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Deep Silicon Amorphization Induced by Femtosecond Laser Pulses up to the Mid-Infrared  
Deep Silicon Amorphization Induced by Femtosecond Laser Pulses up to the Mid-Infrared
Scientists from the Institute of Optics (Laser Processing Group), from the Universidad Aix-Marseilley and del Department of Applied Physics of the Autonomous University of Madrid They have published in the journal Advanced Optical Materials a work in which they have managed to double the depth achieved so far of waveguides Directly laser engraved on silicon.

These waveguides serve to transmit information using light instead of electricity, just like optical fibers but within an integrated circuit, and thus facilitate the transition from light to electricity still necessary within communications equipment, such as the router of our houses.

Applications


In recent years, the amount of information transmitted over communication networks has increased exponentially. This need for greater bandwidths means that electronic processing in large communication nodes is reaching its limit and new technologies are needed to perform these functions using light instead of electricity and thus avoid the opto-electronic conversion that is currently performs. Silicon-embedded waveguides advance in that direction.

Silicon photonic circuits


Silicon is a key material for the electronics industry, mainly due to its abundance on Earth (it is the most abundant element in the Earth's crust behind oxygen).
It is a semiconductor that can occur in two structurally different solid phases (crystalline and amorphous) with very different physical properties.
The advent of silicon photonics further expanded the scope of this material It is used for the coupling of silicon chip components to fiber optic telecommunications systems that typically operate around the 1550 nm wavelength, where silicon is transparent . However, to allow the propagation of confined light (a path or guide through which light circulates), it was necessary to develop the concept of silicon on insulator that allows the manufacture of optical circuits with lithographic methods similar to those used for electronic systems.
Initially this concept of silicon on insulator represents a great advantage to take advantage of the huge investment made by the semiconductor industry in these technologies. However, its drawbacks are the lack of flexibility in the designs you can do and the long processing cycles required to achieve the desired architectures. Therefore, it would be highly desirable to develop a versatile direct write technology for optical engineering in silicon.

To overcome this limitation, the scientists propose to take advantage of the laser writing of amorphous silicon surface channels. When crystalline silicon is irradiated with an ultra-fast laser, it melts and resolidifies in the form of amorphous silicon, thus being able to form "paths" through which light is transmitted.
This provides a unique opportunity for custom fabrication of the waveguides that each project needs.

Direct laser writing of amorphous lines on crystalline silicon has the potential to become a flexible alternative to silicon-on-insulator technology in photonic integrated circuits. However, the maximum amorphous layer thickness achieved so far is 60 nm, which is below the requirements for waveguide in telecommunications. In this work, the authors present different strategies to bring the thickness of the layer to twice the current limit.

Our work


In view of previous research, there appeared to be a natural upper limit to the thickness of an amorphous layer that can be formed by ultrafast laser irradiation, at least for the range of parameters explored so far. To overcome this limit, they have explored a much wider range of parameters in laser wavelengths, ranging from 515 nm to 4.0 µm, paying special attention to the infrared region where silicon is transparent, in order to achieve deeper amorphization. They have also studied the influence of the number of pulses, the orientation of the silicon crystal and the placement of a thick covering layer of silicon dioxide on top of the material to be treated, to dissipate the heat of the deposited laser energy in a more efficient way.

The largest thickness value reached in the study is t = 128 nm, using laser pulses of λlaser = 3 µm on silicon covered with a one micron thick layer of SiO2. Despite being twice as thick as what has been achieved so far, it is still insufficient for asymmetric surface waveguides operating at telecommunication wavelengths.
For future research, they propose the deposition or union of a Si covering layer after writing the waveguide, which should allow a single-mode operation for the thickness values achieved in the present study.

Link to the scientific article

This work is a collaboration of the Institute of Optics, the Aix-Marseilley University and the Department of Applied Physics of the Autonomous University of Madrid.
 
Investigación financiada por el Ministerio de Ciencia e Innovación y la Agencia Estatal de Investigación
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