Impact of Cr3+ substitution on the nephelauxetic ratio and Racah parameter of Cr-Mn-Zn nanoferrites

For the first time, we present the ligand field splitting (10Dq) nephelauxetic ratio (β), Racah parameter (B) and stability of Cr3+ within Mn–Zn nanoferrite which synthesized using the citrate route. These parameters were determined via optical absorption spectra. The obtained bands in the significance range of optical absorption spectra are wide as well as asymmetric; therefore the deconvolution process becomes a necessary task. The values of 10Dq and B have been estimated from the produced bands. The values of 10Dq are observed to increase from 16366.61 to 16447.37 cm-1 whereas, B values are observed to decrease from 829.81 to 760.80 cm-1 with additional Cr3+ substitution. The increasing 10Dq and decreasing B values, evidence for bonds between Cr3+ ions with their environments became a further covalent (less ionic). Moreover, the values of β are observed to decrease for further Cr3+ substitution. This decrease of β produces less stability between Cr3+ and its ligand. Furthermore, Dq/B values declare that Cr3+ centers are in the low-field sites by a slight tendency towards the high-field sites with further Cr/Fe substitution process. Therefore, it was suggested that Cr3+ substituted Mn–Zn nanoferrites are excellent candidates for diverse optical applications such as a tunable laser.

Nonlinear Absorption and Refraction of Picosecond and Femtosecond Pulses in HgTe Quantum Dot Films

We report measurements of the saturated intensities, saturable absorption, and nonlinear refraction in 70-nm thick films containing 4 nm HgTe quantum dots. We demonstrate strong nonlinear refraction and saturable absorption in the thin films using tunable picosecond and femtosecond pulses. Studies were carried out using tunable laser pulses in the range of 400–1100 nm. A significant variation of the nonlinear refraction along this spectral range was demonstrated. The maximal values of the nonlinear absorption coefficients and nonlinear refractive indices determined within the studied wavelength range were −2.4 × 10−5 cm2 W−1 (in the case of 28 ps, 700 nm probe pulses) and −3 × 10−9 cm2 W−1 (in the case of 28 ps, 400 nm probe pulses), respectively. Our studies show that HgTe quantum dots can be used in different fields e.g., as efficient emitters of high-order harmonics of ultrashort laser pulses or as laser mode-lockers.

Tunable laser induced periodic surface structures in Ge2Sb2Te5 thin films

Planar photonic structures, such as gratings and metasurfaces, are routinely used for beam steering, waveguide coupling, and light localization. However, conventional fabrication techniques that involve lithography are demanding in terms of time and cost. Much cheaper and simpler methods for surface patterning and formation of periodic surface structures are enabled by direct laser processing. Here, we demonstrate low-cost rapid fabrication of high-quality phase gratings based on the formation of laser induced periodic surface structures (LIPSS, or ripples) in Ge2Sb2Te5 (GST) thin films. Due to unique phase change properties of GST, the structures demonstrate strong modulation of refractive index with period controlled by the wavelength of laser irradiation. We study the formation of phase change LIPSS in a broad range of excitation wavelengths and observe transition between regimes with different orientations of generated ripples with respect to laser polarization.

A Stable 1550nm WGM Laser Generated by Yb3+/Er3+ Co-doped Silica Microspheres under μm ASE Source Pumping

The amplified spontaneous emission (ASE) light source at 1 μm was used to excite Yb3+/Er3+ co-doped silica (YEDS) microspheres by evanescent-wave coupling through a taper optical fiber (TOF) to generate a stable 1550 nm laser. Inevitably, the experimental process was accompanied by a frequency up-conversion luminescence whose fluorescence spectrum was collected and analyzed. Pumped by the ASE light source with different power, the system of YEDS microsphere and TOF also generated single-mode laser and multi-longitudinal mode laser. Their peak wavelength, output power of the peak laser, full width at half maximum (FWHM), and side mode suppression ratio (SMSR) were respectively measured. Moreover, the characteristics of a 1550 nm whispering gallery mode (WGM) laser excited by a 1 μm ASE pumping and a tunable laser (TLS) pumping were compared under different variable conditions. It was demonstrated that the ASE pump source has polarization in all directions to excite WGMs in the microsphere cavity, which was different from and superior to the TLS. Therefore, the laser generated by the ASE pump source as an excitation source was not affected by vibration and temperature change. The results show that the 1550 nm laser pumped by ASE can stably output the microsphere WGM mode laser under the interference of vibration and temperature change, which is more suitable for the actual application environment.