This PDF file contains the front matter associated with the SPIE Proceedings Volume 11122, including the title page, copyright information, table of contents, and author and conference committee lists.

Active devices and nanoantennas are promising research area of nanophotonics. They can be used to build high-speed elements, optical switches, and sensors. The ultrafast all-optical switches can be represented as semiconductor metasurfaces and nanoantennas, which scattering properties are controlled using femtosecond laser pulses in the pump-probe technique. In this work, the ultrafast dynamics of the light scattering is experimentally investigated for phased arrays of asymmetric sub-wavelength GaAs super-cells consisting of resonators of the various sizes. Energy reallocation of the incident radiation into the different diffraction orders, controlled by the design of the metasurface, was obtained. This effect appears only for the resonant polarization for the structure, as well as at wavelengths close to the optimized value of 800-815 nm. Such energy reallocation is a sign of the phase-manipulation behavior of the metasurface. GaAs metasurfaces are studied by Fourier plane imaging microscopy, in which pump and probe signals of different diffraction orders can be measured independently. The transmission coefficient modulation ∆𝑇⁄𝑇 of the probe pulse in the first diffraction order is shown to be ~15% at a pump fluence of 0.02 μJ/cm^2. The femtosecond relaxation time of free carriers in the GaAs metasurface is ~150 fs. These properties indicate that asymmetric GaAs nanoantennas can be used as all-optical switches.

The correlated polar semimetal Ca3Ru2O7 exhibits a rich phase diagram including two magnetic transitions (TN =56 K and TC =48 K) with the appearance of an insulating-like pseudogap (at TC ). In addition, there is a crossover back to metallic behavior at T∗=30 K, the origin of which is still under debate. We utilized ultrafast optical pump optical probe spectroscopy to investigate quasi- particle dynamics as a function of temperature in this enigmatic quantum material. n conjunction with density functional theory, our experimental results synergistically reveal the origin of the T-dependent pseudogap. Further, our data and analysis indicate that the T∗ emerges as a natural consequence of T-dependent gapping out of carriers, and does not correspond to a separate electronic transition. Our results highlight the value of low fluence ultrafast optics as a sensitive probe of low energy electronic structure, thermodynamic parameters, and transport properties of Ruddlesden-Popper ruthenates.

Lead-halide perovskite nanocrystals (NCs) have just emerged as a novel type of semiconductor nanostructure possessing great potentials in the optoelectronic, photovoltaic and quantum-information-processing applications. This renders it extremely necessary to have a comprehensive understanding of their electronic energy-level structures, which mysteriously exhibit either a doublet or a triplet exciton peak at the single-particle level. Here we show that transition from doublet to triplet excitons in single CsPbI3 NCs can be triggered by reinforcing quantum confinement in the same batch of sample upon being stored in the ambient environment. Besides size reduction and blue-shifted emission, this enhanced quantum confinement is also manifested by the suppressed emission of multiple and charged excitons in single CsPbI3 NCs with a triplet-exciton configuration. We propose that the doublet and triplet excitons should correspond respectively to the weak and strong quantum confinement regimes of single CsPbI3 NCs, with the electron-hole exchange interaction and the Rashba effect determining the exact energy-level alignments and the fine-structure splitting values.

Förster resonance energy transfer (FRET) is considered as a molecular ruler to quantify protein-protein interactions and structural conformation in a wide range of biomolecules in both controlled environments and in living cells. Here, we have employed integrated fluorescence spectroscopy methods to characterize the energy transfer efficiency and donor-acceptor distance for novel genetically engineered mCerulean3–linker– mCitrine environmental sensors. Based on the amino acids sequences of the linker region, these sensors can be sensitive to either macromolecular crowding or the ionic strength of the surrounding environment. These hetero-FRET sensors also enable us to develop new spectroscopic approaches for quantifying the energy transfer efficiency and the donor-acceptor distance as a means of elucidating the underlying mechanisms for environmental sensing. Ensemble averaging approaches using time-resolved fluorescence and time-resolved fluorescence polarization anisotropy of G12 sensor are highlighted. Our findings in control environments so far are currently being used for complementary studies in living cells.

We studied the use of vibrationally resonant, third-order sum-frequency generation (TSFG) for imaging of biological samples. We found that laser-scanning TSFG provides vibrationally sensitive imaging capabilities of lipid droplets and structures in sectioned tissue samples. Although the contrast is based on the infrared-activity of molecular modes, TSFG images exhibit a high lateral resolution of 0.5 μm or better. We observed that the imaging properties of TSFG resemble the imaging properties of coherent anti-Stokes Raman scattering (CARS) microscopy, offering a nonlinear infrared alternative to coherent Raman methods. TSFG microscopy holds promise as a high-resolution imaging technique in the fingerprint region where coherent Raman techniques often provide insufficient sensitivity.

We study the second-order nonlinear optical properties of several 2D materials through second harmonic generation (SHG) and sum frequency generation (SFG). SHG signals from 2D transition metal dichalcogenides (TMD) pumped at multiple fundamental wavelengths are measured and compared with theoretical analysis. We also use polarization-resolved second harmonic generation to characterize 2D materials and explore their biological applications. Using a narrow-band femtosecond laser beam and a supercontinuum, we measure the SFG of TMDs to characterize their second-order nonlinear susceptibility over a range of wavelengths.

In principle, nonlinear multi-photon microscopy is straight forward to attain increased imaging resolution by √2 or more, in which the signal is only generated at the very center of the focal spot of laser. Label-free and nonlinear CARS and SRS microscopies are applicable to this spot reduction effect. However, their excitation laser wavelengths are limited to near infrared (NIR), partially because NIR is the wavelength only commercially available for typical femtosecond laser. Thus, the potential improvement in spatial resolution is completely compromised by longer wavelengths adopted for imaging. To fully utilize nonlinear advantage to defeat resolution limit, we reduced the wavelengths of our femtosecond lasers to visible region and demonstrate hyperspectral blue SRS microscopy with resolution about ~100 nm. Moreover, the electronic pre-resonance condition was reported to enhance sensitivity of SRS imaging as the absorption of NIR dyes match the laser frequencies. In our concept, we gained SRS sensitivity by actively tuning the wavelengths of pump and Stokes lasers to near resonant to electronic transition of endogenous biomolecules (e.g. DNA and proteins). Additionally, to achieve specificity to biomolecules, we developed single optical fiber based spectral focusing technology, and demonstrated high-resolution hyperspectral SRS imaging of intact tissues.

Resonance Raman offers a significant increase in Raman signal levels. We show how this can be used to select a specific molecule within a complex biosystem to study, in our case to determine if hemoglobin survives in ancient fossils. Key to this ability is the fact that the vibration must be on the same molecule as the absorption. Further, we show that the Raman fingerprint, or changes to it, can provide further selectivity or identify changes in that molecule based upon the particular sample. In our case, we find that the iron in the hemoglobin has oxidized into FeOOH, but still attached to both its porphyrin-like heme group and the protein network that gives the hemoglobin absorption. Very narrow Raman resonances are found in molecules with symmetry-forbidden, phonon-allowed absorptions. We show several in biologically relevant materials including that methylated-DNA (m-DNA) can be distinguished from non-methylated (n-DNA) with nano-bowtie- and resonance-enhanced Raman spectra. These efiects are retained when plasmon resonances are used to enhance a local region of the sample, but find that the overall signal from a uniformly distributed specimen is not increased significantly by the enhancement of a small region, so is not recommended unless the sample can be concentrated into that region.

Underwater spectroscopy is always a challenge, especially when fighting organic pollutants, which cause fluorescence when looking at plankton in sea water. Using a free space coherent Raman spectroscopy we have obtained key parameters helping us to build our fibre laser spectroscopy system, which we deploy on a boat to measure in-situ, in real time seawater nutrients. We have developed a near background free 2-color coherent anti-Stokes Raman spectroscopy system (CARS), which is based on specially treated photonic crystal fibre supercontinuum in the near femtosecond regime. We will present a full system characterization including the choice of the appropriate wavelengths of the Stokes and pump pulse, optimization of the Stokes supercontinuum in the temporal and spectral domain, as temporal adjustment of our optimized home built fibre laser. Finally, for the first time, we present vessel based spectra of living plankton underwater around New Zealand’s coastal region via a specially designed Raman probe head.

Nonlinear and ultrafast microscopy techniques enable label-free chemical imaging with high sensitivity, specificity, and optical resolution. However, reliance on specialized high-intensity femtosecond laser sources makes these techniques expensive and introduces a risk for sample damage. Simpler linear imaging methods, such as reflectance confocal microscopy, only sense variations in refractive index, and lack clear contrast provided by nonlinear techniques. But in the case of biological samples, where there is often a structure-function relationship (e.g. a cell’s mitochondrial network dynamically rearranges itself in response to metabolic activity), the kind of chemical information picked up by nonlinear techniques might be inferred from linear reflectance texture. If such a mapping can be learned, multiphoton-like contrast could be synthesized with images from much simpler instrumentation. Our approach to synthetic nonlinear microscopy employs machine learning technique involving convolutional neural network, namely U-net, that has demonstrated promising performance in segmentation of biomedical images. We have incorporated a nonlinear laser-scanning microscope with a confocal detection channel in order to acquire a training dataset of co-registered reflectance and nonlinear images. Results will be presented, along with a discussion on how well we can expect the trained network to generalize to new specimens.

Light, in many ways, is an ideal form of electromagnetic waves to probe and treat biological tissues. But biomedical optical techniques encounter an inevitable trade-off between resolution and penetration depth due to the strong scattering of light in tissue; existing microscopic optical modalities seldom can see beyond the so-called optical diffusion limit (~1 mm for human skin). In this talk, we summarize our endeavors in the past years of using the synergy of light and sound to achieve high-resolution optical imaging, focusing, and neuron activation in thick biological tissue based on the synergy of light and sound and optical wavefront shaping. Limitations, potential applications, and further direction are also discussed. The work has been supported by the National Natural Science Foundation of China (no. 81671726 and no. 81627805), the Hong Kong Research Grant Council (no. 25204416), the Hong Kong Innovation and Technology Commission (no. ITS/022/18), and the Shenzhen Science and Technology Innovation Commission (no. JCYJ20170818104421564).

Single-pixel imaging (SPI) technology has garnered great interest within the last decade because of its ability to record high-resolution images using a single-pixel detector. The imaging speed of SPI is governed by the response time of digital micromirror devices (DMDs) and the amount of compression of acquired images, leading to low (ms-level) temporal resolution. Consequently, it is particularly challenging to investigate fast dynamic phenomena. Recently, a novel method called time-encoded single-pixel imaging based on photonic time-stretch to achieve high-speed SPI has been reported. It achieves a frame rate far beyond that can be reached with conventional single-pixel cameras.

In non-degenerate two-photon excitation (ND-TPE), electronic transition of fluorophores happens via absorption of two photons with different energies. This contrasts with conventional - or degenerate - two-photon excitation (D-TPE), where two photons with identical energies are absorbed. ND-TPE can improve performance of two-photon microscopy by extending the excitation wavelength range, reducing out-of-focus excitation, and increasing resolution and penetration depth. However, a systematic study of fluorophore performance under ND-TPE is missing, which is critical for the selection of optimal excitation wavelength combinations. It is a well-known fact that degenerate two-photon absorption spectra often deviate from theoretical predictions based on one-photon absorption spectra. Therefore, it is not clear whether non-degenerate two-photon absorption spectra are predictable from the corresponding degenerate spectra. Using our sensitive fluorescence-based spectroscopy technique, we measured non-degenerate two-photon absorption cross-sections (ND-TPACS) of several commonly used fluorophores and generated 2-dimensional ND-TPACS maps. We observed that the shape of the measured ND-TPACS spectra follows the spectra of the degenerate two-photon absorption cross-sections (D-TPACS). However, ND-TPACS are higher in magnitude, which is predicted by the “resonant enhancement” phenomenon. Therefore, we show that ND-TPACS spectra are predictable from the corresponding degenerate D-TPACS spectra under consideration of resonant enhancement. Predictability of ND-TPACS spectra is an important finding that helps choosing the optimal combination of wavelengths for ND-TPE of a given fluorophore without prior experimental measurement of ND-TPACS.

We present Doppler Raman, a novel detection technique for coherent Raman scattering microscopy that offers improved sensitivity and readily detects low frequency modes from 10cm-1 to 1500cm-1 for use in studying label-free biological systems.

A compact thulium-doped fiber chirped pulse amplifier (FCPA) is presented and used to pump a ZrF4-BaF2-LaF3-AlF3- NaF (ZBLAN) fiber for mid-infrared (MIR) supercontinuum generation. The FCPA is seeded by Raman soliton, which is generated by femtosecond erbium doped fiber laser and a piece of highly nonlinear fiber (HNLF) via soliton self frequency shift (SSFS) process. Additional with fiber stretcher and three stages thulium-doped fiber amplifier, the laser source delivers up to 10 W average power at repetition rate of 10 MHz with pulse width of 252 fs after grating compressor. Supercontinuum generation is realized by coupling the pulses into 10 m ZBLAN fiber with an aspheric lens. Since the laser source operates at anomalous dispersion regime of ZBLAN fiber, the spectral broadening is mainly dominanted by SSFS and stimulated Raman scattering (SRS) in this dispersion regime. Maximum output power of 800 mW corresponding to pulse energy of 80 nJ has been obtained with 5 dB spectrum ranging from 2 μm to 3.5 μm, which is limited by the low pump coupling efficiency of ~10% at this time. This fiber based supercontinuum source may have a wide variety of applications such as medical imaging, micro-spectroscopy and gas detection.

We have experimentally reported an ultrabroadband midinfrared (MIR) supercontinuum (SC) generation with high coherence property in chalcogenide tapered fiber with all normal dispersion (ANDi). The fibers, fabricated by an isolated extrusion method, made of Ge₂₀As₂₀Se₁₅Te₄₅ core and Ge₂₀As₂₀Se₂₀Te₄₀ cladding glasses. A homemade tapering platform allows us to accurately control the core diameters of the tapered fibers to realize ANDi characteristic. A coherent MIR SC spectrum spanning from 1.7–12.7 Μm was generated in a 7 cm long tapered fiber pumped at 5.5 μm. And high coherence property of the generated SC spectrum was investigated and verified by the simulation.