Black Phosphorus–Rhenium Disulfide Heterojunction Delivers Stable Pulses in Two-Micron Lasers

A two-dimensional sandwich of black phosphorus and rhenium disulfide has been tested as a pulse-shaping element inside a two-micron solid-state laser, and the results, published in Frontiers of Optoelectronics, show that the two preparation methods studied produced different laser performance. The liquid-phase version, thinner and optically cleaner, pushed the laser to a maximum average output power of 528 milliwatts, a narrowest pulse width of 366 nanoseconds and a peak power of 28.85 watts at a pump power of 6.37 watts. The mechanically exfoliated twin, thicker and lossier, managed 331.4 milliwatts and pulses of 671.25 nanoseconds under identical conditions.

Passive Q-switching is the most mature route to high-peak-power pulses at this wavelength: a saturable absorber sits inside the resonant cavity and stays transparent once intracavity intensity rises high enough, dumping the stored energy in a short burst. Two-micron pulses are prized because the wavelength sits in the eye-safe band, is strongly absorbed by gas molecules and suits applications from light detection and ranging to medical surgery and materials processing. The catch has always been the absorber itself. Traditional saturable absorber materials for the two-micron band are scarce, and single two-dimensional films suffer from limited damage thresholds, stability problems and shortened service lives when confronted with the large pulse energies of solid-state cavities.

Heterojunctions are the proposed fix. Stacking two layered materials on top of one another can preserve the broadband, nonlinear optical response of each component while reinforcing stability and damage resistance. Black phosphorus brings a directly tunable bandgap that ranges from 0.3 electron-volts in bulk crystals to 2.0 electron-volts in a monolayer, along with high carrier mobility, which has made it a favorite for mid-infrared pulse modulation. Its weakness is environmental instability. Rhenium disulfide, a transition metal dichalcogenide with a distorted 1T phase, holds a direct bandgap of roughly 1.6 electron-volts and keeps its physicochemical stability in both two- and three-dimensional forms. Its interlayer van der Waals coupling is unusually weak, which lowers the barriers to forming clean, high-quality stacks. Earlier work by Cao and colleagues in 2018 had already shown that BP/ReS2 heterojunctions possess excellent interfacial quality without lattice mismatch issues, but their nonlinear saturable absorption and use in mid-infrared pulse modulation had not been reported until this study.

The resulting heterojunction has lattice constants of 6.6 and 22.35 angstroms and an optimal interlayer spacing of 4.13 angstroms after relaxation.

Transmission electron microscopy then confirmed the real structures. In the liquid-phase sample, energy-dispersive spectroscopy mapped coexisting phosphorus- and rhenium-based nanosheets, and high-resolution imaging showed individual BP sheets with an interplanar spacing of 0.2584 nanometers and ReS2 sheets at 0.2466 nanometers, both oriented along the same crystal direction. Where the two overlapped, both spacings converged to 0.2548 nanometers and the ReS2 orientation rotated, evidence of lattice distortion driven by the mismatch between the sheets. Atomic force microscopy put the average thicknesses at 4 nanometers for the liquid-phase sample and 10.5 nanometers for the mechanically exfoliated one. Raman peaks at 352.7, 428.9 and 457.1 per centimeter matched unshifted BP modes, while peaks at 141.9 and 201.9 per centimeter matched ReS2, indicating vertical stacking with good structural match.

Fitting the transmittance curves gave a modulation depth of 11.50 percent for the liquid-phase absorber against 14.90 percent for the mechanically exfoliated one, with saturation intensities of 3.89 and 4.02 megawatts per square centimeter respectively. The decisive difference was the non-saturated loss: 7.15 percent for the liquid-phase film versus 19.70 percent for its taped counterpart. The authors attribute the gap to thickness and to the mechanical exfoliation process itself, which inevitably traps defects and adsorbates between the two materials and drives up the loss. Lower non-saturation loss means less light is wasted before the absorber bleaches, which translates directly into more usable output power inside a laser cavity.

When the two absorbers were placed near the output mirror of the Tm:YAP cavity, both produced stable Q-switching from a pump threshold of 3.13 watts. Output power rose linearly with pump power for both. Without any absorber, the cavity delivered 1212 milliwatts of continuous-wave output at 6.37 watts of pump, a slope efficiency of 23.1 percent. With the liquid-phase absorber inserted, the maximum average output power fell to 528 milliwatts at a slope efficiency of 8.9 percent; the mechanically exfoliated absorber, penalized by its higher insertion loss, yielded 331.4 milliwatts at 5.8 percent. As pumping increased from 3.13 to 6.37 watts, pulse widths shortened from 507 to 366 nanoseconds for the liquid-phase device and from 1256.5 to 671.25 nanoseconds for the other, while repetition rates climbed from 33.3 to 50 kilohertz and from 45.5 to 74.5 kilohertz, the classic signature of Q-switched pulse generation. At equal pump power, the liquid-phase absorber produced the larger single-pulse energy and the higher peak power.

Stability, the property that motivated the heterojunction design in the first place, held up in the measurements. Pulse sequences recorded at 6.37 watts of pump showed good stability for both devices, with the mechanically exfoliated absorber slightly steadier. Over ninety minutes of monitoring in ten-minute intervals, output fluctuations stayed below 2 percent for both lasers, ranging from 524.7 to 533.4 milliwatts for the liquid-phase system and from 327.7 to 336.9 milliwatts for the mechanically exfoliated one. Both systems sustained stable Q-switched operation for at least three hours per day across five days. The emission spectrum peaked at 1932 nanometers, and the choice of preparation method left the spectrum essentially unchanged. Against a comparison table of other two-dimensional saturable absorbers, the BP/ReS2 work reports a small pulse width and the highest peak power among published two-dimensional-absorber passively Q-switched mid-infrared solid-state lasers, and it outperforms single BP or single ReS2 absorbers in single-pulse energy and peak power.

Liquid-phase exfoliation yields thinner, lower-loss films but cannot precisely control where the heterojunction forms; mechanical exfoliation gives larger modulation depth and better pulse-to-pulse steadiness at the cost of insertion loss. The authors conclude that BP/ReS2 heterojunctions, combining the tunable bandgap and high carrier mobility of black phosphorus with the stability and photoresponsivity of rhenium disulfide, hold significant potential as saturable absorbers for mid-infrared pulsed lasers. What remains to be shown, on the evidence presented, is how these laboratory devices scale: the reported results come from a single cavity geometry, a fixed pump configuration and relatively short stability windows, so longer-term damage testing and optimization of the exfoliation routes will determine whether this heterojunction moves from promising demonstration to routine component in eye-safe two-micron laser systems.

Subject of Research: Nonlinear saturable absorption of BP/ReS2 heterojunctions and their use as saturable absorbers in two-micron passively Q-switched solid-state lasers

Article Title: Nonlinear saturable absorption properties of BP/ReS2 heterojunction and its application in 2 μm all-solid-state lasers

Article References: Li, H., Tang, W., Shan, Y., Wang, J., Jiang, K., Fan, M., Chen, T., Zhou, C., & Xia, W. (2025). Nonlinear saturable absorption properties of BP/ReS2 heterojunction and its application in 2 μm all-solid-state lasers. Frontiers of Optoelectronics, 18(1), Article 14. https://doi.org/10.1007/s12200-025-00157-3

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00157-3

Keywords: black phosphorus, rhenium disulfide, heterojunction, saturable absorber, passive Q-switching, Tm:YAP laser, two-micron laser, liquid-phase exfoliation, mechanical exfoliation, nonlinear optics, mid-infrared lasers, two-dimensional materials