (~40 μm), restricting their utility as gain media. There is a need for a moisture-resistant, low-phonon host that supports high doping concentrations and can be grown in bulk form.

Methodology
The study investigates Pr³⁺-doped Lu₂S₃, a binary sesquisulfide, as a potential low-phonon laser host.

• Crystal Growth: Pr:Lu₂S₃ single crystals were grown using a modified micro-pulling-down technique under an inert argon atmosphere. Two doping levels were prepared: 0.05 at.% and 0.5 at.% Pr/Lu (nominal melt concentration). Due to the high melting point (~1750°C), a graphite crucible was used, which introduced some graphite inclusions into the samples.
• Spectroscopic Characterization:
  • Raman Spectroscopy: Performed to determine the maximum phonon energy of the Lu₂S₃ matrix.
  • Excitation Spectra: Measured using a tunable laser (Ekspla NT252) to identify absorption maxima, as direct absorption measurements were distorted by sample inclusions. Excitation covered ranges from 440 nm to 2280 nm.
  • Luminescence Spectra: Emission was recorded from 460 nm to 5.4 μm using various detectors (Si, PbS, PbSe) and spectrometers (StelarNet BlueWave, DwarfStar, ORIEL monochromator). Selective excitation was applied to specific Pr³⁺ energy levels (³P₂, ¹D₂, ¹G₄, ³F₄).
• Theoretical Analysis: The 4f wavefunctions were optimized using the RELIC 1.0 software package. Experimental barycenter energies derived from excitation spectra were fitted to an optimized Hamiltonian matrix to determine electrostatic (F², F⁴, F⁶) and spin-orbit (ζ) interaction parameters. These parameters were used to calculate squared reduced-matrix elements for tensor operators U(k) and L + gS to assist in assigning spectral transitions.

Key Results

• Phonon Energy: Raman spectroscopy identified a maximum phonon energy of approximately 312 cm⁻¹ for the Lu₂S₃ matrix. This value is comparable to established mid-IR hosts like ZnSe (250 cm⁻¹) and PbGa₂S₄ (350 cm⁻¹), confirming its suitability as a low-phonon material.
• Spectral Coverage: The study successfully identified 26 luminescence transitions spanning a wavelength range of ~0.49 μm to 5.5 μm.
• Excitation and Emission:
  • Visible/Near-IR: Sharp absorption lines corresponding to ³H₄ → ³P₀,₁,₂ and ³H₄ → ¹D₂ transitions were identified at 456, 498, and 595 nm.
  • Mid-IR Absorption: Absorption maxima suitable for fluorescence excitation were found at 1022 nm (³H₄ → ¹G₄), 1556/1638 nm (³H₄ → ³F₃,₄), and 2057 nm (³H₄ → ³F₂).
  • Emission: Upon selective excitation, emission was observed across the visible and mid-IR spectrum. Notably, excitation of the ³F₄ level yielded emission up to 5.4 μm. The authors note that transitions potentially extending to ~7 μm (e.g., ³F₃ → ³F₂) are theoretically possible given the small energy gap between these levels relative to the phonon energy, though they were not directly observed in this study due to detector limitations.
• Wavefunction Optimization: Optimized interaction parameters (F² = 296.0 cm⁻¹, F⁴ = 46.0 cm⁻¹, F⁶ = 4.66 cm⁻¹, ζ = 739 cm⁻¹) were derived. These yielded squared reduced-matrix elements that differed significantly (by tens of percent) from literature values for some transitions, highlighting the necessity of matrix-specific calculations for accurate intensity predictions.
• Sample Quality: The crystals exhibited black inclusions (likely graphite) and potential defects such as sulfur vacancies or oxygen substitution, which contributed to a background "pedestal" in the luminescence spectra and prevented precise measurement of fluorescence lifetimes.

Significance and Claims
The paper claims that Pr:Lu₂S₃ represents a unique candidate for mid-IR laser sources. The authors assert that Lu₂S₃ is the only known material that simultaneously combines:

1. Low phonon energy (~312 cm⁻¹).
2. Deep infrared transparency.
3. Non-hygroscopic nature.
4. Tolerance to high lanthanide doping concentrations.
5. Availability in bulk form (unlike KLuS₂).

While the current sample quality limits the determination of fluorescence lifetimes and absorption cross-sections, the spectroscopic data confirms that the Lu₂S₃ host matrix supports a broad range of Pr³⁺ transitions. The study provides the first detailed description of Pr:Lu₂S₃ excitation and luminescence spectra up to 5.4 μm. The authors conclude that with improved growth technology (e.g., alternative crucibles to reduce inclusions), Pr:Lu₂S₃ has significant potential as an active medium for mid-infrared solid-state lasers.