The opposition between the fs-laser-induced negative refractive index change and the positive refractive index change due to the FLIBGS can result in a zero refractive index change at specific wavelengths, which theoretically enable invisibility. Moreover, preliminary results show potential invisibility applications. This type of inscription has several advantages over structures based solely on the stress induced by damage tracks traditionally inscribed in crystals 5 or glasses 6– 8 using high-energy laser pulses (i.e., the so-called type III modifications 13). Using this FLIGBS phenomenon, we demonstrate that the sign of the refractive index contrast can be inverted, which allows for the direct inscription of smooth waveguides (i.e., type I-positive refractive index change) in crystals. For the remainder of the paper, the stated wavelengths refer to light propagating in the waveguides. Note that the propagating wavelengths are studied near the resonance, which are not to be confused with the wavelength used to process the material with the fs laser (producing the FLIGBS) that is far from the resonance (fixed at 795 nm in this work). For the first time, to the best of our knowledge, the effect of the FLIBGS in transparent materials is studied. However, for propagating wavelengths approaching the electronic resonance, we show that the refractive index change exponentially increases owing to a fs-laser-induced band-gap shift (FLIBGS). Away from resonances, the compaction and rarefaction of the structural network (affecting the number of charged particles per volume unit) and other mechanisms such as color centers, a change in the fictive temperature, and defect-induced density changes largely dominate the refractive index change in fs-laser-processed photonic circuits 1, 6, 12. In most applications of photonics, the propagating wavelengths are far from the material resonances to minimize the optical losses the fiber-optic communication window around 1550 nm in fused silica is a good example. In fact, many applications, such as waveguide lasers 9, electro-optic modulators 10, and frequency converters 11, require multi-scan-depressed cladding structures, which complicate or impede the fabrication or their guiding circuits. Another important limitation is the decrease in the refractive index that occurs in most crystals 5 and in a wide variety of glasses 6– 8. In particular, the miniaturization of many fs-laser-processed photonic devices is limited by the minimum bend radius of waveguides, which in turn depends on the magnitude of the induced refractive index contrast. However, a severe limitation of fs-laser inscription is related to the relatively low photoinduced refractive index contrast that is achievable 3, 4. One of the most relevant advantage is the micrometer-scale processing of complex three-dimensional structures, owing to the nonlinear nature of the laser absorption that precisely confines structural changes to the focal volume. Finally, the effect of the FLIBGS can compensate for the fs-laser-induced negative refractive index change, resulting in a zero refractive index change at specific wavelengths, paving the way for new invisibility applications.įemtosecond (fs) laser inscription in transparent materials has unique advantages 1, 2. We also demonstrate that the refractive index contrast can be switched from negative to positive, allowing direct waveguide inscription in crystals. First, we demonstrate waveguide bends down to a submillimeter radius, which is of great interest for higher-density integration of fs-laser-written quantum and photonic circuits. Supported by theoretical calculations, based on a modified Sellmeier equation, the Tauc law, and waveguide bend loss calculations, we experimentally show that several applications could take advantage of this phenomenon. We propose to address this issue by employing a femtosecond-laser-induced electronic band-gap shift (FLIBGS), which has an exponential impact on the refractive index change for propagating wavelengths approaching the material electronic resonance, as predicted by the Kramers–Kronig relations. However, the magnitude of the refractive index change is rather limited, preventing the technology from being a tool of choice for the manufacture of compact photonic integrated circuits. Multiphoton absorption via ultrafast laser focusing is the only technology that allows a three-dimensional structural modification of transparent materials.
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