Laser Pluses Research Articles

Nonlinear optical properties of gold with intense femtosecond laser excitations above the damage threshold

The nonlinear optical properties of gold thin films have been studied under the irradiation of destructive laser pluses. The transmissivity and the reflectivity of a 100-nm-thick gold film are measured with femtosecond laser irradiations up to 106 J/m2 fluence, which is much higher than the damage threshold of gold. The nonlinearity is calculated with Fresnel loss equations. The real part of the complex refractive index is calculated to have a peak at ∼10 kJ/m2, which is close to the damage threshold. The complex third-order nonlinear susceptibility χ3 is estimated to be (4.95 − 2.02i) × 10−21 m2/V2, which agrees with the trend in previous reports that it decreases as the pulse duration decreases. This nonlinearity is further studied with a plasmon-photon exchange (PPE) model, where laser induced plasmons and absorbed photons are strongly correlated. The result of the simulation with the PPE model is in good agreement with the experimental transmissivity above the damage threshold. The model predicted an interaction time between photons and plasmons to be about 500 fs. This model is found to be essential in understanding such nonlinear optical processes under both extremely high and conventionally low laser irradiations.

Analysis on leading-fiber-induced Doppler noise in interferometric FBG sensor arrays using polarization switching and PGC hybrid processing method.

The random disturbance in the leading fiber is considered as a vital noise source in the practical interferometric fiber Bragg grating (FBG) sensor array, which is usually interrogated by periodic laser pulse pair. As the two interrogation laser pluses propagate through the leading fiber in a time-sharing manner, the leading fiber disturbance could cause undesired demodulated phase noises to both the polarization state and the pulse-interval, which are summarized as the polarization fading induced noise and the Doppler noise, respectively. This paper focused on the Doppler noise under the demodulation scheme of polarization switching (PS) and phase generated carrier (PGC) hybrid processing method. A model describing the transformation from arbitrary leading fiber stretching to sensor phase background was presented. The complexity was that the Doppler noise was coupled with the birefringence states, as verified by both simulation and experiment. In response to this issue, a two-stage Doppler noise suppression method was proposed, which is based on the PS and PGC hybrid processing and a reference sensor. A processing procedure was presented where the polarization synthesis must be performed before and the reference sensor was considered. Otherwise, the suppression algorithm will be completely invalid due to the mutual coupling of the Doppler noise and the birefringence. Experimental results showed that only after the first stage of polarization synthesis, identical Doppler noise in the two TDM channels could be obtained, with an amplitude error of 0.02 dB. The second stage involved non-sensitive reference sensor subtraction, which achieved a maximum suppression of about 30 dB, which was the highest to be best of our knowledge. The two-stage Doppler noise suppression method was tested for sinusoidal and wideband leading fiber disturbances, providing a solution for practical interferometric FBG array applications.

Generation of tunable ultraviolet femtosecond pulse in MgO crystal by cascaded four wave mixing

Ultraviolet femtosecond laser pulse is an important tool in studying ultrafast chemical and physical processes. Realizing broadband ultraviolet laser pluses with a wide tunable range would significantly facilitate the study of ultrafast processes. As an effective and convenient method, the cascaded four-wave mixing (CFWM) has been widely adopted to generate broadband and tunable ultraviolet femtosecond laser pulses. In this work, we carry out CFWM in MgO crystal by using two 400-nm pulses to generate tunable ultraviolet femtosecond pulse. The MgO crystal is chosen due to its high third-order nonlinear susceptibility, large band gap and high transmittance in the ultraviolet region. In the experiment, nine frequency up-converted and five frequency down-converted sidebands are observed. The measured wavelength and scattering angle of each sideband are consistent with the CFWM theory predictions. The wavelength range of the sidebands covers 350–450 nm. The total conversion efficiency of the ultraviolet sidebands is 1.2%, which is higher than the reported values with visible/near infrared driven lasers. Meanwhile, the spectra of the high-order sidebands present a Gaussian profile and can support a Fourier-transform-limited pulse duration of less than 50 fs. Besides, the central wavelengths of the sidebands can be effectively tuned by adjusting the time-delay between the two pre-chirped pump pulses. Our study provides an efficient and convenient scheme to generate short ultraviolet femtosecond pulses with a wide tunable range.

Pseudo-topotactic growth of diamond nanofibers

We report pseudo-topotactic growth of single-crystal diamond fibers by nanosecond laser melting of amorphous carbon nanofibers (CNFs) and crystalline multi-wall carbon nanotubes (MWCNTs). A rapid laser melting in a super undercooled state and subsequent quenching convert the tips of CNFs and MWCNTs into phase-pure <110> nanodiamonds along the growth directions. Subsequent laser pluses melt regions below <110> nanodiamonds that provide seeds for epitaxial growth. By repeating this process, the length of <110> nanodiamond fibers can be increased, as each pulse results in ∼50 nm nanodiamond region, depending upon the initial size of CNFs and MWCTs. This conversion process can be carried at ambient temperature and pressure in air. The epitaxial nature of <110> nanodiamond fibers has been confirmed by systematic electron-back-scatter-diffraction studies along the fiber in high-resolution scanning electron microscopy, and high-resolution TEM imaging and diffraction. The nature of C–C bonding characteristics was studied by high-resolution electron-energy-loss spectroscopy to establish the formation of diamond phase by the characteristic peak at 292 eV for sp3 bonding (σ∗), and absence of 284 eV peak for sp2 (π∗) graphitic bonding. The characteristic diamond Raman peak at 1332 cm−1 is found to downshift to 1321 cm−1 because of phonon confinement in nanodiamonds associated with nanofibers. These nanodiamond structures can be doped with both n- and p-type dopants with concentrations far higher than thermodynamic solubility limit due to solute trapping during quenching from the liquid phase. Thus, these nanodiamond structures provide ideal platform for nanosensing, computing and communication, including efficient field emitting devices.

Imaging Ultrafast Plasmon Dynamics within a Complex Dolmen Nanostructure Using Photoemission Electron Microscopy**Supported by the National Basic Research Program of China under Grant No 2013CB922404, the National Natural Science Foundation of China under Grant Nos 11474040, 11474039, 61605017 and 61575030, and the Project of Changchun Science and Technology Bureau under Grant No 14KP007.

We report direct nanoscale imaging of ultrafast plasmon in a gold dolmen nanostructure excited with the 7 fs laser pulses by combining the interferometric time-resolved technology with the three-photon photoemission electron microscopy (PEEM). The interferometric time-resolved traces show that the plasmon mode beating pattern appears at the ends of the dimer slabs in the dolmen nanostructure as a result of coherent superposition of multiple localized surface plasmon modes induced by broad bandwidth of the ultrafast laser pulses. The PEEM measurement further discloses that in-phase of the oscillation field of two neighbor defects are surprisingly observed, which is attributed to the plasmon coupling between them. Furthermore, the control of the temporal delay between the pump and probe laser pluses could be utilized for manipulation of the near-field distribution. These findings deepen our understanding of ultrafast plasmon dynamics in a complex nanosystem.

Four-photon-excited fluorescence resonance energy transfer in an aqueous system from ZnSe:Mn/ZnS quantum dots to hypocrellin A.

In this work, we established a fluorescence resonance energy transfer (FRET) system between ZnSe:Mn/ZnS quantum dots and Hypocrellin A (HA, a photosensitizer used for photodynamic therapy of cancer) in aqueous solution, excited by four-photon. Here, the QDs are the donors and the HA are the acceptors. The four-photon-excited fluorescence resonance energy transfer spectrum was obtained under 1300nm femtosecond laser pluses. The experimental results indicated that the highest efficiency of FRET can reach up to 61.3%. Furthermore, the viability test in cancer cells was further demonstrated for biological applications of FRET system. When FRET occurs the cell killing rate of the cancer cells will reach to 84.8% with the 1mM concentration of HA. Our work demonstrates that while the four-photon excited FRET system is promising in both optics and biological applications, is also needs further investigation.

Optical measurement of the temporal delay between two ultra-short and focussed laser pluses.

Temporal overlapping of ultra-short and focussed laser pulses is a particularly challenging task, as this timescale lies orders of magnitude below the typical range of fast electronic devices. Here we present an optical technique that allows for the measurement of the temporal delay between two focussed and ultra-short laser pulses. This method is virtually applicable to any focussing geometry and relative intensity of the two lasers. Experimental implementation of this technique provides excellent quantitative agreement with theoretical expectations. The proposed technique will prove highly beneficial for high-power multiple-beam laser experiments.

Functional role of induced gamma oscillatory responses in processing noxious and innocuous sensory events in humans

Gamma time–frequency responses (TFRs) induced by painful laser in the contralateral primary somatosensory (SI) cortex have been shown to correlate with perceived pain-intensity in human. Given the functional roles of gamma TFRs in the cortical spaces, it remains unclear whether such a relationship is sustained for other brain regions where the laser-evoked potentials (LEPs) are presented. In this study, we delivered the painful laser pluses at random pain-intensity levels (i.e. strong, medium and weak) in a single train to the dorsal hand of six patients with uncontrolled epilepsy. The laser stimulus produced a painful pinprick sensation by activating nociceptors located in the superficial layers of the skin. For each patient, arrays of >64 subdural electrodes were implanted directly covering the contralateral SI, parasylvian (PS) and medial frontal (MF) cortices to study the stimulus related gamma (TFRs) in the neocortex. In addition, using the same stimulation paradigm, the modality specificity of gamma TFRs was further examined by applying innocuous vibrotactile stimuli to the same regions of the dorsal hand in a separated group of five patients. Our results showed that gamma TFRs are not modality specific, but the largest gamma TFRs were consistently found within the SI region and noxious laser elicited significantly stronger gamma TFRs than innocuous nonpainful vibratory stimuli. Furthermore, stronger pain induced stronger gamma TFRs in the cortices of SI (r=0.4, p<0.001) and PS (r=0.29, p=0.005). Given that potentially harmful noxious stimulus would automatically capture greater attention than the innocuous ones, our results support the hypothesis that the degree of SI and PS gamma TFRs is associated with an attentional drive provoked by painful stimuli.

E14 プリズムを用いたフェムト秒レーザー干渉加工によるマイクロ周期構造制御(OS9 レーザ応用加工(1))

We study laser ablation of solid surface by two interfered femto second laser. Periodic microstructure formed by two interfered femtosecond laser pluses improve tribology properties of solid materials in fluid lubrication. This is because wedge effect occurs by periodic microstructure, and tribology properties depend on shape of microscopic structures. Moreover biprism enable easy manufacturing structure, and by adjustment of laser power, angle of prism, or scanning number of time, we can shape control of stuructures. This method is expected for the application to various products. We report the control method of the shape of periodic microscopic structures and tribology properties change.

Metalloporphyrin bonded SiO2 organic-inorganic materials and their strong nonlinear refractive index

Copper metalloporphyrin was bonded to 3-aminopropltriethoxysilane (NH2(CH2)3Si(OC2H5)3, KH550) by the chemical reaction between the carboxyl group of Cu(Ⅱ) meso-Tertra(4-carboxyphenyl)porphine (Cu(Ⅱ)-TCPP) and amino-group of KH550. The copper metalloporphyrin was connected to gel network after the hydrolysis and condensation of the product. The reaction product of different concentrations was hybridized with 3-glycidoxypropltrimethoxysilane (CH2OCHCH2O(CH2)3Si(OCH3)3, KH560) with sol-gel processing to form sol-gel inorganic material, which has good physical-chemical and optical properties. The cross-linkage of Cu(Ⅱ)-TCPP and KH550 was estimated and confirmed with FT-IR spectroscopy. The state of metalloporphyrin molecules was investiaged by UV/VIS spectra. It was found that the Cu(Ⅱ)-TCPP exists mainly in the state of monomer. Nonlinear optical properties of samples was studied by Z-scan technique using Ti:Sapphire femto-second laser pluses. The nonlinear refractive index n2 reached -1.1161×10-16 m2/W.

Generation of hyperentangled states between remote noninteracting atomic ions

We propose a scheme of generating four-qubit hyperentangled states between a pair of remote noninteracting atomic ions with a $\ensuremath{\Lambda}$ configuration that are confined in Paul traps. These hyperentangled states, different from the normal entangled states that are entangled in a single degree of freedom, are entangled in both spin and motion degrees of freedom. In our proposal, the entanglement is first generated in spin degrees of freedom using linear optics and then transferred to the motion degree of freedom using a sequence of laser pluses, including the stimulated Raman carrier transitions and sideband transitions. The proposal is completed with regenerating entanglement in spin degrees of freedom using linear optics.

Fabrication of platinum particles by intense, femtosecond laser pulse irradiation of aqueous solution

Platinum nano-particles were directly fabricated through molecular dissociation process induced by tightly focused femtosecond laser pluses in hydrogen hexachloroplatinate (IV) hexahydrate (H 2PtCl 6·6H 2O) aqueous solution. After irradiation, it was found that UV–visible absorption spectrum of the solution was changed and a large number of small platinum particles were synthesized. The variation of absorbance in the spectrum and the yield the particle synthesis depended on the concentration of the solution. The most effective condition for particle synthesis was 0.1 wt%. Black precipitates of platinum particles were observed in the bottom of the vessel after irradiation in some cases. The dispersibility of the particles was greatly improved by the addition of polyvinylpyrrolidone (PVP) as a dispersant, and no precipitates were found in the solution with 1.0 × 10 −2 wt% PVP. Consequently, highly monodispersed platinum nano-particles with the mean size of about 3 nm were successfully fabricated.

Autler–Townes Splitting in Photoelectron Spectrum ofThree-Level Li2 Molecule in Ultrashort Pulse Laser Fields

The Autler–Townes (AT) splitting in femtosecond photoelectron spectrumof three-level Li2 molecules is theoretically investigated usingtime-dependent quantum wave packet method. With proper femtosecondlaser pulses, three peaks of the AT splitting can be observed in thephotoelectron spectrum. The AT splitting stems from rapid Rabioscillation caused by intense ultrashort laser pluses. The effects oflaser parameters on the molecular ionization dynamics are alsodiscussed.

Modulation instability of fiber Bragg gratings with Gaussian apodization

The coherently coupled nonlinear Schrdinger (NLS) equation of the propagation of light pulse in fiber Bragg gratings has been utilized to study the modulation instability of laser pluses in fiber Bragg gratings with Gaussian apodization function, not only in the anomalous dispersion regime, but also in the normal dispersion regime. The results show that modulation instability can occur in both regimes. The regular gain spectra are observed obviously when input power reaches a definite value in the anomalous dispersion regime. The modulation instability can increase from a fixed value to infinity when the input power is in the region of modulation instability in normal dispersion regime. Furthermore, the gain spectra are restrained by Gaussian apodization function in both regimes.

Linear and nonlinear luminescence properties of bithiophene based materials

Abstract We synthesized three bithiophene based organic luminescence materials named as 5,5′-bis(p-N,N-dimethylaminostyryl)-2,2′-bithiophene (BMSBT), 5,5′-bis(p-N,N-diethylaminostyryl)-2,2′-bithiophene; (BESBT) and 5,5′-bis(p-N-carbazoylstyryl)-2,2′-bithiophene (BCSBT). Their single-photon excited fluorescence locate in the range; of 520–530 nm with the quantum yield around 20%. When excited by 200 fs laser pluses, these compounds can emit strong two-photon excited fluorescence. The two-photon absorption cross sections of BMSBT, BESBT and BCSBT are detected to be 54 × 10−50, 102 × 10−50 and 124 × 10−50 (cm4 s)/photon respectively.

Lamellar refractive surgery with scanned intrastromal picosecond and femtosecond laser pulses in animal eyes.

To evaluate the use of scanned intrastromal picosecond and femtosecond laser pulses in lamellar refractive surgical procedures. Intrastromal corneal photodisruption was performed in fresh porcine and primate cadaver eyes with a solid-state femtosecond laser. Laser pulses were focused 150 to 200 microns below the epithelial surface and scanned in a spiral pattern to create a plane. A flap was made by scanning an arc pattern from the plane of the spiral to the surface of the cornea. Tissue plane separation was graded using a standard scale, while internal surfaces were analyzed by scanning electron microscopy. Comparison was made to a picosecond laser system using the same delivery system device. Creation of a stromal lenticule for in situ keratomileusis was also demonstrated and compared with both laser systems. For femtosecond pulses, tissue separation was achieved best with pulse energies from 4 to 8 microJ and spot separations from 10-15 microns. Picosecond pulses accomplished less complete separations with pulse energies of 25 microJ and spot separations from 10 to 20 microns. Surface quality corresponded to dissection results, with high-grade dissections resulting in a smooth surface appearance, versus a more irregular surface for low-grade dissections. Although high-grade dissections could be created with picosecond pulses (with optimal parameters) in ex vivo porcine eyes, only femtosecond parameters produced similar results in ex vivo primate eyes. In contrast to previous attempts using picosecond lasers which require additional mechanical dissection, high precision lamellar refractive surgery may be practical with femtosecond laser pulses.