Control of the spatial characteristics of femtosecond laser filamentation in glass via feedback-based wavefront shaping with an annular phase mask.

We propose a feedback-based wavefront shaping with an annular phase mask to control the spatial characteristics of femtosecond laser filamentation in K9 glass. A closed-loop feedback driven by a genetic algorithm was used to search for the optimal phase profile for generating the specified filaments. We demonstrate the flexibility of this method to extend or shorten filaments, improve continuity, and simultaneously control the position of filaments with specified lengths. Our approach offers a flexible regulation of the spatial characteristics of femtosecond laser filamentation for its potential applications.

Temporal visualization of femtosecond laser pulses with single-edge transport in turbid media via Monte Carlo simulation.

We study the propagation of femtosecond laser pulses with a single (front or rear) edge or dual edge through turbid media via Monte Carlo simulation. The results show that both the transmitted pulses spread on the basis of the incident pulse width ${t_{p}} = {{100}}\;{\rm{fs}}$, arising from the scattering effect. Further, the broadening width of the incident laser with a dual-edge pulse is wider than that of the incident laser width a single-edge pulse. The effect of the scattering particles on the front edge and the rear edge of the femtosecond laser can be distinguished in the time domain for femtosecond laser pulses through turbid media with the optical depth (OD) less than 10. In this scattering regime, the front-edge pulse scattered by the particles contributes more to diffused photons, but the effect of the scattering particles on the front edge and the rear edge of the femtosecond laser cannot be discriminated in turbid media with the OD more than 10, where the scattering is dominated by multiple scattering.

Diagnosis of liquid-gas mixed sprays in the near-field region using femtosecond laser induced supercontinuum imaging method.

Direct femtosecond shadowgraphy and supercontinuum (SC)-illumination imaging methods for diagnosing liquid-gas mixed sprays in the near-field region of spray nozzles were compared. Some big spray structures can be captured using femtosecond shadowgraphy which can freeze the motion of the sprays. But the speckles caused by the interference of multi-scattered photons erode the edges of ligaments and conceal many fine droplets. SC-illumination imaging can not only freeze the motion of the sprays but also significantly suppressing the speckles, presenting a more realistic spray pattern. Based on the SC imaging technology, the effects of various factors such as flow ratio of gas to liquid (GLR), total flow and nozzle size on the spray were studied.

Comparison of femtosecond shadowgraphy and optical Kerr gated ballistic imaging for measurements of spray structures.

Two femtosecond methods for imaging moderate liquid-gas sprays in the near-field region of spray nozzles - shadowgraphy and optical Kerr gated (OKG) ballistic imaging - were compared. Most spray structures can be captured using femtosecond shadowgraphy that can freeze the motion of the sprays. Femtosecond OKG ballistic imaging can distinguish finer structures and more-realistic liquid sheets by the filtering of multiple scattered photons. To compensate for the high-spatial-frequency components of the spray structures filtered by OKG ballistic imaging, differential OKG (DOKG) ballistic imaging was demonstrated. For dilute sprays, femtosecond shadowgraphy with a relatively simple experimental setup is recommended. For dense sprays, femtosecond OKG (or DOKG) ballistic imaging is more suitable.

Ballistic imaging through an intense scattering medium using a supercontinuum with a roundabout spatial gate.

We propose a new ballistic imaging method that is capable of imaging an object through an intense scattering medium. In this method, a femtosecond supercontinuum and a roundabout spatial gate were used to suppress speckles and filter background noise, respectively. The roundabout spatial gate extracts ballistic light and avoids low-pass spatial filtering to ensure the high resolution of images. The experimental results showed that even when the optical depth of the scattering medium reached 17, the images extracted by the method had improved identifiability and contrast.

Speckle-suppressed full-field imaging through a scattering medium using a supercontinuum.

We demonstrate speckle-suppressed full-field imaging through a scattering medium using incoherent supercontinuum (SC) illumination. The patterns in images obtained using SC illumination were found to be more easily identifiable than those in images acquired using coherent direct laser illumination due to the speckle suppression. Even when the optical depth reached 12.3, the patterns remained identifiable. As one of the potential applications, we also demonstrated the imaging for a high-pressure diesel spray using SC illumination.

Optimum heterodyning angle for heterodyned optical Kerr gated ballistic imaging.

We perform heterodyned optical Kerr gated (HOKG) ballistic imaging of an object hidden behind a turbid medium using a femtosecond laser. The experimental results show that an optimum heterodyning angle should be selected to acquire the highest spatial resolution of the HOKG imaging system. The optimum heterodyning angle depends on the scattering parameters of the turbid media, and it decreases with increasing optical density or decreasing thickness of the turbid medium.

Sharpness-enhanced ultrafast imaging by using a biased optical Kerr gate.

We proposed a novel biased optical Kerr gated imaging (BOKGI) method for ultrafast imaging. The imaging performance of the BOKGI system has been investigated. Experimental results showed that by using the BOKGI, the high spatial frequency components of the detected object could be effectively retrieved, which are often filtered by the photo-induced soft aperture in a conventional OKGI system. Comparing with the conventional OKGI method, the BOKGI method could enhance the sharpness of images and provide a higher spatial resolution of the imaging system. In addition, the influence of the biased angle on the BOKGI performance has been also investigated.

High contrast ballistic imaging using femtosecond optical Kerr gate of tellurite glass.

We investigated the ballistic imaging technique using femtosecond optical Kerr gate of a tellurite glass. High contrast images of an object hidden behind turbid media were obtained. Compared to the conventional femtosecond optical Kerr gate using fused quartz, the optical Kerr gate using tellurite glass has more capacity to acquire high quality images of the object hidden behind a high optical density turbid medium. The experimental results indicated that the tellurite glass is a good candidate as the optical Kerr material for the ballistic imaging technique due to its large optical nonlinearity.

Visualization of Hot Carrier Dynamics in a Single CsPbBr3 Perovskite Microplate Using Femtosecond Kerr-Gated Wide-Field Fluorescence Spectroscopy.

Lead halide perovskites (LHPs) have excellent semiconductor properties. They have been used in many applications such as solar cells. Recently, the hot carrier dynamics in this type of material have received much attention as they are useful for enhancing the performance of optoelectrical devices fabricated from it. Here, we study the ultrafast hot carrier dynamics of a single CsPbBr3 microplate using femtosecond Kerr-gated wide-field fluorescence spectroscopy. The transient photoluminescence spectra have been measured under a variety of excitation fluences. The temporal evolution of bandgap renormalization and the competition between hot carrier cooling and the recovery of the renormalized bandgap are clearly revealed.

Single-shot decoherence polarization gated imaging through turbid media.

We propose a method for imaging through a turbid medium by using a single-shot decoherence polarization gate (DPG). The DPG is made up of a polarizer, an analyzer, and a weakly scattering medium. Contrary to intuition, we discover that the preferential utilization of sparsely scattered photons by introducing weakly scattering mediums can lead to better image quality. The experimental results show that the visibilities of the images acquired from the DPG imaging method are obviously improved. The contrast of the bar can be increased by 50% by the DPG imaging technique. Furthermore, we study the effect of the volume concentration of the weakly scattering medium on the speckle suppression and the enhancement of the visibilities of the images. The variances of the contrasts of the image show that there exists an optimum optical depth (∼0.8) of the weakly scattering medium for DPG imaging through a specific turbid medium.

Ultrafast Charge Carrier Dynamics in InP/ZnSe/ZnS Core/Shell/Shell Quantum Dots.

The excellent performance of InP/ZnSe/ZnS core/shell/shell quantum dots (CSS-QDs) in light-emitting diodes benefits from the introduction of a ZnSe midshell. Understanding the changes of ultrafast carrier dynamics caused by the ZnSe midshell is important for their optoelectronic applications. Herein, we have compared the ultrafast carrier dynamics in CSS-QDs and InP/ZnS core/shell QDs (CS-QDs) using femtosecond transient absorption spectroscopy. The results show that the ZnSe midshell intensifies the electron delocalization and prolongs the in-band relaxation time of electrons from 238 fs to 350 fs, and that of holes from hundreds of femtoseconds to 1.6 ps. We also found that the trapping time caused by deep defects increased from 25.6 ps to 76 ps, and there were significantly reduced defect emissions in CSS-QDs. Moreover, the ZnSe midshell leads to a significantly increased density of higher-energy hole states above the valence band-edge, which may reduce the probability of Auger recombination caused by the positive trion. This work enhances our understanding of the excellent performance of the CSS-QDs applied to light-emitting diodes, and is likely to be helpful for the further optimization and design of optoelectronic devices based on the CSS-QDs.

Ultrafast Electron Transfer in InP/ZnSe/ZnS Quantum Dots for Photocatalytic Hydrogen Evolution.

InP/ZnS core/shell quantum dots have shown extraordinary application potential in photocatalysis. In this work, we demonstrated by ultrafast spectroscopy that the electron transfer ability of InP/ZnSe/ZnS core/shell/shell quantum dots was better than that of InP/ZnS quantum dots, because the introduction of ZnSe midshell resulted in improved passivation and greater exciton delocalization. The temperature-dependent PL spectra indicate that the exciton-phonon coupling strength and exciton binding energy of InP/ZnSe/ZnS quantum dots are smaller than those of InP/ZnS quantum dots. Further photocatalytic hydrogen evolution testing revealed that the photocatalytic activity of InP/ZnSe/ZnS quantum dots was significantly higher than that of InP/ZnS quantum dots, and InP/ZnSe/ZnS quantum dots even demonstrated improved stability. This research deepened our understanding of carrier dynamics and charge separation of InP/ZnSe/ZnS quantum dots, especially highlighting the application potential of InP/ZnSe/ZnS quantum dots in photocatalytic hydrogen evolution.

Space-selective precipitation of Ge crystalline patterns in glasses by femtosecond laser irradiation.

Crystalline Ge was induced space selectively inside a borosilicate glass by 800 nm, 250 kHz femtosecond laser irradiation. Micro-Raman spectra and x-ray diffraction analysis confirmed that the laser-induced crystals were cubic Ge. A periodic structure consisting of Ge crystalline lines was inscribed in the glass sample by continuously moving the focal point of the laser beam. Large third-order nonlinear optical properties and ultrafast response time were observed from the crystallization region owing to highly optical nonlinearity of Ge crystals. These results may find some applications in fabrication of functional optical and photonic devices, such as optical circuits.

Acquisition of gated spectra from a supercontinuum using ultrafast optical Kerr gate of lead phthalocyanine-doped hybrid glasses.

We demonstrated the selection of the chirped supercontinuum using an ultrafast optical Kerr gate (OKG) of lead phthalocyanine (PbPc)- doped hybrid glasses. Using the OKG, narrow bandwidth and symmetrical gated spectra were obtained continuously from the chirped supercontinuum generated in a sapphire plate with a femtosecond laser. Experimental results show that the obtained Kerr-gated spectra using the PbPc-glass have many advantages comparing with that using CS(2).

Long-working-distance microscopic imaging in a turbid medium by use of an ultrafast optical Kerr gate.

We demonstrate a long-working-distance microscopic imaging of hidden objects in a turbid medium by use of an ultrafast optical Kerr gate (OKG). The results show that the working distance and the spatial resolution of the long-working-distance microscopic imaging system have been increased simultaneously compared with those of the conventional 4f OKG imaging systems. A compound lens consisting of a long-focus achromatic doublet and a microscope objective is used to increase the long working distance and ensure the sufficient spatial resolution. The microscopic OKG imaging system with a working distance of 245 mm and a maximal spatial resolution of approximately 7 µm has been performed.