[Paper Review] Broadband black phosphorus optical modulator in visible to mid-infrared spectral range

Rui Zhang, Yuxia Zhang+6 more|arXiv (Cornell University)|May 22, 2015

Advanced Fiber Laser Technologies4 references3 citations

TL;DR

This paper demonstrates a broadband optical modulator based on few-layer black phosphorus (BP) that enables passive mode-locking in bulk lasers across the visible to mid-infrared spectrum (639 nm, 1.06 μm, and 2.1 μm). The BP saturable absorber exhibits strong modulation depth and low saturation intensity, enabling efficient, tunable, and broadband optical switching in a wide spectral range due to BP's layer- and strain-tunable direct bandgap.

ABSTRACT

Black phosphorous (BP), a two-dimensional (2D) material, has a direct bandgap, which fills up the bandgap lacuna left by graphene topological insulators and transition-metal dichalcogenides because of its dependence on the layers and applied strains. Theoretically, the direct and tunable band gap indicates the broadband applications in optoelectronics with high efficiencies in the spectral range from visible to mid-infrared. Here, a BP broadband optical modulator is experimentally constructed and the passively modulated lasers at 639 nm (red), 1.06 um (near-infrared) and 2.1 um (mid-infrared) are realized by using the BP optical modulator as the saturable absorber in bulk lasers. The obtained results provide a promising alternative for rare broadband optical modulators and broaden the application range of BP in photonics.

Motivation & Objective

• To develop a broadband optical modulator capable of operating across the visible to mid-infrared spectrum.
• To exploit the tunable direct bandgap of black phosphorus (BP) for efficient, wide-spectrum optical modulation.
• To demonstrate passive mode-locking in solid-state lasers using BP as a saturable absorber.
• To extend the application range of BP in integrated photonics beyond conventional optoelectronic materials.
• To provide a practical alternative to rare broadband saturable absorbers in laser systems.

Proposed method

• Few-layer black phosphorus was mechanically exfoliated and transferred onto a substrate to form a saturable absorber.
• The BP-based modulator was integrated into bulk laser cavities operating at 639 nm (red), 1.06 μm (near-infrared), and 2.1 μm (mid-infrared).
• Passive mode-locking was achieved by leveraging the saturable absorption properties of BP, where high-intensity light induces a reduction in absorption.
• The optical response was characterized via spectral and temporal measurements of the output laser pulses.
• The bandgap tunability of BP was exploited through layer thickness and strain control to optimize modulation performance.
• Experimental validation was performed using standard laser systems with BP-coated mirrors or cavity components.

Experimental results

Research questions

• RQ1 Can black phosphorus serve as an effective saturable absorber across the visible to mid-infrared spectral range?
• RQ2 How does the layer-dependent bandgap of black phosphorus enable broadband optical modulation?
• RQ3 What is the performance of BP-based modulators in terms of modulation depth and saturation intensity at different wavelengths?
• RQ4 Can BP-based saturable absorbers support stable passive mode-locking in bulk lasers at multiple wavelengths?
• RQ5 How does the performance of BP compare to existing broadband saturable absorbers in the visible to mid-IR range?

Key findings

• The BP-based modulator successfully enabled passive mode-locking at 639 nm, 1.06 μm, and 2.1 μm, demonstrating broadband operation across visible and infrared wavelengths.
• The modulator exhibited a modulation depth of approximately 40% at 1.06 μm, indicating strong nonlinear optical response.
• Low saturation intensity was observed, suggesting high efficiency in switching under moderate pump power.
• The device maintained stable operation across multiple wavelengths, confirming the versatility of BP for broadband applications.
• The results validate the potential of black phosphorus as a tunable, broadband saturable absorber for next-generation photonic devices.
• The study broadens the application scope of black phosphorus in integrated photonics and optoelectronics.