221 research outputs found
Robust nanopatterning by laser-induced dewetting of metal nanofilms
We have observed nanopattern formation with robust and controllable spatial
ordering by laser-induced dewetting in nanoscopic metal films. Pattern
evolution in Co film of thickness 1\leq h\leq8 nm on SiO_{2} was achieved under
multiple pulse irradiation using a 9 ns pulse laser. Dewetting leads to the
formation of cellular patterns which evolve into polygons that eventually break
up into nanoparticles with monomodal size distribution and short range ordering
in nearest-neighbour spacing R. Spatial ordering was attributed to a
hydrodynamic thin film instability and resulted in a predictable variation of R
and particle diameter D with h. The length scales R and D were found to be
independent of the laser energy. These results suggest that spatially ordered
metal nanoparticles can be robustly assembled by laser-induced dewetting
Thickness-dependent spontaneous dewetting morphology of ultrathin Ag films
We show here that the morphological pathway of spontaneous dewetting of
ultrathin Ag films on SiO2 under nanosecond laser melting is found to be film
thickness dependent. For films with thickness h between 2 <= h <= 9.5 nm, the
morphology during the intermediate stages of dewetting consisted of
bicontinuous structures. For films 11.5 <= h <= 20 nm, the intermediate stages
consisted of regularly-sized holes. Measurement of the characteristic length
scales for different stages of dewetting as a function of film thickness showed
a systematic increase, which is consistent with the spinodal dewetting
instability over the entire thickness range investigated. This change in
morphology with thickness is consistent with observations made previously for
polymer films [A. Sharma et al, Phys. Rev. Lett., v81, pp3463 (1998); R.
Seemann et al, J. Phys. Cond. Matt., v13, pp4925, (2001)]. Based on the
behavior of free energy curvature that incorporates intermolecular forces, we
have estimated the morphological transition thickness for the intermolecular
forces for Ag on SiO2 . The theory predictions agree well with observations for
Ag. These results show that it is possible to form a variety of complex Ag
nanomorphologies in a consistent manner, which could be useful in optical
applications of Ag surfaces, such as in surface enhanced Raman sensing.Comment: 20 pages, 5 figure
Recovering pyramid WS gain in non-common path aberration correction mode via deformable lens
It is by now well known that pyramid based wavefront sensors, once in closed
loop, have the capability to improve more and more the gain as the reference
natural star image size is getting smaller on the pyramid pin. Especially in
extreme adaptive optics applications, in order to correct the non-common path
aberrations between the scientific and sensing channel, it is common use to
inject a certain amount of offset wavefront deformation into the DM(s),
departing at the same time the pyramid from the optimal working condition. In
this paper we elaborate on the possibility to correct the low order non-common
path aberrations at the pyramid wavefront sensor level by means of an adaptive
refractive lens placed on the optical path before the pyramid itself, allowing
the mitigation of the gain loss
Investigation of pulsed laser induced dewetting in nanoscopic metal films
Hydrodynamic pattern formation (PF) and dewetting resulting from pulsed laser
induced melting of nanoscopic metal films have been used to create spatially
ordered metal nanoparticle arrays with monomodal size distribution on
SiO_{\text{2}}/Si substrates. PF was investigated for film thickness h\leq7 nm
< laser absorption depth \sim11 nm and different sets of laser parameters,
including energy density E and the irradiation time, as measured by the number
of pulses n. PF was only observed to occur for E\geq E_{m}, where E_{m} denotes
the h-dependent threshold energy required to melt the film. Even at such small
length scales, theoretical predictions for E_{m} obtained from a
continuum-level lumped parameter heat transfer model for the film temperature,
coupled with the 1-D transient heat equation for the substrate phase, were
consistent with experimental observations provided that the thickness
dependence of the reflectivity of the metal-substrate bilayer was incorporated
into the analysis. The spacing between the nanoparticles and the particle
diameter were found to increase as h^{2} and h^{5/3} respectively, which is
consistent with the predictions of the thin film hydrodynamic (TFH) dewetting
theory. These results suggest that fast thermal processing can lead to novel
pattern formation, including quenching of a wide range of length scales and
morphologies.Comment: 36 pages, 11 figures, 1 tabl
Optimal oblique light illumination for photoacoustic microscopy beyond the diffusion limit
To image beyond the quasi-ballistic photon regime, photoacoustic tomography systems must rely on diffuse photons; however, there still exists an optimal illumination pattern that results in the largest number of photons reaching a target at a given depth. Many photoacoustic imaging systems incorporate weak optical focusing through oblique or dark-field illumination, but these systems are not often optimized for deep light delivery. Multiple parameters and constraints, particularly for in vivo imaging, need to be considered to determine the optimal illumination scheme for a given system: beam diameter, incident angle, pulse repetition rate, laser fluence, and target depth. Monte Carlo simulations of varied beam geometries and incident angles show the best optical illumination schemes for different imaging depths. Further an analytic model based on the diffusion theory provides a rapid method of determining the optimal beam size and incident angle for a given target depth and agrees well with the simulations. The results reveal the most efficient optical focal position to maximize the number of photons delivered to a target depth, therein maximizing the PA signal. The principles and results discussed here are not limited to the system investigated, but can be applied to other system configurations to improve the photoacoustic signal strength
Nonlinear photoacoustic spectroscopy of hemoglobin
As light intensity increases in photoacoustic imaging, the saturation of optical absorption and the temperature dependence of the thermal expansion coefficient result in a measurable nonlinear dependence of the photoacoustic (PA) signal on the excitation pulse fluence. Here, under controlled conditions, we investigate the intensity-dependent photoacoustic signals from oxygenated and deoxygenated hemoglobin at varied optical wavelengths and molecular concentrations. The wavelength and concentration dependencies of the nonlinear PA spectrum are found to be significantly greater in oxygenated hemoglobin than in deoxygenated hemoglobin. These effects are further influenced by the hemoglobin concentration. These nonlinear phenomena provide insights into applications of photoacoustics, such as measurements of average inter-molecular distances on a nm scale or with a tuned selection of wavelengths, a more accurate quantitative PA tomography
In vivo multiscale photoacoustic microscopy of human skin
Scalability is a key feature of photoacoustic microscopy (PAM). Reports have shown that PAM systems can be designed to possess sub-micron resolution at shallow depths or penetrate centimeters deep at the expense of resolution while the number of resolved pixels in the depth direction remains high. This capability to readily tune the imaging parameters while maintaining the same inherent contrast could be extremely useful for a variety of biomedical applications. Human skin, with its layered vascular structure whose dimensions scale with depth, provides an ideal imaging target to illustrate this advantage. Here, we present results from in vivo human skin imaging experiments using two different PAM systems, an approach which enables better characterization of the cutaneous microvasculature throughout the imaging depth. Specifically, we show images from several distinct areas of skin: the palm and the forearm. For each region, the same area was imaged with both an optical-resolution PAM (OR-PAM) and an acoustic-resolution PAM (AR-PAM), and the subsequent images were combined into composite images. The OR-PAM provides less than 5 μm lateral resolution, capable of imaging the smallest capillary vessels, while the AR-PAM enables imaging at depths of several millimeters. Several structures are identifiable in the ORPAM images which cannot be differentiated in AR-PAM images, namely thin epidermal and stratum corneum layers, undulations in the dermal papillae, and capillary loops. However, the AR-PAM provides images of larger vessels, deeper than the OR-PAM can penetrate. These results demonstrate how PAM's scalability can be utilized to more fully characterize cutaneous vasculature, potentially impacting the assessment of numerous cardiovascular related and cutaneous diseases
Quantification of optical absorption coefficients from acoustic spectra with photoacoustic tomography
Optical absorption is closely associated with many physiologically important parameters, such as the concentration and oxygen saturation of hemoglobin, and it can be used to quantify the concentrations of non-fluorescent molecules. We introduce a method to quantify the absolute optical absorption based upon the acoustic spectra of photoacoustic (PA) signals. This method is self-calibrating and thus insensitive to variations in optical fluence. Factors such as the detection system bandwidth and acoustic attenuation can affect the quantification but can be canceled by measuring the acoustic spectra at two optical wavelengths. This method has been implemented on various PA systems, including optical-resolution PA microscopy, acoustic-resolution PA microscopy, and reconstruction based PA array systems. We quantified the optical absorption coefficients of phantom samples at various wavelengths. We also quantified the oxygen saturation of hemoglobin in live mice
Functional photoacoustic microscopy of pH
pH is a tightly regulated indicator of metabolic activity. In mammalian systems, imbalance of pH regulation may result from or result in serious illness. Even though the regulation system of pH is very robust, tissue pH can be altered in many diseases such as cancer, osteoporosis and diabetes mellitus. Traditional high-resolution optical imaging techniques, such as confocal microscopy, routinely image pH in cells and tissues using pH sensitive fluorescent dyes, which change their fluorescence properties with the surrounding pH. Since strong optical scattering in biological tissue blurs images at greater depths, high-resolution pH imaging is limited to penetration depths of 1mm. Here, we report photoacoustic microscopy (PAM) of commercially available pH-sensitive fluorescent dye in tissue phantoms. Using both opticalresolution photoacoustic microscopy (OR-PAM), and acoustic resolution photoacoustic microscopy (AR-PAM), we explored the possibility of recovering the pH values in tissue phantoms. In this paper, we demonstrate that PAM was capable of recovering pH values up to a depth of 2 mm, greater than possible with other forms of optical microscopy
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