Shuting Lei

Self-controlled formation of microlenses by optical breakdown inside wide-band-gap materials

By repeatedly illuminating fused silica slabs with focused femtosecond pulses, we permanently decrease the local refractive index without increasing the linear absorption or scattering. This progressively forms a biconvex lens in the prefocal region. With linearly polarized light, the index change reaches several percent and is associated with the formation of an array of planar nanocracks. We analyze the polarization-dependent focusing power of the subwavelength periodic structure. While the detailed material modification changes, spontaneous defocusing lens formation is a common feature of every wide-band-gap transparent materials that we have studied (SiO2, BK7, LiF, sapphire, and mica).

Limitations to laser machining of silicon using femtosecond micro-Bessel beams in the infrared

We produce and characterize high-angle femtosecond Bessel beams at 1300-nm wavelength leading to nonlinearly ionized plasma micro-channels in both glass and silicon. With microjoule pulse energy, we demonstrate controlled through-modifications in 150-μm glass substrates. In silicon, strong two-photon absorption leads to larger damages at the front surface but also a clamping of the intensity inside the bulk at a level of ≈4 × 1011 W cm−2 which is below the threshold for volume and rear surface modification. We show that the intensity clamping is associated with a strong degradation of the Bessel-like profile. The observations highlight that the inherent limitation to ultrafast energy deposition inside semiconductors with Gaussian focusing [Mouskeftaras et al., Appl. Phys. Lett. 105, 191103 (2014)] applies also for high-angle Bessel beams.

Near-infrared femtosecond laser machining initiated by ultraviolet multiphoton ionization

We report on the experimental study of microstructures fabricated on the surface of fused silica by two femtosecond laser pulses, a tightly focused 266 nm beam followed by a loosely focused 800 nm beam. By setting the fluence of each pulse below the damage threshold, visible microstructures are fabricated using the combined beams. Our results suggest that the ultraviolet pulse generates seed electrons through multiphoton absorption, and the near-infrared pulse utilizes these electrons to cause damage by avalanche ionization.

Assessment of Microgrooved Cutting Tool in Dry Machining of AISI 1045 Steel

This paper studies the performance of microgrooved cutting tool in dry orthogonal machining of mild steel (AISI 1045 steel) using advantedge finite element simulation. Microgrooves are designed on the rake face of cemented carbide (WC/Co) cutting inserts. The purpose is to examine the effect of microgroove textured tools on machining performance and to compare it with nontextured cutting tools. Specifically, the following groove parameters are examined: groove width, groove depth, and edge distance (the distance from cutting edge to the first groove). Their effects are assessed in terms of the main force, thrust force, and chip–tool contact length. It is found that microgrooved cutting tools generate lower cutting force and thrust force, and consequently lower the energy necessary for machining. The groove width, groove depth, and edge distance all have influence on cutting force in their own ways.

Femtosecond laser nanomachining initiated by ultraviolet multiphoton ionization

We report on the experimental results of 300 nm features generated on fused silica using a near-infrared (IR) femtosecond laser pulse initiated by an ultraviolet (UV) pulse. With both pulses at a short (~60 fs) delay, the damage threshold of the UV pulse is only 10% of its normal value. Considerable reduction of UV damage threshold is observed when two pulses are at ± 1.3 ps delay. The damage feature size of the combined pulses is similar to that of a single UV pulse. A modified rate equation model with the consideration of defect states is used to help explain these results. This concept can be applied to shorter wavelengths, e.g. XUV and X-ray, with the required fluence below their normal threshold.

Finite element investigation of friction and wear of microgrooved cutting tool in dry machining of AISI 1045 steel

This article studies the tribological performance of microgrooved cutting tool in dry orthogonal machining of mild steel (AISI 1045 steel) using finite element simulation. The purpose is to examine the effects of microgrooves on friction and wear of the textured tools and to compare it with non-textured cutting tools. For the cemented carbide (WC/Co) cutting inserts, microgrooves are designed on the rake face. Specifically, the following groove parameters are examined: groove width, groove depth, and edge distance (the distance from the cutting edge to the first groove). Their effects are assessed in terms of friction coefficient and wear on the rake face. It is found that microgrooved cutting tools can significantly reduce friction and wear in machining, which is attributed mainly to the reduced chip–tool contact length.

Carrier-envelope-phase stabilized terawatt class laser at 1 kHz with a wavelength tunable option

We demonstrate a chirped-pulse-amplified Ti:Sapphire laser system operating at 1 kHz, with 20 mJ pulse energy, 26 femtosecond pulse duration (0.77 terawatt), and excellent long term carrier-envelope-phase (CEP) stability. A new vibrational damping technique is implemented to significantly reduce vibrational noise on both the laser stretcher and compressor, thus enabling a single-shot CEP noise value of 250 mrad RMS over 1 hour and 300 mrad RMS over 9 hours. This is, to the best of our knowledge, the best long term CEP noise ever reported for any terawatt class laser. This laser is also used to pump a white-light-seeded optical parametric amplifier, producing 6 mJ of total energy in the signal and idler with 18 mJ of pumping energy. Due to preservation of the CEP in the white-light generated signal and passive CEP stability in the idler, this laser system promises synthesized laser pulses spanning multi-octaves of bandwidth at an unprecedented energy scale.

Study of laser beam propagation in microholes and the effect on femtosecond laser micromachining

In femtosecond (fs) laser micromachining, such as microhole drilling, the hole created by previous laser pulses may act like a waveguide, whose sidewall may significantly affect the beam profile of subsequent laser pulses propagating into the hole. This effect is very important for both the fundamental study and the practical applications of fs laser micromachining, but has not been well studied in literature. The effect is investigated in this paper by numerically solving the transient Maxwell’s wave equation. The study reveals how microholes affect laser intensity profile for different laser and hole parameters, and the implied effects on fs laser micromachining. The study has provided a good fundamental physical explanation for the elliptical hole shape obtained in fs laser micromachining with linearly polarized laser beams.

Fabricating nanostructures on fused silica using femtosecond infrared pulses combined with sub-nanojoule ultraviolet pulses

Circular craters with diameters of 500 nm are fabricated on the surface of fused silica by femtosecond ultraviolet-infrared (UV-IR) pulse trains with 0.8 nJ UV pulse energy. UV damage thresholds at different IR energies and UV-IR delays are measured. Diameters and depths of the ablated craters can be modified by adding the IR pulse and varying the UV-IR delays. These results demonstrate the feasibility of nanomachining using short wavelength lasers with pulse energy far below normal damage thresholds.

Spatial characterization of Bessel-like beams for strong-field physics

We present a compact, simple design for the generation and tuning of both the spot size and effective focal length of Bessel-like beams. In particular, this setup provides an important tool for the use of Bessel-like beams with high-power, femtosecond laser systems. Using a shallow angle axicon in conjunction with a spherical lens, we show that it is possible to focus Bessel-like modes to comparable focal spot sizes to sharp axicons while maintaining a long effective focal length. The resulting focal profiles are characterized in detail using an accurate high dynamic range imaging technique. Quantitatively, we introduce a metric (R0.8) which defines the spot-size containing 80% of the total energy. Our setup overcomes the typical compromise between long working distances and small spot sizes. This is particularly relevant for strong-field physics where most experiments must operate in vacuum.