Band structure tuning of g-C3N4 via sulfur doping for broadband near-infrared ultrafast photonic applications

Dechun Li,Ying Li,Shengzhi Zhao,Shiping Xu,Hongwei Chu,Li Dong

doi:10.1515/nanoph-2021-0549

Abstract

Abstract Graphitic carbon nitride (g-C3N4) featuring a stable heptazine ring structure and high polymerization degree, was indexed as a high thermochemical stability material, attracting rising research enthusiasm for diverse applications. However, the poor near-infrared (NIR) optical absorption and resulting limited NIR applications were pronounced for g-C3N4 due to its large bandgap of 2.7 eV. In the present work, sulfur-doping was manifested by first-principles calculations to introduce impurity level and result in anisotropic spin splitting in g-C3N4 for enhancing broadband nonlinear optical characteristics in NIR regime. The modified sulfur-doped g-C3N4 (S-C3N4) exhibited the maximum effective nonlinear absorption coefficient to be −0.82 cm/GW. Pulse duration within hundred nanoseconds was realized with high modulation stability employing S-C3N4 as saturable absorber in Q-switching operations. Moreover, broadband ultrafast photonics properties were successfully demonstrated in constructed ytterbium-doped and erbium-doped fiber lasers, generating highly stable dissipative soliton and traditional soliton mode-locking pulses. The presented S-C3N4 nanomaterial with remarkable nonlinear optical performances might explicitly boost the development and application of g-C3N4 materials in advanced optoelectronic and ultrafast photonic devices.

Highlights

Nonlinear optical (NLO) materials, as a flourishing fundamental constructing block of advanced laser optics, optoelectronics and optical communication, have set off a huge research boom [1,2,3,4]
To quantitatively determine the nonlinear absorption capacity of the sulfur-doped graphitic C3N4 (g-C3N4) (S-C3N4) sample, the effective nonlinear absorption coefficient was employed and it could be extracted by the following equations [42]: T
To deeply investigate the intrinsic mechanism of sulfur doping on the broadband NIR optical response of g-C3N4, the structure and band gap variation was simulated by first-principle calculation, as implemented in VASP

Summary

Introduction

Nonlinear optical (NLO) materials, as a flourishing fundamental constructing block of advanced laser optics, optoelectronics and optical communication, have set off a huge research boom [1,2,3,4]. Massive studies have demonstrated that sulfur doping could distinctly improve the optical and electronic characteristics of g-C3N4 by narrowing down the bandgap structure, improving nearinfrared (NIR) laser absorbance, and accelerating charges separation and carrier mobility [21, 29, 30]. What’s more, the density functional theory calculation demonstrated that sulfur-doping introduced defects level and caused anisotropic spin splitting in g-C3N4 beneficial to the nonlinear optical absorption characteristics of S-C3N4 in NIR regime. The additional peaks were ascribed to the stretching modes of the C–N and C=N bonds By comparing it with the Raman spectra of S-C3N4, almost all peaks were observed on S-C3N4, which demonstrated the well-preserved atomic structure after sulfur doping. The characterization results demonstrated the successful synthesis of S-C3N4

Broadband NLO responses

Third-order nonlinear refractive index and susceptibility

Broadband S-C3N4 based PQS lasers application

Broadband ultrafast photonics applications

Theoretical calculation and discussion

Conclusions