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Reversible control over the microparticle motion using light excites interesting applications in optofluidics, microswimmers, artificial optical matter, and biomedical engineering. The dielectric microspheres swim towards the near infrared pulsed laser in response to the backaction force mediated by photonic nanojet. Hereby, we report that the backaction force exhibits hysteretic behaviour owing to the distinguishable responses of the temperature rise inside the nanojet and the temperature rise of the liquid ensemble. Accordingly, the magnitude of backaction force at the same laser power varies between power increase and decrease stages. In order to develop multidimensional manipulation tool, we studied the possibility of using lasers with different spatiotemporal profiles to mediate the backaction force, and developed the counterpropagating beam scheme for reversible control of the particle motion directions. We further harness the hysteresis to reverse the direction of backaction force on dielectric particles in presence of a constant force from a counter-propagating beam with broadband supercontinuum spectrum. In contrast to the microsphere caught in the single beam gradient trap, the microsphere encounters augmented Brownian motion at higher balanced power level. The microsphere would eventually escape from the common region of the paired beams, enabling high throughput morphology analysis for cancer cell classification, biopsy, and diagnosis.

Optical radiation force points to the same direction as the photon flux, while the direction reversal is quite challenging and necessitates special efforts. Herein, we present the observation of a photonic nanojet-mediated backaction of dielectric particles owing to the local heating of solvent molecules inside the nanojet in a purely dielectric system. Such backaction has been verified to be photothermal, without the influence from thermally induced turbulence. Our findings have been theoretically corroborated by statistical analysis on the size-dependent force and speed. In addition to the increase with laser power, the backaction force is a competing effect between the absorption and thermal conductivity of the immersion medium and is also affected by the photothermally raised background temperature. The backaction force exerts on dielectric particles with a broad size range. Since most biological particles are dielectric, with a refractive index greater than that for the surrounding medium, this work could inspire applications in biophotonics, such as cell sorting and classification.

Metasurface provides miniaturized devices for integrated optics. Here, we design and realize a meta-converter to transform a plane-wave beam into multiple Laguerre-Gaussian (LG) modes of different orders at various diffraction angles. The metasurface is fabricated with Au nano-antennas, which vary in length and orientation angle for modulation of both the phase and the amplitude of a scattered wave, on a silica substrate. Our error analysis suggests that the metasurface design is robust over a 400 nm wavelength range. This work presents the manipulation of LG beams through controlling both radial and azimuthal orders, which paves the way in expanding the communication channels by one more dimension (i.e., radial order) and demultiplexing different modes.

Solitons can self-assemble into stable bound states, also denoted as soliton molecules that exhibit molecule-like dynamics. Soliton molecules have been predominantly investigated in the anomalous dispersion mode-locked fiber lasers. However, the soliton molecule dynamic evolution is still largely unexplored in the normal dispersion regime. We reveal here that, in the normal dispersion regime, the buildup and dissociation dynamics of soliton molecules. Our theoretical prediction indicates that, under different transmission functions of a saturable absorber (SA), a pair of solitons can be formed directly from background noise and then evolve into a soliton molecule through intense repulsive interaction, or a soliton molecule can be triggered to dissociate into a single soliton with transient annihilation and energy transfer. The experimental observation of short-time soliton molecule buildup and a new soliton molecule dissociation process corroborate the theoretical prediction. Furthermore, a long-time soliton molecule buildup ( ∼ 900 m s ) is discovered with single soliton splitting and soliton pair attraction. The buildup time is over four orders of magnitude longer than that of the short-time soliton molecule ( ∼ 21 µ s ). Our work unveils new perspectives into the ultrafast transient process and the interaction dynamics of soliton molecules in complex nonlinear systems.

Optical coherence tomography (OCT) relies on the reflection of light from structures in different layers to interferometrically reconstruct the volumetric image of the sample. However, light returned from multiple layers suffers from imbalanced attenuation owing to the optical path difference and inhomogeneous tissue absorption. We report an optimization algorithm to improve signal strength in deep tissue for swept-source (SS)-OCT imaging. This algorithm utilizes the attenuation coefficient of consecutive layers within the sample and combines them to compensate for the signal intensity loss from deep tissue. We stacked 170-µm thick cover slides as a standard sample for benchmark testing. The optimized OCT image provides a 30% increase in signal intensity in the deep structure compared with the conventional images. We applied this method for pearl inspection, whose layered structure demonstrates a great application for our optimized OCT imaging. In contrast to X-ray micro-CT scan and scanning electron microscope (SEM) imaging modalities, the optimized OCT imaging provides great potential for pearl quality inspection. The proposed improvement algorithm for SS-OCT could also be applied to diverse biomedical imaging scenarios, including label-free tissue imaging.

We report the first, to the best of our knowledge, demonstration of Grüneisen relaxation photoacoustic microscopy (GR-PAM) of lipid-rich tissue imaging at the 1.7 µm band, implemented with a high-energy thulium-doped fiber laser and a fiber-based delay line. GR-PAM enhances the image contrast by intensifying the region of strong absorbers and suppressing out-of-focus signals. Using GR-PAM to image swine-adipose tissue at 1725 nm, an 8.26-fold contrast enhancement is achieved in comparison to conventional PAM. GR-PAM at the 1.7 µm band is expected to be a useful tool for label-free high-resolution imaging of lipid-rich tissue, such as atherosclerotic plaque and nerves.