Common-path Interferometry Research Articles

Nonlinear detection of phase difference in optical interference using computer generated hologram assisted common-path three-beam interferometry

Nonlinear detection of phase difference in optical interference using computer generated hologram assisted common-path three-beam interferometry

Absorption Measurement in Ultrapure Crystalline Quartz with the Eliminated Influence of Ambient Air Absorption in the Time-Resolved Photothermal Common-Path Interferometry Scheme

We demonstrate measurements of the absorption coefficient α ≈ 2.5 × 10−7 cm−1 in synthetic crystalline quartz at a wavelength of 1071 nm with a signal-to-noise ratio of 10/1 using the Time-resolved photothermal common-path interferometry (TPCI) scheme. It utilized cells filled with flowing argon and eliminated the influence of ambient air absorption. The scheme elements limiting the sensitivity of measurements at the level of ≈7.8 × 10−8 cm−1 were revealed. When these elements are replaced by better ones in terms of their thermal influence, the sensitivity of absorption coefficient measurements in crystalline quartz is ~10−8 cm−1. The calculation of the correction due to these optical elements of the values of the measured absorption coefficients is also described, which makes it possible to achieve the same sensitivity without replacing the elements. The improved scheme confirms the presence of the spatial inhomogeneity of absorption with a minimum coefficient of 2.5 × 10−7 cm−1 in synthetic crystalline quartz. The discrepancy of the absorption coefficient values in different regions of the crystal in the presented series of experiments was 2.5 × 10−7 cm−1 to 4 × 10−6 cm−1. Taking into account the ratio of thermo-optical parameters and the heat diffusion effect, the calculation shows that for quartz glasses the corresponding sensitivity of the absorption coefficient measurements equals ≈1.5 × 10−9 cm−1.

Measurement of stress characteristics in flexible multi-thin-film substrates using tomography common optical path interferometry

Flexible displays are lightweight, thin, flexible, and impact-resistant, making them a major trend in the display industry. However, their short lifespan is a significant challenge. Thus, improving process parameters and extending product life through advanced measurement technology is crucial. Therefore, this study proposes using tomography common optical path interferometry (TCOPI) to measure and analyze the tomography stress characteristics of multi-thin-film flexible substrates in the folding direction. For polyethylene terephthalate (PET) and polyimide (PI), three thicknesses of indium tin oxide (ITO) were coated on the PET, graphene/PET, and graphene/PI flexible film substrates, and an automatic sliding-folding test platform was used to fold each flexible film substrate. Moreover, this study used TCOPI to measure and analyze the change in residual stress and the sheet resistance of the samples before and after folding. Additionally, this study conducted inward and outward folding tests on graphene/PET/graphene and graphene/PI/graphene substrates to analyze tomographic stress and sheet resistance in the folding direction. After 11,000 folding tests, the graphene/PET/graphene sample exhibited smaller stress and sheet resistance change rates compared to the graphene/PI/graphene sample, as measured by TCOPI. Consequently, the graphene/PET/graphene sample demonstrated better resistance to folding stress in this batch manufacturing process. Specifically, for the graphene layer of the graphene/PET/graphene sample, the minimum absolute value of the stress change rate before and after folding reached 5%. At the same time, the minimum sheet resistance value change rate was 8%. Based on the tomography stress measurements, suggestions for fine-tuning process parameters are provided to improve the flexible film substrate’s folding endurance.

Phase-resolved measurement of entangled states via common-path interferometry

We propose and experimentally demonstrate a method to directly measure the phase of biphoton states using an entangled mode as a collinear reference. The technique is demonstrated with entangled photonic spatial modes in the Laguerre–Gaussian basis, and it is applicable to any pure quantum system containing an exploitable reference state in its entanglement spectrum. As one particularly useful application, we use the new methodology to directly measure the geometric phase accumulation of entangled photons.

Problem of Measuring Absorption Using Time-Resolved Photothermal Common-Path Interferometry under Conditions of Developed Heat Diffusion

We present a study of the problem of measuring ultra-low absorption in quartz materials using the time-resolved photothermal common-path interferometry (TPCI) method, which we proposed and elaborated, with allowance for developed heat diffusion in the samples. This task is related to the implementation of one of the ways of increasing the sensitivity of measurements, namely, increasing the energy of the heating pulse by increasing its duration. Analytical formulas for the time dependence of the power of the time-varying component of the probe radiation are obtained in the Gaussian approximation for the laser beams. A correction factor that takes into account the heat diffusion effect is calculated theoretically and used during calibration. The rate at which the power of the time-varying component decreases when the sample is cooled after the end of the heating pulse is calculated. When measuring in crystalline quartz, quartz glass, and also in air, the calculated rate coincided with the experimental one, which is additional evidence for the reliability of the calculations of the correction factor for calibrating the measurements. When the duration of the heating pulse is increased to 5 ms, the calculated sensitivity of the scheme for measuring absorption in quartz glasses is 2 × 10−9 cm−1.

Poster Session II: Investigating photoreceptor function in disease-affected retinas using optoretinography.

Assessing the functional response of photoreceptors is vital in understanding retinal disease progression. Traditional subjective methods like visual acuity and visual fields, and objective ones like electroretinography, have limitations. An ideal complement to these techniques is optoretinography (ORG), which images the retina and tests its function at once. ORG utilizes the phase of the optical coherence tomography (OCT) signal to quantify nanometer-scale changes, measuring subtle photoreceptor responses to stimuli. Efforts to observe stimulus-evoked responses in human cone photoreceptors began with adaptive optics (AO) and common path interferometry, enabling the resolution and tracking of individual cells. Advances in OCT systems with cellular resolution through AO or digital aberration correction successfully measured ORG responses from single cones and rods. This method tracks phase differences between outer segment tips (COST or ROST) and the inner-outer segment junction (IS/OS) to assess individual cell responses. A novel velocity-based method recently demonstrated the feasibility of measuring ORG signals with clinical-grade OCT systems. In the present work, we implemented this technique on disease-affected human retinas, revealing lower magnitudes of response compared to healthy retinas, and highlighting its potential clinical applications.

Real-time detection of virus antibody interaction by label-free common-path interferometry

Viruses have a profound influence on all forms of life, motivating the development of rapid and minimally invasive methods for virus detection. In this study, we present a novel methodology that enables quantitative measurement of the interaction between individual biotic nanoparticles and antibodies in solution. Our approach employs a label-free, full-field common-path interferometric technique to detect and track biotic nanoparticles and their interactions with antibodies. It is based on the interferometric detection of light scattered by viruses in aqueous samples for the detection of individual viruses.We employ single-particle tracking analysis to characterize the size and properties of the detected nanoparticles, and to monitor the changes in their diffusive mobility resulting from interactions. To validate the sensitivity of our detection approach, we distinguish between particles having identical diffusion coefficients but different scattering signals, using DNA-loaded and DNA-devoid capsids of the Escherichia coli T5 virus phage.In addition, we have been able to monitor, in real time, the interaction between the bacteriophage T5 and purified antibodies targeting its major capsid protein pb8, as well as between the phage SPP1 and nonpurified anti-SPP1 antibodies present in rabbit serum. Interestingly, these virus-antibody interactions are observed within minutes. Finally, by estimating the number of viral particles interacting with antibodies at different concentrations, we successfully quantify the dissociation constant Kd of the virus-antibody reaction using single-particle tracking analysis.

DMD-based single-pixel off-axis interferometry for wavefront reconstruction of a biological sample

Single-pixel detecting is suitable for wavefront reconstruction in some special wavelengths where array detectors are immature or even unavailable and/or under low light conditions. However, most of the demonstrations are generally realized by multi-step phase-shifting interferometry with a liquid crystal spatial light modulator (LC-SLM) that has a slow modulation speed, which limits the reconstruction speed, thus restricting practical applications of the technique. Here, we propose to use a digital-micromirror-device-(DMD)-based single-pixel off-axis common-path interferometry (SOCI) for faster wavefront reconstruction. The method utilizes passive detection based on the DMD to realize SOCI for accelerating wavefront reconstruction. As compared to the LC-SLM-based phase-shifting techniques, since the modulation speed of the DMD is hundreds of times faster than that of the LC-SLM, the DMD-based SOCI accelerates several folds of reconstruction speed further, which, thus, makes the final wavefront reconstruction three orders of magnitude faster. The effectiveness and advantages of our method are experimentally demonstrated by quantitatively reconstructing the amplitude and phase images of a biological sample.

Vortex phase deterioration common-path interferometry

Common-path interferometers (CPIs) are significant due to their compactness and vibration resistance. The usual challenge with CPIs arises due to a very small separation between reference and sample beams, where sending a reference beam through a sample is considered to be a limitation. However, this limitation also makes it difficult to probe the interaction of beams with materials as a function of their phase structure. This study paves the way for a new kind of interferometry that can provide unique phase signatures to study the sample. This paper proposes and demonstrates a novel approach based on thermo-optic refraction, to send both beams through the sample and probe the phase deterioration due to the relative interaction of beams in the material medium. Here, thermo-optic refraction interferometry allows the superposition of a higher-order vortex beam with a non-vortex beam through the phenomenon of thermal lensing. The non-vortex beam is made to expand in a controlled fashion by another laser. The relative interaction between the expanding non-vortex beam and the vortex beam within the sample, results in the output interferogram. The phase deterioration analysis of the output interferogram elucidates medium-driven phase changes. This technique is demonstrated using milk samples by recording the root-mean-square azimuthal phase deterioration of the orbital angular momentum beam.

Quantitative Determination of Surface Morphology of Red Blood Cell

In this study, the determination of the surface morphology of red blood cell (RBC) from interferogram image obtained by quantitative phase imaging (QPI) method is presented. QPI, is an optical measurement method frequently used in recent years, allows to obtain quantitative data for different samples (cell, thin film surface, etc.). Many measurement setups at the micrometer level and with nanometer precision have been designed for quantitative surface determination. Among these, white light diffraction phase microscopy (WDPM) is a design that combines the advantages of off-axis holography-specific speed and phase sensitivity associated with common path interferometry. Interferogram image of RBCs have been formed by the WDPM setup. Analysis of this image has been carried out by Fourier transform. As a result of this analysis, three-dimensional (3D), dynamic (observable from all angles) and height-known profiles of RBCs have been created. From the height profiles, the parameters related to the morphology of RBCs as the projected surface area (PSA), diameter (D), mean corpuscular volume (MCV) and total surface area occupied by the cell (SA), have been determined quantitatively. In addition, two-dimensional images, obtained by examining blood samples with light microscopy and scanning electron microscopy (SEM), have been compared with the data achieved by WDPM. The advantages and disadvantages of WDPM and light microscopy and SEM, which are commonly used in biomedical measurements, are discussed through the results. In this way, it was possible to see the difference between QPI and traditional methods used to imaging the cell surface.

System analysis of a common-path heterodyne polarimeter

We proposed a polarimeter combining the common-path heterodyne interferometry (CPHI) with a half-wave plate. When the azimuth of the wave plate is at 22.5 degrees, it has the best resolution for optical rotation (OR) measurement. The purpose of this paper is to analyze all possible sources of error in the system. The retardation closer to the true half wavelength, the higher the resolution will be. A large phase change of the test signal relative to the reference signal can be found by a lock-in amplifier. The system phase error is approximately 0.06∘. The accuracy of optical rotation is 1.0 × 10−3 degrees for the phase retardation is 178.08∘. The concentration resolution can reach 1.6×10−3[gdl] for fructose solutions. The magnitude of the phase error depends on the stability of the optical system and whether the two interfering lights coincide exactly. If the phase error can be reduced to 0.01∘, the resolution of OR can be increased to 1.3×10−4∘.

Characteristics of a Gaussian focus embedded within spiral patterns in common-path interferometry with phase apertures

A phase-only method is proposed to transform an optical vortex field into desired spiral diffraction–interference patterns. Double-ring phase apertures are designed to produce a concentric high-order vortex beam and a zeroth-order vortex beam, and the diffracted intensity ratio of two beams is adjustable between 0 and 1. The coherent superposition of the two diffracted beams generates a brighter Airy spot (or Poisson spot) in the middle of the spiral pattern, where the singularity for typical vortex beam is located. Experiments employing circular, triangular, and rectangular phase apertures with topological charges from 3 to 16 demonstrate a stable, compact, and flexible apparatus for vortex beam conversion. By adjusting the parameters of the phase aperture, the proposed method can realize the optical Gaussian tweezer function and the optical vortex tweezer function simultaneously along the same axis or switch the experimental setup between the two functions. It also has potential applications in light communication through turbulent air by transmitting an orbital angular momentum-coded signal with a concentric beacon laser.

Heterostructure Films of SiO2 and HfO2 for High-Power Laser Optics Prepared by Plasma-Enhanced Atomic Layer Deposition

Absorption losses and laser-induced damage threshold (LIDT) are considered to be the major constraints for development of optical coatings for high-power laser optics. Such coatings require paramount properties, such as low losses due to optical absorption, high mechanical stability, and enhanced damage resistance, to withstand high-intensity laser pulses. In this work, heterostructures were developed by sub-nanometer thin films of SiO2 and HfO2 using the plasma-enhanced atomic layer deposition (PEALD) technique. Thin-film characterization techniques, such as spectroscopic ellipsometry, spectrophotometry, substrate curvature measurements, X-ray reflectivity, and Fourier transform infrared spectroscopy, were employed for extracting optical constants, residual stress, layer formation, and functional groups present in the heterostructures, respectively. These heterostructures demonstrate tunable refractive index, bandgap, and improved optical losses and LIDT properties. The films were incorporated into antireflection coatings (multilayer stacks and graded-index coatings) and the LIDT was determined at 355 nm wavelength by the R-on-1 method. Optical absorptions at the reported wavelengths were characterized using photothermal common-path interferometry and laser-induced deflection techniques.

Spinning disk interferometric scattering confocal microscopy captures millisecond timescale dynamics of living cells.

Interferometric scattering (iSCAT) microscopy is a highly sensitive imaging technique that uses common-path interferometry to detect the linear scattering fields associated with samples. However, when measuring a complex sample, such as a biological cell, the superposition of the scattering signals from various sources, particularly those along the optical axis of the microscope objective, considerably complicates the data interpretation. Herein, we demonstrate high-speed, wide-field iSCAT microscopy in conjunction with confocal optical sectioning. Utilizing the multibeam scanning strategy of spinning disk confocal microscopy, our iSCAT confocal microscope acquires images at a rate of 1,000 frames per second (fps). The configurations of the spinning disk and the background correction procedures are described. The iSCAT confocal microscope is highly sensitive-individual 10 nm gold nanoparticles are successfully detected. Using high-speed iSCAT confocal imaging, we captured the rapid movements of single nanoparticles on the model membrane and single native vesicles in the living cells. Label-free iSCAT confocal imaging enables the detailed visualization of nanoscopic cell dynamics in their most native forms. This holds promise to unveil cell activities that are previously undescribed by fluorescence-based microscopy.

Cross-polarized common-path temporal interferometry for high-sensitivity strong-field ionization measurements.

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 1013 cm-2 if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent'ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.

Measurement of natural frequencies and mode shapes of transparent insect wings using common-path ESPI.

In this study, a common-path electronic speckle pattern interferometry system which upholds the natural property of transparency of insect's wings has been developed to measure the wings' natural frequencies and mode shapes for the first time. A novel base-exciting method was designed to enable the simultaneous application of sinusoidal and static forces to excite wings and introduce an additional phase. The moiré effect induced by the amplitude modulation was employed to accurately recognize the resonance state. Subsequently, the mode shapes were visualized by phase-shifting and real-time frame subtraction. Eight pairs of forewings from cicadas were investigated. The first three order natural frequencies of the wings are approximately 145 Hz, 272 Hz and 394 Hz, respectively, which are dispersed to prevent modal coupling. The cambered mode shapes exhibit a strongly spanwise-chordwise anisotropy flexural stiffness distribution, generally dominated by bending and twisting deformation. The details of the high-order mode shapes show that the tip exhibits distinct deformation, indicating more flexibility to cope with external impact load, and the nodal lines usually comply with the direction of the wing veins in higher modes, substantiating the fact that the veins play an important role as stiffeners of the membrane. The results are in excellent agreement with the dynamic performance of previous studies, which will potentially affect a broader community of optical measurement specialists and entomologists to enhance our understanding of time-averaged interferograms and insect flights.

Investigation on phase-modulation characteristics and transmission of the liquid crystal device under continuous-wave laser irradiation

Liquid crystal devices (LCDs) are widely used in several applications involving high-power lasers, such as laser processing, laser shaping, laser communication, laser radar and so on. Investigation on the performance of the LCDs working in high-power laser has always been of concern. In this study, a modified nearly common-path interferometry technique is proposed to investigate the phase modulation characteristics of the LCD irradiated by 1064 nm continuous-wave (CW) laser with Gaussian beam profile and an effective area of 0.1 cm2. In addition, the temperature rise of the LCD induced by laser irradiation was synchronously measured with an infrared thermal imager. A quantitative and dynamic investigation of the relation between the thermal effect caused by the laser and the phase-modulation variation is reported. Experimental results showed that the trend of temperature increase of the LCD was similar to that of phase shift variation. The theoretical phase shift calculated using the temperature-dependent LC refractive index coincided with the experimental results. This indicated that the refractive index change induced by temperature was the main reason for the phase-modulation variation of the LCD under laser irradiation. When the laser power was further increased, interference fringe distortion appeared, and morphological changes in the LC were observed. This implied that the laser-induced temperature rise had reached the clearing point of the LC and that the LCD was inactive. The temperature simulation along the laser incidence direction implied that, under the irradiation CW laser, indium tin oxide (ITO) layer absorbs laser energy, resulting in a temperature rise and a small temperature gradient between the ITO, polyimide (PI) and LC layers. The rising temperature is too low to induced damage of ITO layer and PI layer, but influences the refractive index of the LC. Compared with other layers of the LCD, the LC material will fail first when the irradiated laser power is further risen. The transmission variation of an LCD induced by laser irradiation was found. Possibly owing to the refractive index change of the LC as well as the multilayer film structure of the LCD, there seemed to be a cyclical transmission variation.

In Situ Metrology for Pad Surface Monitoring in CMP Using a Common-Path Phase-Shifting Interferometry: A Feasibility Study

In the fabrication of semiconductors, chemical mechanical polishing (CMP) is an essential wafer-planarization process. For optimal CMP, it is crucial to monitor the texture of the polishing pad; this leads to homogenous planarization of wafers. Hence, we present a new interferometric approach for in situ evaluation of the CMP pad surface based on a common-path phase-shifting interferometry, with which a series of phase-modulated interference signals immune to external perturbation can be recorded. A nanoscopic surface topology can then be reconstructed to estimate surface roughness using the recorded interference images. The surface mapping performance of the proposed method was tested by retrieving a topology of a vibrating nanostructure in immersion, of which height profiles were consistent with the result from atomic force microscopy (AFM). The method was also validated by examining the surface of a used CMP pad in simulated conditions.

Curved-Mechanical Characteristic Measurements of Transparent Conductive Film-Coated Polymer Substrates Using Common-Path Optical Interferometry

This study proposes a method for measuring curved-mechanical characteristics based on a whole-folding test for transparent conductive film-coated polymer substrates using common-path optical interferometry. Accordingly, 80-, 160-, and 230-nm indium tin oxide films coated on 40 × 40 mm 125-μm-thick polyethylene terephthalate (PET) substrates, and monolayer graphene films coated on 40 × 40 mm 250-μm-thick PET substrates are inspected and analyzed under the curving conditions of 50-, 30-, 20-, and 10-mm radii before and after an 11,000 whole-folding cycle test based on a 10-mm folding radius. This study utilizes the changes in the phase retardations of transparent conductive film-coated polymer substrates under different curving conditions before and after 11,000 whole-folding cycles to analyze the substrates’ residual stress characteristics that were the direct result of manufacturing process parameters. The results from this study of curved-mechanical characteristic measurements of flexible transparent conductive substrates can provide designers with improved product development and can assist manufacturers in improving the manufacturing design of enhanced coating processes.

Bond-selective interferometric scattering microscopy

Interferometric scattering (iSCAT) microscopy has been a very promising technology for highly sensitive label-free imaging of a broad spectrum of biological nanoparticles from proteins to viruses in a high-throughput manner. Although it can reveal the specimen’s size and shape information, the chemical composition is inaccessible in interferometric measurements. Infrared (IR) spectroscopic imaging provides chemical specificity based on inherent chemical bond vibrations of specimens but lacks the ability to image and resolve individual nanoparticles due to long IR wavelengths. Here, we describe a bond-selective iSCAT microscope where the mid-IR induced photothermal signal is detected by a visible beam in a wide-field common-path interferometry configuration. A thin film layered substrate is utilized to reduce the reflected light and provide a reference field for the interferometric detection of the weakly scattered field. A pulsed mid-IR laser is employed to modulate the interferometric signal. Subsequent demodulation via a virtual lock-in camera offers simultaneous chemical information about tens of micro- or nano-particles. The chemical contrast arises from a minute change in the particle’s scattered field in consequence of the vibrational absorption at the target molecule. We characterize the system with sub-wavelength polymer beads and highlight biological applications by chemically imaging several microorganisms including Staphylococcus aureus, Escherichia coli, and Candida albicans. A theoretical framework is established to extend bond-selective iSCAT microscopy to a broad range of biological micro- and nano-particles.