Conductive graphitic wires generation in diamond by means of pulsed Bessel beam micromachining

. We present the fabrication of transverse graphitic microelectrodes in a 500  m thick synthetic diamond bulk by means of pulsed Bessel beams. By suitably placing the elongated focal length of the Bessel beam across the entire sample, the graphitic wires grow from the bottom surface up to the top during multiple shot irradiation. The morphology of the microstructures generated and the micro-Raman spectra are studied as a function of the laser parameters and the diamond crystal orientation. We show the possibility to generate high conductivity microelectrodes, which are crucial for the application of electric fields or current transport/collection in various chips and detectors.


Introduction
Ultrafast laser micromachining has emerged as an excellent tool for fabrication of microstructures in different dielectrics and crystals using picosecond and femtosecond pulses thanks to the non-linear absorption processes involved in the radiation-matter interaction, such as multiphoton absorption and avalanche ionisation [1].Recently, non-conventional beams such as nondiffracting Bessel beams, which are characterised by an intense central peak surrounded by weak rings and an elongated focal region, have been used for in-bulk modification of different transparent materials without the need for sample translation [2].Indeed, the nondiffracting Bessel zone obtained thanks to the conical energy reservoir provided by the lateral rings and sustaining the central core reconstruction along the propagation direction, can be orders of magnitude larger than the standard Rayleigh range of Gaussian beams with same focal spot transverse dimension.
Diamond is widely used in high energy particle detectors, integrated photonic chips and microfluidic systems due to extreme hardness, high thermal conductivity, biocompatibility and top-notch chemical resistivity [3].The microfabrication of 3D conductive/graphitic electrodes in diamond are crucial since these structures act as an electric field source in all the applications mentioned above.In this work, in-bulk conductive microelectrodes have been fabricated perpendicular to the surfaces of monocrystalline CVD diamond samples (5 by 5 by 0.5 mm), using pulsed Bessel beams and without any sample translation.Micro-Raman spectroscopy has been used to evaluate the crystallinity modification after the laser irradiation, and the electrical characterization of the fabricated graphitic microelectrodes have been performed through current-voltage (IV) measurements.

Graphitic wires generation
The microfabrication studies were performed by means of a 20-Hz Ti:Sapphire amplified laser system delivering 40-fs transform-limited pulses at 790 nm wavelength in the mJ pulse energy range, and tunable in duration up to the ps regime.A conical lens (axicon) was used to reshape the Gaussian beam into a finite energy Bessel beam (BB) and a telescopic system allowed us to demagnify it and to obtain the final Bessel beam featured by a 3 m core size and by a 700 m non diffracting length at the sample position.The position of the beam with respect to the diamond was optimized in such a way to have the BB focal length crossing the sample thickness.
The micromachining was performed in a multiple shot regime, in two different diamond samples ((100) and ( 110) oriented samples respectively) as a function of pulse duration (from 200 fs till 10 ps) and pulse energy, and the laser pulses were injected orthogonally to the sample surface.No sample translation is needed along the beam propagation direction, and in contrast to other works presented in the literature the optimization of the fabrication has been done without the use of techniques for aberration corrections.
The length of the wires generated depends on the number of Bessel pulses injected, controlled using an optical shutter.The morphology strongly depends on the pulse duration and the pulse energy chosen, but also on the crystal orientation.Our results confirmed that a higher pulse duration (in particular 10 ps in our case) favours better conductivity.

Conductivity measurements
The current-voltage characterization was conducted with a 2-probe configuration in an IVT chamber.Different electrodes were fabricated and characterized in order to analyse the dependence of the resistivity as a function of different laser parameters such as pulse duration and energy.

Micro-Raman characterization
In order to understand the crystalline structure of the laser-written graphitic electrodes created at different pulse durations, micro-Raman spectroscopy was performed at the top of the wires (a few micrometer below the sample surface in order to maximize the signal).Raman spectra were acquired using a LabRAM Aramis (Horiba Jobin-Yvon) instrument with a spectral resolution of ~1 cm −1 .In figure 3 we present the spectra recorded in the central part of the wire cross-sections, corresponding to two different micro-electrodes fabricated with 200 fs and 10 ps pulses respectively.Results highlight the better transformation of diamond into graphite in the latter regime.

Conclusions
We have shown that graphitic micro-structures can be fabricated in 500 m thick monocrystalline CVD diamond samples without sample translation, using pulsed Bessel beams featured by a 3 m core size and a non-diffracting zone of 700 m.We have obtained high quality conductive electrodes with a resistivity of 0.04 Ω cm which, to the best of our knowledge, is one of the lowest values compared to the literature results using laser micromachining and the lowest using Bessel beams.These results shine light into various potential applications such as incorporating conductive electrodes in diamond based detectors, microfluidic chips and integrated photonic circuits.

Fig. 1 .
Fig. 1.Example of graphitic microstructures formed in diamond samples by Bessel beam machining (9000 pulses, 5 J pulse energy and 200 fs pulse duration).In a) the top surface of the sample has a (100) orientation, and in b) a (110) orientation.

Fig. 2 .
Fig. 2. IV graph obtained on the best conductivity microelectrode (shown in inset) fabricated with 10 ps pulse duration and 6 J pulse energy.

Fig. 3 .
Fig. 3. Micro-Raman spectra of two graphitic wires fabricated with a 200 fs and 10 ps pulse duration respectively.