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Optimization of ultrafast laser parameters for 3D micromachining of fused silica
摘要： We present an optimization study on laser parameters for 3D micromachining of fused silica to achieve critical goals for practical applications including high surface quality, high volume production, and complex surfaces by ultrafast laser direct writing assisted chemical etching. We conducted experiments on laser pulse width of 300 fs and 1 ps, pulse energy ranging from 0.1 μJ to 1.6 μJ, three different polarizations (circular, parallel and perpendicular) and number of overlapped pulses from 3 to 10,000 at 1030 nm with up to 2 MHz repetition rate to investigate their effect on nanogratings and one dimensional (1D) channel and two dimensional (2D) planar surface selective etching on 1 mm thick fused silica. In one configuration, we achieved 21.8 nm RMS surface roughness with 80 μm Gaussian filtering and in another configuration, we estimated the maximum writing speed to be 1.25 m/s for given 2 MHz repetition rate with less than 400 nm filtered root mean square (RMS) surface roughness at a 1 mm2 area which covers the thickness of the glass.
关键词： 3D microfabrication,Direct writing,Roughness,Chemical etching,Selectivity,Ultrafast laser
更新于2025-09-11 14:15:04
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Optimized uni-traveling carrier photodiode and mushroom-mesa structure for high-power and sub-terahertz bandwidth under zero- and low-bias operation
摘要： In this paper, physically-based simulations are carried out to investigate and design uni-traveling carrier photodiode (UTC-PD) for high-power sub-terahertz wave generation at zero- and low-bias operation. The reliability of the physically-based simulation is demonstrated by comparing with our experimental result. Both the bandwidth and RF output power of the proposed UTC-PD is significantly improved by careful design the built-in electric field distribution under high-power input. For the optimized UTC-PD with the mesa diameter of 5 μm, its 3dB bandwidth large than 100 GHz even if the photocurrent reaches 6 mA under zero-bias operation. The device can reach a high bandwidth of 92.4 GHz, 105 GHz, and 119.5 GHz under the reverse bias of 0.5 V, 1 V, and 2 V, respectively, even the input photocurrent as high as 18.2 mA. The peak output-power of the device has enhanced at least 7 dB even at 170 GHz and zero- or low-bias operation. Besides, a novel design of mushroom-mesa UTC-PD (MM-UTC-PD) is proposed which with 4.3% improved high-speed performance. The MM-UTC-PD can trade-off between the external quantum-efficiency and bandwidth when miniaturized junction size is required.
关键词： ultrafast optics,sub-terahertz,optical communications,high-power photodiodes,optical receivers
更新于2025-09-11 14:15:04
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Controlling energy distribution of fast ions and X-ray emission via target reliefs in ultrafast and relativistic laser plasma interaction
摘要： Secondary emission from laser produced plasma is governed by the electron distribution function. Therefore, its control is of utmost importance to steer the emission, e.g., of ultrashort bursts of high energy photons and ions for decisive application. Maximum gain is achieved if the laser light absorption by plasma is also maximized. In our theoretical analysis including comparison to recent experiments, we follow this route and study how the energy is transferred from a short laser pulse to the energy of fast ions and X-rays. We make use of ion and K-a emissions, which respond differently to branches of the electron distribution function when we optimize the laser light absorption via structuring of the target surface. Our investigation comprises laser intensities up to 5 (cid:2) 1020 W/cm2 produced with femtosecond near infrared laser pulses and titanium foil targets of a few micrometer thicknesses. In particular, we reveal an energy relaxation process of hot electrons, which determines the observed laser intensity dependence of secondary emission and points to the bene?t of target surface structuring in different optimization scenarios.
关键词： laser plasma,ultrafast laser,fast ions,electron distribution function,target reliefs,relativistic laser plasma interaction,X-ray emission
更新于2025-09-11 14:15:04
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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) - Power-Scaling Nonlinear-Mirror Modelocked Thin-Disk Lasers
摘要： High-power ultrafast laser sources constitute a key technology to a wide variety of scientific and industrial applications that benefit from the combination of high average power and sub-ps pulse duration. While coherently combined amplifier systems now deliver kW-level average powers, ultrafast thin-disk lasers (TDLs) remain of considerable interest as compact, low-noise laser sources delivering multi-hundred-Watts with excellent beam quality. These oscillators are usually modelocked using either of two well-established techniques: SESAM or Kerr-lens modelocking (KLM). Recently, we demonstrated the first TDL modelocked using the frequency-doubling nonlinear mirror (NLM). This technique relies on the combination of an intracavity χ(2) crystal used for second-harmonic generation (SHG) and a dichroic output coupler mirror (OC) that is highly reflective for the second harmonic (SH) and partially reflective for the fundamental wave (FW). The NLM device (SHG crystal + dichroic OC) thus provides a saturable reflectivity enabling modelocked operation. Our first NLM-modelocked TDL delivered <30 W with pulse durations >323 fs at a repetition rate of 17.8 MHz. Here we present a power-scaled NLM-modelocked TDL delivering 66 W and 430 fs pulses at 9.3 MHz, and up to 87 W and 586 fs pulses at 8.9 MHz. We thereby improve the output average power by a factor ≈3 and the peak power by a factor ≈5. This corresponds to 3 orders of magnitude more peak power than NLM results based on bulk-crystal lasers. We furthermore identify stable modelocking regimes that avoid Q-switching instabilities.
关键词： modelocking,thin-disk lasers,ultrafast lasers,nonlinear mirror,power scaling
更新于2025-09-11 14:15:04
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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) - Airy Plasmon Pulses Investigated by Multiphoton Photoemission Electron Microscopy (PEEM)
摘要： Airy wave packets are a special class of non-diffractive solutions to the wave equation which accelerate along a bend trajectory. Their self-healing property makes them particularly interesting for integrated nanophotonic applications. Surface plasmon polaritons (SPPs), in contrast, have the beneficial property to be bound to the two-dimensional surface of a noble metal and enable sub-wavelength confinement. In two dimensions, Airy wave packets are the only non-diffracting solution of the wave equation. The high intrinsic bandwidth of a plasmonic excitation makes it ideal for ultrafast photonics. Photoemission Electron Microscopy (PEEM) represents an ideal tool to deliver images of these processes. The method has been successfully applied by us in the past to investigate the transient behavior of an optical nanoantenna emitting Hankel plasmon pulses. We have used PEEM in combination with a variable wavelength excitation from an optical parametric chirped pulse amplifier system (OPCPA) to experimentally investigate Airy plasmon pulses emitted by an excitation grating. The OPCPA system allows us to experimentally investigate the performance of the excitation in the wavelength range between 670 and 840 nm, where a 3-photon-process is necessary to emit an electron. In this way, we can sample the spectral response which is necessary to determine the ultrafast characteristics of the Airy plasmon pulse. We find that the excitation of stationary Airy plasmons is possible over a large bandwidth. However, the creation of an ultrafast hotspot requires also a matching of the spectral phase which is hard to fulfill in strong dispersing systems. We therefore accompany our measurements with rigorous finite-difference time domain simulations, a suitable nonlinear electron yield model, and analytic calculations to determine the modal purity of the Airy plasmon excitation based on reciprocity. Results support our experimental data and suggest that further improvements of the excitation scheme are necessary to obtain spatio-temporal hotspots.
关键词： PEEM,Surface plasmon polaritons,SPPs,ultrafast photonics,Multiphoton Photoemission Electron Microscopy,Airy plasmon pulses
更新于2025-09-11 14:15:04
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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) - All-Optical Background-Free Detection of Ring Currents by Dynamical Symmetry Breaking High Harmonic Spectroscopy
摘要： Excited atoms and molecules can carry long-lived currents that circulate in the microscopic media. From a quantum mechanical perspective, these currents can be understood as a coherent wave-packet comprising a superposition of bound- states that oscillates in time [1–3]. When the wave-packet has a non-zero angular momentum expectation value, ring-currents circulate in the medium. For instance, a hydrogen atom excited to a 2p-state with non-zero magnetic quantum number m (e.g. by interaction with circularly polarized light) carries a steady-state ring current [2]. More complex systems can also carry persistent ring currents, e.g. spin-orbit wave-packets in Xenon [4], or multi-electron wave-packets in larger molecules [1]. This phenomenon is general to any quantum system and is especially interesting because it occurs on the natural time-scale of electronic motion – attoseconds to femtoseconds. Understanding ring currents is thus fundamentally important for manipulating and controlling ultrafast processes on the nanoscale, including chemical bond formation and topologically protected surface currents [5], as well as for the generation of intense attosecond-duration magnetic fields [1,6]. However, ring currents are very difficult to detect, particularly in a time-resolved manner. Only very recently were ring currents directly experimentally resolved in Argon through pump-probe angularly-resolved incidence photoelectron spectrum measurements [3]. Here we propose and theoretically explore an all-optical technique for ultrafast time-resolved detection of ring currents in atoms, molecules and solids, based on high harmonic generation (HHG). In this technique a microscopic medium interacts with a bi-chromatic (ω-2ω) bi-elliptical laser pulse, generating high harmonic photons. We show that the harmonic spectra emitted from current-carrying media differs from that of current-free media. We use dynamical symmetry (DS) considerations [7] to derive conditions for a maximal (background-free) signal in the harmonic ellipticity, which occurs when the pump beams are cross-linearly polarized [8]. In this configuration the bi-chromatic laser field exhibits a dynamical reflection symmetry that leads to linearly polarized harmonic selection rules [7]; however, this selection rule is broken when the medium carries a current, because ring-currents are not reflection-symmetric (similar to chiral systems [9], but where the current can be described in 2D). Thus, current-carrying media emit elliptically polarized harmonics, where the harmonic ellipticity is correlated to the intensity and sign of the current in the system. We numerically demonstrate the approach by pump-probe HHG calculations from Neon noble gas atoms, and from both aligned and un-oriented aromatic molecules (benzene and furan), using a non-interacting electron quantum model, and time-dependent density functional theory calculations. The presented work could be useful for ultrafast spectroscopy of current-carrying processes (chemical reactions, topological currents, etc.), as well as for manipulation and control of ring currents, paving the way for their table-top all- optical detection.
关键词： high harmonic generation,ultrafast spectroscopy,ring currents,dynamical symmetry breaking
更新于2025-09-11 14:15:04
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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) - High Power, Ultrafast Yellow Laser using Fourth-Harmonic Generation of Ultrafast Mid-IR Cr <sup>2+</sup> :ZnS Laser in MgO:PPLN Crystal
摘要： The ultrafast optical radiation in the yellow spectral region is of great interest for variety of scientific and technological applications including astronomy, spectroscopy and medicine, with potential to revolutionize eye surgeries through exquisite accuracy [1, 2]. However, it is relatively difficult to access the yellow region, especially when good beam quality, high power and high efficiency are required. Till date, various types of yellow lasers have been reported [3, 4] mostly in CW regime and nanosecond timescales. Here, we report on a compact, high- power, high repetition-rate, ultrafast source of yellow radiation. Using an ultrafast Cr2+:ZnS laser of average power of 4.5 W with spectral width of Δλ=138 nm centered at 2.36 μm, producing output pulses of width ~40 fs at a repetition rate of 80 MHz, and two stage single-pass second harmonic generation (SHG), we have generated ultrafast radiation of power 1 W tunable across 577-589 nm. The schematic of the experimental setup is shown in Fig. 1(a). The single-pass SHG of Cr2+:ZnS laser in a 2 mm long 5% mol doped MgO:PPLN multi-grating crystal (Λ=33.15 - 35.25 μm), giving second harmonic (SH) pulses of 90 fs at 1.18 μm, with a maximum average power of 2.3 W at a single-pass efficiency as high as 65%. Further the 1.18 μm radiation is frequency-doubled in a 1 mm long and 0.5×1 mm2 in aperture, MgO:PPLN crystal (Λ=8.9 -9.45 μm) producing yellow radiation. The acceptance bandwidth (FWHM) and temporal walk-off length (LT) of the second crystal are calculated to be 3 nm and 288 μm, respectively. In order to optimize the focusing condition for maximum fourth harmonic generation (FHG) efficiency, we pumped the crystal at constant power of 1.3 W at 1.18 μm and recorded the fourth harmonic (FH) power as a function of focusing parameter, ξ =L/b, where, L is the length of the crystal and b=2πnw0 2/λ, w0 is the beam waist radius, n is the refractive index of the crystal and λ is the input wavelength.
关键词： Cr2+:ZnS laser,MgO:PPLN crystal,fourth-harmonic generation,ultrafast yellow laser
更新于2025-09-11 14:15:04
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Theoretical Design of a Pump-Free Ultrahigh Efficiency All-Optical Switching Based on a Defect Ring Optical Waveguide Network
摘要： A theoretical design of a defect ring optical waveguide network is proposed to construct a pump-free ultrahigh efficiency all-optical switch. This switch creates ultrastrong photonic localization and causes the nonlinear dielectric in the defect waveguide to intensely respond. At its ON state, this material defect without Kerr response helps to produce a pair of sharp pass bands in the transmission spectrum to form the dual channel of the all-optical switch. When it is switched to its OFF state, the strong Kerr response induced refractive index change in the high nonlinear defect waveguide strongly alters the spectrum, leading to a collapse of the dual channels. Network equation and generalized eigenfunction method are used to numerically calculate the optical properties of the switch and obtain a threshold control energy of about 2.90 zJ, which is eight orders of magnitude lower than previously reported. The switching efficiency/transmission ratio exceeds 3×1011, which is six orders of magnitude larger than previously reported. The state transition time is nearly 108 fs, which is approximately two orders of magnitude faster than the previously reported shortest time. Furthermore, the switch size can be much smaller than 2.6 μm and will be suitable for integration.
关键词： ultrafast optics,pump-free,all-optical switching,ultrahigh efficiency,nonlinear optics,waveguide network
更新于2025-09-11 14:15:04
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Applications of Silicon Photonics in Sensors and Waveguides || Introductory Chapter: Unique Applications of Silicon Photonics
摘要： Current technological demands require two key components: miniaturization of devices and integration of multifunctional components onto a single chip offered at low cost. The continuous improvement in meeting the demands of integrated circuits has been enabled by incremental efforts of miniaturization of the transistor. Moore’s law states that the minimum feature size shrinks by a factor of 0.7 every 2 years. There has been tremendous growth in the areas of semiconductors and electronics to meet these requirements. However, the research on silicon photonics started only in the 1980s. The advantage of silicon is that its properties can be tailored by doping, which makes it suitable for applications both in electronics and photonics. For useful applications, the technology also plays a major role along with the material. Here, a few applications in photonics domain have been demonstrated.
关键词： Waveguides,Silicon Photonics,Miniaturization,Sensors,Integration,Ultrafast Lasers
更新于2025-09-11 14:15:04
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Ultrafast laser-annealing of perovskite films for efficient perovskite solar cells
摘要： Perovskite solar cells have attracted much attention recently for their high efficiency, ease of preparation and low cost. Here, we report a novel laser-annealing method for perovskite films at a low substrate temperature by scanning laser spots on the film surfaces. An ultrafast crystallization process within a few seconds is realized under a laser with a high intensity and a fast scanning speed. Because the crystalline perovskite phase has a stronger light absorption than the amorphous phase, the fast laser annealing can induce a higher temperature in the former and lead to the selective growth of large perovskite grains. Under optimum conditions, perovskite films with high crystallinity are successfully fabricated, resulting in perovskite solar cells with high power conversion efficiency and good stability. Moreover, a faster laser-annealing process of perovskite films is achieved by using a linear laser beam, which is expected to be a promising technique for the mass production of large-scale perovskite solar cells.
关键词： Perovskite solar cells,ultrafast crystallization,laser-annealing,large-scale production,power conversion efficiency
更新于2025-09-11 14:15:04