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Although organic–inorganic hybrid perovskites (OIHPs) are considered frontline candidates owing to their intriguing optoelectronic properties, their unstable surfaces are prone to degradation, significantly affecting device performance and hence limiting their commercialization. Additive engineering has become the most effective strategy to improve the quality of 3D perovskite single crystals (PSCs). Herein additive engineering employing various aliphatic, aromatic, and chiral ammonium salts results in high-quality transparent methylammonium lead bromide (MAPbBr3) PSCs, which are demonstrated for application in terahertz (THz) devices, besides being sources for high-order harmonics generation (HHG). Remarkably the engineered crystal surfaces showed resistance to surface degradation compared to pristine ones, thereby enhancing the transmission/reflection of THz pulses. Moreover, a noticeable blue shift in harmonics (both nano second (ns) and pico second (ps) laser induced plasmas (LIPs)) was achieved compared to pristine MAPbBr3 (MA). The current research is anticipated to open up new sources of additive-based SCs to be used entirely for THz devices and tunable high-order harmonic generation avenues. Exploring these inanimate materials via additive inclusion in MA SCs opens up new avenues to achieve blue-shifted UV spectra for diverse applications in attosecond physics and nonlinear spectroscopy, including tunable THz devices because of the easy tunability of the THz absorption properties, i.e. their phonon modes in an OIHPSC. [ABSTRACT FROM AUTHOR]

We study high-order harmonics generation from plasmas generated from graphite, fullerenes, carbon nanotubes, carbon nanofibers, diamond nanoparticles, and graphene. Our approach utilizes a heating nanosecond laser pulse to produce plasmas that serve as the media for high harmonic generation from a subsequent driven femtosecond laser pulse. High harmonics are generated at different time delays following the plasma formation, which allows us to analyze the spreading of species with different masses. We analyze the harmonic yields from species of single carbon atom, 60 atoms (fullerene), 106 atoms (diamond nanoparticles), 109 atoms (CNTs and CNFs), and even much larger species of graphene sheets. The harmonic yields are analyzed in the range of 100 ns–1 ms delays. The harmonic yields were significantly higher within the 200 ns–0.5 µs range, but no harmonic is observed between 10 µs–1 ms. Our observations show that, at the optimal ablation of atoms and clusters, the laser-induced plasmas produced on the surfaces of different carbon-contained species spread out from targets with the comparable velocities.

Silver (Ag) is one of the important targets in the studies of high harmonic generation (HHG) in the laser-induced plasma due to the relatively high conversion generation efficiencies and cut-off energy. In this work, we study the dependence of HHG in plasma plumes of Ag as a function of various laser parameters, including intensity of ablation pulse, intensity and ellipticity of driving pulse, and the delay between the two pulses. We identify the type of ion that dominates the measured HHG spectra by comparing the experimental data with strong-field approximation simulations and classical calculations for Ag atoms, Ag+, and Ag2+ ions. We also perform a comparative HHG study between the plasmas of bulk Ag target and the plasmas of a nano-powders target. It is found that the harmonic yields in the latter case are higher over a wide range of the aforementioned laser parameters. The results also indicate that the number of nanoparticles in the plasma generated from a nano-powder sample is indeed significantly greater than that in the plasma from the bulk sample. [ABSTRACT FROM AUTHOR]

The transition metal dichalcogenides have instigated a lot of interest as harmonic generators due to their exceptional nonlinear optical properties. Here, the molybdenum disulfide (MoS2) molecular structures with dopants being in a plasma state are used to demonstrate the generation of intense high-order harmonics. The MoS2 nanoflakes and nickel-doped MoS2 nanoflakes produced stronger harmonics with higher cut-offs compared with Mo bulk and MoS2 bulk. Conversely, the MoS2 with nickel nanoparticles and carbon nanotubes (MoS2-NiCNT) produced weaker coherent XUV emissions than other materials, which is attributed to the influence of phase mismatch. The influence of heating and driving pulse intensities on the harmonic yield and cut-off energies are investigated in MoS2 molecular structures. The enhanced coherent extreme ultraviolet emission at ~32 nm (38 eV) due to the 4p-4d resonant transitions is obtained from all aforementioned molecular structures, except for MoS2-NiCNT. [ABSTRACT FROM AUTHOR]

Gold films are widely used for different applications. We present the results of third- and high-order nonlinear optical studies of the thin films fabricated from Au nanoparticle solutions by spin-coating methods. These nanoparticles were synthesized by laser ablation of bulk gold in pure water using 200 ps, 800 nm pulses. The highest values of the nonlinear absorption coefficient (9 × 10−6 cm W−1), nonlinear refractive index (3 × 10−11 cm2 W−1), and saturation intensity (1.3 × 1010 W cm−2) were achieved using 35 fs, 400 nm pulses. We also determined the relaxation time constants for transient absorption (220 fs and 1.6 ps) at 400 nm. The high-order harmonic generation was studied during propagation of 35 fs, 800 nm pulses through the plasma during the ablation of gold nanoparticle film and bulk gold. The highest harmonic cutoff (29th order) was observed in the plasma containing gold nanoparticles.

Organometallic lead bromide and iodide perovskite single crystals (PSCs) are potential candidates for terahertz applications. Herein, we performed terahertz time-domain spectroscopy (THz-TDS) in the frequency range of 0.1–3.0 THz on different thicknesses of MAPbBr3 (0.3, 0.6, and 0.8 mm) and MAPbI3 (0.6, 0.8, 0.9, 1.3, and 2.3 mm). The measurements were carried out with respect to the position (along the focal area), azimuthal rotation of the PSCs, and incidence angles of the reference THz pulse on the PSCs' surface. Based on the transmitted THz pulses from PSCs from the above measurements, we calculated the real and imaginary parts of the refractive index, dielectric constants, absorption coefficients, and dark conductivity. These optical parameters tend to increase with decreases in the PSCs' thicknesses. The transmission spectra of the terahertz electric field indicate that the measured optical properties do not vary significantly with the position and orientation of PSCs. The real parts of the refractive index and dielectric constants are higher than the imaginary values for both PSCs. On the other hand, a slight blueshift in the optical phonon vibrations corresponding to Pb-Br/I-Pb and Pb-Br/I bonds is observed with an increase in thickness. Interestingly, the phonon vibrations do not vary with the incidence angle of the THz pulses on the same crystal's surface. The optical parameters based on THz-TDS reveal that the PSCs satisfy the requirement for tunable THz devices which need suitable, sensitive, and stable absorption properties between 0.1 and 3 THz. [ABSTRACT FROM AUTHOR]

Organic–inorganic lead halide perovskite nanocrystals (NCs) have received the phenomenal attention of researchers in the last few years due to their advanced optical properties and developments in simple synthetic procedures. In this work, we demonstrate the effect of Ni-dopant on the ultrafast third-order nonlinear optical (NLO) properties of two-dimensional (2D) CsPbBr3 NCs using 60 fs laser pulses at a wavelength of 400 nm. Based on the probe pulse intensity, pure CsPbBr3 NCs exhibited saturable absorption and reverse saturable absorption (RSA). The doping of NCs by Ni at different concentrations significantly modified the nonlinear absorption mechanism where the RSA was followed by the SA with growing input laser intensity. The nonlinear refractive index of Ni-doped CsPbBr3 NCs decreased when compared with the pure CsPbBr3. In addition, the NLO properties of these 2D NCs are correlated with the efficiency of the high-order harmonics generated during propagation of the two-color (800 + 400 nm) and single-color (800 nm) chirp-free (35 fs) and negatively/positively chirped pulses through the laser-induced plasmas produced on the surface of undoped and Ni-doped perovskite NCs. The ablation of Ni-doped NCs allowed increasing the harmonic yield compared to undoped CsPbBr3 NCs. Our studies undoubtedly demonstrate the advanced lower and higher order NLO properties of Ni-doped 2D CsPbBr3 NCs. [ABSTRACT FROM AUTHOR]

We report the two-photon absorption and nonlinear refraction studies of the nickel nanoparticles (NPs) in deionized water using femtosecond laser pulses at 400 and 800 nm. The laser ablation technique was employed for the synthesis of nickel NPs using nanosecond, picosecond, and femtosecond pulses. The largest values of nonlinear absorption coefficients and nonlinear refractive indices were determined to be 9 × 10−11 cm W−1 and 2 × 10−15 cm2 W−1 for smaller sized nickel NPs suspension. Third harmonic generation from the Ni plasma which contained NPs was analyzed using picosecond and femtosecond pulses as heating radiation and femtosecond probe pulses. [ABSTRACT FROM AUTHOR]

Mixed-halide perovskite materials (MHSCs) hold significant interest in photonics applications owing to their inherent advantages, including tunable bandgap properties, remarkable defect tolerance characteristics, and facile processability. These attributes position MHSCs as up-and-coming materials for various applications. However, the commercialization of these materials is severely affected by external factors, such as humidity and oxygen. The current work studies change in higher harmonics generation (HHG) in MAPbBr 3- x Cl x single crystals (MHSC) with changing nitrogen-based additives. These additives act as a passivating layer and improve the nanolevel crystallinity. The additive engineering strategy impacts morphological and optical properties, depending on the additive's interaction.

We demonstrate high-order harmonic generation in Ni-doped CsPbBr 3 perovskite nanocrystals ablated by nanosecond pulses using chirp-free 35 fs, and chirped 135 fs pulses in the case of single-color pump (800 nm) and a two-color pump (800 and 400 nm). We analyzed the spectral shift, cut-off, and intensity distribution of harmonics in the case of chirped drving pulses compared to chirp-free pulses. It is shown that the presence of Ni dopants and CsPbBr 3 plasma components improves the harmonics emission. Also, we measured the third-order nonlinear optical (NLO) properties of these nanocrystals using 800 nm, 60 fs, 1 kHz pulses. The variations of measured NLO parameters of CsPbBr 3 perovskite nanocrystals containing different concentrations of nickel correlate with variations of generated high-order harmonics from laser induced plasmas of studied nanocrystals in terms of harmonics intensity, cut-off, and spectral shift (in case of chirped driving pulses). The spectral shift of the harmonics generated from the Ni-doped CsPbBr 3 perovskite nanocrystals can be used to form tunable extreme ultraviolet sources., (© 2022 IOP Publishing Ltd.)