High-quality manipulable fiber-microsphere for super-resolution microscopy.

Despite the gain in resolution brought by microsphere (MS)-assisted microscopy, it has always faced several limitations, such as a limited field of view, surface defects, low contrast, and lack of manipulability. This Letter presents a new type of MS created at the tip of an optical fiber, which we call a fiber microsphere (fMS). The fMS is made from a single-mode or coreless fiber, molten and stretched, ensuring high homogeneity and a sphere diameter smaller than the fiber itself. In addition, the connection between the fMS and the fiber makes scanning the sample a simple task, offering a solution to the difficulties of handling. The fabrication procedure of the fMS and the optical system used in the study are detailed. Our measurements show a clear superiority of the fMS over the soda-lime MS in resolving power and imaging performance.

Molded high curvature core-aligned micro-lenses for single-mode fibers.

A polymer-based fiber micro-lens molding fabrication technique with, to our knowledge, unprecedented performances is presented along with its advantages and applications. This technique is a fast and affordable tool to achieve a wide variety of possible spherical and aspherical micro-lens sizes and curvatures. The alignment of the micro-lens mold with the fiber core receiving the lens is done optically, which allows high precision. Using the proposed technique, different micro-lenses are fabricated. Then the output beams of two different micro-lenses on single-mode fibers are characterized. Fiber micro-lenses with curvature as small as 5 µm are achieved. This shows how low curvature micro-lenses can achieve collimation and how high curvature ones on single-mode fibers lead to high focusing (FWHM=λ) that is much smaller than what most conventional commercial techniques can reach. Given that this technique imposes no stress and causes no damage to the fiber receiving the micro-lens, it presents a significant potential for compatibility with non-silica-based and micro-structured fibers such as photonic crystal and quasi-crystal fibers.

Over-wavelength pitch sized diffraction gratings for augmented reality applications.

Waveguide based optical combiners for augmented reality (AR) glasses are integrating several surface relief gratings (SRG) whose pitch sizes can be as small as 200 nm for the blue wavelength. All SRG components exploit the first diffraction order to couple in and out or to deviate the light. We present SRG using higher diffraction orders featuring over-wavelength pitch sizes. Our gratings use the edge wave (EW) diffraction phenomenon to steer light in the preferred far field direction.

Direct imaging of a photonic jet at shaped fiber tips.

This Letter presents, to our knowledge, the first direct measurement of the three-dimensional distribution of photonic jets (PJs) generated by shaped-tip multimode optical fibers. A PJ at the distal end of optical fibers makes it easier to scan a sample, for lithography or optical analysis, for example, with a spot smaller than the diffraction limit. The backscattered light can also be easily collected. In this study, the volume of the PJ has been reconstructed using a stack of image planes and compared to numerical simulations. For the first time, the power distribution of the non-fundamental mode around the PJ has been observed, giving a better understanding of PJ-based laser etching using multimode optical fibers. An original 50/125 fiber with a microlens fitting just on its core has made it possible to strongly reduce the power spread compared to the thermoformed 100/140 fibers used in our previous works.

Illumination guidelines for ultrafast pump-probe experiments by serial femtosecond crystallography.

Time-resolved crystallography with X-ray free-electron lasers enables structural characterization of light-induced reactions on ultrafast timescales. To be biologically and chemically relevant, such studies must be carried out in an appropriate photoexcitation regime to avoid multiphoton artifacts, a common issue in recent studies. We describe numerical and experimental approaches to determine how many photons are needed for single-photon excitation in microcrystals, taking into account losses by scattering.

Long focal length high repetition rate femtosecond laser glass welding.

A long focal length focusing device is proposed for the process of glass welding by femtosecond laser pulses at high repetition rate and we report on the significant advantages. The study is performed using a 100 mm focusing length F-theta lens. The results are compared to those obtained with a high numerical aperture microscope objective. The long focal length with the associated Rayleigh length method allows a robust high process speed: welding at 1000 mm/s has been achieved, several orders of magnitude larger compared to what was reported until now. Moreover, for one pulse, the same energy on a larger laser spot leads to a lower thermic gradient increase. As a result, with the proposed parameters, the heat accumulation process reduces the residual stress in the welding seams, preventing the formation of fractures inside the seams: mechanical resistance at 30 MPa has been measured.

Large-mode-area optical fiber for photonic nanojet generation.

The photonic nanojet (PNJ) generated by a shaped optical fiber tip is an attractive technology for laser micro-machining. The working distance has the same order of size as the fiber core diameter; therefore, multimode (MM) fibers are generally preferred. However, the PNJ is due to the fundamental mode and, therefore, the energy coupled on the high-order modes does not contribute to the process. We demonstrate the benefit of a large-mode-area (LMA) optical fiber in the generation of the PNJ. A homemade 40 µm mode field diameter LMA fiber is compared with a 100/140 MM-shaped fiber tip. Similar micro-peaks are obtained, and an energy gain is demonstrated. The coupled energy required was eight times less intense with the LMA fiber, which may open new possibilities for laser micro- and nano-processing.

Photonic jet lens.

Microsphere-assisted microscopy currently benefits from a considerable interest in the microscope-research community. Indeed, this new imaging technique enables the lateral resolution of optical microscopes to reach around λ/5 through a full-field and a far-field acquisition while being label-free. Despite the photonic jet clearly not being a relevant concept to justify the super-resolution phenomenon, we show here how it can be used to predict imaging formation and performance such as the image position and the microsphere magnification. This study allows a better understanding of the experimental measurements that have been observed over the last decade and that will be observed in coming years, through numerical simulations using different optical and geometrical parameters.

Illumination conditions in microsphere-assisted microscopy.

White-light microsphere-assisted microscopy is a full-field and label-free imaging promising technique making it possible to achieve a subdiffraction lateral resolution. However, performance of this technique depends not only on the geometrical parameters but also on the illumination conditions of the optical system. In the present work, experimental measurements and computer simulations have been performed in air in order to determine the influence of the two diaphragm apertures of the Köhler arrangement and the spectral width of the light source on both the depth-of-focus of the microsphere and the optimization of the imaging contrast. Furthermore, the super-resolution phenomenon is demonstrated and the cumulated optical aberrations are shown through the measurement of the optical transfer function for the different arrangements of the illumination part.

Ultra-narrow photonic nanojets through a glass cuboid embedded in a dielectric cylinder.

A glass cuboid, embedded inside a dielectric cylinder is studied when illuminated with a monochromatic plane wave. A photonic nanojet (PNJ) with a full-width at half-maximum (FWHM) waist of around 0.25λ0 is obtained outside the external surface of the cuboid. The influence of the parameters of a square section cuboid is studied. Three particular phenomena can be obtained and are discussed: an ultra-narrow PNJ on the external surface of the cuboid, a long photonic jet and the excitation of whispering gallery modes (WGMs). A parametric study, over the width and the height of a rectangular section cuboid, shows that these parameters can be used to control the photonic jet properties. We also study several other geometries of the insert, which shows that the key parameter is the refractive index of the inserted material. Finally, we show that by changing the incident angle we can obtain a curved photonic jet.

Microsphere-assisted phase-shifting profilometry.

In the present work, we have investigated the combination of a superresolution microsphere-assisted 2D imaging technique with low-coherence phase-shifting interference microscopy. The imaging performance of this technique is studied by numerical simulation in terms of the magnification and the lateral resolution as a function of the geometrical and optical parameters. The results of simulations are compared with the experimental measurements of reference gratings using a Linnik interference configuration. Additional measurements are also shown on nanostructures. An improvement by a factor of 4.7 in the lateral resolution is demonstrated in air, thus giving a more isotropic nanometric resolution for full-field surface profilometry in the far field.

Photonic jet: key role of injection for etchings with a shaped optical fiber tip.

We demonstrate the key role of the laser injection into a multimode fiber to obtain a photonic jet (PJ). PJ, a high concentrated propagating beam with a full width at half-maximum smaller than the diffraction limit, is here generated with a shaped optical fiber tip using a pulsed laser source (1064 nm, 100 ns, 35 kHz). Three optical injection systems of light are compared. For similar etched marks on silicon with diameters around 1 µm, we show that the required ablation energy is minimum when the injected light beam is close to the fundamental mode diameter of the fiber. Thus, we confirm experimentally that to obtain a PJ out of an optical fiber, light injection plays a role as important as that of the tip shape and, therefore, the role of the fundamental mode in the process.

3D Super-Resolution Optical Profiling Using Microsphere Enhanced Mirau Interferometry.

We present quantitative three dimensional images of grooves on a writable Blu-ray Disc based on a single objective Mirau type interferometric microscope, enhanced with a microsphere which is considered as a photonic nanojet source. Along the optical axis the resolution of this microsphere assisted interferometry system is a few nanometers while the lateral resolution is around 112 nm. To understand the physical phenomena involved in this kind of imaging we have modelled the interaction between the photonic jet and the complex disc surface. Agreement between simulation and experimental results is demonstrated. We underline that although the ability of the microsphere to generate a photonic nanojet does not alone explain the resolution of the interferometer, the nanojet can be used to try to understand the imaging process. To partly explain the lateral super-resolution, the potential role of coherence is illustrated. The presented modality may have a large impact on many fields from bio-medicine to nanotechnology.

Photonic jet subwavelength etching using a shaped optical fiber tip.

We demonstrate that photonic jets (PJs) can be obtained in the vicinity of a shaped optical fiber and that they can be used to achieve subwavelength etchings. Only 10% of the power of a 30 W, 100 ns, near-infrared (1064 nm) Nd:YAG laser, commonly used for industrial laser processing, has been required. Etchings on a silicon wafer with a lateral feature size close to half-laser wavelength have been achieved using a shaped-tip optical fiber. This breakthrough has been carried out in ambient air by using a multimode 100/140 µm silica fiber with a shaped tip that generates a concentrated beam at their vicinity, a phenomenon referred to as a PJ, obtained for the first time without using microspheres. PJ achieved with a fiber tip, easier to manipulate, opens far-reaching benefits for all PJ applications. The roles of parameters such as laser fluence, tip shape, and mode excitation are discussed. A good correlation has been observed between the computed PJ intensity distribution and the etched marks' sizes.

Additive manufacturing of a monolithic optical force sensor based on polarization modulation.

One of the specific interests of optical sensors is their compatibility with harsh environments. The polarization modulated force sensor we propose offers this advantage, in addition to low cost and ease of manufacturing thanks to its acrylate 3D printed monolithic design. All the polarization control is indeed achieved using the geometry of a single component making unnecessary future alignments. The complex geometry of the transducer is obtained thanks to the 3D printing process. This process and the resulting material optical properties are described. The sensor concept and the fabrication method are experimentally investigated. A monolithic force sensor in the required range of 20 N is exhibited for application in the field of MR-compatible robotics. The potentiality of 3D printing for optical application in the design of the force sensor is illustrated.

Photonic jet breakthrough for direct laser microetching using nanosecond near-infrared laser.

Nanosecond near-IR lasers are commonly used for industrial laser processing. In this paper, we demonstrate that a 70 µm diameter beam generated from a 5 W, 28 ns, near-IR (1064 nm) Nd:YAG laser can etch a silicon wafer with a lateral feature size as small as 1.3 µm. Surprisingly with this laser, microetching can also be achieved on glass, despite the low absorption of this material at this wavelength. This breakthrough is carried out in ambient air by using glass microspheres with diameters between 4 and 40 µm that generate a concentrated beam at their vicinity, a phenomenon referred to as a photonic jet. The roles of parameters such as laser fluence, pulse number, microsphere diameter, and distance between the microsphere and the sample are discussed. A good correlation has been observed between the computed photonic jet intensity distribution and the etched marks' geometry.

Mode couplings and elasto-optic effects study in a proposed mechanical microperturbed multimode optical fiber sensor.

A step index multimode optical fiber with a perturbation on a micrometer scale, inducing a periodic deformation of the fiber section along its propagation axis, is theoretically investigated. The studied microperturbation is mechanically achieved using two microstructured jaws squeezing the straight fiber. As opposed to optical fiber microbend sensors, the optical axis of the proposed transducer is not bended; only the optical fiber section is deformed. Further, the strain applied on the fiber produces a periodical elliptical modification of the core and a modulation of the index of refraction. As a consequence of the micrometer scale perturbation period, the resulting mode coupling occurs directly between guided and radiated modes. To simulate the transmission induced by these kinds of perturbations, simplified models considering only total mode couplings are often used. In order to investigate the range of validity of this approximation, results are compared to the electromagnetic mode couplings rigorously computed for the first time, to our knowledge, with a large multimode fiber (more than 6000 linear polarized modes) using the Marcuse model. In addition, in order to have a more complete modeling of the proposed transducer, the anisotropic elasto-optic effects in the stressed multimode fiber are considered. In this way, the transmission of the microperturbed optical fiber transmission and, therefore, the behavior of the transducer are physically explained and its applications as a future stretching sensor are discussed.

Simultaneous strain and coherent imaging using coupled photorefractive holography and shearography through scattering media.

The direct simultaneous acquisition of coherent imaging and strain information is of particular importance in the biomechanical characterization of biological tissue. This type of simultaneous information acquisition can be accomplished using a coupled photorefractive holography and shearography system for imaging and strain measurements, respectively. Optical scattering in a conventional speckle shearing interferometer rapidly reduces the contrast of the shearing fringes, thereby limiting the use of such interferometers with opaque surfaces. By coupling photorefractive holography with speckle shearing interferometry, properties of the photorefractive effect (spatial high-pass filtering and temporal low-pass filtering) combine to restore the shearing fringe contrast and enable strain imaging in diffusing media. This effect is demonstrated using synthetic scattering phantoms built from suspensions of silica spheres in water.

Photonic jet driven non-linear optics: example of two-photon fluorescence enhancement by dielectric microspheres.

The two-photon excited fluorescence from a dye solution is enhanced when a small amount of micro-meter sized silica beads are added. This observation is made in the simple scattering regime (inter-sphere distance four times larger than their radius) and is shown to depend on the concentration of the silica spheres. For a solution of rhodamine B, the enhancement can reach more than 30 %. As complementary experiments show that the fluorescence efficiency is unchanged, we argue that the non-linear absorption is enhanced due to focussing of the incident beam in the near-field of the spheres, a situation previously referred to as photonic (nano-)jets [3]. Our calculations indeed show that for the parameters of the spheres studied near-field focussing leads to an intensity concentration close to the sphere surface. We suggest that these photonic jets could be used to enhance other non-linear optical effects.

Properties of a three-dimensional photonic jet.

By focusing light with a sphere several wavelengths in diameter, we can obtain a photonic nanojet [Opt. Express 13, 526 (2005)]: if light is focused on the surface of the sphere, the width of the beam stays smaller than the wavelength along a distance of propagation of approximately two wavelengths and reaches a high intensity. We use the rigorous Mie theory to analyze the basic properties of the photonic jet in the general three-dimensional polarized case. This fast algorithm allows us to determine the influence of the radius and the refractive index of the sphere on the photonic jet. The polarization response is also studied. We observe that high-intensity concentrations and subwavelength focusing are two different effects. Their basic properties are analyzed, and explanations are proposed.