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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Novel InN-Based SESAMs with Ultra-Short Time Response
摘要： Semiconductor saturable absorbers are becoming a matter of interest since they are keystone elements of pulsed lasers, leading to ultrashort pulses with high peak intensities and wide optical spectra. This kind of ultrafast lasers are empowering new applications in the fields of optical telecommunications and nonlinear optics. In order to improve the radiation source with regard to pulse energy and temporal duration, new saturable absorbers are still under development. In this work, we study semiconductor saturable absorbers (S1 and S2) consisting of an active InN layer grown by molecular beam epitaxy directly on a 10-μm-thick GaN-on-sapphire template. We compare structures with an InN thickness of 400 nm (S1) and 900 nm (S2). The energy band gap of the samples extracted from optical transmission measurements is Eg1 ～ 0.69 eV (1782 nm) and Eg2 ～ 0.70 eV (1767 nm) for samples S1 and S2, respectively. The nonlinear absorption of the samples at 1550 nm has been measured by the Z-scan method, using a radiation source which delivers 250 fs pulses with an average power of 35 mW and a repetition rate of 5.2 MHz. To increase the power density impinging the sample, the laser is focused with a lens with a focal length of 3 cm in the Z-scan scheme, attaining an energy fluence of E = 5.2 mJ/cm2. The nonlinear optical response of the samples is obtained by measuring the optical transmittance as a function of the incident peak intensity on the sample. In both samples, a huge nonlinear change is observed. The linear transmittance at 1550 nm has been estimated in Tlin = 17 % for S1 and Tlin = 2.6 % for S2, and the modulation depth is ΔT (S1) = 22.6 % and ΔT (S2) = 21.8 %, respectively. Thus, sample S1 exhibits a nonlinear transmittance change (Tnl/Tlin) above ～ 240% at the maximum peak intensity (optical bleaching), whereas for S2 the total change in transmittance reaches 815%. These results reveal that the thickness of S2 is particularly adapted for the fabrication of saturable absorbers at 1550 nm, since it is thick enough to get a low linear transmittance but keeping the maximum modulation depth. Semiconductor saturable mirrors (SESAMs) have been implemented by deposition of an aluminium mirror directly on the InN sample surface. Optical fiber mode-locked lasers have been developed with these SESAMs. The AC traces and optical spectra for lasers using samples S1 and S2 are plotted. The traces have been fitted to Gaussian curves with a temporal width of 210 fs for S1 and 150 fs for S2. In conclusion, we present a new SESAM technology based on InN thin films which displays huge nonlinear effects. Optimized InN layer thickness results in more than 800% change in transmittance, which makes it possible to generate ultrashort pulses with temporal duration as low as 150 fs.
关键词： Semiconductor saturable absorbers,InN,ultrafast lasers,SESAMs,nonlinear optics
更新于2025-09-12 10:27:22
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Excited state structural dynamics of 4-cyano-4′-hydroxystilbene: deciphering the signatures of proton-coupled electron transfer using ultrafast Raman loss spectroscopy
摘要： The photo-initiated proton-coupled electron transfer (PCET) process plays a crucial role in the context of light harvesting in various biological and chemical systems. Molecular model systems are typically employed to understand the mechanisms underlying the functioning of complex biological systems. Some molecular dyads based on the PCET property have been particularly designed to achieve efficient sunlight-to-fuel production. Organic photoacids are potential sources for such applications since they exhibit an enhancement in their acidity upon photoexcitation, facilitating the mimicking of some of the biological processes. p-Hydroxybenzylideneimidazolinone (p-HBI), an organic photoacid, is a key chromophore in green fluorescent protein, which exhibits green emission due to excited state proton transfer. Herein, we investigate the structural changes and dynamics of 4-cyano-40-hydroxystilbene (CHSB), an analogue of p-HBI, in the presence of an external base, t-butylamine (TBA), using the techniques of ultrafast transient absorption, emission and ultrafast Raman loss spectroscopy.
关键词： excited state dynamics,Raman loss spectroscopy,proton-coupled electron transfer,photoacids,ultrafast spectroscopy
更新于2025-09-12 10:27:22
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Optical microfiber-based ultrafast fiber lasers
摘要： Optical microfibers drawn from conventional fibers have attracted considerable interests and have found many novel applications. Here, we review recent advances in ultrafast fiber lasers based on optical microfibers. Starting with characteristics and fabrication of optical microfibers, which are closely related to ultrafast fiber lasers, we show that characteristics of large portion of evanescent field, tailorable dispersion, high optical nonlinearity, very low optical loss and full compatibility with conventional fibers are greatly beneficial to novel ultrafast fiber lasers. We then highlight recent works on ultrafast fiber lasers based on optical microfibers in terms of fast saturable absorbers made from optical microfiber-supported nanomaterials, dispersion management and high optical nonlinearity, as well as some other novel ultrafast fiber lasers. Finally, we briefly discuss future opportunities for optical microfiber-based ultrafast fiber lasers, such as high-order dispersion management, nonlinearity management and applications for sensing and measurement.
关键词： Dispersion management,Saturable absorbers,Ultrafast fiber lasers,Optical microfibers,Optical nonlinearity
更新于2025-09-12 10:27:22
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Investigation of interactions between ultrafast laser beams and screen-printed silver nanopaste films
摘要： This study aims to investigate the interaction between ultrafast laser beams and screen-printed silver (Ag) nanopaste ?lms for strain sensors. After single pulse ablation of Ag nanopaste ?lms with a thickness of 4.58 μm, the ablation threshold was approximately 0.2 J/cm2. By increasing single pulse laser ?uences from 0.78 J/cm2 to 1.87 J/cm2, the laser-ablated line widths and depths of Ag nanopaste ?lms were ranging from 22 ± 0.1 μm and 4 ± 0.09 μm to 44.8 ± 1.4 μm and 4.2 ± 0.05 μm, respectively. The sheet resistance of laser-processed Ag nanopaste ?lms was measured by a four-point probe instrument. When the single pulse laser ?uences set from 4.7 mJ/cm2 to 1.97 J/cm2, the measured sheet resistances near the laser-ablated line edge of 0.5 mm were 59.52 ± 0.76 mΩ/sq and 115.83 ± 6.11 mΩ/sq, respectively. The optimal ultrafast laser processing parameters consisting of the areal laser ?uence of 47.4 J/cm2, the laser pulse repetition rate of 300 kHz, the scanning speed of galvanometers of 500 mm/s, and the overlapping rate of laser spots of 96.1% were used to pattern strain sensors with transversal and longitudinal electrode structures on Ag ?lm/glass substrates.
关键词： Sheet resistance,Silver (Ag) nanopaste ?lms,Single pulse ablation,Strain sensor,Ultrafast laser
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Label-Free Multiphoton Microscopy in Human Tissue Enabled by an Er:Fiber-Laser Based Tunable Source
摘要： Multiphoton microscopy (MPM) is an important bio-imaging tool. Different modalities can serve as a contrast agent, such as second-/third-harmonic generation (SHG/THG) and two-/three-photon excitation fluorescence (2PEF/3PEF). Ultrafast lasers with flexible wavelength tunability are crucial for driving MPM bio-imaging, and the conventional solution relies on ultrafast Ti:sapphire lasers plus an optical parametric oscillator/amplifier. Recently, we have demonstrated that ultrafast fiber lasers are a potential solution to implementing compact, robust, and wavelength tunable femtosecond sources for driving MPM. To realize wavelength tunability we employ self-phase modulation (SPM) in optical fibers to broaden a narrowband input spectrum of Yb-/Er-doped fiber lasers (YDFLs/EDFLs) up to >400-nm wide with well-isolated spectral lobes; filtering the leftmost/rightmost lobes leads to nearly transform-limited pulses [1–6]. Such a SPM-enabled spectral selection (SESS) allows us to obtain wavelength widely tunable femtosecond pulses for MPM [2,5,6]. In this submission, we representatively demonstrate label-free harmonic generation microscopy (HGM) in human skin and brain tissues. Figure 1(a) depicts a scanning microscope driven by an EDFL-based SESS source. The EDFL operates at 31-MHz repetition rate and generates 290-fs pulses centered at 1550 nm. The narrowband EDFL [blue curve in Fig. 1(a)] is coupled into 9-cm optical fiber (10-μm mode-field diameter and -10 fs2/mm group-velocity dispersion at 1550 nm). The output spectrum is shown as the red curve in Fig. 1(b) under 85-nJ coupled pulse energy. We use optical filters to select the leftmost spectral lobe peaking at 1250 nm, which leads to 11.7-nJ, 47-fs pulses. Then we employ these pulses to drive a scanning microscope and conduct HGM in human skin and brain tissues. Figure 1(c) shows the dermal papilla at the junction of epidermis and upper dermis in human skin. Basal cells are visualized by THG (cyan hot) due to optical inhomogeneity at the interface (e.g., cell membrane); SHG (red hot) originates from the non-centrosymmetric structure of collagen fibers. In Fig. 1(d), neural network and brain vasculature in human brain tissue can be visualized by THG and SHG, respectively [Fig. 1(d)]. THG contrast inside the vasculature shows also red blood cells. In conclusion, we report on MPM deep-tissue imaging enabled by an EDFL-based SESS source. It is noteworthy that besides HGM excited by 1250-nm femtosecond pulses demonstrated here, the SESS source also supports 1300-/1700-nm illumination for 3PEF of green/red fluorescent protein (GFP/RFP) [7,8]. Such a fiber-based solution can be applied to many important applications, such as histopathology, morphology, and neuroscience.
关键词： ultrafast fiber lasers,Multiphoton microscopy,self-phase modulation,bio-imaging,harmonic generation microscopy
更新于2025-09-12 10:27:22
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Single-Shot Multi-Stage Damage and Ablation of Silicon by Femtosecond Mid-infrared Laser Pulses
摘要： Although ultrafast laser materials processing has advanced at a breakneck pace over the last two decades, most applications have been developed with laser pulses at near-IR or visible wavelengths. Recent progress in mid-infrared (MiR) femtosecond laser source development may create novel capabilities for material processing. This is because, at high intensities required for such processing, wavelength tuning to longer wavelengths opens the pathway to a special regime of laser-solid interactions. Under these conditions, due to the λ2 scaling, the ponderomotive energy of laser-driven electrons may significantly exceed photon energy, band gap and electron affinity and can dominantly drive absorption, resulting in a paradigm shift in the traditional concepts of ultrafast laser-solid interactions. Irreversible high-intensity ultrafast MIR laser-solid interactions are of primary interest in this connection, but they have not been systematically studied so far. To address this fundamental gap, we performed a detailed experimental investigation of high-intensity ultrafast modifications of silicon by single femtosecond MiR pulses (λ = 2.7–4.2 μm). Ultrafast melting, interaction with silicon-oxide surface layer, and ablation of the oxide and crystal surfaces were ex-situ characterized by scanning electron, atomic-force, and transmission electron microscopy combined with focused ion-beam milling, electron diffractometry, and μ-Raman spectroscopy. Laser induced damage and ablation thresholds were measured as functions of laser wavelength. The traditional theoretical models did not reproduce the wavelength scaling of the damage thresholds. To address the disagreement, we discuss possible novel pathways of energy deposition driven by the ponderomotive energy and field effects characteristic of the MIR wavelength regime.
关键词： ultrafast laser,ablation,silicon,damage threshold,mid-infrared
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Soliton Self-Compression and UV Dispersive Wave Emission in Compact Hollow Capillary Systems
摘要： Soliton dynamics underlie a wide range of phenomena in nonlinear fibre optics. In particular, higher-order solitons in gas-filled hollow-core photonic crystal fibre (HC-PCF) have been applied to self-compression of ultrafast laser pulses [1] and the generation of widely tuneable resonant dispersive waves (RDW) from the vacuum ultraviolet (VUV) to the visible spectral range [2]. We recently demonstrated that by moving to long, large-core hollow capillary fibres (HCF) and shorter driving pulses, these effects can be scaled by up to three orders of magnitude in pulse energy, providing unprecedented peak power in ultrafast VUV pulses as well as a route towards terawatt-scale optical attosecond pulses [3]. Here we show that by further decreasing the initial pulse duration, high-energy soliton dynamics can be obtained in HCF as short as 35 cm.
关键词： Soliton dynamics,ultrafast laser pulses,hollow-core photonic crystal fibre,nonlinear fibre optics,resonant dispersive waves
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Birefringent Surface Gratings for Ultrafast Spin-VCSELs
摘要： The data rates required for optical interconnection in datacenters continue to increase. On the other hand, intensity-modulated vertical-cavity surface-emitting lasers (VCSELs), which are the preferred light source for this task, have a limited bandwidth of at present about 35 GHz [1]. This limit can be overcome with spin-VCSELs that show much faster polarisation and modulation dynamics [2]. In these devices, spin-polarised electrons are injected, by which the degree of circular polarisation of the emitted light can be changed and thus information be encoded. The characteristic eigenfrequency of the laser, corresponding to the resonance frequency in conventional laser diodes, is determined by the birefringence splitting B, namely the frequency difference between the two fundamental polarisation modes of a single-mode VCSEL. Recently a polarisation oscillation frequency of 212 GHz was demonstrated [2]. In that laser the birefringence was induced by bending the sample and employing the elasto-optic effect. Alternatively we have shown that the cavity birefringence can be tuned by mounting VCSELs on piezoelectric substrates or by asymmetric heating, which, however, leads to much lower B. Up to now no truly integrated B-inducing technique existed for a spin-VCSEL that would allow to maintain great similarity to the successful high-efficiency device platform of present oxide-confined VCSELs. In this work, for the first time, we present a VCSEL in which high B of almost 100 GHz stems from a tailored integrated surface grating.
关键词： ultrafast modulation,spin-VCSELs,optical interconnection,surface gratings,birefringence
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Nonlinear Generation of Ultrafast, Higher Order Vector Vortex Beams
摘要： Light beams carrying both spin (polarization) and orbital angular momentum (OAM) can be represented by mapping the total angular momentum onto a higher-order Poincaré sphere [1]. Due to the non-separability of the polarization and OAM degrees-of-freedom, these beams, commonly known as vector vortex beams, find varieties of applications in scientific and technological fields including photon entanglement [2], material processing [3] and microscopy [4]. Such beams are commonly generated using q-plates [5] and liquid crystal based spatial light modulators [6]. However, the limitation of low power handling and restricted wavelength coverage of such techniques can be addressed through the vector vortex beam generation using nonlinear parametric processes. Unfortunately, the dependence of phase-matching of the parametric processes on the polarization of the interacting beams puts the major hindrance to the nonlinear generation of vector vortex beams. Here we report a novel experimental scheme to generate vector vortex beams using single pass second harmonic generation (SHG) of ultrafast higher order vector vortex beams.
关键词： ultrafast,higher order,nonlinear generation,second harmonic generation,vector vortex beams
更新于2025-09-11 14:15:04
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Graphdiyne for Ultrashort Pulse Generation in an Erbium-Doped Hybrid Mode-Locked Fiber Laser
摘要： An erbium-doped hybrid passively mode-locked ?ber laser based on few-layer graphdiyne (GDY) saturable absorber (SA) has been investigated for the ?rst time. Hybrid mode-locked ?ber laser is composed of non-linear polarization rotation (NPR) technology and GDY-SA. The central wavelength, pulse width and repetition rate of the output pulse are 1530.7 nm, 690.2 fs and 14.7 MHz, respectively. Compared with the passively mode-locked pulse laser with GDY-SA or NPR technology alone, the output pulse width of hybrid passively mode-locked ?ber laser is reduced more than 50 fs. It is demonstrated that the performance of GDY can be potentially applied in ultrafast laser.
关键词： saturable absorber,?ber laser,ultrafast photonics,mode-locked,graphdiyne
更新于2025-09-11 14:15:04