The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific, medical and industrial applications. However, its potential is limited by the low speeds at which material can be removed and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk.

Silicon is an excellent material for microelectronics and integrated photonics1–3 with untapped potential for mid-IR optics4. Despite broad recognition of the importance of the third dimension5,6, current lithography methods do not allow fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realised with techniques like reactive ion etching. Embedded optical elements, like in glass7, electronic devices, and better electronic-photonic integration are lacking8. Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1 µm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has a different optical index than unmodified parts, which enables numerous photonic devices. Optionally, these parts are chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface, i.e., “in-chip” microstructures for microfluidic cooling of chips, vias, MEMS, photovoltaic applications and photonic devices that match or surpass the corresponding state-of-the-art device performances.

Holography is the most promising route to true-to-life 3D projections, but the incorporation of complex images with full depth control remains elusive. Digitally synthesised holograms 1 – 7 , which do not require real objects to create a hologram, offer the possibility of dynamic projection of 3D video 8 , 9 . Extensive efforts aimed 3D holographic projection 10 – 17 , however available methods remain limited to creating images on a few planes 10 – 12 , over a narrow depth-of-field 13 , 14 or with low resolution 15 – 17 . Truly 3D holography also requires full depth control and dynamic projection capabilities, which are hampered by high crosstalk 9 , 18 . The fundamental difficulty is in storing all the information necessary to depict a complex 3D image in the 2D form of a hologram without letting projections at different depths contaminate each other. Here, we solve this problem by preshaping the wavefronts to locally reduce Fresnel diffraction to Fourier holography, which allows inclusion of random phase for each depth without altering image projection at that particular depth, but eliminates crosstalk due to near-orthogonality of large-dimensional random vectors. We demonstrate Fresnel holograms that form on-axis with full depth control without any crosstalk, producing large-volume, high-density, dynamic 3D projections with 1000 image planes simultaneously, improving the state-of-the-art 12 , 17 for number of simultaneously created planes by two orders of magnitude. While our proof-of-principle experiments use spatial light modulators, our solution is applicable to all types of holographic media.

Since their first demonstration, modelocked thin-disk lasers have consistently surpassed other modelocked oscillator technologies in terms of achievable pulse energy and average power by several orders of magnitude. Surprisingly, state-of-theart results using this technology have so far only been achieved in modelocking regimes where soliton pulse shaping is dominant (i.e., soliton modelocking with semiconductor saturable absorber mirrors or Kerr lens modelocking), in which only small nonlinear phase shifts are tolerable, ultimately limiting pulse energy scaling. Inspired by the staggering success of novel modelocking regimes applied to overcome these limitations in modelocked fiber lasers, namely the similariton (self-similarly evolving pulses) and dissipative soliton regimes, here, we explore these nonlinearity-resistant regimes for the next generation of high-energy modelocked thindisk lasers, whereby millijoule pulse energies appear to be realistic targets. In this goal, we propose two possible implementations. The first is based on a passive multipass cell and designed to support dissipative solitons in an all-normal dispersion cavity. The second incorporates an active multipass cell and is designed to support similaritons. Our numerical investigations indicate that this is a very promising path to increase the pulse energy achievable directly from modelocked oscillators toward the millijoule level while additionally simplifying their implementation by eliminating the need for operation in cumbersome vacuum chambers.

We report exploitation of ablation cooling, a concept well-known in rocket design, to remove materials, including metals, silicon, hard and soft tissue. Exciting possibilities include ablation using sub-microjoule pulses with efficiencies of 100-µJ pulses.OCIS codes: 140. 3390, 060.1510, 320.7090 Femtosecond pulses hold great promise for high-precision material and tissue processing. It is well-known that use of such pulse durations allows very precise and virtually thermal-damage free material removal under appropriate conditions. However, two drawbacks remain. The first is the limited ablation rates, which is particularly limiting for biological tissue removal. The second is that at several microjoules, but often 10's to 100's of microjoules of pulse energy are required. Regarding speed of ablation, the physics of the laser-material interaction precludes a straightforward scaling up of the removal rate simply by sending pulses more frequently, since this leads to collateral damage due to heat accumulation.Here, we exploit the same physics to circumvent this limitation. We apply rapid succession of ultrafast pulses from a burst-mode fibre laser to ablate the target before residual heat deposited by the previous pulse can diffuse away from the interaction region. In addition to being able to process thermally sensitive materials, such as brain tissue, at high speed and without thermal damage, our approach reduces the required pulse energy by orders of magnitude, opening the route to highly efficient material removal with oscillator-level pulse energies.For a long time, it was commonly assumed that heat effects are nearly completely eliminated through the use of ultrafast pulses. Heat damage can indeed occur during ultrafast pulse processing as a result of pulse-to-pulse accumulation of residual heat that is deposited around the border of the ablated region by each pulse. While deposition of some residual heat by each pulse is unavoidable, we present here a laser system that can catch much of this heat before it can diffuse beyond the volume to be ablated by the next incoming pulse by operating at very high repetition rates. This brings three interrelated advantages: (1) Most of the residual heat left by the previous pulse has not yet diffused out of the volume to be ablated by the next pulse. Thus, each pulse targets an already hot volume, which lowers the required ablation energy and peak power with numerous side benefits, such minimizing plasma shielding, reducing shock wave, cavitation bubble formation and self-focusing. In addition, the quantity of residual heat is proportional to the pulse energy, thus reducing the magnitude of the problem to be solved. (3) Finally and most importantly, much of this residual heat is then carried away from the tissue in the form of ablated matter when ablated by the next pulse, reducing the build-up of heat from pulse to pulse. This is known as ablation cooling, which is very well known in the context of atmospheric entry of meteorites or reusable space rockets ...