Sb₄O₅Cl₂ for 34^th-order-harmonic mode locking

Highlights

• •We have systematically studied the structure characteristic and nonlinear optics property of Sb₄O₅Cl₂.

• The saturation intensity and modulation depth are 402.5 MW/cm² and 1.3% measured by experiment.

• Harmonic mode locking with 850-fs pulse duration, 202.3-MHz maximum repetition rate is generated based on Sb₄O₅Cl₂ SA.

• We think this work opens novel avenues toward optoelectronics device based on Sb₄O₅Cl₂.

Abstract

Due to the excellent electrochemical characteristics, Sb₄O₅Cl₂ has been potentially used in flame retardant, catalyst, and anode materials, etc. At the same time, it has broad application prospects in optics and optoelectronics because of its strong inoxidizability, high damage threshold and relatively high-photoelectric conversion efficiency. However, the application of Sb₄O₅Cl₂ in ultra-fast photonics has not been reported so far. In this work, the structure characteristic and nonlinear optics property of Sb₄O₅Cl₂ have been systematically studied. The result indicates saturation intensity and modulation depth are 402.5 MW/cm² and 1.3%, which prove Sb₄O₅Cl₂ could be utilized as a saturable absorber (SA) in ultrafast photonics. Especially, harmonic mode-locking with 850-fs pulse duration, 2.86-nm 3-dB bandwidth, 202.3-MHz maximum repetition rate in optical communication region is generated based on a tapered fiber incorporated with Sb₄O₅Cl₂ for the first time to our knowledge. This work opens novel avenues toward optoelectronics device based on Sb₄O₅Cl₂ in the fields of optical modulation, optical detection and optical communication, etc.

Introduction

Saturable absorbers (SAs) are materials which reduce light absorption with the increase of light intensity [[1], [2], [3], [4], [5], [6], [7], [8]]. In the past few years, SAs such as, graphene, transition metal disulfides (TMDs), carbon nanotubes, bismuthene and black phosphorus have been extensively studied because of their excellent optical properties [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]]. But more stable and cheaper SAs are worth further to be explored. Sb is a nitrogen group element (V main group). Its compounds were used as cosmetics or decorations in ancient times. However, it was not until about the 17^th century that Sb was known to be a separate chemical element. In recent years, Sb has attracted great interest as an alloy materials such as lead alloy for battery grids and thermoelectric alloys, or as a stuffing element of III–V semiconducting materials for example, GaSb, InSb and AlSb [[23], [24], [25], [26]]. Antimony oxides such as Sb₂O₃ are widely used in plastics, fibers, rubber, ceramics, flame retardants, catalysts, batteries, and other chemical industries [27]. However, the research of Sb compounds in the field of ultrafast optics mainly concentrated on Sb₂Se₃ [28]. Compared to Sb, Bi is also an important element of the V main group. Compounds of Bi, such as Bi₂Te₃ and Bi₂Se₃, have been widely studied in ultrafast optics [[29], [30], [31]]. Sb₄O₅Cl₂ is a kind of antimony oxide with stable performance and lower manufacturing cost than other materials. It can be synthesized by electrochemical method with Sb anode, chloride salt and different metal cathodes dissolved in HCl or hydrothermal approach with SbCl₃, NaOH, and ethylene glycol [32]. According to reports, three-dimensional bulk Sb₄O₅Cl₂ can be used as flame retardant, lithium storage materials, and photo catalysts. But there are few applications in optics.

In addition, ultrafast pulses, with pulse width in picosecond and femtosecond range, are widely used in communication, medical treatment, mechanical processing, and other fields [33,34]. As a technique to generate ultrashort pulses from lasers, mode-locking can be divided into active mode-locking and passive mode-locking. Compared with active mode-locking, passive mode-locking can generate higher-quality pulses. In recent years, the performances of passively mode-locked lasers, such as repetition frequency, pulse width and peak power, etc, have been continuously improved through changing structures and devices. Among them, the most advanced one is fiber laser. The peak power, pulse width, spectrum, central wavelength of fiber laser can exceed 10 kW [35], narrower than 10 fs [36,37], broadened over 100 nm, extend to different bands [[38], [39], [40]], respectively. Moreover, fiber laser can be converted between different output states in the same cavity, which has been successfully realized in the experiment [41,42]. However, Sb₄O₅Cl₂ for the applications in optics, especially in ultrafast optics, has not been reported yet [[43], [44], [45]].

In this work, nonlinear optical and ultrafast photonics properties of Sb₄O₅Cl₂ are investigated for the first time. The saturable absorption of Sb₄O₅Cl₂ is measured by a double balance detector, the saturation intensity and modulation depth are 402.5 MW/cm², 1.3% respectively. When Sb₄O₅Cl₂ SA is inserted into an Er-doped fiber laser, a harmonic mode-locked all-fiber laser based on evanescent field interaction with Sb₄O₅Cl₂ is achieved. The pulse duration is 850-fs with central wavelength of 1563.1 nm. When the pump power is above 214 mW, a 34^th-order-harmonic mode-locked pulse (corresponding to repetition of 202.3 MHz) has been obtained. Our work provides an effective scheme for the development of Sb₄O₅Cl₂ in nonlinear optics. The obtained results show that Sb₄O₅Cl₂ has outstanding saturable absorption properties and valuable potential applications.