Coherent Optical Methods Research Articles

Optical Extinction-Based 3D Nano-Imaging of WS2 on Gold.

Broadband nanoextinction images recorded in tip-enhanced optical spectroscopy geometry track the 3D topography of a single layer of WS2 on Au substrate. The described nano-optical method is complementary to conventional atomic force microscopy and offers additional information about the buried material-metal interface that is not accessible using conventional topographic imaging. Beyond 3D optical imaging, we observe large variations in the junction plasmon resonance on the nanoscale. The latter is important to understand and account for in tip-enhanced Raman and photoluminescence studies that target low-dimensional materials specifically. Our observations and (coherent) optical scattering-based method are also relevant to emerging efforts aimed at exploring strong coupling and Fano interferences in hybrid plasmonic-low dimensional quantum material systems.

Super-sensitivity full-field measurement of structural vibration with an adaptive incoherent optical method

Full-field measurements of structural vibrations have been achieved by incoherent optical methods with video cameras such as digital image correlation and optical flow. In contrast to coherent optical methods such as scanning laser vibrometers, incoherent optical methods are low-cost and easy to set up. However, the sensitivity (minimum measurable displacement) of incoherent optical methods is generally lower than that of coherent optical methods. Typically, the sensitivity of incoherent optical methods is essentially limited by the finite bit depth of the digital camera due to the quantization with round-off errors. Quantitatively, this theoretical sensitivity limit is determined by the bit depth B as δp=1/(2B−1) [pixel] which corresponds to a displacement causing an intensity change of one gray level. Fortunately, natural dithering may be leveraged to overcome the quantization and exceed the sensitivity limit, achieving super-sensitivity.In this work, we first study the mechanism of the sensitivity limit induced by the quantization and the critical limitation of existing super-sensitivity methods in the full-field measurement of structural vibration. Addressing such a limitation, we present a general adaptive weighted averaging method with dithering, achieving super-sensitivity incoherent optical measurement of full-field deformational vibration of flexible structures. Specifically, both spatial and temporal samplings of the pixels are leveraged simultaneously to adaptively identify the motion (deformation) shape of the structure by principal component analysis (PCA) of spatiotemporal pixel intensities. Then, the identified deformation shape is used to perform a deformation-shape-weighted averaging over spatial pixels with dithering, revealing the sub-sensitivity displacement while retaining spatial resolution at full field. Numerical simulations and laboratory experiments are performed for principle explanation and method validation. Particularly, we derive a mathematical model of the minimum measurable vibration amplitude A∗ for the developed super-sensitivity method, which is found to be determined by the number of spatial pixels for averaging Ns and the noise level σn in the imaging system. Thus, this work provides, for the first time, a rigorous quantification of the sensitivity of incoherent optical measurement for structural vibration, and a new quantitative method to achieve super-sensitivity full-field measurement of structural vibration.

Theoretical analysis on coherent optical super-resolution method for inspection of functional micro-structured surfaces with complex-amplitude-response distribution

In case of in-line inspection of optical functional micro-structured surfaces with phase response as well as amplitude response, higher resolution than the diffraction limit and phase measurability are needed. We proposed a novel coherent super-resolution method named complex-amplitude-response SIM (structured illumination microscopy). An equation is derived that enables extraction of frequencies higher than the diffraction limit of the complex-amplitude response distribution of a sample from multiple acquired images, which had not been obtained previously. This equation was validated through theoretical analysis by using images calculated by numerical optical simulation that were assumed to be acquired. Numerical analyses using the derived equation and simulations of image acquisition revealed that the proposed method is able to measure phase- and amplitude-response distributions of samples with a resolution higher than the diffraction limit. It was demonstrated that the proposed method is able to separate two points at a submicron distance of 500 nm under conditions of visible light and long W.D. (NA 0.2) and measure the phase difference with a maximum error of approximately 0.03 rad (∼π/100). When the amplitude response distribution of a sample is uniform and consists only of phase responses, the structure of the sample cannot be observed by standard optical microscopy. Numerical simulations revealed that the proposed method is able to visualise such samples with fine periodic phase responses on the submicron scale (∼500 nm) beyond the diffraction limit. Furthermore, we have shown that the proposed method is able to detect phase defects in ultra-fine optical functional structures with periods of less than 100 nm, which is expected to characterise the next generation metasurfaces.

Observation of optical coherence in a disordered metal-molecule interface by coherent optical two-dimensional photoelectron spectroscopy

Coherent multidimensional optical spectroscopy methods overcome some of the limitations found in their one-dimensional counterparts and allow us, for example, to resolve overlapped spectral features, to separate homogeneous and inhomogeneous broadening, and to track the energy transfer kinetics and coherent dynamics in complex quantum systems. In their most common configurations, signal detection is achieved by optical means, making these techniques widespread in studies of bulk systems but less common in the surface and interface sciences. In this paper, we demonstrate an inherently surface-sensitive two-dimensional coherent spectroscopy scheme, based on photoelectron detection, by studying the interface formed between tris(8-hydroxyquinolinato)aluminium (${\mathrm{Alq}}_{3}$) and a ferromagnetic Co surface. Despite the inhomogeneous linewidth broadening (\ensuremath{\approx}800 meV) in the ensemble of disordered molecules, we resolve two narrow resonances (\ensuremath{\approx}10 meV linewidth) with an energy spacing of \ensuremath{\approx}80 meV. By combining experimental data and simulations, using the Lindblad master equation, we identify these resonances as lowest unoccupied molecular orbital (LUMO) to $\mathrm{LUMO}+1$ transitions in chemically decoupled second-layer ${\mathrm{Alq}}_{3}$ molecules and deduce related optical coherence lifetimes of at least 120 and 240 fs. These observations establish that ${\mathrm{Alq}}_{3}$ molecules in a disordered adsorbate layer exhibit well-defined and rather homogeneous internal electronic transitions, although the absolute energetic positions of the involved states with respect to the substrate reference are significantly affected by the disorder. The results indicate that inhomogeneous line broadening in a disordered ${\mathrm{Alq}}_{3}$ layer and pure dephasing have only a minor impact on optical transitions in individual molecules. This opens interesting opportunities for coherent control schemes and emphasizes the importance of optical coherences for all electron dynamics, even in disordered hybrid metal-molecule interfaces.

Realizing negative index of refraction in an ensemble of ground-state polar molecules with lasers

We propose a coherent optical method for creating negative refractive index (NRI) for a gaseous ensemble of ground-state polar molecules possessing both permanent electric and magnetic moments. Exploiting the pure rotational transition between the two lowest rotational levels of the ground vibrational state one can generate two dressed states of mixed parity using a microwave laser. These dressed states are then used as the two lower states of a Λ-type three-level scheme using two infrared lasers to couple them to another ro-vibrational level in the ground-state manifold. One of the infrared lasers is used as a weak probe and the other as a control field with a fixed detuning. We take into account local-field effects on the dielectric response of the medium at the probe frequency in terms of Clausius–Mossoti relation. We extract magnetic response out of purely dielectric response and calculate the magnetic permeability of the medium in terms of dielectric susceptibility. Our results show that there is a small frequency window near the two-photon resonance where both electric permittivity and magnetic permeability are negative with vanishingly small absorption. The figure of merit for the medium is shown to exceed unity. We interpret our results in terms of the proximity of EIT and quantum interference. We discuss the possible realization of our method using cold polar molecules that are recently experimentally produced.

Application of coherent-optical methods for early diagnosis of chlamydia

The article considers the possibility of using a scanning speckle microscope for fast 2D scanning of clinical samples containing chlamydia. Modern methods of laboratory diagnosis of chlamydia do not confirm the presence of a viable pathogen in the early stages of infection. The use of coherent-optical methods allows one to automatically detect even single C. trachomatis cells in virtually any sample. &nbsp

Imaging Hidden Objects with Spatial Speckle Intensity Correlations over Object Position.

We present a coherent optical method for wavelength-resolution imaging of moving objects hidden within thick randomly scattering media. Spatial speckle intensity correlations as a function of object position are shown to provide access to the spatially dependent dielectric constant of the moving object. This speckle correlation imaging method yields field-based information previously inaccessible in heavily scattering environments. Proof of concept experimental results show excellent agreement with the theory. This new imaging approach will be valuable in high resolution imaging in tissue and other scattering environments where natural motion occurs or the object position can be controlled.

Single-shot, planar infrared imaging in flames using polarization spectroscopy.

Planar infrared visualization of species in flames is challenging due to the severe thermal radiation background and relatively weak fluorescence quantum yields from ro-vibration transitions. In this express, we report imaging of molecular species in a flame via an absorption-based coherent optical method, namely infrared polarization spectroscopy (IRPS). Single-shot, planar imaging of hydrogen fluoride (HF) has been achieved in a premixed CH(4)/O(2) Bunsen flame, being seeded with a small amount of SF(6). The HF molecule was excited through a rovibrational transition at around 2.5 µm, which belongs to the fundamental vibration band. High spatial resolution was guaranteed using an orthorgonal pump-probe geometry, and an effective suppression of thermal background emission was achieved owing to the coherent nature of the demonstrated two-dimensional IRPS. Other advantages, e.g. high temporal resolution and species-specificity, are also features of this laser-based technique, which make it suitable for imaging of non-fluorescent but infrared active gaseous molecules in harsh environments.

Optical Negative Feedback for Linewidth Reduction of Semiconductor Lasers

We propose a coherent optical negative feedback method for reducing linewidth of single-mode semiconductor lasers, which requires only an optical filter. Results of numerical analysis indicate that the power spectral density of FM noise can be reduced by more than 40 dB and the subkilohertz spectral linewidth can be obtained with our method when the spectral linewidth is 10 MHz under a free running condition. From a proof-of-concept experiment, we confirmed that the spectral linewidth of a single-mode semiconductor laser is reduced when using our method from 6.4 MHz to 6.5 kHz. This result confirms that the spectral linewidth can be reduced by at least 30 dB, which proves the validity of our method.

Phase imaging from noisy interferometric observations via sparse coding of phase and amplitude

Phase-shifting interferometry is a coherent optical method that combines high accuracy with high measurement speeds. However, despite its advantageous properties, the inference of the object amplitude and the phase, herein termed wavefront reconstruction is not a trivial task owing to, namely, the Poissonian noise associated with the measurement process and to the 2π phase periodicity of the observation mechanism. In this paper, we formulate the wavefront reconstruction (phase and amplitude) as an inverse problem where the amplitude and the absolute phase are assumed to admit sparse linear representations in suitable sparsifying transforms. We introduce the sparse phase and amplitude reconstruction (SPAR) algorithm. SPAR takes into full consideration the Poissonian (photon counting) measurements and uses data adaptive BM3D frames as a sparse representation for amplitude and absolute phase.

Wavefront reconstruction in phase-shifting interferometry via sparse coding of amplitude and absolute phase.

Phase-shifting interferometry is a coherent optical method that combines high accuracy with high measurement speeds. This technique is therefore desirable in many applications such as the efficient industrial quality inspection process. However, despite its advantageous properties, the inference of the object amplitude and the phase, herein termed wavefront reconstruction, is not a trivial task owing to the Poissonian noise associated with the measurement process and to the 2π phase periodicity of the observation mechanism. In this paper, we formulate the wavefront reconstruction as an inverse problem, where the amplitude and the absolute phase are assumed to admit sparse linear representations in suitable sparsifying transforms (dictionaries). Sparse modeling is a form of regularization of inverse problems which, in the case of the absolute phase, is not available to the conventional wavefront reconstruction techniques, as only interferometric phase modulo-2π is considered therein. The developed sparse modeling of the absolute phase solves two different problems: accuracy of the interferometric (wrapped) phase reconstruction and simultaneous phase unwrapping. Based on this rationale, we introduce the sparse phase and amplitude reconstruction (SPAR) algorithm. SPAR takes into full consideration the Poissonian (photon counting) measurements and uses the data-adaptive block-matching 3D (BM3D) frames as a sparse representation for the amplitude and for the absolute phase. SPAR effectiveness is documented by comparing its performance with that of competitors in a series of experiments.

Going ultrafast

A new asynchronous coherent optical sampling method allows for the direct visualization of actively mode-locked quantum cascade laser pulses at terahertz wavelengths.

Coherent Optical Method for Studies of Nanoscale Objects in Liquid Media Based upon Spatial Averaging of Data

Correlation method for processing dynamic light scattering patterns observed when laser radiation passes through liquid media was developed. The method is based on procedure of spatial averaging of the data contained measured values of correlation functions of intensity of light scattered by moving small particles. The method allows one to measure particles diameters from 30 to 750 nm with measurement error less then 10%. The method makes it possible to decrease correlation functions measurement time down to 1 ms and provides opportunity to carry out measurements for different scattering angles simultaneously. The method offers a way of recognizing speckle pattern transformations caused by harmonic, translatory and Brownian motions of particles, and allows one to register influence of uncontrolled forces such as monochromatic and wideband acoustic fields upon particles size measurements results.

Optical image compression and encryption methods

Over the years extensive studies have been carried out to apply coherent optics methods in real-time communications and image transmission. This is especially true when a large amount of information needs to be processed, e.g., in high-resolution imaging. The recent progress in data-processing networks and communication systems has considerably increased the capacity of information exchange. However, the transmitted data can be intercepted by nonauthorized people. This explains why considerable effort is being devoted at the current time to data encryption and secure transmission. In addition, only a small part of the overall information is really useful for many applications. Consequently, applications can tolerate information compression that requires important processing when the transmission bit rate is taken into account. To enable efficient and secure information exchange, it is often necessary to reduce the amount of transmitted information. In this context, much work has been undertaken using the principle of coherent optics filtering for selecting relevant information and encrypting it. Compression and encryption operations are often carried out separately, although they are strongly related and can influence each other. Optical processing methodologies, based on filtering, are described that are applicable to transmission and/or data storage. Finally, the advantages and limitations of a set of optical compression and encryption methods are discussed.

Speckle Reduction in OCT using Massively-Parallel Detection and Frequency-Domain Ranging

Speckle noise significantly limits the information content provided by coherent optical imaging methods such as optical coherence tomography and its recent derivative, optical frequency-domain imaging (OFDI). In this paper, we demonstrate a novel OFDI system that simultaneously acquires hundreds of angularly resolved images, which can be compounded to reduce speckle noise. The system comprises an InGaAs line-scan camera and an interferometer, configured so that the elements of the detector array simultaneously capture light spanning a backscattering angular range of 32 degrees. On successive read-outs of the array, the wavelength of the laser source was stepped through a range of 130 nm centered at 1295 nm to concurrently generate 400 angle-resolved OFDI images. A theory of angle-resolved OFDI and the design equations of the system are presented. Incoherent averaging of the angle-resolved data is shown to yield substantial speckle reduction (as high as an 8 dB SNR improvement) in images of a tissue phantom and esophageal tissue ex vivo.

A Mobile Shearography System for Non-Destructive Testing of Industrial and Artwork Components

By reason of their sensitivity, accuracy and non-contact as well as non-destructive characteristics, modern optical methods such as digital speckle shearography have found an increasing interest for NDT applications on factory floors. With new carbon fiber technologies and other lightweight constructions in aircraft and automotive manufacturing, adapted examination designs and especially developed testing methods are necessary. Shearography as a coherent optical method has been widely accepted as a useful NDT tool. It is a robust interferometric method to determine locations with maximum stress on various material structures. However, limitations of this technique can be found in the bulky equipment components, the interpretation of complex shearographic result images and at the work with non-cooperative surfaces such as dark absorber and bright shining reflectors. We report a mobile shearography system that was especially designed for investigations at aircraft and automotive construction sites. It is also useful to find out the current conditions of artworks.

Speckle diagnostics of relaxation processes in non-stationary scattering systems

Coherent optical method of the study of non-stationary mass transfer in scattering systems оn the basis of statistical analysis оf spatial-temporal fluctuations of speckle intensity is considered. Non-stationary mass transfer in the case оf saturating liquid phase evaporation from a disordered porous layer and structure modification of IR-laser-mediated cartilage tissue are discussed as the possible applications of the developed speckle-diagnostical technique. For liquid phase evaporation from а porous layer, the specific feature such as the anomalous broadening of spectra of speckle intensity fluctuations with decrease оf the liquid phase evaporation rate was found out. This feature is caused by peculiarities оf development of the fractal-like interface between liquid and gaseous phases in а porous layer. In the case оf the thermally induced structure modification оf а cartilage tissue, the hysteresis-like form оf dependence of the time-averaged contrast оf speckle-modulated tissue image оn the tissue temperature inside the treatment zone is characteristical. The effect оf cartilage thermal modification is presumably caused by the «bound-to-free water» transition in the proteoglycane aggregates аs one оf the basic components of the tissue structure.

Full Field Tensile Strain Shearography of Welded Specimens

AbstractDigital shearography is a coherent optical method used in conjunction with digital image processing. It allows the shearogram, which depicts directly displacement derivatives, in real time to be observed and to be evaluated numerically. Strains are functions of the displacement derivatives. Thus, the shearogram contains the strain information, but usually it includes both the in‐plane strain, e.g. ∂u/∂x, and the out‐of‐plane component, e.g. ∂w/∂x. In order to get the pure in‐plane strain as well as the pure out‐of‐plane component, two linearly independent directions of illumination (usually the same but mutual and sequential illuminations) are introduced in the measuring device. The shearograms for each illuminating direction are evaluated by applying the phase shifting technique. The result by subtracting the phase maps of the two shearograms yields a new fringe pattern depicting the pure in‐plane strain component and the result by adding the phase maps corresponds to the pure out‐of‐plane component. The theory and an application to non‐destructive tensile testing are demonstrated in this paper. Its applications for determining the strain distribution of the homogeneous and inhomogeneous material and an error analysis are presented.

Controlling optical properties of biological tissues: II. Coherent optical methods for studying the tissue structure

Correlation and polarization properties of dynamic speckle fields formed upon scanning in vitro human sclera samples subjected to immersion agents with a focused laser beam were studied. The immersion agent diffusion into the tissue reduces the relative refractive index of structure-forming elements (collagen fibrils) with respect to the base sclera substance and thus stimulates the passage from multiple to single scattering of a probing beam. Experimental data are presented on the evolution of the autocorrelation function of intensity fluctuations and the intensity ratio between polarized and depolarized components of laser radiation passed through a sclera layer due to artificial clarification of the tissue samples analyzed. Possible applications of the developed method to the tissue structure diagnostics are discussed.

Determination of strain distribution by means of digital shearography

Digital shearography is a coherent optical method in conjunction with digital image processing. It allows the shearogram, which depicts directly displacement derivatives, in real time to be observed and to be evaluated numerically. Strains are functions of the displacement derivatives. Thus, the shearogram contains the strain information, but usually it includes both the in-plane strain, e.g. ∂ u/∂ x, and the out-of-plane component, e.g. ∂ w/∂ x. In order to get the pure in-plane strain as well as the pure out-of-plane component, two linearly independent directions of illumination (usually the same but mutual illuminations) are introduced in the measuring device. The shearograms for each illuminating direction are evaluated by applying the phase shifting technique. The result of subtracting the phase maps of the two shearograms yields a new fringe pattern depicting the pure in-plane strain component, and the result of adding the phase maps corresponds to the pure out-of-plane component. This theory and experimental procedure are demonstrated in this paper. Its applications for determining the strain distribution of homogeneous and inhomogeneous materials are presented.