Gholamreza Shayeganrad

We demonstrate the feasibility of femtosecond laser pulses to tune refractive index of SiNx, enabling direct writing waveguides in SiNx and reconfiguring microring resonators. The refractive index increases with pulse energy and pulse density.

EPJ Web of Conferences, open-access proceedings in physics and astronomy

We report on multi-wavelength generation through simultaneous second-order and third-order nonlinear parametric processes following cascaded stimulated Raman scattering (SRS) in silica fibers. The fiber system consists of a short standard step-index silica fiber and a microfiber tapered from it. When this system is pumped with a 130 ps laser at 1040 nm, multiple new wavelengths in the UV (340–370 nm) and green (507–547 nm) bands arise through four-wave mixing (FWM)/sum-frequency generation (SFG) from the pump and its Raman signals. The evolution of these wavelengths with pump power is demonstrated in detail. Matching of frequencies and propagation constants in the involved processes is verified. This work provides a compact and simple method of multi-wavelength generation with fibers.

We demonstrate that the temporal contrast of femtosecond light pulses is a critical parameter in laser writing inside transparent dielectrics, allowing different material modifications. In particular, anisotropic nanopores in silica glass are produced by high-contrast of 10⁷ femtosecond Yb:KGW laser pulses rather than low-contrast of 10³ Yb fiber laser pulses. The difference originates in the fiber laser storing a third of its energy in a post-pulse of up to 200 ps duration. The absorption of this low-intensity fraction of the pulse by laser-induced transient defects with relatively long lifetime and low excitation energy, such as self-trapped holes, drastically changes the kinetics of energy deposition and the type of material modification. We also demonstrate that low-contrast pulses are effective in creating lamellar birefringent structures, possibly driven by a quadrupole nonlinear current.

In ultrafast laser writing, and light‐matter interaction in general, it has been widely accepted that the higher the energy density, the stronger material changes occur, unless thermal effects are involved. Here, this belief is challenged by demonstrating that a decreased energy density—achieved through increased scanning speed and without thermal accumulation—surprisingly leads to a more significant modification of silica glass, i.e, a higher increase in the isotropic refractive index or greater birefringence of nanopore‐mediated modification. This counterintuitive phenomenon is attributed to the nonlocality of light‐matter interaction at tight focusing, where the intensity gradient of the light beam and associated diffusion of charge carriers play a critical role in increasing material modification. A tenfold increase in the writing speed of the polarization multiplexed data storage with the potential to achieve MB s^‐1 using high transmission nanopore‐based modification.

This article investigates the quadrupole excitation of a trapped atom exposed to the tightly focused Laguerre-Gaussian (LG) beams with parallel and antiparallel spin angular momentum (SAM) and orbital angular momentum (OAM) under nonparaxial conditions. The Rabi frequency profile of allowed quadrupole transition channels, modified by SAM and OAM interaction, in the focal plane is provided. In the case of antiparallel SAM and OAM, the excitation probability undergoes substantial modification due to the considerable contribution of longitudinal intensity variations in tightly focused condition. The findings offer insights into controlling localized atom transition, including OAM transfer, with potential applications in qudit-based technologies.

We demonstrate that counterintuitively, silicon wafer stealth dicing can be performed with femtosecond laser pulses at 1030-nm, where linear absorption predominates. A 1.3-NA oil-immersion objective mitigated plasma defocusing and delocalization before the focal point.

Increase in modification strength induced by ultrafast laser writing at high scanning speed in silica glass is demonstrated. Counterintuitively, despite lower energy density, stronger modification is produced, which is beneficial for 5D optical data storage.

Photosensitivity in nature is commonly associated with stronger light absorption. It is also believed that artificial optical anisotropy to be the strongest when created by light with linear polarization. Contrary to intuition, ultrafast laser direct writing with elliptical polarization in silica glass, while nonlinear absorption is about 2.5 times weaker, results in form birefringence about twice that of linearly polarized light. Moreover, a larger concentration of anisotropic nanopores created by elliptically polarized light pulses is observed. The phenomenon is interpreted in terms of enhanced interaction of circularly polarized light with a network of randomly oriented bonds and hole polarons in silica glass, as well as efficient tunneling ionization produced by circular polarization. Applications to multiplexed optical data storage and birefringence patterning in silica glass are demonstrated.

Birefringent patterning by ultrafast laser nanostructuring in silica glass has been used for space-variant birefringent optics with high durability and high optical damage threshold. We demonstrate that the oblate-shaped birefringent modification (type X) with ultrahigh optical transmission has higher optical damage resistance, comparable to pristine silica glass. The lower damage threshold of nanogratings based modification (type 2) following thermal annealing at 900°C for an hour is improved from 0.96 J/cm² to 1.62 J/cm² for 300 fs laser pulses and approaches the optical damage threshold of type X (1.56 J/cm²). This opens the door to utilize these optical elements for high power laser applications where optical transmission and damage threshold are the key parameters. The lower damage threshold of type 2 modification is related to the relatively high concentration of defects, such as E’ centers and oxygen-deficiency centers (ODCs).