Combined soliton pulse compression and plasma-related frequency upconversion in gas-filled photonic crystal fiber

Generation and characterization of frequency tunable sub-15-fs pulses in a gas-filled hollow-core fiber pumped by a Yb:KGW laser

We investigate soliton self-compression and photoionization effects in an argon-filled antiresonant hollow-core photonic crystal fiber pumped with a commercial Yb:KGW laser. Before the onset of photoionization, we demonstrate self-compression of our 220 fs pump laser to 13 fs in a single and compact stage. By using the plasma driven soliton self-frequency blueshift, we also demonstrate a tunable source from 1030 to ∼700 nm. We fully characterize the compressed pulses using sum-frequency generation time-domain ptychography, experimentally revealing the full time-frequency plasma-soliton dynamics in hollow-core fiber for the first time.

Ionization-induced adiabatic soliton compression in gas-filled hollow-core photonic crystal fibers

We investigate in the experiments the ionization-induced adiabatic soliton compression process in a short length of He-filled single-ring photonic crystal fiber. We observe that the plasma-driven blueshifting solitons show little residual light near the pump wavelength in a certain pulse energy region, leading to a high-efficiency frequency upconversion process. In contrast, at high pulse energy levels, we observe that the quality of the frequency upshifting process is impaired due to the existence of a dynamical loss channel induced by the coupling of the soliton to linear modes near the pump wavelength. In addition, through adjusting the input pulse energy, the central wavelength of blueshifting solitons can be continuously tuned over 300 nm. These experimental results, confirmed by numerical simulations, not only offer a deep insight into ionization-induced soliton-plasma dynamics in gas-filled hollow-core photonic crystal fibers, but also develop highly tunable ultrafast light sources at visible wavelengths, which may have many applications in ultrafast spectroscopy.

Highly-tunable, visible ultrashort pulses generation by soliton-plasma interactions in gas-filled single-ring photonic crystal fibers

Ultrashort laser pulses, featuring remarkable spectral tunability, are highly demanded for nonlinear light-matter interactions in a variety of molecules. Here, we report on the generation of soliton-plasma-driven ultrashort pulses with both bandwidth- and wavelength-tunability in the visible spectral region. Using He-filled single-ring photonic crystal fiber (SR-PCF), we demonstrate in the experiments that the spectral bandwidths of blueshifting solitons can be manipulated by adjusting the input pulse energy, gas pressure and core diameter of the SR-PCF, while the central wavelengths of these solitons can be continuously tuned over 200 nm. We found that in a large-core SR-PCF (24.6-µm core diameter), the bandwidths of blueshifting solitons can be effectively broaden to near 100 nm, pointing out the possibility of generating few-cycle, wavelength-tunable visible pulses using this set-up. In addition, we observed in the experiments that in a small-core SR-PCF (with a core diameter of 17 µm), the blueshifting solitons show little residual light near the pump wavelength, resulting in a high-efficiency frequency up-conversion process. These experimental results, confirmed by numerical simulations, pave the way to a new generation of compact, ultrashort light sources with excellent tunability at visible wavelengths, which may have many applications in the fields of time-resolved spectroscopy and ultrafast nonlinear optics.

Continuously wavelength-tunable blueshifting soliton generated in gas-filled photonic crystal fibers

We experimentally report the generation of wavelength-tunable blueshifting soliton in the visible spectral region through a gas-filled single-ring photonic crystal fiber (SR-PCF). In particular, in a He-filled SR-PCF, we observed a sharp narrow-band spectral peak at the first resonant spectral region of the SR-PCF, which results from phase-matched nonlinear processes. To the best of our knowledge, this is the first time investigating the influence of the core-cladding resonance on the blueshifting soliton. In addition, when Ar gas was filled into the SR-PCF, some interference fringes on the blueshifting soliton were observed at high pulse-energy levels due to plasma-induced pulse fission. These two experimental observations are confirmed by numerical simulations. Furthermore, through properly adjusting input pulse energy, we found that the blueshifting soliton can obtain a high conversion efficiency (∼84%) and its wavelength can be tuned over hundreds of nanometers (∼240  nm).

Near-octave intense mid-infrared by adiabatic down-conversion in hollow anti-resonant fiber

We show that adiabatic down-conversion can be made the dominant four-wave mixing process in an anti-resonant hollow-core fiber for nearly a full octave of mid-infrared bandwidth with energy exceeding 10 μJ, allowing the generation of energetic and shapeable two-cycle pulses. A numerical study of a tapered fiber with an applied gas pressure gradient predicts the efficient conversion of a 770-860 nm near-infrared frequency band to 3-5 μm, while a linear transfer function allows pre-conversion pulse shaping and simple dispersion management. Our proposed system may prove to be useful in diverse research topics employing nonlinear spectroscopy or strong light-matter interactions.

Wavelength-tunable few-cycle pulses in visible region generated through soliton-plasma interactions

We numerically investigate the generation of wavelength-tunable few-cycle pulses in the visible spectral region through soliton-plasma interactions. We found that in a He-filled single-ring photonic crystal fiber (SR-PCF), soliton-plasma interactions could shift the optical spectra of pulses propagating in the fiber to shorter wavelengths. Through adjusting the single pulse energy launched into the fiber, the central wavelength of these blueshifting pulses could be continuously tuned over hundreds of nanometers, while maintaining a high energy conversion efficiency of >57%. Moreover, we observed that during the nonlinear pulse propagation in the SR-PCF, soliton self-compression effects enhanced the plasma density in the fiber at high pulse energies, which could modulate the phase-matching condition of ultraviolet (UV) dispersive wave (DW) generation. Furthermore, we employed the recently-developed model to study numerically the loss and dispersion of the SR-PCF in its resonant and anti-resonant spectral regions, and demonstrated the remarkable influence of the core-cladding resonance on the process of soliton-plasma interactions. The numerical results demonstrated here pave the way to develop wavelength-tunable, few-cycle light sources in the visible region, which may have considerable application potential in pump-probe spectroscopy and strong-field physics.

PHz-Wide Spectral Interference Through Coherent Plasma-Induced Fission of Higher-Order Solitons

We identify a novel regime of soliton-plasma interactions in which high-intensity ultrashort pulses of intermediate soliton order undergo coherent plasma-induced fission. Experimental results obtained in gas-filled hollow-core photonic crystal fiber are supported by rigorous numerical simulations. In the anomalous dispersion regime, the cumulative blueshift of higher-order input solitons with ionizing intensities results in pulse splitting before the ultimate self-compression point, leading to the generation of robust pulse pairs with PHz bandwidths. The novel dynamics closes the gap between plasma-induced adiabatic soliton compression and modulational instability.

Optimal Sensitizer Concentration in Single Upconversion Nanocrystals

Each single upconversion nanocrystal (UCNC) usually contains thousands of photon sensitizers and hundreds of photon activators to up-convert near-infrared photons into visible and ultraviolet emissions. Though in principle further increasing the sensitizers' concentration will enhance the absorption efficiency to produce brighter nanocrystals, typically 20% of Yb³⁺ ions has been used to avoid the so-called "concentration quenching" effect. Here we report that the concentration quenching effect does not limit the sensitizer concentration and NaYbF₄ is the most bright host matrix. Surface quenching and the large size of NaYbF₄ nanocrystals are the only factors limiting this optimal concentration. Therefore, we further designed sandwich nanostructures of NaYbF₄ between a small template core to allow an epitaxial growth of the size-tunable NaYbF₄ shell enclosed by an inert shell to minimize surface quenching. As a result, the suspension containing 25.2 nm sandwich structure UCNCs is 1.85 times brighter than the homogeneously doped ones, and the brightness of each single 25.2 nm heterogeneous UCNC is enhanced by nearly 3 times compared to the NaYF₄: 20% Yb³⁺, 4% Tm³⁺ UCNCs in similar sizes. Particularly, the blue emission intensities of the UCNCs with the sandwich structure in the size of 13.6 and 25.2 nm are 1.36 times and 3.78 times higher than that of the monolithic UCNCs in the similar sizes. Maximizing the sensitizer concentration will accelerate the development of brighter and smaller UCNCs as more efficient biomolecule probes or photon energy converters.

Tunable frequency-up/down conversion in gas-filled hollow-core photonic crystal fibers

Based on the interplay between photoionization and Raman effects in gas-filled photonic crystal fibers, we propose a new optical device to control frequency conversion of ultrashort pulses. By tuning the input-pulse energy, the output spectrum can be either down-converted, up-converted, or even frequency-shift compensated. For low input energies, the Raman effect is dominant and leads to a redshift that increases linearly during propagation. For larger pulse energies, photoionization starts to take over the frequency-conversion process and induces a strong blueshift. The fiber-output pressure can be used as an additional degree of freedom to control the spectrum shift.

Strong Raman-induced noninstantaneous soliton interactions in gas-filled photonic crystal fibers

We have developed an analytical model based on the perturbation theory to study the optical propagation of two successive solitons in hollow-core photonic crystal fibers filled with Raman-active gases. Based on the time delay between the two solitons, we have found that the trailing soliton dynamics can experience unusual nonlinear phenomena, such as spectral and temporal soliton oscillations and transport toward the leading soliton. The overall dynamics can lead to a spatiotemporal modulation of the refractive index with a uniform temporal period and a uniform or chirped spatial period.

Raman-induced temporal condensed matter physics in gas-filled photonic crystal fibers

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means.