Encoding of arbitrary micrometric complex illumination patterns with reduced speckle.

In nonlinear microscopy, phase-only spatial light modulators (SLMs) allow achieving simultaneous two-photon excitation and fluorescence emission from specific region-of-interests (ROIs). However, as iterative Fourier transform algorithms (IFTAs) can only approximate the illumination of selected ROIs, both image formation and/or signal acquisition can be largely affected by the spatial irregularities of the illumination patterns and the speckle noise. To overcome these limitations, we propose an alternative complex illumination method (CIM) able to generate simultaneous excitation of large-area ROIs with full control over the amplitude and phase of light and reduced speckle. As a proof-of-concept we experimentally demonstrate single-photon and second harmonic generation (SHG) with structured illumination over large-area ROIs.

Single-pixel imaging of the retina through scattering media.

Imaging the retina of cataractous patients is useful to detect pathologies before the cataract surgery is performed. However, for conventional ophthalmoscopes, opacifications convert the lens into a scattering medium that may greatly deteriorate the retinal image. In this paper we show, as a proof of concept, that it is possible to surpass the limitations imposed by scattering applying to both, a model and a healthy eye, a newly developed ophthalmoscope based on single-pixel imaging. To this end, an instrument was built that incorporates two imaging modalities: conventional flood illumination and single-pixel based. Images of the retina were acquired firstly in an artificial eye and later in healthy living eyes with different elements which replicate the scattering produced by cataractous lenses. Comparison between both types of imaging modalities shows that, under high levels of scattering, the single-pixel ophthalmoscope outperforms standard imaging methods.

Low-cost single-pixel 3D imaging by using an LED array.

We propose a method to perform color imaging with a single photodiode by using light structured illumination generated with a low-cost color LED array. The LED array is used to generate a sequence of color Hadamard patterns which are projected onto the object by a simple optical system while the photodiode records the light intensity. A field programmable gate array (FPGA) controls the LED panel allowing us to obtain high refresh rates up to 10 kHz. The system is extended to 3D imaging by simply adding a low number of photodiodes at different locations. The 3D shape of the object is obtained by using a non-calibrated photometric stereo technique. Experimental results are provided for an LED array with 32 × 32 elements.

High sampling rate single-pixel digital holography system employing a DMD and phase-encoded patterns.

A single-pixel digital holography system with phase-encoded illumination using a digital micromirror device (DMD) as a spatial light modulator (SLM) is presented. The enhanced switching rate of DMDs, far exceeding the stringent frame-rate of liquid crystal SLMs, allows recording and reconstruction of complex amplitude distributions in just a few seconds. A single amplitude binary modulation device is used for concurrently displaying the phase-encoded sampling patterns, compensating the distortion of the wavefront, and applying phase-shifting, by means of computer generated holograms. Our detection system consists of a simple photodiode that sequentially records the irradiance fluctuations corresponding to the interference between object and reference beams. The system recovers phase and amplitude information even when a diffuser is placed in front of the photodiode.

Generation of multifocal irradiance patterns by using complex Fresnel holograms.

We experimentally demonstrate Fresnel holograms able to produce multifocal irradiance patterns with micrometric spatial resolution. These holograms are assessed from the coherent sum of multiple Fresnel lenses. The utilized encoded technique guarantees full control over the reconstructed irradiance patterns due to an optimal codification of the amplitude and phase information of the resulting complex field. From a practical point of view, a phase-only spatial light modulator is used in a couple of experiments addressed to obtain two- and three-dimensional distributions of focal points to excite both linear and non-linear optical phenomena.

Real-time acquisition of complex optical fields by binary amplitude modulation.

We describe, through simulations and experiments, a real-time wavefront acquisition technique using random binary amplitude masks and an iterative phase retrieval algorithm based on the Fresnel propagator. By using a digital micromirror device, it is possible to recover an unknown complex object by illuminating with this set of masks and simultaneously recording the resulting intensity patterns with a high-speed camera, making this technique suitable for dynamic applications.

Single-pixel digital holography with phase-encoded illumination.

We demonstrate imaging of complex amplitude objects through digital holography with phase-structured illumination and bucket detection. The object is sampled with a set of micro-structured phase patterns implemented onto a liquid-crystal spatial light modulator while a bucket detector sequentially records the irradiance fluctuations corresponding to the interference between object and reference beams. Our reconstruction algorithm retrieves the unknown phase information from the full set of photocurrent measurements. Interestingly, the sampling functions can be codified onto the reference beam, so they can be nonlocal with respect to the object. Finally, we show that the system is well-fitted for transmission of the object information through scattering media.

Characterization of Tin/Ethylene Glycol Solar Nanofluids Synthesized by Femtosecond Laser Radiation.

Solar energy is available over wide geographical areas and its harnessing is becoming an essential tool to satisfy the ever-increasing demand for energy with minimal environmental impact. Solar nanofluids are a novel solar receiver concept for efficient harvesting of solar radiation based on volumetric absorption of directly irradiated nanoparticles in a heat transfer fluid. Herein, the fabrication of a solar nanofluid by pulsed laser ablation in liquids was explored. This study was conducted with the ablation of bulk tin immersed in ethylene glycol with a femtosecond laser. Laser irradiation promotes the formation of tin nanoparticles that are collected in the ethylene glycol as colloids, creating the solar nanofluid. The ability to trap incoming electromagnetic radiation, thermal conductivity, and the stability of the solar nanofluid in comparison with conventional synthesis methods is enhanced.

In situ decoration of graphene sheets with gold nanoparticles synthetized by pulsed laser ablation in liquids.

The demand for nanocomposites of graphene and carbonaceous materials decorated with metallic nanoparticles is increasing on account of their applications in science and technology. Traditionally, the production of graphene-metal assemblies is achieved by the non-environmentally friendly reduction of metallic salts in carbonaceous suspensions. However, precursor residues during nanoparticle growth may reduce their surface activity and promote cross-chemical undesired effects. In this work we present a laser-based alternative to synthesize ligand-free gold nanoparticles that are anchored onto the graphene surface in a single reaction step. Laser radiation is used to generate highly pure nanoparticles from a gold disk surrounded by a graphene oxide suspension. The produced gold nanoparticles are directly immobilized onto the graphene surface. Moreover, the presence of graphene oxide influences the size of the nanoparticles and its interaction with the laser, causes only a slight reduction of the material. This work constitutes a green alternative synthesis of graphene-metal assemblies and a practical methodology that may inspire future developments.

Diffractive control of 3D multifilamentation in fused silica with micrometric resolution.

We show that a simple diffractive phase element (DPE) can be used to manipulate at will the positions and energy of multiple filaments generated in fused silica under femtosecond pulsed illumination. The method allows obtaining three-dimensional distributions of controlled filaments whose separations can be in the order of few micrometers. With such small distances we are able to study the mutual coherence among filaments from the resulted interference pattern, without needing a two-arm interferometer. The encoding of the DPE into a phase-only spatial light modulator (SLM) provides an extra degree of freedom to the optical set-up, giving more versatility for implementing different DPEs in real time. Our proposal might be particularly suited for applications at which an accurate manipulation of multiple filaments is required.

Second-harmonic illumination to enhance multispectral digital lensless holographic microscopy.

Multispectral digital lensless holographic microscopy (MDLHM) operating with second-harmonic illumination is shown. Added to the improvement of the spatial resolution of the previously reported MDLHM operating with near-infrared illumination, this second-harmonic MDLHM shows promise as a tool to study the behavior of biological samples under a broad spectral illumination. This illumination is generated by focusing a highly spatially coherent ultrashort pulsed radiation into an uncoated Type 1 ß-BaB2O4 (BBO) nonlinear crystal. The second-harmonic MDLHM allows achieving multispectral images of biological samples with enhanced micrometer spatial resolution. The illumination wavelength of the second-harmonic MDLHM can be tuned by displacing a focusing optics with respect to a pinhole; spatially resolved information at different wavelengths of the sample can then be retrieved.

Image transmission through dynamic scattering media by single-pixel photodetection.

Smart control of light propagation through highly scattering media is a much desired goal with major technological implications. Since interaction of light with highly scattering media results in partial or complete depletion of ballistic photons, it is in principle impossible to transmit images through distances longer than the extinction length. Nevertheless, different methods for image transmission, focusing, and imaging through scattering media by means of wavefront control have been published over the past few years. In this paper we show that single-pixel optical systems, based on compressive detection, can also overcome the fundamental limitation imposed by multiple scattering to successfully transmit information. But, in contrast with the recently introduced schemes that use the transmission matrix technique, our approach does not require any a-priori calibration process that ultimately makes the present method suitable to use with dynamic scattering media. This represents an advantage over previous methods that rely on optical feedback wavefront control, especially for short speckle decorrelation times.

Encoding complex fields by using a phase-only optical element.

We show that the amplitude and phase information from a two-dimensional complex field can be synthesized from a phase-only optical element with micrometric resolution. The principle of the method is based on the combination of two spatially sampled phase elements by using a low-pass filter at the Fourier plane of a 4-f optical system. The proposed encoding technique was theoretically demonstrated, as well as experimentally validated with the help of a phase-only spatial light modulator for phase encoding, a conventional CMOS camera to measure the amplitude of the complex field, and a Shack-Hartmann wavefront sensor to determine its phase.

On-axis non-linear effects with programmable Dammann lenses under femtosecond illumination.

We demonstrate the utilization of Dammann lenses codified onto a spatial light modulator (SLM) for triggering non-linear effects. With continuous wave illumination Dammann lenses are binary phase optical elements that generate a set of equal intensity foci. We theoretically calculate the influence of ultrashort pulse illumination on the uniformity of the generated pattern, which is affected by chromatic aberration for pulses with temporal widths lower than 100 fs. The simulations also indicate that acceptable uniformity can be achieved for pulses of several fs by shortening the distance among foci which can be easily modified with the SLM. Multifocal second-harmonic generation (SHG) and on-axis multiple filamentation are produced and actively controlled in ß-BaB2O4 (BBO) and fused silica samples, respectively, with an amplified Ti: Sapphire femtosecond laser of 30 fs pulse duration. Experimental results are in very good agreement with theoretical calculations.

Diffractive digital lensless holographic microscopy with fine spectral tuning.

We experimentally demonstrate an all-diffractive optical setup for digital lensless holographic microscopy with easy wavelength line selection and micrometric resolution. In the proposed system, an ultrashort laser pulse is focused with a diffractive lens (DL) onto a pinhole of diameter close to its central wavelength to achieve a highly spatially coherent illumination cone as well as a spectral line with narrow width. To scan the complete spectrum of the light source the DL is displaced with respect to the pinhole plane. The proposed microscopy setup allows us to spectrally separate contributions from different sections of a sample, which may be attractive for several applications in life sciences.

Compressive holography with a single-pixel detector.

This Letter develops a framework for digital holography at optical wavelengths by merging phase-shifting interferometry with single-pixel optical imaging based on compressive sensing. The field diffracted by an input object is sampled by Hadamard patterns with a liquid crystal spatial light modulator. The concept of a single-pixel camera is then adapted to perform interferometric imaging of the sampled diffraction pattern by using a Mach-Zehnder interferometer. Phase-shifting techniques together with the application of a backward light propagation algorithm allow the complex amplitude of the object under scrutiny to be resolved. A proof-of-concept experiment evaluating the phase distribution of an ophthalmic lens with compressive phase-shifting holography is provided.

Femtosecond digital lensless holographic microscopy to image biological samples.

The use of femtosecond laser radiation in digital lensless holographic microscopy (DLHM) to image biological samples is presented. A mode-locked Ti:Sa laser that emits ultrashort pulses of 12 fs intensity FWHM, with 800 nm mean wavelength, at 75 MHz repetition rate is used as a light source. For comparison purposes, the light from a light-emitting diode is also used. A section of the head of a drosophila melanogaster fly is studied with both light sources. The experimental results show very different effects of the pinhole size on the spatial resolution with DLHM. Unaware phenomena on the field of the DLHM are analyzed.

Dispersion management in two-photon microscopy by using diffractive optical elements.

We demonstrate efficient generation of wide-field fluorescence signals in two-photon microscopy exploiting diffractive optical elements and short pulses by using a dispersion-compensated beam delivery optics module. Computer-generated holograms are codified onto a phase-only spatial light modulator, which allows for arbitrary single-shot patterning of the sample. Spatiotemporal shaping of the pulse is mandatory to overcome spatial chirp and pulse-front tilt effects that spread both in space and time the irradiance patterns, thus limiting not only the spatial resolution but also the signal-to-noise ratio in two-photon microscopy. By using a multipass amplifier delivering 30 fs, 0.8 mJ pulses at 1 kHz repetition rate, we experimentally demonstrated arbitrary single-shot fluorescence irradiance patterns in Rhodamine B.

Programmable quasi-direct space-to-time pulse shaper with active wavefront correction.

We experimentally demonstrate an extremely compact and programmable pulse shaper composed of a single phase mask encoded into a spatial light modulator. Its principle of operation is similar to the previously theoretically introduced quasi-direct space-to-time pulse shaper [Opt. Express16, 16993 (2008)], which is based on diffractive optics. The proposed pulse shaper exhibits not only real-time temporal modulation, but also high-efficiency output pulses thanks to an active correction of the wavefront aberrations.

Synthesis of fractal light pulses by quasi-direct space-to-time pulse shaping.

We demonstrated a simple diffractive method to map the self-similar structure shown in squared radial coordinate of any set of circularly symmetric fractal plates into self-similar light pulses in the corresponding temporal domain. The space-to-time mapping of the plates was carried out by means of a kinoform diffractive lens under femtosecond illumination. The spatio-temporal characteristics of the fractal pulses obtained in this way were measured by means of a spectral interferometry technique assisted by a fiber optics coupler (STARFISH). Our proposal allows synthesizing suited sequences of focused fractal femtosecond pulses potentially useful for several current applications, such as femtosecond material processing, atomic, and molecular control of chemical processes or generation of nonlinear effects.