Light Intensity Enhancement Factor Research Articles

Optimization of High-Density Fe-Au Nano-Arrays for Surface-Enhanced Raman Spectroscopy of Biological Samples.

The method of realizing nanostructures using porous alumina templates has attracted interest due to the precise geometry and cheap cost of nanofabrication. In this work, nanoporous alumina membranes were utilized to realize a forest of nanowires, providing a bottom-up nanofabrication method suitable for surface-enhanced Raman spectroscopy (SERS). Gold and iron were electroplated through the straight channels of the membrane. The resulting nanowires are, indeed, made of an active element for plasmonic resonance and SERS as the hexagonal distribution of the nanowires and the extreme high density of the nanowires allows to excite the plasmon and detect the Raman signal. The method to reduce the distance between pores and, consequently, the distance of the nanowires after electrodeposition is optimized here. Indeed, it has been predicted that the light intensity enhancement factor is up to 1012 when the gap is small than 10 nm. Measurements of Raman signal of thiol groups drying on the gold nanowires show that the performance of the device is improved. As the thiol group can be linked to proteins, the device has the potential of a biosensor for the detection of a few biomolecules. To assess the performance of the device and demonstrate its ability to analyze biological solutions, we used it as SERS substrates to examine solutions of IgG in low abundance ranges. The results of the test indicate that the sensor can convincingly detect biomolecules in physiologically relevant ranges.

The impurity study of MRF processed fused silica surface

MRF (magnetorheological finishing) will leave impurities and affect the LIDT (laser induced damage threshold) performance of fused silica components. This manuscript first measured the impurity distribution after MRF. Then three-dimensional FDTD (finite-difference time-domain) simulation was employed to study the light field distribution of impurities under laser irradiation and the relationship between the dielectric constant, the size of the impurities and LIEF (light intensity enhancement factor). At last the HF etching was verified to remove the impurities. Studies have shown that carbonyl iron and cerium oxide impurities exist after MRF. FDTD simulation showed for impurities of the same size, the LIEF value first decreases and then increases with the dielectric constant; while for impurities of the same dielectric constant, the LIEF value increases as the size increases. Finally, HF etching can effectively remove impurities on MRF processed fused silica components.

Three dimensional numerical simulation of modulation by scratches on fused silica surface

The scratches generated during optical manufacturing influence the LIDT (laser induced damage threshold) performance of fused silica components. In order to get a better understanding of this influence, the three dimensional FDTD (finite-difference time-domain) simulation is employed to study the light field distribution of scratches under laser irradiation and the relationship between the location, geometry and distribution of scratches and LIEF (light intensity enhancement factor). The results show that scratches located on the exit surface are more likely to induce damage. The geometric size of a single scratch greatly affects its LIEF value. The LIEF of a scratch with the same geometry increases as the width increases. While if the width of the scratch is given, its LIEF increases firstly, then decreases, eventually it tends to be stable as the geometry varies. For Hertzian scratches, the LIEF of adjacent scratches is larger than that of a single scratch due to the superposition of field strength; when the distance between Hertzian scratches becomes farther, the superposition of field strength gradually weakens, and finally reaches the same level as that of a single scratch. These results can help controlling the scratches to improve the quality of optical components.

Investigation of subsurface damage density and morphology impact on the laser-induced damage threshold of fused silica.

The laser-induced damage threshold (LIDT) of fused silica is affected by laser field intensity modulation and laser energy absorption. In this paper, the subsurface damage (SSD) density and morphology are detected by the small-angle taper polishing method. The modulation effect of SSD morphology on the incident laser/electric field is analyzed by the finite difference time domain (FDTD) simulation. Finally, the LIDT of the taper polished surface is tested to analyze the relationship among LIDT, SSD density, and SSD morphology, and the results show a high correlation. A reliable regression model is obtained based on the results, which shows that LIDT is inversely proportional to SSD density and the light intensity enhancement factor (LIEF).

Light intensification effect of trailing indent crack in fused silica subsurface

A finite-difference time-domain algorithm was applied to solve Maxwell’s equations to obtain the redistribution of an electromagnetic plane wave in the vicinity of a trailing indent crack (TIC). The roles of five geometrical parameters playing in light intensification were calculated numerically under the irradiation of a 355-nm normal incidence laser. The results show that the light intensity enhancements between the nearest neighbor pits were remarkable, which may lead to damage. The calculated results reveal that the light intensity enhancement factor (LIEF) can be up to 11.2 when TIC is on the rear-surface. With the increase of the length as well as the depth of pits, LIEF increased. Conversely, with the increase of the axis of pits, LIEF gradually declined to a stable status. It was observed that there exists an optima width or gap, which enables LIEF to be increased dramatically and then decreased gently. By comparison, results suggest that the worst cases occur when the depth and the length are both very large, especially if the width equals to 2 l and the gap equals the width. This work provides a recommended theoretical criterion for defect inspection and classification.

Rear-surface light intensification caused by a Hertzian-conical crack in 355-nm silica optics

Theoretical studies show that a Hertzian-conical crack can be considered to be composed of double cone faces for simplicity. In the present study, the three-dimensional finite-difference time-domain method is employed to quantify the electric-field distribution within the subsurface in the presence of such a defect under normal incidence irradiation. Both impurities (inside the crack) and the chemical etching have been investigated. The results show that the maximum electric field amplitude |E|max is 9.57374 V/m when the relative dielectric constant of transparent impurity equals 8.5. And the near-field modulation will be improved if the crack is filled with the remainder polishing powders or water vapor/drops. Meanwhile, the laser-induced initial damage moves to the glass—air surface. In the etched section, the magnitude of intensification is strongly dependent on the inclination angle θ. There will be a highest modulation when θ is around π/6, and the maximum value of |E|max is 18.57314 V/m. When θ ranges from π/8 to π/4, the light intensity enhancement factor can easily be larger than 100, and the modulation follows a decreasing trend. On the other hand, the modulation curves become smooth when θ > π/4 or θ < π/8.

Simulation of field intensification induced by pit-shaped crack on fused silica rear-surface

Rotating paraboloid model is establishd, and three-dimensional finite-difference time-domain method is used to simulate pit-shaped cracks on fused silica rear-surface. The light intensification with its depth, width, gap distance and etch value are investigated under 355 nm laser incident. Results show that the strongest modulation is located at the connection area between pit and pit, and the modulation become strong with approaching to the surface. The maximum light intensity enhancement factor (LIEF) is 11.53 when the breadth depth ratio ranges from 2.0 to 3.5 and gap distance close to 1/2 width. As gap distance greater than the width, the modulation reduces greatly, which is equal to a single pit. For 60-width, 30-depth and 30-gap distance cracks, the maximum LIEF is 11.0 during the acid etching. As the gap distance is less than 300 nm, the diffraction of the light field makes the neighbor pits connective.

The effects of shape and position on field distribution of discontinuous crack

Elliptic paraboloid model is established, and three-dimensional finite-difference time-domain method has been employed to simulate the field distribution of discontinuous crack with different shapes and positions in fused silica output subsurface under 355 nm laser incidence. Results show that both electric field intensity and the number of enhanced points (with |E| 2.0 V/m) follow a trend of first increasing and then decreasing with the increase of collinear axis length of ellipsoid. The light intensity enhancement factor reaches a maximal value when the axial ratio is 1.1 to 1.2. If parallel collinear axis length of ellipsoid increases, the electric field intensity first increases gradually, and then becomes stable when the axial ratio is 0.53. At the same time, the number of enhanced points grows rapidly.

Three-dimensional numerical simulation of light field modulation in the vicinity of inclusions in silica subsurface

One of the important factors of the low laser induced damage threshold is the defects in the subsurface of fused silica. The three-dimensional model of a spherical inclusion in the subsurface is established in this study. Three-dimensional finite-difference time-domain method is used to calculate and simulate the light field distribution in the vicinity of inclusions. The effects of dielectric constant and inclusion size are analyzed, separately. The results show that the light intensity enhancement factor (LIEF) does not change with the size and the dielectric constant of the inclusions when the dielectric constant is smaller than that of fused silica, where the LIEF is kept at about 4. When the dielectric constant is 6.0, the LIEFs are 50.1588, 73.3904 and 102.9953 for the inclusions with sizes of 1.5λ, 2λ and 2.5λ respectively. When the inclusion size is constant, the LIEF will increase with the increase of dielectric constant. The light enhancement for the round inclusions is much higher than that for the ellipsoidal inclusions. Therefore, the round inclusions with large size and dielectric constant significantly enhance the electric field.

Simulation of Light Intensification Induced by Defects of Polished Fused Silica

Light intensity distribution in the vicinity of inclusions and etched cracks in polished fused silica at wavelength scale are simulated by using the finite-difference time-domain algorithm. Light intensity enhancement factor as functions of diameter and refractive index of inclusions are investigated, more than 10 times that of incident beam is obtained in the simulation. We model the etched crack in close proximity to a real structure, which is characterized by AFM. We find that the peak light intensity of the crack is a function of its cross sectional breadth depth ratio, providing good hints for the effective processing of fused silica samples to improve the damage threshold.

Role of light intensification by cracks in optical breakdown on surfaces.

The intensity distribution of an initially plane light wave incident on planar and conical surface cracks is calculated numerically by using a wave propagation computer code. The results show that light intensity enhancements caused by interference of internal reflections at the crack and the surface are very sensitive to the light polarization, the beam angle of incidence, and the crack geometry (e.g., crack width and orientation with the surface). The light intensity enhancement factor (LIEF) can locally reach 2 orders of magnitude for conical cracks of ideal shape. The electric field direction relative to the crack surfaces determines the light intensity profile around the crack. For normal-incidence illumination on the output surface, total internal reflection at the crack and the surface can occur and leads to higher LIEFs. For identical geometry and illumination conditions, a crack located on the entrance surface of an optic generates electric field enhancements that are weaker than those on the exit surface. As cracks on polished surfaces are randomly oriented, the probability for large intensity enhancements to occur is high. The model is able to predict quantitatively the magnitude of surface laser-induced damage threshold drop and damage propagation enhancement in dielectric materials that are due to cracks.