Nonlinear refractive index of optical crystals

Stable Vortex Solitons Sustained by Localized Gain in a Cubic Medium

We propose a simple dissipative system with purely cubic defocusing nonlinearity and nonuniform linear gain that can support stable localized dissipative vortex solitons with high topological charges without the utilization of competing nonlinearities and nonlinear gain or losses. Localization of such solitons is achieved due to an intriguing mechanism when defocusing nonlinearity stimulates energy flow from the ringlike region with linear gain to the periphery of the medium where energy is absorbed due to linear background losses. Vortex solitons bifurcate from linear gain-guided vortical modes with eigenvalues depending on topological charges that become purely real only at specific gain amplitudes. Increasing gain amplitude leads to transverse expansion of vortex solitons, but simultaneously it usually also leads to stability enhancement. Increasing background losses allows creation of stable vortex solitons with high topological charges that are usually prone to instabilities in conservative and dissipative systems. Propagation of the perturbed unstable vortex solitons in this system reveals unusual dynamical regimes, when instead of decay or breakup, the initial state transforms into stable vortex solitons with lower or sometimes even with higher topological charge. Our results suggest an efficient mechanism for the formation of nonlinear excited vortex-carrying states with suppressed destructive azimuthal modulational instabilities in a simple setting relevant to a wide class of systems, including polaritonic systems, structured microcavities, and lasers.

Buried Depressed-Cladding Waveguides Inscribed in Nd3+ and Yb3+ Doped CLNGG Laser Crystals by Picosecond-Laser Beam Writing

Buried depressed-cladding waveguides were fabricated in 0.7-at.% Nd:Ca3Li0.275Nb1.775Ga2.95O12 (Nd:CLNGG) and 7.28-at.% Yb:CLNGG disordered laser crystals grown by Czochralski method. Circular waveguides with 100 μm diameters were inscribed in both crystals with picosecond (ps) laser pulses at 532 nm of 0.15 μJ energy at 500 kHz repetition rate. A line-by-line writing technique at 1 mm/s scanning speed was used. Laser emission at 1.06 μm (with 0.35 mJ pulse energy) and at 1.03 μm (with 0.16 mJ pulse energy) was obtained from the waveguide inscribed in Nd:CLNGG and Yb:CLNGG, respectively, employing quasi-continuous wave pumping with fiber-coupled diode lasers. The waveguide realized in RE3+-doped CLNGG crystals using ps-laser pulses at high repetition rates could provide Q-switched or mode-locked miniaturized lasers for a large number of photonic applications.

Femtosecond pulse generation from a SESAM mode-locked Tm,Ho:SrF2 laser at 2.08 µm

We report on a passively mode-locked Tm,Ho:SrF2 laser employing a SESAM as saturable absorber (SA), delivering nearly Fourier-transform-limited 246 fs pulses at 2084nm without any additional intra- or extra-cavity dispersion compensation elements. This represents, to the best of our knowledge, the shortest pulses generated from the mode-locked fluoride bulk lasers in the 2-µm spectral range. Such compact femtosecond laser can be a potential seed source for large-sized fluoride bulk amplifier systems with exact gain match, enabling the generation of ultrashort intense pulses around 2 µm.

Electrical, Thermal Lens and Optical Study of Fluorescein Film for Application As Organic Photovoltaic Devices

This article is devoted to the study of various dielectric and optoelectrical parameters nonlinear optic behaviors, thermal lens and self-diffraction parameters of Fluorescein (FLs) doped polymethyl methacrylate (PMMA) films. The films were prepared with 60 mM. These studies are based on the calculated values of refractive, absorption coefficient, energy gap, extinction coefficient and nonlinear Refraction index ( n 2 ) . The polymer films were prepared using the casting technique. All samples were previously investigated by UV-Vis-NIR spectrophotometric measurements and Optical microscopy SEM and ATM. Utilizing thermal lens spectrometry, an investigation of the thermo-optical characteristics as well as the nonlinear refractive index was carried out. In this method, a pump beam and a probe beam were brought into collinear alignment with one another. To determination the nonlinear Refraction index ( n 2 ) . High values of nonlinear refractive index predict a bright future for materials in optical applications. These results indicate that the new dye is a promising candidate for applications in nonlinear optical devices. Investigations were carried out on organic photovoltaic devices in addition to devices consisting of active layers with conducting polymer of PHPP:P3HT film and PHPP:P3HT/Fls. The methods of polymer and dyes synthesis are presented and their physical properties are given.

Refractiveindex.info database of optical constants

We introduce the refractiveindex.info database, a comprehensive open-source repository containing optical constants for a wide array of materials, and describe in detail the underlying dataset. This collection, derived from a meticulous compilation of data sourced from peer-reviewed publications, manufacturers' datasheets, and authoritative texts, aims to advance research in optics and photonics. The data is stored using a YAML-based format, ensuring integrity, consistency, and ease of access. Each record is accompanied by detailed metadata, facilitating a comprehensive understanding and efficient utilization of the data. In this descriptor, we outline the data curation protocols and the file format used for data records, and briefly demonstrate how the data can be organized in a user-friendly fashion akin to the books in a traditional library.

P T-symmetry breaking in dual-core phosphate-glass photonic crystal fibers

We investigate the properties of a soft glass dual-core photonic crystal fiber for application in multicore waveguiding with balanced gain and loss. Its base material is a phosphate glass in a P2O5-Al2O3-Yb2O3-BaO-ZnO-MgO-Na2O oxide system. The separated gain and loss cores are realized with two cores with ytterbium and copper doping of the base phosphate glass. The ytterbium-doped core supports a laser (gain) activity under excitation with a pump at 1000 nm wavelength, while the CuO-doped is responsible for strong attenuation at the same wavelength. We establish conditions for an exact balance between gain and loss and investigate pulse propagation by solving a system of coupled generalized nonlinear Schrödinger equations. We predict two states of light under excitation with hyperbolic secant pulses centered at 1000 nm: 1) linear oscillation of the pulse energy between gain and loss core (P T-symmetry state), with strong power attenuation; 2) retention of the pulse in the excited gain core (broken P T-symmetry), with very modest attenuation. The optimal pulse energy levels were identified to be 100 pJ (first state) and 430 pJ (second state).

Exploring the Femtosecond Filamentation Threshold in Liquid Media Using a Mach-Zehnder Interferometer

We experimentally studied the supercontinuum induced by femtosecond filamentation in different liquid media. Using a Mach-Zehnder interferometer, we determined the relative filamentation thresholds (Pth) of these media. Research has shown that the value of the filamentation threshold is greater than that of Pcr (critical power for self-focusing), which can mainly be attributed to the strong dispersion effect. Changing the focal length of the focusing lens affects filamentation dynamics, thereby affecting the measured results regarding the filamentation threshold. With shorter focal lengths, the linear focusing (i.e., geometrical focusing) regime dominates, and the measured values of Pth for different liquid media are almost the same; as the focal length becomes larger, self-focusing starts to play a role, making the values of Pth for different media different from each other. This study presents an efficient method for investigating the femtosecond filamentation phenomenon in liquid media, helpful to provide further insights into the physical mechanism of supercontinuum generation via femtosecond filamentation in liquid media.

Polarization Splitting at Visible Wavelengths with the Rutile TiO2 Ridge Waveguide

On-chip polarization control is in high demand for novel integrated photonic applications such as polarization division multiplexing and quantum communications. However, due to the sensitive scaling of the device dimension with wavelength and the visible-light absorption properties, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at visible wavelengths. In this paper, a new polarization-splitting mechanism based on energy distributions of the fundamental polarized modes in the r-TiO₂ ridge waveguide is investigated. The bending loss for different bending radii and the optical coupling properties of the fundamental modes in different r-TiO₂ ridge waveguide configurations are analyzed. In particular, a polarization splitter with a high extinction ratio operating at visible wavelengths based on directional couplers (DCs) in the r-TiO₂ ridge waveguide is proposed. Polarization-selective filters based on micro-ring resonators (MRRs) with resonances of only TE or TM polarizations are designed and operated. Our results show that polarization-splitters for visible wavelengths with a high extinction ratio in DC or MRR configurations can be achieved with a simple r-TiO₂ ridge waveguide structure.

KGW and YVO4: two excellent nonlinear materials for high repetition rate infrared supercontinuum generation

We present an experimental investigation of supercontinuum generation in potassium gadolinium tungstate (KGW) and yttrium vanadate (YVO₄) crystals pumped with 210 fs, 1030 nm pulses from an amplified Yb:KGW laser operating at 2 MHz repetition rate. We demonstrate that compared to commonly used sapphire and YAG, these materials possess considerably lower supercontinuum generation thresholds, produce remarkable red-shifted spectral broadenings (up to 1700 nm in YVO₄ and up to 1900 nm in KGW) and exhibit less bulk heating due to energy deposition during filamentation process. Moreover, durable damage-free performance was observed without any translation of the sample, suggesting that KGW and YVO₄ are excellent nonlinear materials for high repetition rate supercontinuum generation in the near and short-wave infrared spectral range.

Heterogeneous integration of a III-V quantum dot laser on high thermal conductivity silicon carbide

Heat accumulation prevents semiconductor lasers from operating at their full potential. This can be addressed through heterogeneous integration of a III-V laser stack onto non-native substrate materials with high thermal conductivity. Here, we demonstrate III-V quantum dot lasers heterogeneously integrated on silicon carbide (SiC) substrates with high temperature stability. A large T₀ of 221 K with a relatively temperature-insensitive operation occurs near room temperature, while lasing is sustained up to 105°C. The SiC platform presents a unique and ideal candidate for realizing monolithic integration of optoelectronics, quantum, and nonlinear photonics.

Kerr-lens mode-locking of an Yb:CLNGG laser

We report on a Kerr-lens mode-locked laser based on an Yb³⁺-doped disordered calcium lithium niobium gallium garnet (Yb:CLNGG) crystal. Pumping by a spatially single-mode Yb fiber laser at 976 nm, the Yb:CLNGG laser delivers soliton pulses as short as 31 fs at 1056.8 nm with an average output power of 66 mW and a pulse repetition rate of ∼77.6 MHz via soft-aperture Kerr-lens mode-locking. The maximum output power of the Kerr-lens mode-locked laser amounted to 203 mW for slightly longer pulses of 37 fs at an absorbed pump power of 0.74 W, which corresponds to a peak power of 62.2 kW and an optical efficiency of 20.3%.

Polarization spectroscopy of high-order harmonic generation in gallium arsenide

An interesting property of high harmonic generation in solids is its laser polarization dependent nature which in turn provides information about the crystal and band structure of the generation medium. Here we report on the linear polarization dependence of high-order harmonic generation from a gallium arsenide crystal. Interestingly, we observe a significant evolution of the anisotropic response of above bandgap harmonics as a function of the laser intensity. We attribute this change to fundamental microscopic effects of the emission process comprising a competition between intraband and interband dynamics. This intensity dependence of the anisotropic nature of the generation process offers the possibility to drive and control the electron current along preferred directions of the crystal, and could serve as a switching technique in an integrated all-solid-state petahertz optoelectronic device.

Kerr-lens mode-locked Yb:YAlO3 laser generating 24-fs pulses at 1085 nm

We report on a Kerr-lens mode-locked Yb:YAlO₃ laser generating soliton pulses as short as 24 fs at 1085 nm with an average output power of 186 mW and a pulse repetition rate of 87.5 MHz, representing the shortest pulses ever achieved from any mode-locked laser based on Yb³⁺-doped structurally ordered crystal. Optimized for power-scalable operation, the Yb:YAlO₃ laser delivers 1.9 W at 1060 nm at the expense of a longer pulse duration of 44 fs, corresponding to a peak power of 462 kW and an optical efficiency of 43.2%.

Interferometric measurements of nonlinear refractive index in the infrared spectral range

This study presents the development and application of interferometric technique for the measurement of nonlinear refractive index of optical materials, while directly accounting for experimentally determined laser pulse shape and beam profile. The method was employed in a systematic study of nonlinear refractive index on a series of common optical materials used in near and mid-IR spectral range, where experimental data on nonlinear material properties is still scarce. The values of nonlinear refractive index were determined at 1.03 µm, 2.2 µm, and 3.2 µm. The measurement results are compared to the values determined by previous studies (where available), and the influence of cascaded second-order nonlinearities is discussed.

Amplification of femtosecond pulses based on χ(3) nonlinear susceptibility in MgO

We experimentally demonstrate large, widely tunable gain using Kerr instability amplification in MgO. By pumping the crystal near optical damage at 1.4×10¹³W/cm² by a femtosecond Ti:sapphire laser, we amplify visible and near-infrared pulses by factors >5000 or a gain g≈17/mm. We temporally characterize the pulses to show that they are 42 fs in duration, much shorter than the pump pulse. In the non-collinear setup, the angle between the pump and seed selects the amplified wavelength, where we find certain angles amplify both the visible and near-infrared simultaneously. We find that near the maximum pumping intensities, higher-order nonlinearities may play a role in the amplification process.

High-Q microresonators on 4H-silicon-carbide-on-insulator platform for nonlinear photonics

The realization of high-quality (Q) resonators regardless of the underpinning material platforms has been a ceaseless pursuit, because the high-Q resonators provide an extreme environment for confining light to enable observations of many nonlinear optical phenomenon with high efficiencies. Here, photonic microresonators with a mean Q factor of 6.75 × 10⁶ were demonstrated on a 4H-silicon-carbide-on-insulator (4H-SiCOI) platform, as determined by a statistical analysis of tens of resonances. Using these devices, broadband frequency conversions, including second-, third-, and fourth-harmonic generations have been observed. Cascaded Raman lasing has also been demonstrated in our SiC microresonator for the first time, to the best of our knowledge. Meanwhile, by engineering the dispersion properties of the SiC microresonator, we have achieved broadband Kerr frequency combs covering from 1300 to 1700 nm. Our demonstration represents a significant milestone in the development of SiC photonic integrated devices.

Oxacarbon superalkali C3X3Y3 (X = O, S and Y = Li, Na, K) clusters as excess electron compounds for remarkable static and dynamic NLO response

An intriguing class of excess electron oxacarbon superalkali clusters is explored for nonlinear optical response through density functional theory (DFT) methods at CAM-B3LYP/6-311++G(d,p). These superalkali clusters shows noticeable binding energies per atom (E_b) which reveals their thermodynamic stabilities (-86.45 ∼ -119.44 kcal mol^-1). The obtained significant VIPs values also suggest the electronic stability of these clusters. The VIP values range from 2.06 eV to 3.42 eV. These clusters show remarkable electronic properties and their HOMO-LUMO gaps (E_H-L) are significantly reduced. The lowest H-L gap of 0.96 eV is obtained for C₃O₃K₃ while the highest H-L gap of 2.07 eV is calculated for C₃S₃Li₃. The obtained PDOS spectra further provide evidence for the superior electronic properties of these clusters. The clusters show excellent nonlinear optical properties as revealed from remarkable values (1.6 × 10⁶ au) of static first hyperpolarizability. The controlling factors for hyperpolarizability are also explored by using conventional two-level model. The calculated values of β_o are correlated nicely with β_tl. The crucial excitation energy is the key factor in controlling the first hyperpolarizability. In these excess electron clusters, the second hyperpolarizability (γ_o) response increases up to 4.3 × 10⁹ au. Moreover, the calculated scattering hyperpolarizability (β_HRS) values are quite significant in these clusters and the highest value of 1.3 × 10⁶ au is calculated for C₃S₃K₃. Additionally, these clusters also possess larger dynamic nonlinearities. The dynamic second hyperpolarizability with dc-Kerr effect increases up to 1.0 × 10¹¹ au. The remarkable values for refractive index (n₂) also suggest the excellent nonlinearity of these superalkali clusters.

Femtosecond anti-Stokes conical emission in BK-7 glass and O and E-wave calcite

The angle of anti-Stokes conical emission (CE) is experimentally measured in the frequency shift span of 2000cm^-1 to 9000cm^-1. The experiment was performed using a 800 nm 50 fs laser pump in samples of BK-7 glass and calcite in both the O and E-wave configurations. The experimental results of angular emission are then compared to three competing models: the Alfano-Shapiro four wave mixing (FWM) model from 1970, the Luther FWM model from 1994, and the Faccio X-wave model from 2004. Results indicate that in all samples and configurations tested, the original FWM has the best agreement with experimental results in the anti-Stokes span.

Nonlinear Optical Investigation of Microbial Chromoproteins

Membrane-bound or cytosolic light-sensitive proteins, playing a crucial role in energy- and signal-transduction processes of various photosynthetic microorganisms, have been optimized for sensing or harvesting light by myriads of years of evolution. Upon absorption of a photon, they undergo a usually cyclic reaction series of conformations, and the accompanying spectro-kinetic events assign robust nonlinear optical (NLO) properties for these chromoproteins. During recent years, they have attracted a considerable interest among researchers of the applied optics community as well, where finding the appropriate NLO material for a particular application is a pivotal task. Potential applications have emerged in various branches of photonics, including optical information storage and processing, higher-harmonic and white-light continuum generation, or biosensorics. In our earlier work, we also raised the possibility of using chromoproteins, such as bacteriorhodopsin (bR), as building blocks for the active elements of integrated optical (IO) circuits, where several organic and inorganic photonic materials have been considered as active components, but so far none of them has been deemed ideal for the purpose. In the current study, we investigate the linear and NLO properties of biofilms made of photoactive yellow protein (PYP) and bR. The kinetics of the photoreactions are monitored by time-resolved absorption experiments, while the refractive index of the films and its light-induced changes are measured using the Optical Waveguide Lightmode Spectroscopy (OWLS) and Z-scan techniques, respectively. The nonlinear refractive index and the refractive index change of both protein films were determined in the green spectral range in a wide range of intensities and at various laser repetition rates. The nonlinear refractive index and refractive index change of PYP were compared to those of bR, with respect to photonics applications. Our results imply that the NLO properties of these proteins make them promising candidates for utilization in applied photonics, and they should be considered as valid alternatives for active components of IO circuits.

Thermal lensing effects and nonlinear refractive indices of fluoride crystals induced by high-power ultrafast lasers

Thermo-optical and nonlinear property characterization of refractive optical components is essential for endoscopic instrumentation that utilizes high-power, high-repetition-rate ultrafast lasers. For example, ytterbium-doped fiber lasers are well suited for ultrafast laser microsurgery applications; however, the thermo-optical responses of many common lens substrates are not well understood at 1035 nm wavelength. Using a z-scan technique, we first measured the nonlinear refractive indices of CaF₂, MgF₂, and BaF₂ at 1035 nm and found values that match well with those from the literature at 1064 nm. To elucidate effects of thermal lensing, we performed z-scans at multiple laser repetition rates and multiple average powers. The results showed negligible thermal effects up to an average power of 1 W and at 10 W material-specific thermal lensing significantly altered z-scan measurements. Using a 2D temperature model, we could determine the source of the observed thermal lensing effects. Linear absorption was determined as the main source of heating in these crystals. On the other hand, inclusion of nonlinear absorption as an additional heat source in the simulations showed that thermal lensing in borosilicate glass was strongly influenced by nonlinear absorption. This method can potentially provide a sensitive method to measure small nonlinear absorption coefficients of transparent optical materials. These results can guide design of miniaturized optical systems for ultrafast laser surgery and deep-tissue imaging probes.

Explanation of the competition between O- and E-wave induced stimulated Raman and supercontinuum in calcite under ultrafast laser excitation

Key optical properties of calcite were measured to unravel the difference between stimulated Raman scattering (SRS) and self-phase modulation (SPM) for the supercontinuum (SC) for ordinary (O) wave and extraordinary (E) wave. These properties are group velocity dispersion, walk-off, spontaneous Raman spectra and cross section, optical 1086cm^-1 phonon linewidth, nonlinear susceptibility (χ³), steady-state and transient SRS, and SC caused from SPM. These are investigated for O-waves and E-waves from a 2.7 cm thick calcite crystal. Using 390 fs pulses (∼0.8µJ pulse energy) at 517 nm, the O-wave produced a stronger sharp SRS peak at 1086cm^-1 and a weaker SC spectrum in the visible range than the E-wave. The salient difference found between the O- and E-waves for SRS and SPM in calcite is attributed to the larger Raman cross section and the size of nonlinear susceptibility (χ³) for O-waves as compared to E-waves.

Topological photonic crystals: a review

The field of topological photonic crystals has attracted growing interest since the inception of optical analog of quantum Hall effect proposed in 2008. Photonic band structures embraced topological phases of matter, have spawned a novel platform for studying topological phase transitions and designing topological optical devices. Here, we present a brief review of topological photonic crystals based on different material platforms, including all-dielectric systems, metallic materials, optical resonators, coupled waveguide systems, and other platforms. Furthermore, this review summarizes recent progress on topological photonic crystals, such as higherorder topological photonic crystals, non-Hermitian photonic crystals, and nonlinear photonic crystals. These studies indicate that topological photonic crystals as versatile platforms have enormous potential applications in maneuvering the flow of light.

A Seedless Method for Gold Nanoparticle Growth inside a Silica Matrix: Fabrication of Materials Capable of Third-Harmonic Generation in the Near-Infrared

A composite in which gold nanoparticles (AuNPs) approximately 10 nm in size are embedded in amorphous transparent silica matrix has been produced. The synthetic protocol uses HAuCl₄ as the Au ion source, tetraethoxysilane (TEOS) as the SiO₂ precursor, and l-ascorbic acid (AA) as the reducing agent. AA is employed before the sol-gel process in an amount sufficient only for reduction of Au³⁺ ions to Au⁺ . By using a cationic surfactant, benzylcetyldimethylammonium chloride hydrate (BDAC) and/or cetyltrimethylammonium bromide (CTAB), the Au⁺ ions are encapsulated within metalomicelles, which prevents them from being reduced to Au⁰ and enables their homogeneous distribution in the gel. Reduction of Au⁺ to Au⁰ and the growth of the AuNPs occurs at room temperature during the gelation, and arises from the release of EtOH during the hydrolysis of TEOS. The composites contain 0.027 wt % of Au. They exhibit nonlinear optical behavior characterized by the third-order nonlinear refraction index, n₂ , in the range 3.6-5.7×10^-16  cm²  W^-1 at λ=1.030 μm. The composites are capable of effective third-harmonic generation of ultrashort near-IR (210 fs, 1.030 μm) laser pulse through a direct third-order mechanism.

Linear and Nonlinear Absorption of Titanium Dioxide Films Produced by Plasma Ion-Assisted Electron Beam Evaporation: Modeling and Experiments

Titanium dioxide films were prepared by plasma ion-assisted electron beam evaporation. Linear optical properties were investigated in terms of spectrophotometry using the beta-distributed oscillator (ß_do) model as a parametrized dispersion law. The nonlinear two-photon absorption coefficient of titanium dioxide was determined by means of the laser-induced deflection technique at a wavelength of 800 nm. The obtained values of (2–5) × 10−11 cm/W were consistent with published experimental values for rutile as well as for simulations performed in the frames of the ß_do and Sheik–Bahae models.

Synthesis and characterization of LiLuF4:Er3+ and LiLuF4:Yb3+,Er3+ exhibiting upconversion fluorescence pumped by a 1560 nm laser

The UC emission at ∼1 μm could be enhanced by doping Yb³⁺ ions in LiLuF₄:Er microcrystals under 1560 nm excitation.

Ultrafast nonlinear refraction measurements of infrared transmitting materials in the mid-wave infrared

We utilize the conventional Z-scan technique to provide absolute measurements of third-order nonlinear refraction coefficients (n₂) in the mid-wave infrared at 2 µm and 3.9 µm of common optical materials that have transparency windows spanning this regime. We study a variety of narrow band gap and wide band gap semiconductors, fluoride crystals (BaF₂, CaF₂, LiF, and MgF₂) and optical glasses, and a series of chalcogenide glasses. The n₂ is found to span on the order of ∼10^-15 to ∼10^-12 cm²/W for the semiconductors, ∼10^-16 cm²/W for the fluoride crystals and glasses, and ∼10^-14 to ∼10^-13 cm²/W for the chalcogenides. The experimental results are compared to previous measurements of n₂ conducted in the visible and near-infrared along with empirical and theoretical formulations.

Beta-distributed oscillator model as an empirical extension to the Lorentzian oscillator model: physical interpretation of the ${\beta \_{{\rm do}}}$β_do model parameters

The physical sense of the free parameters of the beta-distributed oscillator (${\beta \_{{\rm do}}}$β_do) dispersion model is discussed. It is shown that the set of six model parameters provides information on central wavenumber, oscillator strength, absorption edge position, asymmetry, as well as homogeneous and inhomogeneous linewidth of a complicated absorption feature. For materials satisfying the Moss rule, the number of independent ${\beta \_{{\rm do}}}$β_do parameters decreases down to five. We also show that the Cody absorption edge shape essentially represents a special case of the ${\beta \_{{\rm do}}}$β_do approach. By making use of the generalized Miller rule, we propose a generalization of the ${\beta \_{{\rm do}}}$β_do model to nonlinear refractive indices and absorption coefficients.

High-harmonic generation in solids driven by counter-propagating pulses

We used two 800 nm laser pulses propagating in the opposite directions, to drive the emission of high-order vacuum ultra-violet harmonics off of the surface of an MgO (100) single crystal. We demonstrated the advantages that our approach provides compared to a single beam geometry, in both forward and backward emission.

High-Q microresonators integrated with microheaters on a 3C-SiC-on-insulator platform

We demonstrate, to the best of our knowledge, the first thermally reconfigurable high-Q silicon carbide (SiC) microring resonators with integrated microheaters on a 3C-SiC-on-insulator platform. We extract a thermo-optic coefficient of around 2.67×10^-5/K for 3C-SiC from wavelength shift of a resonator heated by a hot plate. Finally, we fabricate a 40-μm-radius microring resonator with intrinsic Q of 139,000 at infrared wavelengths (∼1550  nm) after integration with a NiCr microheater. By applying current through the microheater, a resonance shift of 30 pm/mW is achieved in the microring, corresponding to ∼50  mW per π phase shift. This platform offers an easy and reliable way for integration with electronic devices as well as great potential for diverse integrated optics applications.

Large optical nonlinearity of ITO/Ag/ITO sandwiches based on Z-scan measurement

Indium tin oxide (ITO)-based sandwich structures with the insertion of silver (ITO/Ag/ITO) show large nonlinear optical enhancement of both nonlinear refraction and saturable absorption. Here optical nonlinearity is measured using a Z-scan experiment with a 1310 nm pulsed laser at normal incidence. The nonlinear refractive index (n₂=15.43×10^-16  m²/W) for an ITO/Ag/ITO sandwich with a 14 nm silver interlayer and the nonlinear absorption coefficient (β=-648×10^-11  m/W) for an ITO/Ag/ITO sandwich with a 10 nm silver interlayer are about 18 and 16 times greater than that of a single-layer ITO, respectively. Meanwhile, a large figure of merit and modulation depth values hinted that ITO/Ag/ITO sandwiches are promising saturable absorber materials to switch continuous laser waves into laser pulses. These novel nonlinear optical properties make ITO/Ag/ITO sandwiches a promising candidate for all-optical modulation devices at optical communication wavelengths.

High-quality factor, high-confinement microring resonators in 4H-silicon carbide-on-insulator

Silicon carbide (SiC) exhibits promising material properties for nonlinear integrated optics. We report on a SiC-on-insulator platform based on crystalline 4H-SiC and demonstrate high-confinement SiC microring resonators with sub-micron waveguide cross-sectional dimensions. The Q factor of SiC microring resonators in such a sub-micron waveguide dimension is improved by a factor of six after surface roughness reduction by applying a wet oxidation process. We achieve a high Q factor (73,000) for such devices and show engineerable dispersion from normal to anomalous dispersion by controlling the waveguide cross-sectional dimension, which paves the way toward nonlinear applications in SiC microring resonators.

Nonlinear refractive index measurement by SPM-induced phase regression

Herewith, we describe how intensity and phase of the ultrashort pulse retrieved with second-harmonic frequency-resolved optical gating (SHG FROG) can be utilized for measurement of the nonlinear refractive index (n ₂). Through comparison with available literature, we show that our method surpasses Z-scan in terms of precision by a factor of two, and thus, constitutes an interesting alternative. We present results for various materials: fused silica, calcite, YVO ₄, BiBO, CaF ₂, and YAG at 1030 nm. Unlike the Z-scan, the use of this method is not restricted to free-space geometry, but due to its characteristics, it can be used in integrated waveguides or photonic crystal fibers as well.

UK, P. P. N, Chandrasekharan K. Effects of substituents on the enrichment of the optical limiting action of novel imidazo[2,1-b][1,3,4]thiadiazole fused thiophene-based small molecules

Schematic of the optical limiting action of a novel imidazo[2,1-b][1,3,4]thiadiazole based small molecule (ThITD3), which blocks high irradiance and transmits low-intensity (less harmful) light.

K. SN, D. NR, K. C. A phenothiazine–silver hybrid system exhibiting switching and photo-induced enhancement in nonlinear optical absorption

A novel photo-responsive hybrid system made of phenothiazine and silver nanoparticles showing enhanced nonlinear absorption and switching.

Millijoule-level, kilohertz-rate, CPA-free linear amplifier for 2 μm ultrashort laser pulses

The generation of millijoule-level ultrashort laser pulses at a wavelength of 2.05 μm in a compact chirped pulse amplification-free linear amplifier based on Holmium-doped YLF gain medium is presented. More than 100 MW of pulse peak power has been achieved. We show the capabilities of this laser amplifier from a 1 kHz to 100 kHz repetition rate. A detailed numerical description supports the experimental work and verifies the achieved results.

Theoretical study of multilayer coatingreflection taking into account third-order optical nonlinearities

A theoretical approach is presented that allows calculating the reflectance of dielectric multilayer coatings taking third-order optical nonlinearities into account. The description is based on third-order optical susceptibility so that nonlinear refraction and two-photon absorption processes are automatically considered in terms of the real and imaginary parts of the susceptibility. Two model systems are calculated in order to demonstrate the physical validity of the approach: a mirror that is optimized for maximum reflectance at a given wavelength and a given incident intensity and a mirror that provides a rather flat dependence of the reflected intensity on the incident one. Model calculations are performed for laser wavelength values of 1064 nm, 800 nm, and 532 nm.

High-Q integrated photonic microresonators on 3C-SiC-on-insulator (SiCOI) platform

We report a high-quality 3C-silicon carbide (SiC)-on-insulator (SiCOI) integrated photonic material platform formed by wafer bonding of crystalline 3C-SiC to a silicon oxide (SiO₂)-on-silicon (Si) substrate. This material platform enables to develop integrated photonic devices in SiC without the need for undercutting the Si substrate, in contrast to the structures formed on conventional 3C-SiC-on-Si platforms. In addition, we show a unique process in the SiCOI platform for minimizing the effect of lattice mismatch during the growth of SiC on Si through polishing after bonding. This results in a high-quality SiCOI platform that enables record high Qs of 142,000 in 40 µm radius SiC microring resonators. The resulting SiCOI platform has a great potential for a wide range of applications in integrated optics, including nonlinear optical devices, quantum optical devices, and high-power optical devices.

Toward Metal Halide Perovskite Nonlinear Photonics

The possibility of controlling light using the nonlinear optical properties of photonic devices opens new points of view in information and communications technology applications. In this Perspective, we review and analyze the potential role of metal halide perovskites in a framework different from their usual one in photovoltaic and light-emitting devices, namely, the one where they can play as nonlinear photonic materials. We contextualize this new role by comparing the few extant results on their nonlinear optical properties to those of other known nonlinear materials. As a result of this analysis, we provide a vision of future developments in photonics that can be expected from this new perspective on metal halide perovskites.

Plasma impact on structural, morphological and optical properties of copper acetylacetonate thin films

The influence of plasma exposure on structural, morphological and optical properties of copper (II) acetylacetonate thin films deposited by thermal evaporation technique was investigated. Copper (II) acetylacetonate as-grown thin films were exposed to the atmospheric plasma for different times. The exposure of as-grown cu(acac)₂ thin film to atmospheric plasma for 5min modified its structural, morphological and optical properties. The effect of plasma exposure on structure and roughness of cu(acac)₂ thin films was evaluated by XRD and AFM techniques, respectively. The XRD results showed an increment in crystallinity due to exposure for 5min, but, when the exposure time reaches 10min, the film was transformed to an amorphous state. The AFM results revealed a strong modification of films roughness when the average roughness decreased from 63.35nm to ~1nm as a result of interaction with plasma. The optical properties of as-grown and plasma exposured cu(acac)₂ thin films were studied using spectrophotometric method. The exposure of cu(acac)₂ thin films to plasma produced the indirect energy gap decrease from 3.20eV to 2.67eV for 10min exposure time. The dispersion parameters were evaluated in terms of single oscillator model for as-grown and plasma exposured thin films. The influence of plasma exposure on third order optical susceptibility was studied.

Synthesis, molecular, electronic structure, linear and non-linear optical and phototransient properties of 8-methyl-1,2-dihydro-4H-chromeno[2,3-b]quinoline-4,6(3H)-dione (MDCQD): Experimental and DFT investigations

Base catalysed ring opening ring closure (RORC) reaction of 6-methylchromone-3‑carbonitrile (1) with 1,3-cyclohexanedione afforded 8-methyl-1,2-dihydro-4H-chromeno[2,3-b]quinoline-4,6(3H)-dione (MDCQD). Theoretical calculations by Density Functional Theory (DFT) at the B3LYP/6-311G (d,p) level of theory was utilized to illustrate the equilibrium geometries of MDCQD. Also, the nonlinear optical properties, simple harmonic vibrational frequencies, thermo-chemical parameters and Mullikan atomic charges were calculated. In addition, the electronic absorption spectra in polar and non polar solvents were discussed on the basis of TD-DFT calculations. A nanofiber-like structure with high aggregation was resolved by using scanning electron microscopy images and its particle sizes were measured by particle size analyzer. The spectroscopic characteristics of the prepared thin film of MDCQD were studied in a wide spectral range of 200-2500nm. The analysis of the absorption edges affords two direct optical band gaps with energies of 1.00 and 2.76eV. A characteristic emission peak of photoluminescence spectrum in the visible region was detected and has a red-shift as a result of solvent polarity. The MDCQD film based heterojunction showed rectification behavior and diode-like characteristics. The photovoltaic characteristics under illumination of 100mW/cm² were studied. The open-circuit voltage and short-circuit current were found to be 0.22V and 4.25×10^-7A/cm², respectively. Moreover, the prepared heterojunction showed remarkable phototransient characteristics which afford the probability for the operation as a photodiode.

Light amplification by seeded Kerr instability

Amplification of femtosecond laser pulses typically requires a lasing medium or a nonlinear crystal. In either case, the chemical properties of the lasing medium or the momentum conservation in the nonlinear crystal constrain the frequency and the bandwidth of the amplified pulses. We demonstrate high gain amplification (greater than 1000) of widely tunable (0.5 to 2.2 micrometers) and short (less than 60 femtosecond) laser pulses, up to intensities of 1 terawatt per square centimeter, by seeding the modulation instability in an Y₃Al₅O₁₂ crystal pumped by femtosecond near-infrared pulses. Our method avoids constraints related to doping and phase matching and therefore can occur in a wider pool of glasses and crystals even at far-infrared frequencies and for single-cycle pulses. Such amplified pulses are ideal to study strong-field processes in solids and highly excited states in gases.

Compact titanium dioxide waveguides with high nonlinearity at telecommunication wavelengths

Dense integration of photonic integrated circuits demands waveguides simultaneously fulfilling requirements on compactness, low loss, high nonlinearity, and capabilities for mass production. In this work, titanium dioxide waveguides with a thick core of 380 nm exhibiting a compact mode size (0.43 μm²) and a low loss (5.4 ± 1 dB/cm) at telecommunication wavelengths around 1550 nm have been fabricated and measured. A microring resonator having a 50 μm radius has been measured to have a loaded quality factor of 53500. Four-wave mixing experiments reveal a nonlinear parameter for the waveguides of 21-34 W^-1 m^-1 corresponding to a nonlinear index around 2.3-3.6 x 10^-18 m²/W, which results in a wavelength conversion efficiency of -36.2 dB. These performances, together with the potentially simple dispersion engineering to the fabricated waveguides by the post processes, yield a strong promise for the titanium dioxide waveguides applied in photonic integrated circuits, especially for nonlinear implementations.

Nonlinear refractive index measurements using time-resolved digital holography

In this Letter, a novel method to evaluate nonlinear refractive index using time-resolved digital holographic microscopy is introduced. To demonstrate the viability of the method, cross-correlative nonlinear refractive index values for sapphire are measured experimentally: 2.75·10^-20  m²/W at 1030 nm and 4.10·10^-20    m²/W 515 nm wavelengths. The obtained results for sapphire are compared to those available in literature obtained by other methods.