Low-power Femtosecond Laser Research Articles

Femtosecond tunable-wavelength photoassisted cold field emission

We demonstrate photoassisted cold field emission (PFE) from a tungsten tip induced by tunable-wavelength low-power femtosecond laser excitation. The emission current from the apex of the (310)-oriented single-crystalline emitter is shown to linearly depend on the incident laser power, while the effective work function is reduced by the respective photon energy. Our results promote the implementation of the linear regime in laser-triggered cold field emission for ultrafast transmission electron microscopy.

Fast and precise chiroptical spectroscopy by photoelectron elliptical dichroism.

The photoionization of chiral molecules by elliptically polarized femtosecond laser pulses produces photoelectron angular distributions which show a strong and enantio-sensitive forward/backward asymmetry along the light propagation direction. We report on high precision measurements of this photoelectron elliptical dichroism (PEELD). Using an optical cavity to recycle the laser pulses and increase the signal-to-noise ratio, we determine enantiomeric excesses with a 0.04% precision with a low-power femtosecond laser (4 W) in a compact scheme. We perform momentum-resolved PEELD measurements in 16 molecules, from volatile terpenes to non-volatile amino acids and large iodoarenes. The results demonstrate the high structural sensitivity of PEELD, confirming the spectroscopic interest of this technique. Last, we show how a convolutional neural network can be used to retrieve the chemical and enantiomeric composition of a sample from the momentum-resolved PEELD maps.

Optimization of low-power femtosecond laser trepan drilling by machine learning and a high-throughput multi-objective genetic algorithm

The geometric quality (taper), recast layer, and processing efficiency of micro-holes are important issues in femtosecond laser trepan drilling (LTD). Although one-step drilling based on low-power femtosecond LTD may be an ideal drilling method, its disadvantages such as processing time and taper quality still need to be improved. To address these issues, machine learning was successfully applied to establish an accurate predictive model for the femtosecond LTD process. Based on the machine learning model, the femtosecond LTD results for a given parameter set were quickly and accurately obtained, avoiding a large number of complex experiments and characterization requirements. Subsequently, through the combination of our established optimal machine learning predictive model and a high-throughput genetic algorithm, optimized solutions were quickly and successfully designed for a wide range of process spaces. Finally, the reliability of the optimized process was verified by experiments. The combination of machine learning and a high-throughput optimization algorithm provided an efficient and low-cost solution for the optimization of complex laser processing technology.

Direct control of store-operated calcium channels by ultrafast laser

Ca2+ channels are essential to cell birth, life, and death. They can be externally activated by optogenetic tools, but this requires robust introduction of exogenous optogenetic genes for expression of photosensitive proteins in biological systems. Here we present femtoSOC, a method for direct control of Ca2+ channels solely by ultrafast laser without the need for optogenetic tools or any other exogenous reagents. Specifically, by focusing and scanning wavelength-tuned low-power femtosecond laser pulses on the plasma membrane for multiphoton excitation, we directly induced Ca2+ influx in cultured cells. Mechanistic study reveals that photoexcited flavins covalently bind cysteine residues in Orai1 via thioether bonds, which facilitates Orai1 polymerization to form store-operated calcium channels (SOCs) independently of STIM1, a protein generally participating in SOC formation, enabling all-optical activation of Ca2+ influx and downstream signaling pathways. Moreover, we used femtoSOC to demonstrate direct neural activation both in brain slices in vitro and in intact brains of living mice in vivo in a spatiotemporal-specific manner, indicating potential utility of femtoSOC.

Experiments of drilling micro-holes on superalloy with thermal barrier coatings by using femtosecond laser

A femtosecond laser was used to process micro-holes on the nickel-based superalloy with thermal barrier coatings. Scanning electron microscope (SEM) was used to observe the morphology of micro-holes inlet, outlet and the side wall. The changes of morphology with laser focusing position and laser power were analyzed. The results showed that the micro-holes processed by femtosecond laser required a certain amount of focus position to optimize the processing quality. The entrance was not round sometimes and the micro-holes outlet was irregular with high power laser machining. A further repair of micro-hole by using low power femtosecond laser was presented. Low laser power modification can improve the quality of micro-holes.

Progress of the research on Yb 3+ -doped femtosecond laser crystals

As widely used in military, medicine, communication, processing and other fields, femtosecond laser has become a research hotspot of the whole laser technology in the 21st century. The pump source used by laser diodes (LDS) has become a new development trend of all-solid-state femtosecond lasers owe to the rapid development of LDs. Yb3＋-doped laser crystal materials gradually become important gain media for LD pumping and generate 1.0 μm femtosecond laser due to their unique energy level structures, broad absorption and emission spectra. In this paper, the research progress of Yb3＋-doped femtosecond laser crystals is summarized in detail, the main problems are analyzed, and two directions for the development of femtosecond laser crystals in the future are proposed: high efficiency and low power femtosecond laser, high power and high energy femtosecond laser. The crystal growth, spectra properties, continuous and femtosecond laser performances of the Yb3＋:Sr3Y2(BO3)4 crystal were studied in detail. And the Yb3＋:Sr3Y2(BO3)4 femtosecond laser with the center wavelength of 1060 nm, pulse width of 116 fs, average output power of 1.08 W and optical conversion efficiency of 33.1% were successfully generated. The experimental results indicate that Yb3＋:Sr3Y2(BO3)4 and its corresponding system crystals are excellent femtosecond laser materials with high optical conversion efficiency.

Research on precise mechanical trimming of a micro shell resonator with T-shape masses using femtosecond laser ablation

This paper presents a novel method to achieve permanent frequency trimming between wineglass modes of micro shell resonator (MSR) with T-shape masses (T-masses) using femtosecond laser ablation. Trimming effects on stiffness and mass distribution around the rim are analyzed by finite element method (FEM). Stiffness and mass trimming can be achieved by grooves ablating in different positions. A systematic frequency trimming process including stiffness and mass modification is proposed. FEM simulation is further applied to estimate the variation of cross coupling and frequency split during the trimming process. High resolution trimming without surface deterioration is achieved by using a low-power femtosecond laser. Finally, frequency trimming experiments were implemented on imperfect resonators. Test results show that frequency split can be effectively reduced to less than 0.5Hz within several steps, demonstrating the effectiveness of this method.

Low power femtosecond tip-based nanofabrication with advanced control

In this paper, we propose an approach to enable the use of low power femtosecond laser in tip-based nanofabrication (TBN) without thermal damage. One major challenge in laser-assisted TBN is in maintaining precision control of the tip–surface positioning throughout the fabrication process. An advanced iterative learning control technique is exploited to overcome this challenge in achieving high-quality patterning of arbitrary shape on a metal surface. The experimental results are analyzed to understand the ablation mechanism involved. Specifically, the near-field radiation enhancement is examined via the surface-enhanced Raman scattering effect, and it was revealed the near-field enhanced plasma-mediated ablation. Moreover, silicon nitride tip is utilized to alleviate the adverse thermal damage. Experiment results including line patterns fabricated under different writing speeds and an “R” pattern are presented. The fabrication quality with regard to the line width, depth, and uniformity is characterized to demonstrate the efficacy of the proposed approach.

Reflection dynamics during surface excitation using low power femtosecond laser beam

Reflection dynamics during surface excitation using low power femtosecond laser beam

Efficient metallic spintronic emitters of ultrabroadband terahertz radiation

Terahertz electromagnetic radiation is extremely useful for numerous applications such as imaging and spectroscopy. Therefore, it is highly desirable to have an efficient table-top emitter covering the 1-to-30-THz window whilst being driven by a low-cost, low-power femtosecond laser oscillator. So far, all solid-state emitters solely exploit physics related to the electron charge and deliver emission spectra with substantial gaps. Here, we take advantage of the electron spin to realize a conceptually new terahertz source which relies on tailored fundamental spintronic and photonic phenomena in magnetic metal multilayers: ultrafast photo-induced spin currents, the inverse spin-Hall effect and a broadband Fabry-P\'erot resonance. Guided by an analytical model, such spintronic route offers unique possibilities for systematic optimization. We find that a 5.8-nm-thick W/CoFeB/Pt trilayer generates ultrashort pulses fully covering the 1-to-30-THz range. Our novel source outperforms laser-oscillator-driven emitters such as ZnTe(110) crystals in terms of bandwidth, terahertz-field amplitude, flexibility, scalability and cost.

Effect of therapeutic femtosecond laser pulse energy, repetition rate, and numerical aperture on laser-induced second and third harmonic generation in corneal tissue.

Clinical therapy incorporating femtosecond laser (FSL) devices is a quickly growing field in modern biomedical technology due to their precision and ability to generate therapeutic effects with substantially less laser pulse energy. FSLs have the potential to produce nonlinear optical effects such as harmonic generation (HG), especially in tissues with significant nonlinear susceptibilities such as the cornea. HG in corneal tissue has been demonstrated in nonlinear harmonic microscopy using low-power FSLs. Furthermore, the wavelength ranges of harmonic spectral emissions generated in corneal tissues are known to be phototoxic above certain intensities. We have investigated how the critical FSL parameters pulse energy, pulse repetition rate, and numerical aperture influence both second (SHG) and third harmonic generation (THG) in corneal tissue. Experimental results demonstrated corresponding increases in HG intensity with increasing repetition rate and numerical aperture. HG duration decreased with increasing repetition rate and pulse energy. The data also demonstrated a significant difference in HG between FSL parameters representing the two most common classes of FSL therapeutic devices.

Ultrafast Electron Pulses from a Tungsten Tip Triggered by Low-Power Femtosecond Laser Pulses

We present an experimental and numerical study of electron emission from a sharp tungsten tip triggered by sub-8-fs low-power laser pulses. This process is nonlinear in the laser electric field, and the nonlinearity can be tuned via the dc voltage applied to the tip. Numerical simulations of this system show that electron emission takes place within less than one optical period of the exciting laser pulse, so that an 8 fs 800 nm laser pulse is capable of producing a single electron pulse of less than 1 fs duration. Furthermore, we find that the carrier-envelope phase dependence of the emission process is smaller than 0.1% for an 8 fs pulse but is steeply increasing with decreasing laser pulse duration.

Precisely introducing defects into periodic structures by using a double-step laser scanning technique

We demonstrate a promising method to precisely introduce desired defects into large-area periodic structures by using a double-step laser scanning technique. A multiexposure two-beam interference technique is first used to create 2D periodic structures. A low power femtosecond laser combined with a high numerical aperture objective lens is then used to map the periodic structures to determine the positions and orientations of air holes or material cylinders without intermediate development. Based on the mapping results, the desired defects are written precisely into these structures by increasing the power of the femtosecond laser to induce a multiphoton polymerization effect. The experimental results show that defects are patterned with accurate positions and orientations. This proposed technique should be useful for fabrication of photonic crystals with well-defined defects.

Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses

We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons.

Localisation of light and spectral broadening of femtosecond laser pulses in a fibre with a minimal-microstructure cladding

Microstructure optical fibres with a cladding consisting of a single cycle of air holes and the minimum core diameter of 1 μm have been fabricated and studied. Guided modes supported by this fibre are characterised by a high light localisation degree and display the C6ν point-group spatial symmetry of the transverse field distribution. A high refractive index step between the core and the cladding in the created fibres strongly confines the light field in the fibre core. The spectral broadening of low-power femtosecond laser pulses in the fibre of this type is experimentally studied.