Saturable Plasmonic Metasurfaces for Laser Mode Locking

Researchers from France, China, and Brazil have developed plasmonic metasurfaces that provide an efficient saturable absorption that can be tuned with the polarization of light.

Researchers from the Laboratoire Interdisciplinaire Carnot de Bourgogne, at Université Bourgogne, France, and the Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University, China, along with co-workers from the Department of Electrotechnology, Federal Institute of Bahia, Brazil, employed planar nanotechnologies to fabricate 2D plasmonic metasurfaces with the size, gap, and orientation, and thus well-controlled plasmonic mode that chemically synthesized counterparts handle less efficiently. The nonlinear saturable absorption under intense laser pumping was systematically investigated by altering the excitation power, the polarization, and the geometrical parameters of the plasmonic metasurfaces. The link between the polarimetric saturable absorption and the plasmonic landscape of the metasurfaces has been quantified. The researchers also implemented the saturable metasurfaces into a fiber laser cavity architecture and achieved a stable self-starting ultrashort laser pulse generation.

The team members investigated different plasmonic landscapes such as nanorods, nanocrosses, and nanorings as saturable absorbers to generate ultrafast laser pulses. They measured the modulation depth of the saturable absorption of such plasmonic metasurfaces as high as 60%.

“Such high modulation depths are uncommon, especially for thin metasurfaces,” said Philippe Grelu, a professor from Université Bourgogne. “A comparison between 2D-saturable absorbers shows that the maximum modulation depth reported is less than 11%, and a similar study with colloidal gold nanorods reports a modulation depth of only around 5%. A typical SESAM [semiconductor saturable absorber mirror] can feature a modulation depth exceeding 30%, but from a much thicker device.”

(a) SEM image of a NR array with a 50-nm gap in the long axis direction (Gy) and a 300-nm gap in the short axis direction (Gx). The horizontal scale bar represents 200 nm. The inset shows a single NR from this array, which has a length (L) of 445 nm and a width (W) of 120 nm. The vertical scale bar represents 100 nm. (b) The experimental transmission (red circles) of the NR array as a function of the input power with the excitation polarization of 18° with respect to the long axis of the NR. The modulation depth Md and typical transmission Tt are defined from the corresponding fittings (blue curve). The transmission is normalized to the value of the nearby blank glass slide. (c) Experimental excitation power and polarization dependent nonlinear transmission of a NRs array. (d) The scheme of home-built ultrafast fiber laser that integrates lithographical NRs as saturable absorber, where LD represents laser diode, WDM wavelength-division multiplexing, EDF erbium-doped fiber, ISO optical isolator, PC polarization controller, C1,2 collimators, and O1,2 objectives. (e) Pulse train shown on the oscilloscope in short (300 ns, lower panel) and long (10 ms, upper panel) time ranges. Courtesy of Jiyong Wang, Aurelien Coillet, Olivier Demichel, Zhiqiang Wang, Davi Rego, Alexandre Bouhelier, Philippe Grelu, and Benoit Cluzel.

“The key point is to find the quantitative relationship between the nonlinear absorption and the specific plasmonic modes, and this might only be achieved by using planar nanotechnologies to fabricate the plasmonic metasurfaces,” said Grelu’s colleague, Benoit Cluzel, such as electron-beam lithography rather than spin-coating the colloidal nanoparticles onto the fiber or dipping the fiber into the nanoparticle solutions.

By integrating the plasmonic metasurfaces within a free-space section of the fiber laser architecture, the researchers obtained a stable self-starting mode-locked laser operation. The typical duration of a single soliton pulse is 729 fs, with a large signal-to-noise ratio of 75 dB in the radio-frequency domain.

“We validated saturable absorption as a general nonlinear optical property of metal nanostructures, a well-known phenomenon for semiconductor,” said Jiyong Wang, lead author of the study. “More importantly, we demonstrated a promising application for nonlinear plasmonics, a method most related studies paid little attention to.”

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-020-0291-2).