800-nm Pulses Research Articles

Enhancement of third harmonic yield in fused filaments due to Gouy shift suppression

We have measured the absolute energy of the third harmonic generated by multiple filaments in the focusing geometry of a 744-nm femtosecond pulse in air. The subdivision of the initial 744-nm beam into several beamlets and their fusion in the vicinity of the geometrical focus breaks up the symmetry of the fluence distribution before and after the focus and suppresses the Gouy shift. As a result, the third harmonic yield as the function of 744-nm pump pulse energy undergoes branching. An order of magnitude higher branch corresponds to 248-nm pulse energy obtained after four, three, or two colliding beamlets. The lower branch corresponds to 248-nm pulse energy for a single beamlet of the same initial energy as the energy of several ones added together. The two branches of the third harmonic yield converge back as the peak power after a single opening reaches the critical power for self-focusing in air.

Control of the polarization direction of isolated attosecond pulses using inhomogeneous two-color fields

We theoretically demonstrate the control of the polarization direction of isolated attosecond pulses (IAPs) with inhomogeneous two-color fields synthesized by an 800-nm fundamental pulse and a 2000-nm control pulse having crossed linear polarizations. The results show that by using the temporally and spatially shaped field, the high-order harmonic generation (HHG) process can be efficiently controlled. An ultra-broad supercontinuum ranging from 150th to 400th harmonics which covers the water window region is generated. Such a supercontinuum supports the generation of a 64-as linearly polarized IAP, whose polarization direction is at about 45° with respect to the x axis. Moreover, we analyze the influence of the inhomogeneity parameters and the relative angle of the fundamental and control pulses on the IAP generation. It is shown that the polarization direction of the IAP can rotate in a wide range approximately from 8° to 90° relative to the x axis when the inhomogeneity parameters and the relative angle vary.

Spectral attenuation of a 400-nm laser pulse propagating through a plasma filament induced by an intense femtosecond laser pulse**Project supported by the National Natural Science Foundation of China (Grant Nos. U1932133, 51733004, 51525303, and 21702085) and the Fundamental Research Funds for the Central Universities, China (Grant Nos. lzujbky-2016-35 and lzujbky-2018-it36).

The spectral attenuation of a 400-nm probe laser propagating through a femtosecond plasma in air is studied. Defocusing effect of the low-density plasma is an obvious effect by examining the far-field patterns of the 400-nm pulse. Besides, the energy of 400-nm pulse drops after interaction with the plasma, which is found to be another effect leading to the attenuation. To reveal the physical origin behind the energy loss, we measure fluorescence emissions of the interaction area. The fluorescence is hardly detected with the weak 400-nm laser pulse, and the line spectra from the plasma filament induced by the 800-nm pump pulse are clearly shown. However, when the 400-nm pulse propagates through the plasma filament, the fluorescence at 391 nm from the first negative band system of is enhanced, while that from the second positive band of neutral N2 at 337 nm remains constant. Efficient near-resonant absorption of the 400-nm pulse by the first negative band system occurs inside the plasma, which results in the enhanced fluorescence. Furthermore, the spectral attenuation of the 400-nm probe laser is measured as a function of the pump–probe time delay as well as the pump-pulse energy.

High-order harmonic generation using quasi-phase matching and two-color pump in the plasmas containing molecular and alloyed metal sulfide quantum dots

As high-order harmonic emitters, quantum dots are produced through laser-induced plasmas. Subsequently, we generate high-order harmonics with 800-nm and 30-fs pulses from laser-produced plasmas containing quantum dots of different metal sulfides (Ag2S, CdS, and Cd0.5Zn0.5S). The high-order harmonic generation is analyzed using different approaches, including two-color (800 nm + 400 nm) pump, application of alloyed quantum dots, and quasiphase matching of interacting waves. We discuss the self-phase modulation induced splitting of harmonics, the difference in the application of thick and thin crystals for second harmonic (400 nm) emission during two-color pumping of the quantum dot plasma, the spatial modulation of the quantum dot plasma for quasiphase matching, and the comparison of harmonic yields from monomer and quantum dot plasmas. This study allows us to determine the mechanisms of coherent extreme ultraviolet radiation generation using a few nanometer-sized emitters as well as optimal methods for further enhancing the high-order harmonic generation efficiency.

Tunable dual-comb from an all-polarization-maintaining single-cavity dual-color Yb:fiber laser.

We demonstrate dual-comb generation from an all-polarization-maintaining dual-color ytterbium (Yb) fiber laser. Two pulse trains with center wavelengths at 1030 nm and 1060 nm respectively are generated within the same laser cavity with a repetition rate around 77 MHz. Dual-color operation is induced using a tunable mechanical spectral filter, which cuts the gain spectrum into two spectral regions that can be independently mode-locked. Spectral overlap of the two pulse trains is achieved outside the laser cavity by amplifying the 1030-nm pulses and broadening them in a nonlinear fiber. Spatially overlapping the two arms on a simple photodiode then generates a down-converted radio frequency comb. The difference in repetition rates between the two pulse trains and hence the line spacing of the down-converted comb can easily be tuned in this setup. This feature allows for a flexible adjustment of the tradeoff between non-aliasing bandwidth vs. measurement time in spectroscopy applications. Furthermore, we show that by fine-tuning the center-wavelengths of the two pulse trains, we are able to shift the down-converted frequency comb along the radio-frequency axis. The usability of this dual-comb setup is demonstrated by measuring the transmission of two different etalons while the laser is completely free-running.

Photoelectron Holographic Interferometry to Probe the Longitudinal Momentum Offset at the Tunnel Exit.

Laser-induced electron tunneling underlies numerous emerging spectroscopic techniques to probe attosecond electron dynamics in atoms and molecules. The improvement of those techniques requires an accurate knowledge of the exit momentum for the tunneling wave packet. Here we demonstrate a photoelectron interferometric scheme to probe the electron momentum longitudinal to the tunnel direction at the tunnel exit by measuring the photoelectron holographic pattern in an orthogonally polarized two-color laser pulse. In this scheme, we use a perturbative 400-nm laser field to modulate the photoelectron holographic fringes generated by a strong 800-nm pulse. The fringe shift offers direct experimental access to the intermediate canonical momentum of the rescattering electron, allowing us to reconstruct the momentum offset at the tunnel exit with high accuracy. Our result unambiguously proves the existence of nonzero initial longitudinal momentum at the tunnel exit and provides fundamental insights into the nonquasistatic nature of the strong-field tunneling.

Transparent capacitive micromachined ultrasonic transducers (CMUTs) for photoacoustic applications

Integration of acoustic and optical techniques prompted the need for transparent ultrasonic transducers to guide the light through the transducer and improve the signal to noise ratio. In the presented paper, capacitive micromachined ultrasound transducers (CMUTs) using glass substrate and indium-tin-oxide electrodes were fabricated by adhesive wafer bonding technique presenting a transparency of up to 82% in the visible range. A receive sensitivity of 65.5 μV/Pa was measured with noise equivalent sensitivity of 95 Pa. Capacity of the produced CMUTs for photoacoustic imaging was also demonstrated by successfully detecting the photoacoustic signal from an aluminum foil target, which was irradiated by a 532-nm pulse laser transmitted through the CMUT. The centre frequency of the detected photoacoustic signal was at 2 MHz with 52.3% -6-dB fractional bandwidth.

Enhancing terahertz generation from a two-color plasma using optical parametric amplifier waste light

We show experimentally that the terahertz (THz) emission of a plasma, generated in air by a two-color laser pulse (containing a near IR frequency and its second harmonic), can be enhanced by the addition of an 800-nm pulse. We observed enhancements of the THz electric field by a factor of up to 30. This provides a widely accessible means for researchers using an optical parametric amplifier (OPA) to increase their THz yields by simply adding the residual pump beam of the OPA to the plasma generating beam. We investigate the dependence of the THz electric field enhancement factor on the powers of the two-color beam and the 800-nm enhancement beam. Numerical calculations using the well-known photocurrent model are in excellent agreement with the experimental observations.

CaF2晶体的不同晶面在355 nm脉冲激光辐照下的损伤特性

CaF2晶体的不同晶面在355 nm脉冲激光辐照下的损伤特性

Using coherent phonons for ultrafast control of the Dirac node of SrMnSb2

${\mathrm{SrMnSb}}_{2}$ is a candidate Dirac semimetal whose electrons near the $Y$ point have the linear dispersion and low mass of a Dirac cone. Here we demonstrate that ultrafast, 800-nm optical pulses can launch coherent phonon oscillations in ${\mathrm{Sr}}_{0.94}{\mathrm{Mn}}_{0.92}{\mathrm{Sb}}_{2}$, particularly an ${A}_{g}$ mode at 4.4 THz. Through first-principles calculations of the electronic and phononic structure of ${\mathrm{SrMnSb}}_{2}$, we show that high-amplitude oscillations of this mode would displace the atoms in a way that transiently opens and closes a gap at the node of the Dirac cone. The ability to control the nodal gap on a subpicosecond timescale could create opportunities for the design and manipulation of Dirac fermions.

Effect of driving pulse properties on the performance of sub-terawatt laser wakefield acceleration

By introducing a sub-terawatt (TW) laser pulse into a high-density gas target, the self-focusing effect and the self-modulation instability can greatly enhance the laser peak power to a level capable of driving the laser wakefield acceleration (LWFA) of electrons. A particle-in-cell model has been developed to study the scheme in which 1030-nm pulses produced from a diode-pumped laser system are introduced into a gas cell with a flat-top density profile, allowing the LWFA to be operated at high frequencies. Because 1030-nm lasers are typically produced with a long duration >200 fs, a spectral broadening technique can be applied to reduce the pulse duration, from which a greater ponderomotive force is acquired to drive LWFA. To understand the dependence of LWFA performance on the driving pulse duration, selected durations, ranging from 200 fs to 10 fs, are assigned for 0.5-TW, 1030-nm pulses in a series of simulations. Results show that a duration around 50 fs can provide the optimal LWFA results, as a compromise between the weak ponderomotive force available from a long pulse >100 fs and the depletion effect which can rapidly diminish a short pulse <25 fs in a dense plasma. When a low laser peak power of 0.25-TW is available, the pulse depletion can be significant at a high target density and render LWFA ineffective. Using a laser pulse with a longer wavelength >2 μm represents a viable route to realize the LWFA with a low laser peak power; in this way, an appropriately selected target density which allows the laser peak power PL ∼ 1.25Pcr of self-focusing critical power is favourable for realizing an efficient LWFA process.

Solid-state Yb : YAG amplifier pumped by a single-mode laser at 920 nm

An optical amplifier scheme for ultrashort 1030-nm pulses is proposed based on an Yb : YAG crystal with axial pumping by a transverse single-mode laser at a wavelength of 920 nm. A small-signal gain up to 40 dB per pass with a high output beam quality is demonstrated. The maximum average power is 14 W with a slope efficiency exceeding 50 %.

Nonlinear absorption of some thiazine, xanthene, and carbocyanine dyes

The saturated and two-photon absorption of various classes of heterocyclic dyes [thiazine (thionine), xanthene (erythrosine), and carbocyanine (3,3′-di-(γ-sulfopropyl)-4,4′,5,5′-dibenzo-9-ethylthiacarbocyaninebetaine pyridinium salt, DEC)] possessing optical absorption in the 500–680nm range were measured at the wavelengths of 1064 and 532nm using nanosecond (10ns) and picosecond (40ps) pulses and z-scan technique. It was shown that reverse saturated absorption unlikely plays important role in those dyes. We defined the two-photon absorption coefficients of thionine and erythrosine at 1064nm using 40ps pulses (0.8×10−11 and 1.0×10−11cmW−1 respectively). The concurrence of saturated absorption and two-photon absorption was analyzed in the case of thionine and erythrosine using various models. It was suggested that dimerization of those dyes significantly suppresses the nonlinear optical response in the case of both 10ns and 40ps pulses of 532nm radiation. At the same time it was found that, in the case of 532nm pulses, the monomers of thionine participate in the processes of saturated absorption and two-photon absorption, while J-aggregates of DEC show predominantly two-photon absorption.

Low temperature degradation of laser patterned 3Y-TZP: Enhancement of resistance after thermal treatment

The aim of this study is to characterize the resistance to low temperature degradation (LTD) of the surface of dental-grade zirconia (3Y-TZP) patterned with a Nd:YAG laser (532nm harmonic and pulse duration of 10ns) employing an interference setup.Laser patterning decreases the resistance to LTD of 3Y-TZP because of the presence of monoclinic phase and residual stresses, induced by the thermal shock during laser-material interaction. A thermal treatment (1h at 1200°C) anneals the affected microstructure and increase the resistance to LTD of laser patterned 3Y-TZP. Transformation delay may be attributed to monoclinic phase reversion, texture in the tetragonal phase and the existence of a net of shallow microcracks on the surface, accommodating autocatalytic transformation.

Resonance enhanced high-order harmonic generation in H2+ by two sequential laser pulses.

We investigate high-order harmonic generation in H2+ by using two sequential laser pulses, which consist of a 800-nm pump pulse and a time-delayed 1600-nm probe pulse. Based on the solution of the time-dependent Schrödinger equation, we demonstrate that the harmonic cutoff in our two-pulse scheme is significantly extended compared to that in the 1600-nm probe pulse alone. Meanwhile, the harmonic efficiency is enhanced by 2-3 orders of magnitude due to charge-resonance-enhanced ionization steered by the 800-nm pump pulse. By using a probe pulse with longer wavelength, our scheme can be used for efficient high harmonic generation in the water window region. In addition, the influence of the intensity of the pump pulse and the relative time delay of the two laser pulses on the harmonic generation are also investigated.

High-Efficiency Generation of 0.12 mJ, 8.6 Fs Pulses at 400nm Based on Spectral Broadening in Solid Thin Plates**Supported by the National Natural Science Foundation of China under Grant Nos 11434016, 11574384 and 11674386, and the National Key Basic Research Program of China under Grant Nos 2013CB922401 and 2013CB922402.

We demonstrate efficient generation of continuous spectrum centered at 400 nm from solid thin plates. By frequency doubling of 0.8 mJ, 30 fs Ti:sapphire laser pulses with a BBO crystal, 0.2 mJ, 33 fs laser pulses at 400 nm are generated. Focusing the 400-nm pulses into 7 thin fused silica plates, we obtain 0.15 mJ continuous spectrum covering 350–450 nm. After compressing by 3 pairs of chirped mirrors, 0.12 mJ, 8.6 fs pulses are achieved. To the best of our knowledge, this is the first time that sub-10-fs pulses centered at 400 nm are generated by solid thin plates, which shows that spectral broadening in solid-state materials works not only at 800 nm but also at different wavelengths.

Characteristics of femtosecond laser pulses propagating in multiply ionized rare gases

A three-dimensional non-adiabatic model in combination with a rate equation model is used to characterize high-intensity (>1016W/cm2) femtosecond laser pulse propagation in atomic gas medium. In these conditions the atoms in the target gas will become multiply charged ions, while the laser pulse will propagate in a medium with high electron concentration created by itself. We obtain pulse characteristics in time, frequency and space domain for two representative cases: 800nm and 267nm (third harmonic) multi-cycle pulses which are both of practical importance. We show that the spatial-temporal variation of the refractive index in the macroscopic medium is the primary reason for pulse temporal/spectral and spatial shaping during propagation.

Generation of isolated attosecond pulses in a multi-cycle inhomogeneous two-color field without CEP stabilization

We theoretically demonstrate the generation of ultrashort isolated attosecond pulses (IAPs) in the water window region with a multi-cycle inhomogeneous two-color field synthesized by an 800-nm fundamental pulse and a 2000-nm control pulse. The results show that the high-order harmonic generation process is effectively controlled in the temporally and spatially shaped field. An ultra-broadband supercontinuum in the water window region with the photon energies ranging from 387 to 620 eV is generated. Such a supercontinuum supports the generation of an ultrashort IAP with a duration as short as 40 attosecond. Moreover, we analyze the influence of the carrier-envelope phases (CEPs) and the relative phase of the two-color field on the IAP generation. It is shown that our two-color scheme is robust for the generation of sub-80 as IAPs against variations of the CEPs and the relative phase.

Photosensitized cleavage of some olefins as potential linkers to be used in drug delivery

A study of photosensitized cleavage of different olefins as potential linkers for drug carrier complexes is reported. The role of singlet oxygen and the kinetic rates for light induced reactions were estimated by time-resolved measurements of singlet oxygen phosphorescence (at 1270nm) obtained via 532nm pulse laser excitation of a photosensitizer. The mixture of each studied olefin with verteporfin (used as photosensitizer) were exposed to low energy visible radiation. The rate constants for singlet oxygen quenching by studied olefins were determined. The irradiated solutions were investigated by FTIR spectroscopy and potential photoproducts were suggested. The experimental results were compared with simulations made by DFT method.

Pulse-shape control of two-color interference in high-order-harmonic generation

We report on calculations of harmonic generation by neon in a mixed (800-nm + time-delayed 400-nm) laser pulse scheme. In contrast with previous studies we employ a short (few-cycle) 400-nm pulse, finding that this affords control of the interference between electron trajectories contributing to the cutoff harmonics. The inclusion of the 400-nm pulse enhances the yield and cutoff energy, both of which exhibit a strong dependence on the time delay between the two pulses. Using a combination of time-dependent $R$-matrix theory and a classical trajectory model, we assess the mechanisms leading to these effects.