Superfluorescence (SF) is a unique quantum mechanical behavior arising from the self-organization between emitters, thus forming a cooperatively coupled assembly. In contrast to isotropic spontaneous emission or normal fluorescence, SF produces a short but intense burst of light, which makes it ideal for a wide variety of applications in photonics, electronics, and optical computing. Due to the prerequisite of cooperative emitter coupling, SF has been conventionally observed under cryogenic conditions in limited systems, such as atomic gases, and a few bulk material systems. Here we show lanthanide-doped upconversion nanoparticles (UCNPs) as a medium to achieve antiStokes shift SF at room temperature. We observe such room temperature upconverted SF in a few nanoparticles assembly, and in a single nanoparticle, the latter of which is the smallest-ever SF media. In particular, we found that under near-infrared light (800 nm) excitation, each lanthanide ion in a single UCNP nanocrystal can be considered as an individual emitter that interact with each other to establish coherence and to enable anti-Stokes shift SF emission. More importantly, when compared to the microsecond scale slow lifetime of typical upconversion luminescence, the upconverted SF has a 10,000-fold accelerated lifetime (τ = 46 ns of SF v.s. τ = 455.8 μs of normal upconversion luminescence). When taken together, the observed ultrafast upconverted SF in both UCNP assembly and single nanocrystals under NIR light excitations, is uniquely well-positioned for applications in on-chip optical computing, and biophotonics, especially in deep tissue ultra-fast dynamic sensing.

The correlated Stokes-anti-Stokes Raman scattering mediated by phonons was introduced in 1977 by Klyshko. In the last two decades, it has been broadly studied experimentally, including results on diamond, graphene and transparent liquid. The theoretical description of non-resonant Stokes-anti-Stokes pair production was shown to be formally similar to the BCS theory of superconductivity, raising attention to the study of pair production as a function of the Raman shift. Intriguingly, the pair production efficiency is not symmetric with respect to the positive versus negative detuning from phonon resonance, a result that was shown for a 180fs pulsed laser, and that remains without a theoretical explanation. Here we show the asymmetry is persistent in diamond measured with lasers of different pulse widths (180 fs and 5 ps) and different wavelengths (633 nm versus 785 nm).

The heterogeneous cellular environment influences a myriad of biological processes. For example, macromolecular crowding affects biochemical reactions, protein-protein interactions, and protein folding. Additionally, the structure-function relationship of biomolecules and enzymatic activities are sensitive to the surrounding ionic strength. In this contribution, we highlight our recent studies on a family of donor–linker– acceptor constructs, which were designed for mapping the macromolecular crowding and ionic strength in living cells. Integrated ultrafast laser spectroscopy methods have been employed to quantify the Förster resonance energy transfer (FRET) and the donor-acceptor distance as a measure of the sensitivity of these constructs to environmental changes. The donor-acceptor FRET pairs are intrinsically fluorescent cyan and yellow proteins, respectively, that can be genetically encoded in living cells. The sensitivity of these constructs to environmental biomimetic crowding and ionic strength was investigated as a function of the sequence and charge of the linker regions, as well as the identity of the donor protein. Integrating noninvasive, quantitative laser-induced fluorescence methods with FRET, as a molecular ruler, provides a powerful tool for cellular studies towards mapping out macromolecular crowding and ionic strength in living cells. Our results are key for the development of rational design strategies for engineering enhanced noninvasive biosensors with better environmental sensitivities. The same sensors were used as a model system for developing new experimental approaches for protein-protein interaction and FRET studies. Importantly, these diagnostic molecular and analytical tools set the stage for understanding the correlation between these environmental factors and cellular functions.

Quantification of molecular colocalization is an essential issue in understanding many biological processes in living organisms. To measure the spatial distribution of multiple biomolecules, an ideal way is to image them one by one in the exact same region of interest and the same focus plane. To achieve this goal, we integrated multiple optical imaging modalities including stimulated Raman scattering (SRS), multiphoton fluorescence (MPF), and second harmonic generation (SHG) all together into one multimodal microscopy. We further combined deuterium oxide probing with stimulated Raman scattering (DO-SRS) for visualizing newly synthesized protein and lipid molecules, in addition to macromolecules (protein and lipid, NADH and Flavin, collagen) imaged with label free SRS, MPF, and SHG. We quantitatively measured the metabolic dynamics in cells and animals under various conditions, including HeLa cells grew in different serine concentrations, Drosophila ovaries in young and old individuals, and two different types of breast cancer tissues from xenograft mouse models. The results show the capabilities and advantages of this multimodal imaging system in accessing the spatial distributions of multiple molecules quantitatively.

We present a transient absorption setup based on a low-power laser system and a photonic crystal fiber for supercontinuum generation. The setup employs an ultrafast erbium-doped fiber laser system that emits at 775 nm with 80 MHz repetition rate which pumps a non-linear photonic crystal fiber that provides a supercontinuum in the NIR/VIS wavelength region for probing. By using an acousto optic modulator for pump-beam modulation and a lock-in amplifier we were able to achieve a detection-limit of 1 μOD. The setup reveals the potential of photonic crystal fibers as broadband sources in combination with low-power lasers.

Ultrafast fiber lasers represent an affordable source for performing non-linear spectroscopies, like transient absorption or coherent antistokes Raman scattering, and can advance these technologies towards commercial devices. We experimentally investigate the generation of white light in photonic crystal fibers at low pulse energies and high repetition rates using 775 nm pulses at 80 MHz from the second harmonic of an Er:fiber laser. Two different fibers were chosen based on non-linear beam-propagation simulations. The generated broadband light was characterized and compared in terms of spectral bandwidth, pulse duration and shot-to shot noise, showing good agreement with the simulations.