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Atomic layer graphene as saturable absorber for ultrafast pulsed lasers  [PDF]
Qiaoliang Bao,Han Zhang,Yu Wang,Zhenhua Ni,Yongli Yan,Ze Xiang Shen,Kian Ping Loh,Ding Yuan Tang
Physics , 2009,
Abstract: The optical conductance of monolayer graphene is defined solely by the fine structure constant. The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, we demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the thickness of graphene. Our results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth and wideband tuneability.
WS2 as a saturable absorber for ultrafast photonic applications of mode-locked and Q-switched lasers  [PDF]
Kan Wu,Xiaoyan Zhang,Jun Wang,Xing Li,Jianping Chen
Physics , 2014,
Abstract: Two-dimensional (2D) nanomaterials, especially the transition metal sulfide semiconductors, have drawn great interests due to their potential applications in viable photonic and optoelectronic devices, such as saturable absorbers (SAs) and optical switches, etc. In this work, tungsten disulfide (WS2) based SA for ultrafast photonic applications was demonstrated. WS2 nanosheets were prepared using liquid-phase exfoliation method and embedded in polyvinyl alcohol (PVA) thin film for the practical usage. Saturable absorption was observed in the WS2-PVA SA at the telecommunication waveband near 1550 nm. By incorporating WS2-PVA SA into a fiber laser cavity, both stable mode locking operation and Q-switching operation were achieved. In the mode locking operation, the laser obtained femtosecond output pulse width and high spectral purity in the radio frequency spectrum. In the Q-switching operation, the laser had tunable repetition rate and output pulse energy of a few tens of nano joule. Our findings suggest that few-layer WS2 nanosheets embedded in PVA thin film are promising nonlinear optical materials for ultrafast photonic applications as a mode locker or Q-switcher.
Surfactant-aided exfoliation of molydenum disulphide for ultrafast pulse generation through edge-state saturable absorption  [PDF]
Richard C. T. Howe,Robert I. Woodward,Guohua Hu,Zongyin Yang,Edmund J. R. Kelleher,Tawfique Hasan
Physics , 2015,
Abstract: We use liquid phase exfoliation to produce dispersions of molybdenum disulphide (MoS2) nanoflakes in aqueous surfactant solutions. The chemical structures of the bile salt surfactants play a crucial role in the exfoliation and stabilization of MoS2. The resultant MoS2 dispersions are heavily enriched in single and few (<6) layer flakes with large edge to surface area ratio. We use the dispersions to fabricate free-standing polymer composite wide-band saturable absorbers to develop mode-locked and Q- switched fibre lasers, tunable from 1535-1565 and 1030-1070 nm, respectively. We attribute this sub-bandgap optical absorption and its nonlinear saturation behaviour to edge-mediated states introduced within the material band-gap of the exfoliated MoS2 nanoflakes.
Monolayer Graphene as Saturable Absorber in Mode-locked Laser  [PDF]
Qiaoliang Bao,Han Zhang,Zhenhua Ni,Yu Wang,Lakshminarayana Polavarapu,Kian Ping Loh,Zexiang Shen,Qing-Hua Xu,Ding Yuan Tang
Physics , 2010,
Abstract: We demonstrate that the intrinsic properties of monolayer graphene allow it to act as a more effective saturable absorber for mode-locking fiber lasers compared to multilayer graphene. The absorption of monolayer graphene can be saturated at lower excitation intensity compared to multilayer graphene, graphene with wrinkle-like defects, and functionalized graphene. Monolayer graphene has a remarkable large modulation depth of 95.3%, whereas the modulation depth of multilayer graphene is greatly reduced due to nonsaturable absorption and scattering loss. Picoseconds ultrafast laser pulse (1.23 ps) can be generated using monolayer graphene as saturable absorber. Due to the ultrafast relaxation time, larger modulation depth and lower scattering loss of monolayer graphene, it performs better than multilayer graphene in terms of pulse shaping ability, pulse stability and output energy.
Ultrafast isomerization dynamics of retinal in bacteriorhodopsin as revealed by femtosecond absorption spectroscopy
YiShi Wu,Sheng Zhong,XiCheng Ai,KunSheng Hu,JianPing Zhang
Chinese Science Bulletin , 2008, DOI: 10.1007/s11434-008-0283-8
Abstract: A femtosecond (fs) broad-band absorption apparatus was used to measure the early photoisomerization process of bacteriorhodopsin’s (BR) photocycle to reveal the character of the important intermediate of J625 and to obtain a deeper understanding of the role of photoisomerization in BR photocycle. Two time constants of 0.5 ps (95%) and 2.0 ps (5%) were brought out by global fitting on thirty curves in the near-infrared region. We suggest that the first time constant results from the decay of I460 intermediate, and the longer component might be associated with BR isomer. The global analysis over 450, 540, 630, 710 and 870 nm traces identified two time constants, ~0.5 and ~3 ps. The slower component can be extracted from the processes of both J625→BR568 (540 nm) and J625→K590 (630 nm), suggesting J-intermediate takes a partial cis configuration. The obvious negative feature in early delay time of 700–780 nm regions was attributed to the radiative transition (stimulated emission) from the Franck-Condon active configuration along the isomerization potential surface of all-trans-retinal.
Observation of Saturable and Reverse Saturable Absorption at Longitudinal Surface Plasmon Resonance in Gold Nanorods  [PDF]
Hendry I. Elim,Jian Yang,Jim-Yang Lee,Jun Mi,Wei Ji
Physics , 2006, DOI: 10.1063/1.2177366
Abstract: Saturable and reverse saturable absorption at longitudinal surface plasmon resonance (SPR) in gold nanorods (Au NRs) have been observed using Z-scan and transient absorption techniques with femtosecond laser pulses. At lower excitation irradiances, the wavelength dispersion of saturable absorption has been determined near the longitudinal mode of SPR with a recovery time determined to be a few ten picoseconds on the SPR resonance. With higher excitation irradiances, reverse saturable absorption occurs and becomes dominant. The underlying mechanisms are discussed. Such reversible saturable absorption makes Au NRs an ideal candidate for optical limiting applications.
Saturable absorption in multi-core fiber couplers  [PDF]
Elham Nazemosadat,Arash Mafi
Physics , 2013, DOI: 10.1364/JOSAB.30.002787
Abstract: The saturable absorption characteristics of two-, three-, and five-core one-dimensional fiber coupler arrays and the seven-core hexagonal fiber coupler array are investigated. It is shown that the performance of all these saturable absorbers are comparable and not much is gained, if anything, by going from a two-core nonlinear coupler geometry to a higher number of cores. This observation is supported by the similarity of the saturable absorption curves, as well as comparable pulse characteristics obtained from the simulation of a generic mode-locked fiber laser cavity.
Saturable absorption and 'slow light'  [PDF]
Adrian C Selden
Physics , 2005,
Abstract: Quantitative evaluation of some recent 'slow light' experiments based on coherent population oscillations (CPO) shows that they can be more simply interpreted as saturable absorption phenomena. Therefore they do not provide an unambiguous demonstration of 'slow light'. Indeed a limiting condition on the spectral bandwidth is not generally satisfied, such that the requirements for burning a narrow spectral hole in the homogeneously broadened absorption line are not met. Some definitive tests of 'slow light' phenomena are suggested, derived from analysis of phase shift and pulse delay for a saturable absorber
Graphene Mode-Locked Ultrafast Laser  [PDF]
Z. Sun,T. Hasan,F. Torrisi,D. Popa,G. Privitera,F. Wang,F. Bonaccorso,D. M. Basko,A. C. Ferrari
Physics , 2009, DOI: 10.1021/nn901703e
Abstract: Graphene is at the center of a significant research effort. Near-ballistic transport at room temperature and high mobility make it a potential material for nanoelectronics. Its electronic and mechanical properties are also ideal for micro and nanomechanical systems, thin-film transistors and transparent and conductive composites and electrodes. Here we exploit the optoelectronic properties of graphene to realize an ultrafast laser. A graphene-polymer composite is fabricated using wet-chemistry techniques. Pauli blocking following intense illumination results in saturable absorption, independent of wavelength. This is used to passively mode-lock an Erbium-doped fibre laser working at 1559nm, with a 5.24nm spectral bandwidth and ~460fs pulse duration, paving the way to graphene-based photonics.
Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber  [PDF]
M. Zhang,G. Hu,G. Hu,R. C. T. Howe,L. Chen,Z. Zheng,T. Hasan
Physics , 2015,
Abstract: We demonstrate a ytterbium (Yb) and an erbium (Er)-doped fiber laser Q-switched by a solution processed, optically uniform, few-layer tungsten disulfide saturable absorber (WS2-SA). Nonlinear optical absorption of the WS2-SA in the sub-bandgap region, attributed to the edge-induced states, is characterized by 3.1% and 4.9% modulation depths with 1.38 and 3.83 MW/cm2 saturation intensities at 1030 and 1558 nm, respectively. By integrating the optically uniform WS2-SA in the Yb- and Er-doped laser cavities, we obtain self-starting Q-switched pulses with microsecond duration and kilohertz repetition rates at 1030 and 1558 nm. Our work demonstrates broadband sub-bandgap saturable absorption of a single, solution processed WS2-SA, providing new potential efficacy for WS2 in ultrafast photonic applications.
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