Threshold Pulse Energy Research Articles

Adaptive optics assisted and optical coherence tomography guided fs-laser system for ophthalmic surgery in the posterior eye.

While fs-lasers are clinically established for surgery in the anterior eye, their use in the posterior eye is impeded by aberrations and focus position errors. We implemented a laboratory system to investigate whether fs-laser surgery in the posterior eye is made more feasible by aberration correction and tomographic image guidance. Aberration correction is obtained by adaptive optics (AO) and the image guidance is accomplished by optical coherence tomography (OCT). System characteristic measurements and cutting experiments were performed inside an eye model. By aberration correction, wavefront errors were reduced from 270 nm root-mean-square (rms) to 64 nm rms, ignoring Zernike terms for tilts and focus. The Strehl ratio of the assigned point spread function is improved from 0.11 to 0.78. The threshold pulse energy of laser-induced optical breakdown in water is lowered from about 3.0 to about 1.3???J measured at the eye model entrance. After laser cutting of a synthetic foil placed 300???m in front of porcine retinal tissue with the corrected system, postoperative three-dimensional OCT imaging showed no lesions in the tissue. Our results corroborate that AO and OCT will be two essential assistive components for possible clinical systems for fs-laser–based surgery in the posterior eye.

Optimal performance of NdAl3(BO3)4 nanocrystals random lasers

Resonant pumping associated with the influence of the quantum-defect between the excitation and the emitted laser photons allow optimal performance of neodymium ions (Nd3+) based random lasers (RLs), as demonstrated here for the first time. The RL emission at 1063.5 nm due to the Nd3+ transition 4F3/2 → 4I11/2 in a powder consisting of NdAl3(BO3)4 nanocrystals, was investigated by exciting the powder at 690 nm, 750 nm, 810 nm, and 884 nm in resonance with the Nd3+ transitions from the ground state (4I9/2) to the 4F9/2, {4F7/2,4S3/2}, {4F5/2,4H9/2}, and 4F3/2 states, respectively. Although the Nd3+ absorption cross-section at 884 nm is smaller than those centered at 810 and 750 nm, excitation at 884 nm, that is in resonance with the emitting level, provided the smaller excitation pulse energy threshold and the larger slope efficiency due to the lower quantum defect.

Effect of femtosecond laser radiation on mammalian oocytes

Nucleolus-like bodies (NLBs) of mammalian oocytes are a convenient object for studying the effects of laser radiation on cellular organelles during laser surgery operations. NLBs in mouse germinal vesicle oocytes (GV oocytes) were subject to nanosurgery by means of tightly focused femtosecond laser pulses at near-infrared wavelength. The damage threshold of the NLB material was one-half orders of magnitude below the threshold for the water breakdown by femtosecond pulses. An incision of NLB material with submicron resolution was obtained at a threshold pulse energy. At the pulse energy one order of magnitude higher than the threshold level, significant deformation and rupture of NLBs were observed. Anisotropic deformations of the NLBs and the dynamics of their relaxation indicate the presence of granular structure in these organelles. Mechanical strain and the formation of cavitation and steam-gas bubbles in the place of impact of laser pulses can be described with the help of a model of the laser breakdown of medium.

Influence of gain material concentration on an organic DFB laser

In this work, we investigate the properties of an organic distributed feedback laser as the concentration of the gain material in the waveguide core is varied across two orders of magnitude, from 5% down to 0.025%. The laser dye DCJTB (4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-9-enyl-vinyl)-4H-pyran) incorporated into a PVK (poly(9-vinylcarbazole)) host matrix provided the gain. The composite layer of PVK:DCJTB was spin-cast onto a silica grating with second order periodicity, and upon nanosecond optical excitation lasing was generated in the wavelength range of 600 nm. The threshold pulse energy for achieving lasing increased as the concentration of DCJTB was reduced, however the threshold excitation density quantified in terms of number of excited molecules per unit area remained nearly constant at 1.3×1013 molecules/cm2. In contrast, the relative slope efficiency for lasing decreased considerably as the gain concentration was reduced. We show that this effect can not be explained by a standard 4-level lasing model, but rather that it is due to optically induced charge separation for the DCJTB molecules situated in the PVK host matrix. Our findings suggest that fast charge separation and long back recombination times can be a significant factor in limiting further reduction of the gain concentration in organic DFB lasers.

Ablation-cooled material removal with ultrafast bursts of pulses.

The use of femtosecond laser pulses allows precise and thermal-damage-free removal of material (ablation) with wide-ranging scientific, medical and industrial applications. However, its potential is limited by the low speeds at which material can be removed and the complexity of the associated laser technology. The complexity of the laser design arises from the need to overcome the high pulse energy threshold for efficient ablation. However, the use of more powerful lasers to increase the ablation rate results in unwanted effects such as shielding, saturation and collateral damage from heat accumulation at higher laser powers. Here we circumvent this limitation by exploiting ablation cooling, in analogy to a technique routinely used in aerospace engineering. We apply ultrafast successions (bursts) of laser pulses to ablate the target material before the residual heat deposited by previous pulses diffuses away from the processing region. Proof-of-principle experiments on various substrates demonstrate that extremely high repetition rates, which make ablation cooling possible, reduce the laser pulse energies needed for ablation and increase the efficiency of the removal process by an order of magnitude over previously used laser parameters. We also demonstrate the removal of brain tissue at two cubic millimetres per minute and dentine at three cubic millimetres per minute without any thermal damage to the bulk.

Coherent octave-spanning mid-infrared supercontinuum generated in As2S3-silica double-nanospike waveguide pumped by femtosecond Cr:ZnS laser.

A more than 1.5 octave-spanning mid-infrared supercontinuum (1.2 to 3.6 μm) is generated by pumping a As2S3-silica "double-nanospike" waveguide via a femtosecond Cr:ZnS laser at 2.35 μm. The combination of the optimized group velocity dispersion and extremely high nonlinearity provided by the As2S3-silica hybrid waveguide enables a ~100 pJ level pump pulse energy threshold for octave-spanning spectral broadening at a repetition rate of 90 MHz. Numerical simulations show that the generated supercontinuum is highly coherent over the entire spanning wavelength range. The results are important for realization of a high repetition rate octave-spanning frequency comb in the mid-infrared spectral region.

Interplay between random laser performance and self-frequency conversions in NdxY1.00−xAl3(BO3)4 nanocrystals powders

Random Laser emission at 1.06μm, self-second-harmonic generation at 0.53μm and self-sum-frequency generation at 0.46μm were investigated in NdxY1.00−xAl3(BO3)4 nanocrystalline powders, for 0.05⩽x⩽1.00, excited by a pulsed laser operating at 808nm, focusing on the interplay between the RL performance and the second-order nonlinear processes. The RL performance, characterized by a figure-of-merit relating the laser slope efficiency and the excitation pulse energy threshold, improved as x increased up to 1.00 while the efficiency of the self-frequency conversion processes reduced for increasing x because of distortions introduced in the crystalline structure of the grains. The RL wavelength was also dependent on the Nd3+ concentration and presented a redshift from 1061.9nm to 1063.5nm for increasing values of x.

Laser-induced Breakdown Spectroscopy and ablation threshold analysis using a megahertz Yb fiber laser oscillator

A LIBS system is demonstrated using a 100m cavity Yb fiber oscillator producing ~70ps, 320nJ clusters of 50–100fs sub-pulses at 2MHz. A new empirical model for femtosecond ablation is presented to explain the LIBS signal intensity's non-linear dependence on pulse fluence by accounting for the Gaussian beam's spatial distribution. This model is compared to experimental data and found to be superior to linear threshold fits. This model is then used to measure the ablation threshold of Cu using a typical amplified Ti:sapphire system, and found to reproduce previously reported values to within ~20%. The ablation threshold of Cu using the Yb fiber oscillator system was measured to be five times lower than on the amplified Ti:sapphire system. This effect is attributed to the formation of nanostructures on the surface, which have previously been shown to decrease the ablation threshold. The plasma lifetime is found to be ~1ns, much shorter than that of nanosecond ablation, further indicating that the decreased threshold results from surface effects rather than laser–plasma interaction. The low threshold and high pulse energy of the Yb fiber oscillator allows the acquisition of LIBS spectra at megahertz repetition rates. This system could potentially be developed into a compact, fiber-based portable LIBS device taking advantage of the benefits of ultrafast pulses and high repetition rates.

Intense ionizing radiation from laser-induced processes in ultra-dense deuterium D(-1)

Nuclear fusion in ultra-dense deuterium D(-1) has been reported from our laboratory in a few studies using pulsed lasers with energy < 0.2 J. The direct observation of massive particles with energy 1–20 MeV u-1 is conclusive proof for fusion processes, either as a cause or as a result. Continuing the step-wise approach necessary for untangling a complex problem, the high-energy photons from the laser-induced plasma are now studied. The focus is here on the photoelectrons formed. The photons penetrating a copper foil have energy > 80 keV. The total charge created is up to 2 μC or 1 × 1013 photoelectrons per laser shot at 0.13 J pulse energy, assuming isotropic photon emission. The variation of the photoelectron current with laser intensity is faster than linear for some systems, which indicates rapid approach to volume ignition. On a permanent magnet at approximately 1 T, a laser pulse-energy threshold exists for the laser-induced processes probably due to the floating of most clusters of D(-1) in the magnetic field. This Meissner effect was reported previously.

Low-density plasma formation in aqueous biological media using sub-nanosecond laser pulses.

We demonstrate the formation of low- and high-density plasmas in aqueous media using sub-nanosecond laser pulses delivered at low numerical aperture (NA = 0.25). We observe two distinct regimes of plasma formation in deionized water, phosphate buffered saline, Minimum Essential Medium (MEM), and MEM supplemented with phenol red. Optical breakdown is first initiated in a low-energy regime and characterized by bubble formation without plasma luminescence with threshold pulse energies in the range of Ep ≈ 4-5 μJ, depending on media formulation. The onset of this regime occurs over a very narrow interval of pulse energies and produces small bubbles (Rmax = 2-20 μm) due to a tiny conversion (η < 0.01%) of laser energy to bubble energy EB. The lack of visible plasma luminescence, sharp energy onset, and low bubble energy conversion are all hallmarks of low-density plasma (LDP) formation. At higher pulse energies (Ep = 11-20 μJ), the process transitions to a second regime characterized by plasma luminescence and large bubble formation. Bubbles formed in this regime are 1-2 orders of magnitude larger in size [Formula: see text] due to a roughly two-order-of-magnitude increase in bubble energy conversion (η ≳ 3%). These characteristics are consistent with high-density plasma formation produced by avalanche ionization and thermal runaway. Additionally, we show that supplementation of MEM with fetal bovine serum (FBS) limits optical breakdown to this high-energy regime. The ability to produce LDPs using sub-nanosecond pulses focused at low NA in a variety of cell culture media formulations without FBS can provide for cellular manipulation at high throughput with precision approaching that of femtosecond pulses delivered at high NA.

Femtosecond laser processing of fiber Bragg gratings with photo-induced gradient-index assisted focusing

Gradient-index modulation prefabrication in the cladding of standard telecom fibers was proposed for femtosecond laser processing of Type II-IR fiber Bragg gratings. The refractive index of the prefabricated region in the cladding had a large gradient across the laser beam, which could act as a cylindrical lens to enhance the focusing of laser beam when writing Type II-IR gratings. With the help of prefabrication, the threshold pulse energy for processing Type II-IR FBGs was lowered from 750 μJ to 520 μJ. The fabricated FBGs showed good thermal stability at temperatures over 900 °C.

Femtosecond laser plasma plume characteristics in the nanojoule ablation regime

Laser ablation of chromium with nanojoule energy UV femtosecond pulses under background pressure conditions between 0.3 Torr and 700 Torr is studied and the corresponding plasma plume images at different times after irradiation are measured. The ablation focal spot is less than or the order of a micron when 170 nJ of laser pulse energy is used. This low pulse energy leads to short lifetimes of the plasma of the order of tens of nanoseconds. The plume shape changes with ambient pressure due to the collision with background gas. An axially stretched plume changes to a more circular plume as the pressure increases. In addition, a separation of the ionic and atomic components is observed at lower pressure. These two components move at significantly different velocities as well. The plasma plume expands at almost constant velocity at very low pressure but exhibits significant deceleration at higher pressure reaching an asymptotic stopping distance. Plume images are also obtained near the ablation threshold pulse energy. The plume characteristics are compared to different models of plume expansion.

Ultrafast heating and magnetic switching with weak external magnetic field

The TbFeCo magneto-optical media with the coercivity of bigger than 1.0 kOe are used for the investigation of ultrafast heating and magnetic switching with the weak external magnetic field. It has been found that the laser-induced active region becomes larger with an external magnetic field because the boundary of the active region is magnetized with the assistance of the external field during the ultrafast heating. According to this physical phenomenon, the so called “mark expansion method” has been proposed for visual observation of ultrafast switching marks. Using this method, the ultrafast magnetic switching in TbFeCo media has been studied using 40 fs laser pulse with linear polarization. The result shows that the ultrafast magnetic switching can be implemented by the laser pulse with assistance of the weak external field of about 0.7 kOe. Further studies show that the area percentage of the magnetic mark expansion relative to its thermal mark decreases with the increasing of the laser pulse energy. There exists the threshold pulse energy that the active region is fully magnetized. The theoretical analysis of electron, spin, and lattice temperatures has been conducted to the active region of the media where the maximum spin temperature is close to the Curie temperature of the media. The result indicates that the media become active at 4.137 ps and the ultrafast heating plays a key role for the ultrafast magnetic switching. The weak external magnetic field provides sufficient driving force to control the magnetization direction in the media.

175 GHz, 400-fs-pulse harmonically mode-locked surface emitting semiconductor laser

We report a harmonically mode-locked vertical external cavity surface emitting laser (VECSEL) producing 400 fs pulses at a repetition frequency of 175 GHz with an average output power of 300 mW. Harmonic mode-locking was established using a 300 µm thick intracavity single crystal diamond heat spreader in thermal contact with the front surface of the gain sample using liquid capillary bonding. The repetition frequency was set by the diamond microcavity and stable harmonic mode locking was achieved when the laser cavity length was tuned so that the laser operated on the 117th harmonic of the fundamental cavity. When an etalon placed intracavity next to the gain sample, but not in thermal contact was used pulse groups were observed. These contained 300 fs pulses with a spacing of 5.9 ps. We conclude that to achieve stable harmonic mode locking at repetition frequencies in the 100s of GHz range in a VECSEL there is a threshold pulse energy above which harmonic mode locking is achieved and below which groups of pulses are observed.

Laser-induced desorption of atomic and molecular fragments from a tin dioxide surface modified by a thin organic covering of copper phthalocyanine

The systematic features of laser-induced desorption from an SnO2 surface exposed to 10-ns pulsed neodymium laser radiation are studied at the photon energy 2.34 eV, in the range of pulse energy densities 1 to 50 mJ/cm2. As the threshold pulse energy 28 mJ/cm2 is achieved, molecular oxygen O2 is detected in the desorption mass spectra from the SnO2 surface; as the threshold pulse energy 42 mJ/cm2 is reached, tin Sn, and SnO and (SnO)2 particle desorption is observed. The laser desorption mass spectra from the SnO2 surface coated with an organic copper phthalocyanine (CuPc) film 50 nm thick are measured. It is shown that laser irradiation causes the fragmentation of CuPc molecules and the desorption of molecular fragments in the laser pulse energy density range 6 to 10 mJ/cm2. Along with the desorption of molecular fragments, a weak desorption signal of the substrate components O2, Sn, SnO, and (SnO)2 is observed in the same energy range. Desorption energy thresholds of substrate atomic components from the organic film surface are approximately five times lower than thresholds of their desorption from the atomically clean SnO2 surface, which indicates the diffusion of atomic components of the SnO2 substrate to the bulk of the deposited organic film.

Laser-Induced Backside Wet Etching Employing Green DPSS Laser and Liquid Metallic Absorber

An indirect laser processing, laser-induced backside wet etching (LIBWE) is one of effective techniques for flexible laser-direct-write (LDW) type micromachining of transparent materials. Micromachining by LIBWE employing a diode-pumped solid state (DPSS) green laser and a galvanometer-based point scanning system was examined with employing organic dye solution and liquid metal as laser-absorbing media. LIBWE using liquid metal absorber showed smaller threshold pulse energies for etching (9.1 μJ/pulse) than that using organic dye solution (33 μJ/pulse). Different morphologies of etched surfaces were observed for different laser-absorbing media, indicating different mechanisms involved in the etching processes. The laser-induced dynamic phenomena were investigated by means of transient reflection measurements. The different contribution of direct and indirect actions of laser pulses was indicated for LIBWE employing different laser-absorbing media.

Comparison of Single and Two-Photon Absorption for Laser Characterization of Single-Event Upsets in SOI SRAMs

The laser pulse energy thresholds for single-event upset measured by single photon and two photon absorption are measured and compared for Sandia SRAMs and DPSRAMs, and IBM 45-nm SRAMs for devices with and without the back substrate removed. These results are also compared to heavy-ion results taken on the same devices. Sandia SRAM data taken on different test dates resulted in considerably different TPA laser pulse energy thresholds even though the TPA system was calibrated using standard techniques each test date. These differences are believed to be due to changes in laser spot size. This shows that it is imperative to develop a calibration procedure that monitors all relevant laser parameters if TPA is to be used as a routine quantitative tool. Removing the back substrate makes a very large difference in TPA laser pulse energy threshold. This large difference is likely due to either displacement currents generated in the back substrate by TPA and/or nonlinear optical effects which can reduce the laser pulse irradiance in the active region. Nevertheless, the mechanism does not appear to affect the qualitative nature of TPA measurements. Both SPA and TPA laser measurements were used to estimate the heavy-ion threshold LETs of the Sandia DPSRAMs and 45-nm IBM SRAMs. Both SPA and TPA overestimated the heavy-ion threshold LET of the IBM 45-nm SRAMs (likely due to the large laser spot size compared to the size of the SRAM cell), but reasonably estimated the threshold LETs of the Sandia DPSRAMs. For the first time, TPA laser pulse energy (squared) is directly compared to SPA laser pulse energy at threshold. There is reasonable quantitative agreement between the charge required to induce upsets by TPA and SPA with the back substrate removed.

Micro-scale patterning of indium tin oxide film by spatially modulated pulsed Nd:YAG laser beam

Here we demonstrate that indium tin oxide (ITO) films deposited on glass can be directly patterned by a spatially -modulated pulsed Nd-YAG laser beam (wavelength=1064nm, pulse width=6ns) incident onto the film. This method utilizes a pulsed laser-induced thermo-elastic force exerting on the film which plays a role to detach it from the substrate. Sharp-edged clean patterns with feature size as small as 4μm could be obtained. The threshold pulse energy density for patterning was estimated to be ∼0.8J/cm2 for 150nm-thick ITO film, making it possible to pattern over one square centimeter by a single pulse with energy of 850mJ. Not only being free from photoresist and chemical etching steps, the presented method can also provide much higher throughput than the tradition photoablation process utilizing a tightly focused beam.

Multi-layered Ag film pattern printed by spatially modulated pulsed laser beam

Ag film solution-deposited on the glass source substrate was selectively transferred onto a receiver substrate by a spatially modulated pulsed ultraviolet laser beam. After printing a line pattern, an additional layer was orthogonally printed over the first layer. It was found that the thickness of the first layer is a crucial factor affecting the mechanical stability of the overall pattern. When the first layer was thicker than 0.7μm, the second layer was cracked at the junction edges regardless of its thickness. This is attributed to the vertical elongation of the second layer at the edge areas. As long as the first layer remains below 0.7μm thick, however, a very thick additional layer could be printed without any cracks. The printed patterns were mechanically robust and exhibited good electrical contact between the layers. The threshold pulse energy density for printing was measured to be 10mJ/cm2 and this threshold level made it possible to print over square centimeters by a single pulse.

Femtosecond laser induced photoluminescence in poly(methyl methacrylate) and three-dimensional optical storage

We report on a femtosecond-laser induced photoluminescence (PL) in poly(methyl methacrylate) and its potential application to three-dimensional optical storage. Irradiation with a focused 800 nm, 1 kHz, 100 fs pulsed laser induced a strong PL change in UV–visible region. Absorption spectra and Fourier-transform infrared spectra before and after laser irradiation indicate the PL may result from the emissive oxidized products of photo-degradation reaction of PMMA. This makes it possible to read out the stored data by detecting the PL change. The pulse energy threshold of the light-induced PL change of PMMA is found to be at ∼2 μJ/pulse and the optimal recording energy is ∼3 μJ/pulse. A ten-layer pattern inside the bulk sample recorded by tightly focusing a pulsed laser beam was read out by a reflection-type fluorescent confocal microscope, which detected the emission in visible range as the signal. High-contract fluorescent images with a much higher signal-to-noise ratio were obtained without crosstalk in comparison with the ordinary reflection mode.