Origin of Waveguiding in Ultrashort Pulse Structured Silicon

Abstract

The origin of waveguiding in the bulk of silicon after sub‐ps laser inscription is investigated. Locally resolved Raman measurements of waveguide cross sections and along the propagation axis reveal highly localized crystal deformations. These modifications consist of highly confined regions of silicon with a disturbed crystal structure accompanied with strain. This transformation is responsible for a local increase of the refractive index allowing localized waveguiding. On the basis of near‐field measurements at an excitation wavelength of 1550 nm, the absolute value of the refractive index change is estimated to be in the range of 10−3.

In‐Volume Laser Direct Writing of Silicon—Challenges and Opportunities

“…Using this approach, longitudinal inscription of waveguides has been demonstrated by inducing a seed on the exit surface. [ 20,23,25,225 ] This offers a unique possibility for simple writing of optical functional devices in silicon. Based on recent efforts concentrating on silicon and parallel advances in ultrafast laser technologies, its seems natural to expect at short term the deployment of laser 3D writing to even narrower band‐gap materials and other important semiconductors such as zinc selenide, [ 226 ] zinc sulfide, [ 227 ] and gallium phosphide.…”

“…focused their attention on the origin of waveguiding by the inscribed structures based on Raman spectroscopy analyses. [ 23 ] As illustrated in Figure , the waveguide region is unambiguously exhibited with this technique in the spectral ranges of 430–500 cm (Figure 49a) and 930–1040 cm (Figure 49b) associated with amorphous and crystalline features, respectively. [ 176,177 ] The amorphous features increase in the first range while the crystalline features decrease in the second one.…”

“…[ 13–15 ] This novel understanding has subsequently offered multiple possibilities for circumventing these limitations, [ 16–20 ] and for producing a wide variety of material changes inside silicon. [ 21–23 ] This new avenue to 3D laser‐writing in the bulk of silicon has consequently led to the inscription of numerous functions including waveguides, [ 19,20,24–26 ] data storage, [ 19 ] holograms, [ 19 ] microfluidic channels, [ 19 ] gratings, [ 27 ] as well as surface texturing. [ 28 ] These first fruits mark the beginning of a new era for the laser functionalization of silicon as a next generation of J‐class mid‐infrared lasers is emerging, [ 29,30 ] offering additional possibilities for investigating this topic.…”

Inscribing diffraction grating inside silicon substrate using a subnanosecond laser in one photon absorption wavelength

“…Compared to the spectra from un-modified spots, the spectra from modified spots are broader and shifted to lower Raman shift. These changes can be explained by polycrystallization 29 , 30 or strain 31 , 32 .…”

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability

“…As a result positive refractive index change has been reported only in a few selected crystals. Examples include a thermally stable type-I waveguides in crystal 9 , colour-centres in LiF 10 , a transverse magnetic polarization guiding in potassium dihydrogen phosphate (KDP) crystal 11 , a change in the spontaneous polarization in that increases the extraordinary refractive index 7 , 12 , exploitation of the higher refractive index of amorphous silicon versus crystalline silicon 13 and ultrafast laser-induced lattice defects in Nd:YCOB crystals 14 . But, those waveguides either suffer from a low refractive index change of the order of or only guide a single polarization.…”