Gabor's holography: seven decades influencing optics [Invited].

Dennis Gabor's seminal idea of a simple all-optical setup aimed at reconstructing the object wavefront stored on a photographic plate gave birth a little over seven decades ago to the field of holography. In 1971 Gabor obtained the Nobel Prize in Physics for this invention. Still, the road in the early days after his two first papers on the subject was one full of obstacles, so his scientific and engineering contemporaries put his idea to rest for more than 10 years, until the invention of the laser. This fact made his holographic concept take off to new and unsuspected applications. This invited review paper is a homage to Dennis Gabor's 50th anniversary of his Nobel Prize accolade. For this purpose, the review departs from the typical common route, i.e., those written following a timeline fashion, and instead is written with the intent to cover only a few of the holography applications in optics while scanning the electromagnetic spectrum. In doing this, the authors are aware that other invited papers for this special issue will tackle other subjects not dealt with in this review non-timeline paper.

Measurement of morphology thickness and refractive index in a melanoma A375 cell line using digital holographic microscopy.

Digital holographic microscopy (DHM) is a technique that has high potential for analyzing biological samples and has been successfully applied to the study of cells and cell lines providing information about important parameters such as refractive index, morphology, and dry mass, among others; it has also found applicability to study the effects of therapeutic treatments. Finding the size and shape of cells is important since they tend to change in the presence of some pathologies. In this research work, we obtain the morphology thickness and refractive index of the A375 melanoma cell line through a slight tilting of the cell in a DHM setup. Further, the development of a novel mathematical expression based on this tilt and in the optical phase difference is presented. We show images of melanoma cells with the refractive index information included, and their morphology thickness as rendered from the holographic phase maps recorded with DHM.

Fish swim water bulk displacement visualization with digital holographic interferometry.

A collimated transmission beam interferometer is used to measure the water motion provoked by the fish swimming through it. An indirect measurement of the fish motion impact in the water contained in a home-type aquarium is detected. Measurements of the whole aquarium are possible due to a large diameter collimated laser beam in the interferometer's object arm. This beam goes through the aquarium, and any perturbation inside it deflects the collimated beam. The interferometer detects a phase difference, i.e., the beam through the disturbed water undergoes different optical paths. This optical phase change was first demonstrated by means of a simple test using spherical steel marbles placed in a cuvette. For this, the small water movements for a single steel marble are detected with the acquired optical phase. Next, the aquarium optical phase results show water movements according to the fishes' size and swimming speed. It is worth mentioning that no additives were added to the aquarium's fresh water during the tests, so the water was crystal clear.

Digital four-step phase-shifting technique from a single fringe pattern using Riesz transform.

A digital four-step phase-shifting method for obtaining the optical phase distribution from a single fringe pattern is proposed in this Letter. By computing the first-, second-, and third-order Riesz transform components for a given fringe pattern, three π/2 phase-shifted fringe patterns are generated from the obtained Riesz components and, finally, the wrapped phase map is extracted. The validity of the proposed method is demonstrated on both the simulated and experimentally obtained fringe patterns. The performance of the proposed method is evaluated by using the image quality index and edge preservation index. Further, the performance of the proposed method is tested on speckled correlation fringes obtained from digital speckle pattern interferometry, and the resulting phase from the proposed method is compared with the phase obtained from three experimentally recorded phase-shifted fringe patterns. The obtained results reveal that the proposed method provides a simple and robust solution for optical phase extraction from a single fringe pattern with good accuracy and, therefore, make it suitable for real-time measurement applications.

Speckle denoising by variant nonlocal means methods.

This study aims to demonstrate the performances of nonlocal means (NLM) and their variant denoising methods, mainly focusing on NLM-shaped adaptive patches and several NLM-reprojection schemes for speckle noise reduction in amplitude and phase images of the digital coherent imaging systems. In the digital coherent imaging systems such as digital speckle pattern interferometry, digital holographic interferometry, etc., the image quality is severely degraded by additive uncorrelated speckle noise, due to the coherent nature of the light source, and therefore limits the development of several applications of these imaging systems in many fields. NLM and its variant denoising methods are employed to denoise the intensity/phase images obtained from these imaging systems, and their effectiveness is evaluated by considering various parameters. The performance comparison of these methods with other existing speckle denoising methods is also presented. The performance of these methods for speckle noise reduction is quantified on the basis of two criteria matrices, namely, the peak-to-signal noise ratio and the image quality index. Based on these criteria matrices, it is observed that these denoising methods have the ability to improve the intensity and phase images favorably in comparison to other speckle denoising techniques, and these methods are more effective and feasible in speckle-noise reduction.

Speckle noise reduction in digital speckle pattern interferometric fringes by nonlocal means and its related adaptive kernel-based methods.

Digital speckle pattern interferometry (DSPI) is widely used in many scientific and industrial applications. Besides its several advantages, one of the basic problems encountered in DSPI is the undesired speckle noise existing in the fringe pattern. In this paper, we demonstrate the performance of nonlocal means (NLM) and its related adaptive kernel-based filtering methods for speckle noise reduction in DSPI fringes. The NLM filter and its related kernel-based filters such as NLM-average, NLM-local polynomial regression, and NLM-shape adaptive patches are implemented first on simulated DSPI fringes, and their performances are quantified on the basis of peak signal-to-noise ratio (PSNR), mean square error (MSE), and quality index (Q). Further, their effectiveness and abilities in reducing speckle noise are compared with other speckle denoising methods. These filtering methods are then employed on experimental DSPI fringes. The obtained results reveal that these filtering methods have the ability to improve the PSNR and Q of the DSPI fringes and provide better visual and quantitative results. It is also observed that the proposed filtering methods preserve the edge information of the DSPI fringes, which is evaluated on the basis of the edge preservation index of the resultant filtered images.

Surface structural damage study in cortical bone due to medical drilling.

A bone's fracture could be produced by an excessive, repetitive, or sudden load. A regular medical practice to heal it is to fix it in two possible ways: external immobilization, using a ferule, or an internal fixation, using a prosthetic device commonly attached to the bone by means of surgical screws. The bone's volume loss due to this drilling modifies its structure either in the presence or absence of a fracture. To observe the bone's surface behavior caused by the drilling effects, a digital holographic interferometer is used to analyze the displacement surface's variations in nonfractured post-mortem porcine femoral bones. Several nondrilled post-mortem bones are compressed and compared to a set of post-mortem bones with a different number of cortical drillings. During each compression test, a series of digital interferometric holograms were recorded using a high-speed CMOS camera. The results are presented as pseudo 3D mesh displacement maps for comparisons in the physiological range of load (30 and 50 lbs) and beyond (100, 200, and 400 lbs). The high resolution of the optical phase gives a better understanding about the bone's microstructural modifications. Finally, a relationship between compression load and bone volume loss due to the drilling was observed. The results prove that digital holographic interferometry is a viable technique to study the conditions that avoid the surgical screw from loosening in medical procedures of this kind.

Resonance properties of Ag-ZnO nanostructures at terahertz frequencies.

Nanoantennas have been fabricated by scaling down traditional antenna designs using nanolithographic techniques and testing them at different optical wavelengths, these particular nanoantennas have shown responses in a broad range of frequencies going from visible wavelengths to the range of the terahertz. Some self-assembled nanostructures exist that exhibit similar shapes and properties to those of traditional antenna structures. In this work the emission and absorption properties of self-assembled nanostructures made of zinc oxide nanorods on silver nanowires, which resemble traditional dipole antennas, were measured and simulated in order to test their antenna performance. These structures show resonant properties in the 10-120 THz range, with the main resonance at 60 THz. The radiation pattern of these nanostructures was also obtained by numerical simulations, and it is shown that it can be tailored to increase or decrease its directivity as a function of the location of the energy source of excitation. Experimental measurements were performed by Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) in order to show existing vibrational frequencies at the resonant frequencies of the nanostructures, measurements were made from ~9 to 103 THz and the results were in agreement with the simulations. These characteristics make these metal-semiconductor Ag/ZnO nanostructures useful as self-assembled nanoantennas in applications such as terahertz spectroscopy and sensing at terahertz frequencies.

Holographic otoscope using dual-shot-acquisition for the study of eardrum biomechanical displacements.

Recently an optoelectronic holography system was deployed in the clinic with the purpose of quantifying the tympanic membrane (TM) displacements of various mammal species, the objective being the understanding of their middle ear biomechanics. The optoelectronic holography system has an in-line configuration where the data gathered is decoded using lensless digital holography with the Fresnel approximation. This paper presents quantitative data obtained from an acoustically excited postmortem chinchilla's TM. To achieve this we used a robust customized windowed unwrapping method to unwrap the noisy optical phase obtained by subtracting phase maps of two recorded holograms and the results were compared with those obtained when using the unwrapping branch-cut algorithm. Additionally, phase maps obtained by the phase-stepping technique were compared applying both unwrapping methods. For in vivo applications particular emphasis is made on post-processing dual-shot-acquisition of holograms as one of various acquisition strategies and algorithms to diminish measurement error due to heartbeat, breathing, and patient's head motion as well as environment induced mechanical disturbances present in a noncontrolled environment, such as in a clinic. By recording only two holograms representing a stationary and deformed state of eardrum, respectively, we can increase the acquisition speed of the camera used to record faster events happening on the TM surface.

Insect wing deformation measurements using high speed digital holographic interferometry.

An out-of-plane digital holographic interferometry system is used to detect and measure insect's wing micro deformations. The in-vivo phenomenon of the flapping is registered using a high power cw laser and a high speed camera. A series of digital holograms with the deformation encoded are obtained. Full field deformation maps are presented for an eastern tiger swallowtail butterfly (Pterourus multicaudata). Results show no uniform or symmetrical deformations between wings. These deformations are in the order of hundreds of nanometers over the entire surface. Out-of-plane deformation maps are presented using the unwrapped phase maps.

Multimode vibration analysis with high-speed TV holography and a spatiotemporal 3D Fourier transform method.

The combination of a high-speed TV holography system and a 3D Fourier-transform data processing is proposed for the analysis of multimode vibrations in plates. The out-of-plane displacement of the object under generic vibrational excitation is resolved in time by the fast acquisition rate of a high-speed camera, and recorded in a sequence of interferograms with spatial carrier. A full-field temporal history of the multimode vibration is thus obtained. The optical phase of the interferograms is extracted and subtracted from the phase of a reference state to yield a sequence of optical phase-change maps. Each map represents the change undergone by the object between any given state and the reference state. The sequence of maps is a 3D array of data (two spatial dimensions plus time) that is processed with a 3D Fourier-transform algorithm. The individual vibration modes are separated in the 3D frequency space due to their different vibration frequencies and, to a lesser extent, to the different spatial frequencies of the mode shapes. The contribution of each individual mode (or indeed the superposition of several modes) to the dynamic behaviour of the object can then be separated by means of a bandpass filter (or filters). The final output is a sequence of complex-valued maps that contain the full-field temporal history of the selected mode (or modes) in terms of its mechanical amplitude and phase. The proof-of-principle of the technique is demonstrated with a rectangular, fully clamped, thin metal plate vibrating simultaneously in several of its natural resonant frequencies under white-noise excitation.

Morphology visualization of irregular shape bacteria by electron holography and tomography.

In the current work, irregular morphology of Staphylococcus aureus bacteria has been visualized by phase retrieval employing off-axis electron holography (EH) and 3D reconstruction electron tomography using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Bacteria interacting with gold nanoparticles (AuNP) acquired a shrunken or irregular shape due to air dehydration processing. STEM imaging shows the attachment of AuNP on the surface of cells and suggests an irregular 3D morphology of the specimen. The phase reconstruction demonstrates that off-axis electron holography can reveal with a single hologram the morphology of the specimen and the distribution of the functionalized AuNPs. In addition, EH reduces significantly the acquisition time and the cumulative radiation damage (in three orders of magnitude) over biological samples in comparison with multiple tilted electron expositions intrinsic to electron tomography, as well as the processing time and the reconstruction artifacts that may arise during tomogram reconstruction.

Very high speed cw digital holographic interferometry.

It is reported for the first time the use of a very high speed camera in digital holographic interferometry with an out of plane sensitivity setup. The image plane holograms of a spherical latex balloon illuminated by a cw laser were acquired at a rate of 4000 frames per second, representing a time spacing between holograms of 250 microseconds, for 512 x 512 pixels at 8 bits resolution. Two types of tests were accomplished for a proof of principle of the technique, one with no constrains on the object which meant random movements due to non controlled environmental air currents, and the other with specific controlled conditions on the object. Results presented correspond to a random sample of sequential digital holograms, chosen from a 1 second exposure, individually Fourier processed in order to perform the usual comparison by subtraction between consecutive pairs thus obtaining the phase map of the object out of plane displacement, shown as a movie.

Endoscopic pulsed digital holography for 3D measurements.

A rigid endoscope and three different object illumination source positions are used in pulsed digital holography to measure the three orthogonal displacement components from hidden areas of a harmonically vibrating metallic cylinder. In order to obtain simultaneous 3D information from the optical set up, it is necessary to match the optical paths of each of the reference object beam pairs, but to incoherently mismatch the three reference object beam pairs, such that three pulsed digital holograms are incoherently recorded within a single frame of the CCD sensor. The phase difference is obtained using the Fourier method and by subtracting two digital holograms captured for two different object positions.

Mapping the magnetic and crystal structure in cobalt nanowires.

Using off-axis electron holography under Lorentz microscopy conditions to experimentally determine the magnetization distribution in individual cobalt (Co) nanowires, and scanning precession-electron diffraction to obtain their crystalline orientation phase map, allowed us to directly visualize with high accuracy the effect of crystallographic texture on the magnetization of nanowires. The influence of grain boundaries and disorientations on the magnetic structure is correlated on the basis of micromagnetic analysis in order to establish the detailed relationship between magnetic and crystalline structure. This approach demonstrates the applicability of the method employed and provides further understanding on the effect of crystalline structure on magnetic properties at the nanometric scale.

Detection of biological tissue in gels using pulsed digital holography.

An out of plane optical sensitive configuration for pulsed digital holography was used to detect biological tissue inside solid organic materials like gels. A loud speaker and a shaker were employed to produce a mechanical wave that propagates through the gel in such a way that it generates vibrational resonant modes and transient events on the gel surface. Gel surface micro displacements were observed between the firing of two laser pulses, both for a steady resonant mode and for different times during the transient event. The biological tissue sample inserted approximately 2 cm inside the gel diffracts the original mechanical wave and changes the resonant mode pattern or the transient wave on the gel surface. This fact is used to quantitatively measure the gel surface micro displacement. Comparison of phase unwrapped patterns, with and without tissue inside the gel, allows the rapid identification of the existence of tissue inside the gel. The results for the resonant and transient conditions show that the method may be reliably used to study, compare and distinguish data from inside homogeneous and in-homogeneous solid organic materials.

Calibration for medium resolution off-axis electron holography using a flexible dual-lens imaging system in a JEOL ARM 200F microscope.

In this work the calibration of a medium resolution off-axis electron holography using a dual-lens imaging system in a JEOL ARM 200F is shown. The objective dual-lens configuration allows adjusting the field of view from 35nm to 2.5µm. Subsequently, the parameters used in phase shift reconstruction were calibrated considering biprism voltage versus fringe spacing (σ) and versus fringe width (W). The reliability of the transmission electron microscope performance using these parameters was achieved using gold nanoparticles of known size and adjusting the excitation voltage of the lenses.

Detection of surface strain by three-dimensional digital holography.

Three-dimensional digital holography with three object-illuminating beams has been successfully used for the detection of surface strain in metallic objects. The optical setup that uses illuminating beams to irradiate the object from three directions means that all three object surface displacement components, x, y, and z, can be independently calculated and used to find the strain gradients on the surface. The results show the conversion of the complete surface displacement field into a surface strain field. The method is capable of measuring microstrains for out-of-plane surface displacements of less than 10 microm.