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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) - Ultrafast All-Optical Switching in III-V Semiconductor Resonant Nanostructures
摘要： All-dielectric high-index resonant nanostructures offer plentiful opportunities for tailoring characteristics of scattered radiation. In fact, the low level of ohmic losses at optical frequencies and the presence of pronounced magneto-dipolar modes in scattering spectra of dielectric nanoresonators pave the way to numerous scattering effects that could be employed for many practical applications [1]. To further advance the performance of dielectric nanophotonic devices, it is crucial to make them reconfigurable, i.e. to provide a method to manipulate optical properties of the material that they are made of. The electron-hole injection, carried out by optical means, is a promising solution for this task (Fig. 1A). Direct-gap semiconductors are perfectly suitable for that purpose, because this class of materials retains advantages typical for dielectrics and, at the same time, is defined by smaller bandgap widths, compared to dielectrics. The latter feature significantly decreases a value of the pumping pulse fluence that is required to noticeably change the optical response of a resonant nanostructure. Indeed, it was numerically shown that the change of the trajectory of the probing pulse constituted 20° for an asymmetric dimer comprised of two silicon (Si) spheres of different radii for the energy fluence of the pumping pulse Φ(cid:3020)(cid:3036) = 1.6 mJ cm(cid:2879)(cid:2870) [2]. On the other hand, there is an experimental study of all-optical switching in gallium arsenide (GaAs) metasurface that indicated the change of its reflection coefficient ?(cid:1844) = 0.35 for the fluence Φ(cid:3008)(cid:3028)(cid:3002)(cid:3046) = 0.31 mJ cm(cid:2879)(cid:2870) [3]. Although there are already some numerical results on the light scattering in the asymmetric dimer, it is still urgent to carry out the corresponding experiment. For that purpose, it is important to change the geometry of the system (cylinders instead of spheres) and its material (GaAs in place of Si). The system has an asymmetric scattering profile in the regular regime owing to its asymmetric geometry. The geometry also stipulates the difference in absorption cross-sections of the cylindrical resonators. Because of this difference in absorption, the change of the material’s optical parameters is not equal for the two cylinders. Therefore, under certain conditions, after the pumping of the nanostructure, the scattering diagram becomes symmetric. Our numeric results demonstrate the symmetrization of the indicatrix for the pumping fluence Φ(cid:2869) = 0.9 mJ cm(cid:2879)(cid:2870) (the corresponding change of the probe scattering direction was calculated to be 7°) for the dimer with following geometrical parameters: radii – 85 nm and 90 nm, height – 200 nm, distance between the centers of the disks – 450 nm. The central wavelength of the pulse spectrum was (cid:2019)(cid:2869) = 820 nm. The dimer nanoantenna is an ultrafast optical switch that can be used, for instance, to distribute optical signals between two waveguides in integrated photonic circuits. The design of the switch makes its fabrication compatible with modern technological methods. The all-optical modulation can be observed not only in single asymmetric nanoantennas, but also in phased-array metasurfaces composed of supercells with resonators of the different size. In particular, we considered a metasurface that could control the intensities (i.e. transmission coefficients) in the diffraction orders that are formed after the scattering on the nanostructure. In our experiments, the most pronounced relative change of the transmission coefficient was detected in the first order: ?(cid:1846) (cid:1846) = 9.7% for the fluence of the pumping pulse Φ(cid:2870) = 0.02 mJ cm(cid:2879)(cid:2870) (Fig. 2B). The metasurfaces of that kind can also be used as all-optical switches.
关键词： ultrafast optics,resonant nanostructures,III-V semiconductor,all-optical switching
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) - Plasmon-Plasmon Coupling Probed by Ultrafast, Strong-Field Photoemission with <7 ? Sensitivity
摘要： The coupling of propagating surface plasmon waves and localized plasmon oscillations in nanostructures is an essential phenomenon determining electromagnetic field enhancement on the nanoscale. With our recently developed experimental method [1], we can measure the maximum plasmonic field enhancement at any nanostructured metal surface. Here we use our method to investigate the fundamental question of plasmon-plasmon coupling and its effect on large field enhancement factors. Coupling is studied on different nanostructured Ag thin films supporting not only propagating plasmons, but also localized plasmon oscillations due to the different surface nanostructures. Ultrashort laser pulses excite propagating plasmons in Kretschmann geometry (Fig. 1 (a)), while localized plasmons are excited on the surface nanostructures via the coupling of propagating and localized surface plasmons. Photoelectron spectra of the electrons photoemitted due to the plasmonic near fields are measured by a time-of-flight spectrometer [1,2]. The analysis of the cutoffs (highest electron energies, Fig. 1 (b)) of the electron spectra yields maximum plasmonic field enhancement values ×21, ×23 and ×31 for surfaces exhibiting 0.8, 1.6 and 4.5 nm average roughness values, respectively. The finite-difference time-domain (FDTD) simulation of the individual rough surfaces not only support the measured field enhancement values, but also reveal the contributions from propagating and localized plasmons (Fig. 1. (c) and (d)). The dependence of the field localization, i. e. the resulting field enhancement values on the grain size is also demonstrated. It is shown, that when resonance conditions are met, a significant portion of the field enhancement can be attributed to the generation of localized plasmons on the grainy surface nanostructures, acting as dipole sources resonantly driven by the propagating plasmon field [3].
关键词： plasmon-plasmon coupling,ultrafast photoemission,field enhancement,nanostructured Ag thin films
更新于2025-09-12 10:27:22
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Supercontinuum Generation from a Thulium Ultrafast Fiber Laser in a High NA Silica Fiber
摘要： A broadband supercontinuum is generated from a thulium ultrafast soliton fiber laser in a high numerical aperture silica fiber with low nJ seed pulse energies. Amplification of a thulium based ultrafast soliton ring fiber laser in two customized thulium doped fiber amplifiers provides pulses varying in energy between 1.8 nJ and 13.5 nJ. Coupling these seed pulses into a high numerical aperture silica fiber UHNA 7 of varying length, a systematic study of the impact of pulse energy, pulse duration and nonlinear fiber length is conducted. Based on the normal dispersion, self-phase modulation dominates for lower pump power values and results in a symmetric spectral broadening process. For higher pulse energies, four-wave mixing and Raman scattering contribute to a strong red-shift of the spectrum beyond 2.3 μm. For a fiber length of 20 m of UNHA 7, a broadband supercontinuum spanning from 1.7 μm to 2.33 μm is generated with a 20 dB spectral bandwidth of 502 nm with an output pulse energy of 4.4 nJ, corresponding to an output power of 92 mW. This represents a low seed energy threshold for efficient flat supercontinuum generation at infrared wavelengths.
关键词： Supercontinuum generation,Ultrafast optics,Fiber lasers,Thulium
更新于2025-09-12 10:27:22
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Ultrafast Melting of Metal–Organic Frameworks for Advanced Nanophotonics
摘要： The conversion of metal–organic frameworks (MOFs) into derivatives with a well-defined shape and composition is considered a reliable way to produce efficient catalysts and energy capacitors at the nanometer scale. Yet, approaches based on conventional melting of MOFs provide the derivatives such as amorphous carbon, metal oxides, or metallic nanoclusters with an appropriate morphology. Here ultrafast melting of MOFs is utilized by femtosecond laser pulses to produce a new generation of derivatives with complex morphology and enhanced nonlinear optical response. It is revealed that such a nonequilibrium process allows conversion of interpenetrated 3D MOFs comprising flexible ligands into well-organized spheres with a metal oxide dendrite core and amorphous organic shell. The ability to produce such derivatives with a complex morphology is directly dependent on the electronic structure, crystal density, ligand flexibility, and morphology of initial MOFs. An enhanced second harmonic generation and three-photon luminescence are also demonstrated due to the resonant interaction of 100–1000 nm spherical derivatives with light. The results obtained are in the favor of new approaches for melting special types of MOFs for nonlinear nanophotonics.
关键词： ultrafast melting,femtosecond laser,nanophotonics,metal–organic framework,derivatives
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) - Optical Repetition Rate Stabilization Techniques of Ultrafast Yb Doped All Fiber Oscillator
摘要： High intensity lasers usually based on OPCPA technology [1] require stable and precisely controlled repetition rate of the oscillator. Common method of repetition rate control is based on movable mirrors mounted on piezoelectric stages in free space [2]. This method has some disadvantages such as sensitivity to changes of ambient conditions and requirement for high voltage driving of piezo actuators. Different approach of fiber laser repetition rate control, which helps to overcome these shortcomings, is based on refractive index modification [3] instead of geometrical oscillator length change. In this work we demonstrate all-fiber concept of precise repetition rate control of an ultrafast ytterbium oscillator passively mode-locked using semiconductor saturable absorber mirror (SESAM). Refractive index change in Er-doped fiber core induced by pump laser power change allowed to lock repetition rate of oscillator to external clock frequency using phase-locked loop (PLL) electronics.
关键词： Refractive Index Modification,Ultrafast Yb Doped All Fiber Oscillator,Optical Repetition Rate Stabilization,Phase-Locked Loop
更新于2025-09-12 10:27:22
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Ultrafast electron transfer through silicon-vacuum interface induced by the action of intense femtosecond laser pulse
摘要： A theoretical study of the influence of a quasistationary electric field formed during emission separation of charges on ultrafast electron transfer near the silicon-vacuum interface irradiated by femtosecond laser pulses was carried out. The values of the strength of the arising quasistationary field during irradiation by femtosecond pulses with fluences near and above the silicon damage thresholds were estimated. The possibility of the formation of a dynamic optically layered structure in the near-surface layers of silicon irradiated by a femtosecond pulse resulting from a modification of its optical properties due to the depletion of electrons in the surface layer was studied. It is suggested that the time-depending dipole moment induced by the emission separation of charges should lead to the generation of electromagnetic radiation from the terahertz frequency range. The basic properties of this radiation were theoretically investigated.
关键词： Coulomb explosion,silicon,ultrafast heating,terahertz radiation,electron emission,electron transfer,femtosecond laser pulse
更新于2025-09-12 10:27:22
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[IEEE 2019 IEEE 69th Electronic Components and Technology Conference (ECTC) - Las Vegas, NV, USA (2019.5.28-2019.5.31)] 2019 IEEE 69th Electronic Components and Technology Conference (ECTC) - Ultrafast Laser Scribe: An Improved Metal and ILD Ablation Process
摘要： Traditional laser scribe which utilizes a nanosecond laser to ablate metal and interlayer dielectric layers (ILD) has been widely adopted by the microelectronics packaging industry as the gold standard for laser scribe processing in die prep singulation. Shrinking device size and thickness are driving increased demand for high die quality or die break strength. Next generation laser scribe tools are being developed to focus on minimizing thermal laser damage by using ultrafast lasers with short pulse widths in the picosecond to femtosecond range. This paper reports a robust ultrafast laser scribe process utilizing a femtosecond ultrafast laser platform developed through ESI and Intel collaboration. This first-of-a-kind equipment and process deliver significant improvement in die break strength and demonstrate a substantial reduction in bulk silicon cracking or voiding as seen in the nanosecond laser scribe heat affected zones (HAZ). Additionally, this ultrafast laser platform offers precise control over beam placement and scribe depth with onboard monitoring capability, key components to delivering a minimally needed scribe depth without sacrificing silicon integrity and processing time. This ultimately leads to a gentle coat-free ablation process that affords a low cost of ownership compare to its peers. Detailed scribe quality characteristics, process controls and overall system manufacturability will be discussed.
关键词： femosecond,ultrafast,laser scribe,ablation,laser grooving
更新于2025-09-12 10:27:22
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High-resolution material structuring using ultrafast laser non-diffractive beams
摘要： Scales in the 100 nm range represent a generic cornerstone for laser material processing, enabling novel size-dependent functions on surfaces and in the bulk and thus a new range of technological applications. On these scales, the processed material acquires optical, transport or contact properties that do not only rely on local e?ects on singular topographic features but involve increasingly collective behaviors. Rapid access to sub-100 nm features with intense coherent light represents nevertheless a challenge in laser structuring in view of the optical di?raction limit. Ultrafast non-di?ractive beams with controllable time envelopes can overcome this limit and achieve super-resolved processing, a prerequisite for the next generation of ?exible and precise material processing tools. They show a remarkable capacity of structuring transparent materials with high degree of accuracy and exceptional aspect ratio. This capacity relies on triggering fast hydrodynamic and material fracture e?ects with characteristic spatial scales in the nm range. Reviewing the present achievements and technical potential, we discuss from a dynamic viewpoint the physical mechanisms enabling structural features beyond di?raction limit achieved using ultrafast Bessel beams and indicate applications of high technical relevance.
关键词： nanotechnology,non-di?ractive beams,Ultrafast lasers,laser processing
更新于2025-09-12 10:27:22
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Experimental exploration of mode-locking evolution mechanism in dual-ring fiber laser
摘要： We explored experimentally the mode-locking (ML) evolution mechanism of a dual-ring fiber laser. Increase of the total pump power up to 860 mW allowed the observation of the state changes from single-ring ML, dual-ring ML, to dual-ring self-starting ML. Moreover, a group of wave plates and a fixed polarization beam splitter were applied in each subring as precise attenuators. Rotation of each wave plate for an entire turn allowed the investigation of the ML evolution of the two subrings based on the intracavity power variation in each subring. The variation periodicity of the output power was analyzed for each subring and was found to be consistent with the theoretical results. Furthermore, the ML detachment threshold in each subring was estimated to explain the ML evolution. Our study provides a deeper understanding of the ML evolution in dual-ring fiber lasers and can aid their future optimal design for dual comb applications.
关键词： Fiber lasers,Mode-locking evolution,Cascaded nonlinear processes,Mode-locked lasers,Ultrafast mechanism
更新于2025-09-12 10:27:22
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Dark sub-gap states in metal-halide perovskites revealed by coherent multidimensional spectroscopy
摘要： Metal-halide perovskites show excellent properties for photovoltaic and optoelectronic applications, with power conversion efficiencies of solar cell and LEDs exceeding 20%. Being solution processed, these polycrystalline materials likely contain a large density of defects compared to melt-grown semiconductors. Surprisingly, typical effects from defects (absorption below the bandgap, low fill factor and open circuit voltage in devices, strong non-radiative recombination) are not observed. In this work, we study thin films of metal-halide perovskites CH3NH3PbX3 (X = Br,I) with ultrafast multidimensional optical spectroscopy to resolve the dynamics of band and defect states. We observe a shared ground state between the band-edge transitions and a continuum of sub-bandgap states, which extends at least 350 meV below the band edge). We explain the comparatively large bleaching of the dark sub-bandgap states with oscillator strength borrowing from the band-edge transition. Our results show that upon valence to conduction band excitation such sub-gap states are instantaneously bleached for large parts of the carrier lifetime and conversely, that almost dark sub-bandgap states can be populated by light excitation. This observation helps unraveling the photophysical origin of the unexpected optoelectronic properties of these materials.
关键词： photovoltaic,defect states,metal-halide perovskites,optoelectronic,ultrafast multidimensional optical spectroscopy
更新于2025-09-12 10:27:22