Laser Pulse Repetition Rate Research Articles

Implementation and characterization of coating pure titanium dental implant with sintered β-TCP by using Nd:YAG laser

ObjectivesThis work presents laser coating of grade 1 pure titanium (Ti) dental implant surface with sintered biological apatite beta-tricalcium phosphate (β-TCP), which has a chemical composition close to bone. Materials and methodsPulsed Nd:YAG laser of single pulse capability up to 70 J/10 ms and pulse peak power of 8 kW was used to implement the task. Laser pulse peak power, pulse duration, repetition rate and scanning speed were modulated to achieve the most homogenous, cohesive and highly adherent coat layer. Scanning electron microscopy (SEM), energy dispersive X-ray microscopy (EDX), optical microscopy and nanoindentation analyses were conducted to characterise and evaluate the microstructure, phases, modulus of elasticity of the coating layer and calcium-to-phosphate ratio and composition.Results showed that the laser power and scanning speed influenced coating adherence. The cross-sectional field-emission scanning electron microscopy images at low power and high speed showed poor adherence and improved as the laser power increased to 2 kW. Decreasing the scanning speed to 0.2 mm/s at the same power of 2 kW increased adherence. EDX results of the substrate demonstrated that the chemical composition of the coat layer did not change after processing. Moreover, the maps revealed proper distribution of Ca and P with some agglomeration on the surface. The sharp peaks on the X-ray diffraction patterns indicated that β-TCPs in the coat layer were mostly crystalline. The elastic modulus was low at the surface and increased gradually with depth to reach 19 GPa at 200 nm; this value was close to that of bone. The microhardness of the coated substrate increased by about 88%. The laser pulse energy of 8.3 J, pulse peak power of 2 kW, pulse duration of 4.3 min, repetition rate of 10 Hz and scanning speed of 0.2 ms−1 yielded the best results. ConclusionBoth processing and coating have potential use for dental implant applications.

高重复频率脉冲激光去除低热导率涂漆

高重复频率脉冲激光去除低热导率涂漆

Optically induced magnetization reversal in [Co/Pt]N multilayers: Role of domain wall dynamics

All-optical switching (AOS) of magnetization in ferri- and ferromagnetic thin films has in recent years attracted a strong interest since it allows magnetization reversal in the absence of applied magnetic field. Here we investigate AOS in ${[\mathrm{Co}/\mathrm{Pt}]}_{N}$ multilayers. The coercivity (${H}_{C}$) of the multilayers was tuned either by varying the bilayer repetition number ($N$) or the sample temperature ($T$). During the AOS experiments, we first illuminated the multilayers by a sequence of femtosecond laser pulses with varying fluence, light polarization, and repetition rate. The optically affected area was then imaged with magneto-optical Kerr microscopy. Our results indicate that the optical pulses can trigger either AOS or initiate an all-optical domain formation (AODF). The laser fluence required for AOS scales linearly with ${H}_{C}$ and depends on a precise tuning of laser pulse fluence, repetition rate, and light polarization. Furthermore, the magnetic response of the samples at a varying ambient temperature (down to 50 K) and for different time intervals between subsequent laser pulses point to the crucial role of domain wall dynamics in optical control of magnetization in ferromagnetic multilayers.

Transvaginal fast-scanning optical-resolution photoacoustic endoscopy.

.Photoacoustic endoscopy offers in vivo examination of the visceral tissue using endogenous contrast, but its typical B-scan rate is , restricted by the speed of the scanning unit and the laser pulse repetition rate. Here, we present a transvaginal fast-scanning optical-resolution photoacoustic endoscope with a 250-Hz B-scan rate over a 3-mm scanning range. Using this modality, we not only illustrated the morphological differences of vasculatures among the human ectocervix, uterine body, and sublingual mucosa but also showed the longitudinal and cross-sectional differences of cervical vasculatures in pregnant women. This technology is promising for screening the visceral pathological changes associated with angiogenesis.

Single-beam resonant spin amplification of electrons interacting with nuclei in a GaAs/(Al,Ga)As quantum well

The dynamic polarization of nuclear spins interacting with resident electrons under resonant excitation of trions is studied in a nominally undoped GaAs/(Al,Ga)As quantum well. Unlike in common time-resolved pump-probe techniques, we used a single beam approach where the excitation light is modulated between the circular and linear polarization states. The time-integrated intensity of the excitation laser reflected from the sample surface, proportional to the optical generation rate and changes due to the pumping of the resident electrons, is detected. Polarized electrons on the other hand transfer their spin to the lattice nuclei via the hyperfine interaction. Exciting the sample with a train of pulses in an external magnetic field leads to resonant spin amplification observed when the Larmor precession frequency is synchronized with the laser pulse repetition rate. Build-up of the nuclear spin polarization causes a shifting of the RSA peaks since the resulting nuclear field alters the strength of the external magnetic field experienced by the electrons. It was established that the nuclear spin polarization time $T_1$ is temperature dependent and owing to the electron localization at lower temperatures becomes shorter. "Locking" of the nuclear field manifested as the limited by the strength of the external field growth of the nuclear field, that is related to the anisotropy of the electron $g$-factor, was observed. The $g$-factor ratio between the in plane $g_{\parallel}$ and out-of-plane $g_{\bot}$ components was estimated to be $g_{\bot}/g_{\parallel}=1.3$.

Influence of deposition parameters on the structure and microstructure of Bi12TiO20 films obtained by pulsed laser deposition

The structure, morphology and surface roughness of Bi12TiO20 (BTO) thin films grown on R-sapphire by pulsed laser deposition (PLD) were studied at different substrate temperatures, target-substrate distances, oxygen pressures and laser-pulse repetition rates. Although the substrate temperature seems to be the most important experimental parameter, the gas pressure and the target–substrate distance played important role on the phase formed and film thickness, with a significant effect of the laser-pulse repetition rate on the films thickness and preferred orientation of the deposited film. Single-phase γ-Bi12TiO20 was obtained on substrates at 650 °C, while several BTO metastable phases were observed in films deposited on substrates at temperatures between 500 and 600 °C. By the first time, thin films of pure and textured δ-Bi12TiO20 were successfully growth on substrates at 450 °C. When annealed, all the films deposited at lower temperatures resulted in the thermodynamically stable γ-Bi12TiO20.

Passively Q-switched and mode-locked Tm-Ho co-doped fiber laser using a WS2 saturable absorber fabricated by chemical vapor deposition

We demonstrate a passively Q-switched and mode-locked Tm-Ho co-doped fiber laser based on a WS2 saturable absorber (SA) fabricated by chemical vapor deposition method. When the pump power increases from 0.32 W to 0.38 W, the repetition rate of the Q-switched laser pulse increases 47.1 kHz to 90.1 kHz and the pulse width decreases from 6.2 μs to 1.7 μs. In addition, the mode-locking operation can also be observed by adjusting the polarization controller (PC) and mode-locked laser pulses at 1883.2 nm can be achieved at a high pump power between 0.5 W and 0.64 W. The pulse duration is 1.07 ns and repetition rate is 23.8 MHz and the pulse is the longest mode-locked pulse with WS2-SA ever reported.

Femtosecond laser induced selective etching in fused silica: optimization of the inscription conditions with a high-repetition-rate laser source.

Femtosecond laser induced selective etching (FLISE) of dielectric materials is a promising technique for fabricating various microfluidic devices. Here we experimentally studied the dependence of the selective etching speed in fused silica glass on laser pulse energy, repetition rate, and inscription speed using a 1030 nm femtosecond laser. The evolution of micromorphology of the laser inscribed lines was revealed with optical microscopy, scanning electron microscopy, as well as anisotropic diffraction of the optical gratings formed by these inscribed lines. A single pulse energy threshold is required to initiate the FLISE. Further, a laser repetition rate window between an upper threshold and a lower threshold was observed, which were limited by the thermal-induced disruption of the nanogratings and by the disconnection of successive pulses modified spots respectively. The synergetic influences of the above factors were evaluated by the exposure laser energy density, which shows a common threshold for different inscription conditions and demonstrates itself to be an excellent criterion for choosing appropriate parameters in FLISE. The formation of continuous nanogratings is confirmed to be the major mechanism of FLISE in fused silica. Our observations not only help one to understand the micro mechanism in FLISE of fused silica, but also are of great use for fabricating large-scale microfluidic circuits.

Effect of laser pulse repetition rate on morphology and magnetic properties of cobalt ferrite films grown by pulsed laser deposition

In this paper, we report a switch of magnetic anisotropy in CoFe2O4 (CFO) films induced by change in laser pulse repetition rate. CFO films were deposited on MgO (0 0 1) substrates using pulsed laser deposition method with 2, 3, 5, 7 and 10 Hz laser pulse repetition rates. Atomic force microscopy images exhibited the film surface aggregation tends to compact shape with large size islands on the surface of the samples deposited by 2 and 3 Hz while the size of islands decreased on the surface of the 5, 7 and 10 Hz CFO films. The hysteresis curve of films indicated magnetic anisotropy and switching of the easy axis direction by changing laser repetition rates. The films grown at 2 and 3 Hz exhibit an in-plane easy axis, while by increasing the laser repetition rate, the easy axis switches to out-of-plane. The Raman spectra showed the rearrangement of Fe3+ and Co2+ cations on octahedral and tetrahedral sites in the structure by changing the laser pulse repetition rate. Change in the strain, morphology evolution and cation distribution are responsible for the change in easy axis direction and existence of shrinking at M–H loops.

High Repetition Rate All-Solid-State Pulsed 2 μm Laser Based on Selenide Molybdenum Saturable Absorber

A kind of saturable absorber (SA) based on layered MoSe2 nanosheets has been fabricated successfully by an ultrasound-assisted liquid phase exfoliation method. The as-prepared MoSe2 SA was characterized and employed as Q-switch in a diode-pumped Tm-doped crystalline all-solid-state laser at around 2 μ m. A stable passively Q-switched 2 μ m laser with a maximum pulse repetition rate of 158 kHz and a minimum pulse width of 520 ns was successfully realized. A maximum average output power of 1.19 W, corresponding to a maximum pulse energy of 12.7 μ J and pulse peak power of 21.8 W, has been obtained. It was the first demonstration of MoSe2 SA in realizing a solid-state 2 μ m pulsed laser, to the best of our knowledge. The results confirmed the excellent ability and promising potential of MoSe2 SA as an optical modulator in generating high repetition rate and short pulsed laser at the midinfrared spectral band.

Nano- and femtosecond high-repetition-rate multipulse laser irradiation of dehydrated bone tissue: role of accumulated heat and model of cooling

A model is proposed to describe the cooling of a biotissue surface after irradiation by a series of laser pulses. The spatial distribution of heat accumulated by the surface of a hard biotissue irradiated by nano- and femtosecond laser pulses is qualitatively estimated as a function of the pulse repetition rate. The obtained results are compared with the data of experimental irradiation of dry bone with both nanosecond and femtosecond laser pulses. The key contribution of the variation of the biotissue optical characteristics to the dependence of heat, accumulated by its surface, on the repetition rate of nanosecond laser pulses is demonstrated. The mechanism of the pulse repetition rate affecting the residual heat in the biotissue is more complex and requires adequate consideration of the change in its optical and thermophysical parameters. The application of the cooling model allowed substantial confirmation of the key role of the accumulated heat in the process of hard tissue carbonisation around the area of laser impact and the necessity of external cooling in the course of high-repetition-rate multipulse laser processing using both nanosecond and femtosecond pulses.

High-power passively Q-switched 20 μm all-solid-state laser based on a MoTe2 saturable absorber

In this article, a high-power diode-end-pumped passively Q-switched (PQS) Tm:YAP laser is reported with novel two-dimension (2D) molybdenum ditelluride (MoTe2) as a saturable absorber (SA). By using the open-aperture Z-scan method, the saturable absorption properties around 2.0 μm was characterized with a saturable fluence of 2.26 μJ∕cm2 and a modulation depth of 6.0% for the as-prepared MoTe2 SA. The band structure of MoTe2 with the introduction of Te vacancies is simulated by the DFT method, and the results indicate that the bandgap can be reduced with the vacancies in a suitable range. The shortest pulse width of 380 ns was obtained with an average output power of 1.21 W at a repetition rate of 144 kHz, corresponding to a maximum single pulse energy of 8.4 µJ and peak power of 22.2 W. It is the first presentation of MoTe2 as the saturable absorber in 2.0 μm solid-state pulse laser generation and the pulse width is the shortest among 2.0 μm solid-state lasers passive Q-switched with transitional metal dichalcogenides (TMDs) SAs to the best of our knowledge. The results indicated that MoTe2 should be an excellent optical modulator for high repetition rate and short pulsed laser generation in a broadband spectral range.

Quantitative analysis of C, Si, Mn, Ni, Cr and Cu in low-alloy steel under ambient conditions via laser-induced breakdown spectroscopy

A diode-pumped solid-state laser (DPSSL) with a high energetic stability and long service life is applied to ablate the steel samples instead of traditional Nd:YAG laser pumped by a xenon lamp, and several factors, such as laser pulse energy, repetition rate and argon flow rate, that influence laser-induced breakdown spectroscopy (LIBS) analytical performance are investigated in detail. Under the optimal experiment conditions, the relative standard deviations for C, Si, Mn, Ni, Cr and Cu are 3.3%–8.9%, 0.9%–2.8%, 1.2%–4.1%, 1.7%–3.0%, 1.1%–3.4% and 2.5%–8.5%, respectively, with the corresponding relative errors of 1.1%–7.9%, 1.0%–6.3%, 0.4%–3.9%, 1.5%–6.3%, 1.2%–4.0% and 1.2%–6.4%. Compared with the results of the traditional spark discharge optical emission spectrometry technique, the analytical performance of LIBS is just a little inferior due to the less stable laser-induced plasma and smaller amount of ablated sample by the laser. However, the precision, detection limits and accuracy of LIBS obtained in our present work were sufficient to meet the requirements for process analysis. These technical performances of higher stability of output energy and longer service life for DPSSL, in comparison to the Q-switch laser pumped by xeon lamp, qualify it well for the real time online analysis for different industrial applications.

Enlarged Range and Filter-Tuned Reception in Photonic Time-Stretched Microwave Radar

We have previously demonstrated a photonic time-stretched coherent radar (PTS-CR) with central-frequency programmable and bandwidth-tailorable agility. The sampling bandwidth in the reception was essentially compressed by the photonic time-stretched process. However, in the PTS-CR, the reception aperture suffering dead zone was hardly tuned by the fiber delay line and the measurement range was restricted by the pulse repetition rate (PRR) of the mode-locked laser. Here, we report a novel scheme to overcome the restrictions. The reception aperture can be simply tuned by adjusting the center wavelength of an optical filter and the PRR is reduced for enlargement of the measurement range by use of a photonic switch in the transmitter. In the experiment, a tenfold enlargement of measurement range is demonstrated to identify two neighboring targets separated by ~4 cm.

Passively Mode-Locked Ytterbium-Doped Fiber Laser With Cylindrical Vector Beam Generation Based on Mode Selective Coupler

We propose and demonstrate, for the first time to the best of our knowledge, a passively mode-locked ytterbium-doped fiber laser generating cylindrical vector beams (CVBs) using a mode selective coupler (MSC) as the transverse mode converter and splitter, and a semiconductor saturable absorber mirror for mode-locking. According to the coupling mode theory and the phase matching principle, the MSC was fabricated by weakly fused technology, having a low loss of about 0.5 dB and achieving LP11 mode with a high purity of >96%. The CVB fiber laser operates at a center wavelength of 1042.3 nm, with a 3 dB spectral width of 1.5 nm. The repetition rate of the mode-locked laser pulses is 18.58 MHz. The radially and azimuthally polarized vector beams can be switched by adjusting the polarization controllers in the fiber ring cavity, with a high mode purity measured to be >95%. The mode-locked CVB ytterbium-doped fiber laser has potential applications in optical tweezers, optical imaging, etc.

Improving osteoblasts cells proliferation via femtosecond laser surface modification of 3D-printed poly-ε-caprolactone scaffolds for bone tissue engineering applications

Synthetic polymer biomaterials incorporating cells are a promising technique for treatment of orthopedic injuries. To enhance the integration of biomaterials into the human body, additional functionalization of the scaffold surface should be carried out that would assist one in mimicking the natural cellular environment. In this study, we examined poly-e-caprolactone (PCL) fiber matrices in view of optimizing the porous properties of the constructs. Altering the porosity of a PCL scaffold is expected to improve the material’s biocompatibility, thus influencing its osteoconductivity and osteointegration. We produced 3D poly-e-caprolactone (PCL) matrices by a fused deposition modeling method for bone and cartilage tissue engineering and performed femtosecond (fs) laser modification experiments to improve the surface properties of the PCL construct. Femtosecond laser processing is one of the useful tools for creating a vast diversity of surface patterns with reproducibility and precision. The processed surface of the PCL matrix was examined to follow the effect of the laser parameters, namely the laser pulse energy and repetition rate and the number (N) of applied pulses. The modified zones were characterized by scanning electron microscopy (SEM), confocal microscopy, X-ray computed tomography and contact angle measurements. The results obtained demonstrated changes in the morphology of the processed surface. A decrease in the water contact angle was also seen after fs laser processing of fiber meshes. Our work demonstrated that a precise control of material surface properties could be achieved by applying a different number of laser pulses at various laser fluence values. We concluded that the structural features of the matrix remain unaffected and can be successfully modified through laser postmodification. The cells tests indicated that the micro-modifications created induced MG63 and MC3T3 osteoblast cellular orientation. The analysis of the MG63 and MC3T3 osteoblast attachment suggested regulation of cells volume migration.

Passively Q-Switched and Mode-Locked Fiber Laser Based on an ReS2 Saturable Absorber

Transition metal dichalcogenides, a family of two-dimensional material with unusual electronic, optical, mechanical, and electrochemical properties, have received much research attention in recent years. Here we demonstrate that, another type of few-layer transition metal dichalcogenides, rhenium disulfide (ReS2) nanosheets display saturable absorption property at 1.55 μm. By incorporating the ReS2 nanosheets with the polyvinyl alcohol (PVA), a film-type ReS2-PVA saturable absorber is fabricated to realize Q-switching and mode locking of erbium-doped fiber lasers. The repetition rate of the Q-switched laser pulses varies from 12.6 to 19 KHz while the duration changes from 23 to 5.496 μs by tuning the pump from 45 to 120 mW. By optimizing the polarization state, the mode-locked operation is also obtained, emitting a train of pulses centered at 1558.6 nm with the duration of 1.6 ps and the fundamental repetition rate of 5.48 MHz. It is demonstrated that ReS2 nanosheets have the similar saturable absorption property as that of MoS2 and WS2, and may find potential applications in pulsed laser, optical modulators, and sensors.

Research on the optoacoustic communication system for speech transmission by variable laser-pulse repetition rates

Abstract For the optoacoustic communication from in-air platforms to submerged apparatus, a method based on speech recognition and variable laser-pulse repetition rates is proposed, which realizes character encoding and transmission for speech. Firstly, the theories and spectrum characteristics of the laser-generated underwater sound are analyzed; and moreover character conversion and encoding for speech as well as the pattern of codes for laser modulation is studied; lastly experiments to verify the system design are carried out. Results show that the optoacoustic system, where laser modulation is controlled by speech-to-character baseband codes, is beneficial to improve flexibility in receiving location for underwater targets as well as real-time performance in information transmission. In the overwater transmitter, a pulse laser is controlled to radiate by speech signals with several repetition rates randomly selected in the range of one to fifty Hz, and then in the underwater receiver laser pulse repetition rate and data can be acquired by the preamble and information codes of the corresponding laser-generated sound. When the energy of the laser pulse is appropriate, real-time transmission for speaker-independent speech can be realized in that way, which solves the problem of underwater bandwidth resource and provides a technical approach for the air-sea communication.

New onset of phrenic nerve palsy after laser-assisted transvenous lead extraction: a single-centre experience.

Phrenic nerve palsy (PNP) after mechanical transvenous lead extraction (TLE) was recently described for the first time. We aimed to analyse our TLE database for the presence of PNP. All consecutive patients referred to our institution were included in this study. Every available post-procedural chest X-ray was compared to the routinely performed pre-procedural radiographs. A newly elevated hemidiaphragm ipsilateral to TLE was considered indicative of PNP. Altogether 255 TLE procedures with extraction of 364 leads were performed. Most common TLE indication was lead malfunction (63%). Complete radiographic success rate was 97.3% with an in-hospital procedure-related major complication rate of 2.4%, including one intra-procedural death (0.4%). We identified five cases with PNP (2%), all occurring after laser-assisted TLE. Clinical presentation varied from subtle and aspecific chest pain/discomfort to severe and acute dyspnoea, with time to diagnosis varying from immediate to several weeks after the procedure. In 80% of cases, the explanted lead was a defibrillator electrode and the median lead dwelling time was 70.2 months (29.3; 1084.9). In four cases, the extraction was performed using high-energy laser (pulse repetition rate 80 Hz). The present study reports the incidence of PNP after laser-assisted TLE. We postulate that the thermal energy generated by laser is not dissipated quickly enough in occluded or heavily calcified lesions, injuring the ipsilateral phrenic nerve. Our findings advise to carefully consider to increase pulse repetition rate at the subclavian level. Larger, possibly prospective studies are needed to evaluate the real incidence through systematic radiological assessment after TLE.

Laser direct writing (LDW) of magnetic structures

Laser direct writing (LDW) has been used to pattern 90nm thick permalloy (Ni81Fe19) into 1-D and 2-D microstructures with strong shape anisotropy. Sub-nanosecond laser pulses were focused with a 0.75 NA lens to a 1.85μm diameter spot, to achieve a fluence of approximately 350 mJ.cm-2 and ablate the permalloy film. Computer-controlled sample scanning then allowed structures to be defined. Scan speeds were controlled to give 30% overlap between successive laser pulses and reduce the extent of width modulation in the final structures. Continuous magnetic wires that adjoined the rest of the film were fabricated with widths from 650 nm - 6.75μm and magneto-optical measurements showed coercivity reducing across this width range from 47 Oe to 11 Oe. Attempts to fabricate wires narrower than 650nm resulted in discontinuities in the wires and a marked decrease in coercivity. This approach is extremely rapid and was carried out in air, at room temperature and with no chemical processing. The 6-kHz laser pulse repetition rate allowed wire arrays across an area of 4 mm x 0.18 mm to be patterned in 85 s.