Nano-enhanced optical delivery into targeted cells (Conference Presentation)

This paper will discuss and compare some recent oceanic test results from the Bahamas Optical Turbulence Exercise (BOTEX) cruise, where vertical profiling was conducted with both time-resolved laser backscatter measurements being acquired via a subsurface light detection and ranging (lidar) profiling instrument, and laser beam forward deflection measurements were acquired from a matrix of continuous wave (cw) laser beams (i.e. structured lighting) being imaged in the forward direction with a high speed camera over a one-way path, with both transmitter and camera firmly fixed on a rigid frame. From the latter, it was observed that when within a natural turbulent layer, the laser beams were being deflected from their still water location at the image plane, which was 8.8 meters distance from the laser dot matrix transmitter. As well as suggesting that the turbulent structures being encountered were predominately larger than the beam diameter, the magnitude of the deflection has been confirmed to correlate with the temperature dissipation rate. The profiling lidar measurements which were conducted in similar conditions, also used a narrow collimated laser beam in order to resolve small-scale spatial structure, but with the added attribute that sub-nanosecond short pulse temporal profile could potentially resolve small-scale vertical structure. In the clear waters of the Tongue of the Ocean in the Bahamas, it was hypothesized that the backscatter anomalies due to the effect of refractive index discontinuities (i.e. mixed layer turbulence) would be observable. The processed lidar data presented herein indicates that higher backscatter levels were observed in the regions of the water column which corresponded to higher turbulent mixing which occurs at the first and second themoclines. At the same test stations that the laser beam matrix and lidar measurements were conducted, turbulence measurements were made with two non-optical instruments, the Vertical Microstructure Profiler (VMP) and a 3D acoustical Doppler velocimeter with fast conductivity and temperature probes. The turbulence kinetic energy dissipation rate and the temperature dissipation rates were calculated from both these setups in order to characterize the physical environments and corroborate with the laser measurements. To further investigate the utility of elastic lidar in detecting small-scale turbulent structures, controlled laboratory experiments were also conducted, with the objective of concurrently acquiring both the laser beam spatial characteristics in the forward direction and the laser backscatter temporal profile from each transmitted sub-nanosecond pulse. An artificial refractive index discontinuity was generated in clear test tank conditions by placing a clean ice-filled carboy above the laser beam propagation path. The results from both field and laboratory experiments confirm our hypothesis that turbulent layers are detectable by lidar sensors, and motivates that more research and lidar instrumentation development is needed to better quantify turbulence, especially for mitigating associated performance degrading effects for the U.S. Navy’s next generation electro-optic (EO) systems, including active laser imaging and laser communications.

Two multi-kilowatts Nd: YAG laser beams are condensed separately and combined at surface of workpiece in order to increase penetration depth. In this system, one or two laser beams are inevitably inclined depending on size of condensing optics, thus, welding characteristics might be fairly complicated than that of a single laser beam welding. Welding characteristics are evaluated by penetration depth and penetration shape in bead-on-plate tests of stainless steel (304). This paper is intended as an investigation on the welding condition of the deepest weld penetration, especially on arrangement of condensing optics. The deepest weld penetration of 6.3 and 7.7 mm are obtained by two CW laser beams and two rectangularly modulated CW laser beams with total average power of four kilowatts at a welding speed of 5 mm/s, respectively. Deepened penetration depths of weld beads by laser beams in combination due to the increase in total laser power are obtained in comparison to penetration depths of 2.8 and 4.3 mm by a single CW laser beam and a single rectangular modulated CW laser beam with average powers of two kilowatts, respectively. Penetration depths of weld beads obtained with symmetric irradiation exceed those with asymmetric irradiation. The angle between two laser beams for the deepest penetration is 60 degrees for CW laser beams, while the angle is 30 degrees for modulated CW laser beams. The difference in the appropriate angles for the deepest penetration might be explained by different effect of keyhole bottom separation on penetration depths depending on waveform of laser beams. With asymmetric beam arrangement, a deeper penetration is obtained in backhand welding direction for the inclined laser beam than that in forehand direction both in welding using two CW laser beams in combination and two modulated CW beams in combination.

In present paper we investigate the transverse dynamics of frequency-modulated cw laser beam propagated in resonance conditions. At modulation periods comparable with the atomic relaxation times the time dependence of the output intensity exhibits the manifestations of resonance sel f-action effects. The influence of resonance self-induced lens and aperture on cw laser beam transverse profile and the boundary conditions of self-action are analyzed by numerical simulation. Keywords: resonance self-action, cw laser beam propagation, numerical simulation, frequency modulation, propagation length, transverse profile of laser beam, Gauss-Laguerre modes. 1. INTRODUCTION Self-action of cw laser beams due to saturated refraction in resonance media has been studied for a long time (see, e.g. [1-4]), most of investigations was related to observation of lensing properties of the media. Under extremely high absorption and saturation conditions the manifestations of self-induced diffraction have been found to mimic self-focusing and self-channelization at zero frequency detuning when the optical strength of the saturation-induced lens is zero [2-4]. From the theoretical point of view it means that the medium response can be described by means of the nonlinear susceptibility. Obviously, the same approach is valid for time-dependent field whose variation is slow compared with atomic relaxation. Faster field changes are expected to cause transient response of the medium and, therefore, transient self-focusing and self-aperturing effects. Experimental manifestations of non-stationary near-resonant self-focusing of frequency-modulated beams have been reported in saturation spectroscopy [5]. The present paper is a continuation of our previous work [6], where we inve stigate the beams initially modulated in frequency. Passing through resonant absorbing medium, the beam gradually acquires intensity modulation, which is simply caused by different absorption at different frequencies. This effect is neither nonlinear, nor transient, it takes place even at low intensity and low frequency of modulation. Note, that in this case it is not affected by the lensing properties of the medium, since they are due to saturation and not to frequency-dependent refraction as such. First, we study how the saturation of absorption and refraction affects the transmission of modulation from phase at the input to the intensity at the output. We take the frequency of modulation low enough to consider the saturation as adiabatically following the field variation in time. The most prominent manifestations of the induced lens can be seen in moderate saturation regime. In this regime we focus our attention on the resonant self-action dynamics and boundary conditions and on the self-action influence on laser beam transverse profile.

We measured the divergence angles of a CW laser beam passing through gold colloids with different concentrations to quantitatively characterize Faraday-Tyndall effect. Our results show that the divergence angles of 532-nm and 785-nm CW laser beams through gold nanoparticles (GNPs) colloid are increased as the concentration of GNPs increases. The increased divergence angle of GNP at 532 nm is more significant than that at 785 nm, because the surface plasmon resonance of GNP is at 549 nm. Moreover, the effects of different-sized GNPs will be studied.

We report on the differential absorption spectra induced by a continuous-wave (cw) laser beam at 405 nm and by a femtosecond pulse for In0.10Ga0.90N multiple quantum wells and epilayers. For a thick InGaN epilayer, even low power cw pumping can cause sizable absorption changes that are observed at 2 ns after high-energy pulse excitation. However, the absorption changes are very small for a multiple quantum well structure. Photogenerated carriers are trapped in deep levels and screen piezoelectric fields. The screening effect leads to absorption spectrum changes around the absorption edge. The piezoelectric fields are screened effectively and larger absorption changes are observed in thicker epilayers.

Two Nd:YAG laser beams were condensed separately and combined at surface of workpiece in order to increase weld penetration depth. Welding characteristics were evaluated by penetration depth and penetration shape in bead-on-plate tests. Transition from penetration with one bottom to penetration with two bottoms occurs as function of welding condition. Penetration with two bottoms might be a sufficient evidence in favor of keyhole bottom separation. A close study of keyhole bottom separation is necessary to understand the increase of weld penetration by multiple laser beams combined. This paper is intended as an investigation on factors of keyhole bottom separation. The authors found that factors of keyhole bottom separation are angle of incidence of laser beams, laser power density, heat input, distance between two laser beams on a surface of workpiece, and welding position. Keyhole bottom separation occurs under conditions determined by high power density of one laser beam and high total heat input, such as 1500 W/mm2 and 104J/cm, respectively, at angle of incidence of 30° in welding using two CW laser beams in combination.

Laser welding with albumin-based tissue solder has been investigated as an alternative to surgical suturing. Many surgical procedures require the soldered tissues to be in a hydrated environment. We have studied the effects of hydration on laser soldered rat dermis and baboon articular cartilage in vitro. The solder is composed of human serum albumin, sodium hyaluronate and indocyanine green. We used a micro-pipette to deposit 2 (mu) l of solder on each tissue specimen. An 808 nm cw laser beam with irradiance of 27 W/cm 2 was scanned 4 times over the same solder area at a constant speed of 0.84 mm/sec. After photo-coagulation, each tissue specimen was cut into two halves at the center of the solder, perpendicular to the direction of the scanning laser beam. One half was reserved as control while the other half was soaked in phosphate buffered saline for a designated hydration period. The hydration periods were 1 hr, 1, 2, and 7 days. All tissue specimens were fixed in glutaraldahyde, then prepared for scanning electron microcopy analysis. For most of the specimens, there was non-uniform coagulation across the thickness of the solder. Closer to the laser beam, the upper solder region formed a more dense coagulum. While the region closer to solder-tissue interface, the solder aggregated into small globules. This non-uniform coagulation was likely caused by non-uniform energy distribution during photocoagulation. The protein globules and coagulum seem to be responsible for the solder attachment from the specimen surface. However, we have noted that the solder detached from the cartilage substrate as early as after 1 hr of hydration. On the other hand, the solder attached to the dermis much better than to cartilage. This may be explained by the difference in surface roughness of the two tissue types. The dermal layer of the skin is composed of collagen matrix which may provide a better entrapment of the solder than the smooth surface of articular cartilage.

The trapping process of the biological micro-particle in the tweezers using CW laser beam has been investigated in series of our works. In the CW regime, the dynamics of polystyrene microsphere linking to λ-phage DNA molecule depending on the optical force, Brown force and its elastic force are simulated and discussed. In this paper we present the finite difference Langevin equation (FDLE) describing the dynamics of trapped bead linking to DNA molecules in an optical tweezers. Using FDLE the dynamics, i.e., the position-time characteristics of polystyrene microsphere linking to λ-phage DNA molecule are simulated in the pulse regime. The obtained results show that all principle parameters as the peak intensity, beam waist radius, and duration of pulsed Gaussian laser beam, well as the setting-up position of DNA molecule influence on the dynamics, especially on the “pulling” and “stability” times of trapped bead. The conditions for λ-phage DNA molecule in stretching state are discussed for experimence.

All-optical switching has been theoretically analyzed in LOV2 phototropin, the blue light plant photoreceptor based on nonlinear intensity-induced excited-state absorption. The transmission of a cw probe laser beam at 660 nm through LOV2 protein can be controlled by a cw or pulsed pump laser beam at 442 nm, respectively. This modulation is sensitive to the small-signal absorption coefficient, absorption cross-section of the excited L-state at pump beam wavelength and sample thickness. It is shown that the unique spectral and kinetic properties of wild-type LOV2 and LOV2-C39A mutant, result in the probe beam getting completely switched off (100% modulation), by the pump laser beam of intensity 50 kW/cm² and 1 kW/cm², respectively. The switching response in LOV2-WT is faster (~μs) than in LOV2-C39A mutant (~ms). The results have been used to design an optically-addressed spatial light modulator and all-optical NOT, and the universal NAND and NOR logic gates. The results show the applicability of this novel plant photoreceptor protein for photonic applications.

Analysis of all-optical light modulation in proteorhodopsin protein molecules

Boron doping patterns characterized by very sharp lateral edge definition and 300–1000-nm linewidths have been written in silicon by localized heating with a diffraction-limited cw laser beam. The tightly focused beam generates free dopant atoms, and simultaneously promotes solid- state diffusion into the substrate. The nonlinear dependence of diffusion rates on temperature results in linewidths substantially narrower than both the temperature and the laser beam profiles on the surface. An enhancement is observed in diffusion rates under the strong thermal gradients created.

In the small-angle approximation, the exact solution of an equation of radiative transfer is cast in a series form suitable for numerical computation. This is used to calculate the radiance produced by a cw laser beam. For the medium with Gaussian phase function, a convergent numerical procedure is described. Explicit results for the scattered radiance, obtained for a wide range of parameters, display the existence of three well-defined regions on the plots of radiance versus transverse distance. A comparison of the exact theory with two approximate approaches is carried out.

The aim of the present paper is to describe incorporation of basic industrial research results into current University study programs in Physics and Optoelectronic Engineering. The Laboratory of Optics and Fluids (LOF) of University Simon Bolivar (USB) leads a research program on applications of Photonics technology in the Petroleum Industry. More precisely, the main research subject at the (LOF) is development of optical procedures allowing determination of conditions of stability of oil-in-water emulsions. In several countries (for example, Canada and Venezuela) there exist important reservoirs of heavy crude oils, whose high viscosity impede their transportation through pipelines. Therefore, emulsions of heavy oils in water were developed in order to allow their commercialization. Though those emulsions are stable in current environmental conditions, high temperature or velocity gradients frequently provoke their coalescence. In typical experiments conducted at the (LOF) temperature gradients are induced in oil-water emulsions and in crude oil samples by irradiation with a CW laser beam. In crude oil samples the strong dependence of the liquid surface tension and refractive index on the local liquid temperature gives rise to long-range deformation of the liquid free surface. The latter cited thus behaves as an interferometrically smooth liquid mirror, which gives rise in turn to phase and intensity variations in the reflected light beam. In emulsion samples the inhomogeneous heating gives rise to thermoconvective flow, which is clearly observed as a moving speckle pattern in the reflected light beam. These are typical phenomena of self-interaction of a laser beam incident upon a material medium. In the present paper we discuss these optical phenomena, first studied in a basic research context, from an educational viewpoint.

ABSTRACTDopant patterns of 250-nm linewidth have been written in silicon by localized heating with a cw laser beam, and then transferred into positive and negative surface-relief patterns by preferential etching. The tightly focused beam both generates free dopant atoms and simultaneously promotes solidstate diffusion into the substrate. Because of the nonlinear dependence of diffusion rates on temperature, the linewidths of the patterns are substantially narrower than those of both the temperature and the laser-beam profiles on tile surface. In addition, an enhancement is observed in diffusion rates under the strong thermal gradients associated with highly localized heating.

The deep penetration laser welding of metals involves the action of a laser beam usually incident normally on a translating work piece. This generates a keyhole in the metal surrounded by a molten region which secures the weld when the metal freezes. The relevant process parameters involved are the power of the laser beam, the translation speed of the metal and the depth of penetration of the laser beam into the metal, together with appropriate physical parameters characterising the metal. It is the aim of mathematical models of these processes to produce a relationship between these quantities with a view to providing an effective simulation of the process as well as a guide to optimal operating conditions both for CW and pulsed laser systems. It is usual when pursuing such an analysis of the process to assume a suitably averaged keyhole structure to exist. The surface of the keyhole is the region of the metal whose coupling of the laser beam to the material takes place and energy is transferred to the metal, usually by the process of Fresnel absorption and by inverse bremsstrahlung and conduction processes in the vapour in the keyhole, particularly in the case of a CO2 laser. Several such models have been produced and mathematically a strictly deterministic relation between the relevant quantities is usually assumed to exist. The assumption of a steady-state keyhole with suitably averaged properties, however, constitutes a considerable idealisation of the process. Actually, the keyhole manifests itself as a writhing, twisting entity whose behaviour as such has been photographed by several investigators. Such behaviour is clearly seen even for the CW laser case. It arises from a number of different instabilities that can and do inevitably come into play in the laser welding process. Such effects represent in essence the reaction of the material being welded to the laser beam. No mathematical model currently is known which attempts to take account of this behaviour which nevertheless has real significant practical effects which both influence the welding process and can lead to its breakdown. It can become even more necessary to consider such reactions of the material to the laser beam when pulsed lasers or CW lasers with pulsed modulation are considered. The details concerning the particular characteristics of specific instabilities are not considered here. Instead, it is the objective of this paper to provide the first stochastic description of some aspects of the laser welding process when the laser operates in the continuous wave case. It is thus intended to attempt to extend the applicability of current mathematical models and provide increased insight into the nature of the laser welding process.