Black phosphorus a new saturable absorber material for ultrashort pulse generation (1504.04731v1)

Abstract: Low-dimensional materials, due to their versatile properties are very interesting for numerous electronics and optoelectronics applications. Recently rediscovered black phosphorus, with a graphite-like structure can be exfoliated up to the single atomic layer. In contrary to graphene it possesses a direct band gap controllable by the number of stacked atomic layers. For those reasons, it is now intensively investigated. Here we demonstrate, that black phosphorus can serve as a broadband saturable absorber and can be used for ultrashort optical pulse generation. The mechanically exfoliated ~300 nm thick layers of black phosphorus were transferred onto the fiber core and under pulsed excitation at 1560 nm wavelength its transmission increases by 4.4%. It was used to generate 272 fs-short pulses at 1550 nm and 739 fs at 1910 nm. The obtained results shows that black phosphorus can be effectively used for ultrashort pulse generation and proves its great potential to future applications.

• The paper demonstrates the feasibility of using mechanically exfoliated black phosphorus as a saturable absorber, achieving ultrashort pulses of 272 fs and 739 fs in Er- and Tm-doped fiber lasers.
• The paper details the experimental method of depositing black phosphorus on fiber cores, revealing nonlinear absorption with a 4.4% modulation depth and a two-photon absorption coefficient of 500 cm/MW.
• The paper indicates that black phosphorus sustains high optical damage thresholds up to 0.5 W, highlighting its potential for scalable, high-power ultrafast laser applications.

Overview of Black Phosphorus as a Saturable Absorber for Ultrashort Pulse Generation

This paper presents an in-depth exploration of black phosphorus as a promising saturable absorber (SA) material for generating ultrashort optical pulses. The work addresses a critical need in the field of ultrafast laser technology, particularly in the development of passive mode-locking devices for lasers operating in various spectral ranges. The authors experimentally demonstrate the capacity of black phosphorus to produce femtosecond-scale pulses when integrated into fiber lasers doped with erbium (Er) and thulium (Tm).

Key Findings

The researchers successfully utilized mechanically exfoliated black phosphorus layers deposited on a fiber core as an SA to generate ultrashort laser pulses. Key outcomes include:

• Pulse Generation: The team achieved pulse durations of 272 fs at 1560 nm in an Er-doped fiber laser and 739 fs at 1910 nm in a Tm-doped system. These results represent the pioneering application of black phosphorus in this domain.
• Saturable Absorption Characteristics: Under pulsed excitation at a 1560 nm wavelength, the SA displayed significant modulation depth with a 4.4% transmission increase, facilitating mode-locking operation.
• Nonlinear Optical Properties: The analysis demonstrated a polarizability of black phosphorus, exhibiting anisotropic linear absorption and nonlinear absorption properties. The two-photon absorption (TPA) in the material was characterized with a TPA coefficient of approximately 500 cm/MW, comparable to bi-layer graphene.
• High Optical Damage Threshold: The SAs demonstrated robustness, sustaining operation at pump powers up to 0.5 W without any signs of optical damage, indicating durability for high-power applications.

Implications and Future Directions

The paper highlights the formidable potential of black phosphorus in ultrafast photonics, primarily due to its unique bandgap properties and nonlinear absorption capabilities. These characteristics make it a highly versatile SA material across a broadband spectral range compared to traditional SA materials like SESAMs with limited bandwidths.

Practically, the integration of black phosphorus offers a cost-effective and scalable alternative for ultrafast laser systems with applications extending from telecommunications to medical imaging and materials processing. The ease of mechanical exfoliation for black phosphorus, akin to graphene, further aids its incorporation into existing photonic devices without the need for complex fabrication techniques.

Theoretically, the results underscore the need for further exploration of two-dimensional materials like black phosphorus in optoelectronics and photonics. The unique properties observed suggest avenues for developing tailored devices with enhanced performance in spectral regions not easily accessible with current technologies.

Conclusion

The outcomes demonstrated in this paper lay a solid foundation for advancing black phosphorus from a theoretical curiosity to a practical workhorse in photonics. While further research is necessary to optimize black phosphorus-based devices and fully explore their capabilities, this work represents a significant milestone in ultrafast optics. The prospect of integrating black phosphorus in a wider variety of optoelectronic applications is promising, potentially mirroring the expansive interest and development seen with graphene.