Denizhan Koray Kesim

Ablation-cooled material removal with ultrafast bursts of pulses

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.

In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon

Silicon is an excellent material for microelectronics and integrated photonics1–3, with untapped potential for mid-infrared optics4. Despite broad recognition of the importance of the third dimension5,6, current lithography methods do not allow the fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realized with techniques like reactive ion etching. Embedded optical elements7, 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 an optical index different to that in unmodified parts, enabling the creation of numerous photonic devices. Optionally, these parts can be chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface—that is, ‘in-chip’—microstructures for microfluidic cooling of chips, vias, micro-electro-mechanical systems, photovoltaic applications and photonic devices that match or surpass corresponding state-of-the-art device performances.By exploiting dynamics arising from nonlinear laser–material interactions, functional microelements and arbitrarily complex 3D architectures deep inside silicon are fabricated with 1 μm resolution, without damaging the silicon above or below.

Ablation-cooled material removal at high speed with femtosecond pulse bursts

We report exploitation of ablation cooling, 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.

Buried waveguides written deep inside silicon

Silicon waveguides are widely used as optical interconnects and they are particularly important for Si-photonics. Si-based devices, along with other optical elements, are entirely fabricated on the top surface of Si wafers. However, further integration of photonic and electronic devices in the same chip requires a new approach. One alternative is to utilize the bulk of the wafer for fabricating photonic elements. Recently, we reported a direct-laser-writing method that exploits nonlinear interactions and can generate subsurface modifications inside silicon without damaging the surface [1]. Using this method, we fabricated several functional optical elements including gratings [1], lenses [2], and holograms [3]. In this work, we demonstrate optical waveguides entirely embedded in Si.

Two-photon excitation of quantum dots in 3D via stacked fresnal hologram algorithm

Quantum dots are engineered to have nanometers dimensions. The ability to specify the diameter and the material of the quantum dots allow us tune the absorption and emission properties. This results in extensive capabilities for imaging [1] and display [2] applications. Further, with two photon absorption and high peak power laser they can be excited at any point in 3D locally [3].

All-Fiber Laser Systems That Can Operate in Burst Mode

Fiber lasers which operate in burst-mode where densely spaced pulses occur inside bursts repeated at much lower repetition rates can be valuable tool for sensing and imaging. We introduce such lasers and propose possible applications.