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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.

Supercontinuum (SC) generation using multiple thin plates is demonstrated with a femtosecond laser pulse. We propose an improved technique to obtain larger spectrum broadening and higher spectral intensity by employing mixed multiple thin plates with different thicknesses and materials. Furthermore, the spectrum has good stability, which is superior to that of the spectrum induced by the traditional single filament in bulk material. Our approach offers a route towards simple and stable SC generation for potential applications.

We performed numerical experiments to investigate the propagation of high-power optical flat-topped beams through a strongly nonlinear medium, accounting for the diffraction, nonlinearity, and multiphoton absorption capabilities of the beams. We found that the uniform irradiance distribution of flat-topped beams can remain constant near the centre axis for a given propagation distance. A series of comparisons were conducted for flat-topped beams with different orders, which demonstrates that increasing the order reduces change of beam characteristic. Furthermore, we investigated the influence of input power on the propagation of flat-topped beams. Moreover, we numerically analysed the dynamics of the inner energy transfer and uniform irradiance decay of the flat-topped beam.

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.

We demonstrated an improved optical Kerr gate with double gate pulses (D-OKG) to realize an ultrashort switching time even with a slow-response optical Kerr medium. A theoretical model of D-OKG was established and the dependence of the D-OKG performance on the relative intensity and the relative delay time of the two gate pulses was investigated. An integrated optimal condition was predicted. The D-OKG was experimentally built and optimized according to the simulation. The dependence of the D-OKG performance on the two gate pulses was confirmed. In our experiment, the achieved values for transmittance and switching time were 45 % and 150 fs, respectively. Compared to the conventional OKG, the D-OKG owns an integrated optimal performance in the femtosecond regime, and has the potential for a wide range of ultrafast optical switch applications.

Directional gated imaging of hidden objects in scattering media is demonstrated using an ultrafast optical Kerr gate (OKG), in which the gating and imaging beams are focused by utilizing a cylindrical lens and a convergent lens, respectively. Owing to the spatial transmission characteristics of the directional gate, a rectangular aperture stop is formed, which leads to different resolution limits for vertical and horizontal structures of an object. By implementing directional gated imaging, the directional feature can be extracted from an original image.

We demonstrate the optical imaging of objects through highly scattering media using a subtractive optical Kerr gate (SOKG). The SOKG extracts ballistic light via a subtraction operation involving a speckle-suppressed image using supercontinuum illumination and a background image captured with the ballistic light temporally excluded. By using this method, the contrast is significantly enhanced, while the sharpness and spatial resolution of the image are also significantly improved relative to traditional optical Kerr gate imaging. This imaging approach shows immense potential for applications involving the observation of the high-speed breakup of liquid cores in strongly scattering fuel sprays.

We demonstrate the control of femtosecond single-filament formation via feedback-based wavefront shaping. It is observed that the optimal phase profile forms a single-filament with closed-loop genetic algorithm. Using this method, the position stability of the filament can be significantly improved, and the position of the single filament can be flexibly adjusted. Additionally, a two-step approach combining the premodulation of pinhole with wavefront shaping is presented for forming a bright single filament rapidly.

We propose a method to suppress the speckle of optical Kerr gated (OKG) imaging in highly turbid medium using an 800-nm femtosecond laser, in which, the 800-nm fundamental frequency light and the supercontinuum (SC) containing the fundamental frequency are used as the gating beam and imaging beam, respectively. A sequence of images of a test chart hidden behind a highly turbid medium is obtained using the OKG technique. Compared with images obtained with OKG imaging using only the fundamental frequency, the image resolution and contrast of images obtained through OKG imaging using the SC-assisted fundamental frequency are both significantly enhanced.

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.

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 f s , 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.

Molecular unidirectional rotation (UDR) induced by femtosecond lasers has attracted significant attention in recent years. We monitored the molecular UDR for nitrogen molecules under ambient conditions in real time by observing time-resolved spectroscopic probes, based on the rotational Doppler effect. The time-dependent alignment degree of the molecular UDR was experimentally observed to differ from that of the 1-D alignment. Moreover, the rotational Doppler shift exhibited different features at various rotational revivals. Finally, we used spectroscopy to demonstrate the manifestation of the quantum nature of the molecular UDR.

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.

Time-resolved imaging of an object in a turbid medium is demonstrated using an ultrafast optical Kerr gate (OKG) of liquid carbon disulfide (CS2). A time-integrated imaging is also obtained by changing the arrangement of the OKG. The gating pulse of the OKG has a long-time tail arising from the molecular reorientation of CS2. We study the effect of the delay time between the imaging pulse and gating pulse on the efficiency of ultrafast OKG of CS2 imaging through a turbid medium. The experimental results show that the visibility and contrast of the time-resolved image can be improved in contrast to those of the steady-state image. The degree of improvement in the quality of the time-resolved image depends on the ratio of three types of photons gated by the OKG. For the OKG of CS2, there exists a relatively wide delay times to obtain a high contrast image, which decreases the difficulty of precise determination of the delay time between the imaging pulse and gating pulse.

The influence of the size parameter of the scatterers on ultrafast optical Kerr gate (OKG) imaging is investigated in highly scattering poly-disperse turbid media. The results show that in a poly-disperse turbid medium, which in our case, is a suspension of two different sized mono-disperse microspheres, the temporal and spatial behaviors of the light pulses transmitted through it are dominated by the smaller microspheres. The contrasts of the OKG images for the poly-disperse microsphere sample are closer to the contrasts of the OKG images for the smaller sized mono-disperse microsphere sample.

We demonstrated the time-gated ballistic imaging technique using a femtosecond optical Kerr gate (OKG) of bismuth–plumbum oxide glass, the nonlinear optical properties of which were also investigated. The third-order nonlinear refractive-index n2 of the bismuth–plumbum oxide glass was measured to be 2.19 × 10−15 cm2/W, and the nonlinear response time was estimated to be shorter than 180 fs. For the time-gated ballistic imaging, the maximum measurable optical density of turbid media using the OKG of bismuth–plumbum oxide glass was 9.3, while only 7.0 for the OKG of quartz glass. And the intensities of the images for the bismuth–plumbum oxide glass were approximately two orders of magnitude higher than that for the quartz glass. The experimental results indicated that the bismuth–plumbum oxide glass was an excellent optical material for nonlinear optical applications.

The feedback-based wavefront shaping method can be used to focus light behind or inside strongly scattering media. In this study, we investigated several characteristics of the focus after optimization to evaluate the enhancement effect of the wavefront shaping method, including the spot size, the intensity, and two types of enhancement factor. In addition, we studied the influences of various experimental parameters on these characteristics, including the number of controlled segments N, diameter of the irradiated area at the front of the scattering medium D, and distance between the expected focusing spot and scattering medium Z. A larger N and smaller Z provided a smaller focus spot. For a brighter focus spot, a larger N and smaller D and Z were required, while for a high-resolution focus spot, larger N, D, and Z were required.

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.

We employ a supercontinuum (SC) illumination to image the high-pressure fuel sprays in the near-nozzle region. The effect of speckles in the images is significantly mitigated using the SC illumination to improve the identifiability of the microstructures in the spray. The microstructures in the near-nozzle region, i.e., lobes, holes, ligaments, and bridges, are clearly imaged for different fuel pressures and nozzle orifice diameters. The shadowgraphs captured in the experiments also show the spray cone angle of spray is strongly dependent on the injection pressures and nozzle orifice diameters.

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.

We have demonstrated the high time-resolved three dimensional (3-D) imaging using a chirped supercontinuum (SC) and an ultrafast optical Kerr gate (OKG) generated with a femtosecond laser, in which a sapphire plate and a Bi2O3-B2O3-SiO2 oxide glass (BI glass) were used to generate the chirped SC and the optical Kerr gate, respectively. The experimental results show that the 3-D imaging using BI glass offers better temporal and longitudinal spatial resolution compared with CS2.

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.

We demonstrate depth-resolved imaging in a ballistic imaging system, in which a heterodyned femtosecond optical Kerr gate is introduced to extract useful imaging photons for detecting an object hidden in turbid media and a compound lens is proposed to ensure both the depth-resolved imaging capability and the long working distance. Two objects of about 15-μm widths hidden in a polystyrene-sphere suspension have been successfully imaged with approximately 600-μm depth resolution. Modulation-transfer-function curves with the object in and away from the object plane have also been measured to confirm the depth-resolved imaging capability of the low-depth-of-field (low-DOF) ballistic imaging system. This imaging approach shows potential for application in research of the internal structure of highly scattering fuel spray.

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).

We demonstrated two ballistic imaging for an object hidden behind turbid media using the optical Kerr gate (OKG) and spatial filtering (SF), respectively. The influence of the scattering parameters of the turbid media on the image contrast was investigated. The experimental results showed that the image contrast of the SF imaging decreased significantly with increasing optical density and scattering particle size of the turbid media. Compared to the SF imaging, the OKG imaging showed a higher and more stable image contrast as scattering photons in the optical gated imaging case were more effectively eliminated.

In this paper, we demonstrated optical imaging of objects hidden behind highly turbid media with a femtosecond heterodyned optical Kerr gate (HOKG). The experimental results showed that when compared with traditional optical Kerr gated (OKG) imaging, the HOKG imaging system provided higher image sharpness and higher spatial resolution. In traditional OKG imaging system, low pass filtering due to a photoinduced soft aperture decreased the image sharpness. When the HOKG was used, the high spatial frequency components of the object could be effectively compensated.

An instantaneous three-dimensional imaging technique using a chirped supercontinuum and an ultra- fast optical Kerr gate, in which a sapphire plate and a TeO2-ZnO-Na2O oxide glass were used to generate the chirped supercontinuum and the ultrafast optical Kerr gate, respectively, is demonstrated. This technique is applicable to ultrafast shape measurement, such as shape imaging of moving objects, or imaging of laser- induced refractive index changes in transparent media. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

The optical limiting (OL) properties of single-layer graphene dispersions at 532 and 1064 nm wavelengths were investigated using a nanosecond laser. The experimental results show that the activating threshold of the single-layer graphene dispersed in chlorobenzene (CB) is lower than that of single-walled carbon nanotubes (SWNTs) by a factor of 10. The nonlinear scattering experiments for the graphene dispersions indicate that the main mechanism of the OL properties is nonlinear scattering effect, whereas nonlinear absorption might contribute to the OL effect of graphene in CB. To enlarge the damage threshold of the limiter, we propose a cascaded optical limiter by combining the advantages of low activating threshold for the graphene and the high damage threshold for the SWNTs. The cascaded optical limiter shows a low activating threshold, a high damage threshold, and broadband OL properties.

In this paper, we demonstrate femtosecond optical Kerr gated imaging of an object hidden behind a highly turbid medium. The influence of turbid medium properties on image contrast has been investigated. Experimental and Monte Carlo simulation results show that for a given optical density, the image contrast of direct imaging without an optical Kerr gate decreases with the increase of the scattering particle size or the decrease of the thickness of the turbid medium. Compared with direct imaging, optical Kerr gated imaging has a better image contrast as it eliminates more scattered photons effectively. Qualitative comparisons between experiments and simulations show good agreement.

We investigated a ballistic imaging technique using a femtosecond optical Kerr gate (OKG) made of SrTiO3 crystal (STO). High-contrast images of a 1.41 line pairs per mm (lp mm−1) section of the resolution test chart placed behind a turbid medium were obtained. The STO OKG had greater capacity to acquire high quality images at a high optical density than the quartz OKG, showing that the STO is a promising OKG medium due to its large nonlinear susceptibility.