Nowadays, picosecond-long terahertz pulses are commonly generated by optical rectification in non-centrosymmetric crystals such as ZnTe, GaP and LiNbO3 pumped by femtosecond laser pulses. Peak electric fields exceeding 1 MV/cm can be obtained in LiNbO3 crystals at the expense of a complex pumping scheme (relying on the tilt of the pump pulse front). Alternatively, organic crystals (e.g. DAST, DTSMS, OH1 etc.) can also be employed to generate terahertz pulses via collinear optical rectification, showing good conversion efficiencies and high peak electric fields when pumped in the spectral region 1300-1500 nm using optical parametric amplifiers. Considering the growing interest of the industrial laser market towards ytterbium-based laser systems, which offer compactness, high output powers and lower costs compared to Ti:Sapphire lasers, we have explored the generation of high-energy, few-cycle terahertz pulses via optical rectification in the organic crystal HMQ-TMS, when pumped by 170-fs-long pulses at 1030 nm from a Yb:KWG amplified laser. The generated pulses have shown an energy value of about 1.1 μJ and a peak electric field higher than 200 kV/cm.

Terahertz pulses shorter than 100 fs (i.e., pulses with a bandwidth wider than 10 THz) with high peak electric fields (>100 kV/cm) are highly desired to investigate optical properties and fingerprints of materials in a wide frequency range, as well as to monitor ultrafast dynamics in condensed-matter systems with high temporal resolution. Such broadband and intense terahertz pulses can be obtained via two-color gas-based plasma sources which do not suffer from low damage thresholds typical of bulk-crystal based sources and are continuously renewable. Due to the very low dispersion of gases, their use as terahertz emitter and detector allows the generated bandwidth to be practically limited only by the laser pulse duration. To effectively generate broadband terahertz radiation via two-color plasma, pump pulse energies greater than 100 µJ and durations shorter than 100 fs are required. Yet, the pulse duration of ytterbium lasers is typically limited to hundreds of fs despite the power upscaling capability, due to the limited gain bandwidth and gain narrowing effects. Post-pulse compression techniques can bring the merits of ytterbium lasers into the few-cycle regime, which is desirable in various ultrafast applications. We have shown that 170 fs-long pulses emitted by an ytterbium amplified laser can be compressed potentially down to the single-cycle level (5.1 fs at 1030 nm) by utilizing a 6-m-long hollow-core fiber (HCF) filled with Ar gas. In this way, high compression factors, high transmission, and high beam quality can be achieved even for average powers higher than 100 W and high pulse energies. These advantages make HCFs, in combination with ytterbium lasers, an ideal tool for the generation of high-average-power, high-field and extremely broadband THz radiation via two-color-excited air plasma. Exploiting this pulse compression strategy, we have then demonstrated extremely broadband terahertz generation (up to 60 THz), with peak electric fields higher than 50 kV/cm, starting from the initial 170-fs-long pump laser pulses (∼2.6 THz of bandwidth full-width at half maximum). Our approach opens a path for the next generation of table-top terahertz time-domain setups combining high terahertz average powers (up to the W-level) and broadband operation, which can be a simpler alternative to terahertz sources relying on large facilities such as synchrotrons or free-electron lasers.

Lanthanide (Ln3+)‐doped upconverting nanoparticles (UCNPs) offer the possibility of converting near‐infrared (NIR) excitation light to higher energies, spanning the UV, visible, and NIR regions, which makes them versatile for applications in a wide range of fields including bioimaging and photovoltaics. In the quest for schemes to further enhance the upconversion process, one of the proposed solutions relies in coupling UCNPs with plasmonic nanostructures to enhance their emission. indeed, if the excitation frequency is set at the surface plasmon resonance, the local field in the plasmonic structure surrounding becomes much larger than the driving field. In this work, we have contributed, via numerical simulations, to the investigation of a combined system of gold nanorods and NaGdF4:Er3+/Yb3+ upconverting nanoparticles, which feature the double functionality of luminescence enhancement and monitored heating. The paired nanostructures could become an excellent optical heater with a thermal probe incorporated. Gold nanorods create a localized electromagnetic field that enhances the emission intensity from upconverting nanoparticles. At the same time, evidence of strong collective heating from the gold nanorods is demonstrated. Combining these nanostructures provides an all‐optical heating system with improved emission intensity that can be used to monitor the temperature achieved via luminescence thermometry.

Surface plasmon resonance (SPR) effects have been widely used to build photocatalysts that are active in the visible spectral region. Such plasmonic photocatalysts usually comprise a semiconductor material transparent in the visible range (such as TiO2) and plasmonic nanostructures like Au nanoparticles (Au NPs). Specific SPRs, though, cover the visible spectrum only partially and feature weak light absorption. Here, we have explored the unique role played by whispering gallery mode (WGM) resonances in the expression of the photocatalytic activity of plasmonic photocatalysts. Using numerical simulations, we have demonstrated that, by solely exploiting a proper geometrical arrangement and WGM resonances in a TiO2 sphere, the plasmonic absorption can be extended over the entire visible range and can be increased by more than 40 times. Furthermore, the local electric field at the Au–TiO2 interface can also be considerably enhanced. These results were experimentally validated by means of absorption spectroscopy and Raman measurements. Accordingly, such WGM-assisted plasmonic photocatalysts, when employed in water splitting experiments, exhibit enhanced activity in the visible range. Our results show a promising and straightforward way to design full solar spectrum photocatalysts.

Photoinitiation of radical polymerization is widely used in a plethora of technological processes, including in 3D-printing. Known photoinitiators have a limited absorption spectrum which renders the photo-polymerization of thick and/or pigmented samples challenging. In this work, we have contributed to the demonstration of a full solar-spectrum heterogeneous photocatalyst as a panchromatic photoinitiator, that is to say a compound capable of triggering polymerization with visible or near-infrared light, thus covering the entire solar spectrum. In order to ensure a sufficient radical flux, the heterogeneous photocatalyst has been designed to include Ag and Au plasmonic nanoparticles, as well as upconverting material and BiFeO3 semiconductor shells within a core@triple-shell structure.  Through this, rationally-designed full solar-spectrum photocatalysts have be shown to represent promising panchromatic photoinitiators.

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.