Photoelastic Modulator Research Articles

Experimental determination of the Stokes parameters using a dual photoelastic modulator system

We develop a more general methodology for a dual photoelastic modulator (PEM) system that is used for the determination of the Stokes parameters of an arbitrary light beam. This allows for a degree of arbitrariness in the setting of the retardation amplitudes of both PEMs, thus permitting a robust and effective optimization of the detection system. Various experimental issues are considered and a calibration procedure is described that ensures accurate measurement of the absolute values of the Stokes parameters. Measurements of the Faraday rotation of a CoPt multilayer film are provided as a demonstration of the sensitivity of the dual-PEM system.

Imaging and Absolute Extinction Cross-Section Measurements of Nanorods and Nanowires through Polarization Modulation Microscopy

This paper describes an experimental technique for measuring the extinction cross-section of anisotropic nanomaterials. The experiments are performed by focusing a laser beam to a diffraction-limited spot under an optical microscope, and using a photoelastic modulator (PEM) to rotate the polarization of the light beam. Monitoring the transmitted beam with a lock-in amplifier referenced to twice the resonant frequency of the PEM yields the difference in extinction for light polarized parallel and perpendicular to the optical axis of the nanostructure. These experiments are the single particle analog of polarization modulation microscopy (PMM). Experimental results for gold nanorods and CdSe nanowires are presented that demonstrate the sensitivity and properties of this technique. In particular, we show that by collecting images at two laser polarizations separated by 45° it is possible to construct an extinction cross-section image. The advantages and disadvantages of the PMM technique compared to existing ways of measuring the extinction of nanoparticles (photothermal heterodyne imaging and spatial modulation spectroscopy) are discussed. Results from experiments where we collected simultaneous absorption and emission images are also presented for different shaped CdSe nanowires. These measurements provide insight into how the structure of the nanowire affects its photophysics.

Fourier transform approach in modulation technique of experimental measurements

An application of Fourier transform approach in modulation technique of experimental studies is considered. This method has obvious advantages compared with traditional lock-in amplifiers technique--simple experimental setup, a quickly available information on all the required harmonics, high speed of data processing using fast Fourier transform algorithm. A computationally simple, fast and accurate Fourier coefficients interpolation (FCI) method has been implemented to obtain a useful information from harmonics of a multimode signal. Our analysis shows that in this case FCI method has a systematical error (bias) of a signal parameters estimation, which became essential for the short data sets. Hence, a new differential Fourier coefficients interpolation (DFCI) method has been suggested, which is less sensitive to a presence of several modes in a signal. The analysis has been confirmed by simulations and measurements of a quartz wedge birefringence by means of the photoelastic modulator. The obtained bias, noise level, and measuring speed are comparable and even better than in lock-in amplifier technique. Moreover, presented DFCI method is expected to be promised candidate for using in actively developing imaging systems based on the modulation technique requiring fast digital signal processing of large data sets.

Zeeman polarimetry measurement for edge current density determination using Li-beam probe on JT-60U

Zeeman polarimetry system using Li-beam probe has been developed for the edge current density measurement in the JT-60U tokamak, which measures the polarization angle alpha (related to the pitch angle of the magnetic field) by means of photoelastic modulators, etalons, and phase sensitive detection using digital lock-in amplifiers with the accuracy in the alpha of Delta alpha approximately 0.1 degrees. The diagnostic has 20-channel viewing chords covering the plasma peripheral region of normalized minor radius r/a approximately 0.8-1 with a spatial resolution of up to approximately 1 cm. Li-beam injection with beam current of up to approximately 5 mA has been achieved. A new tuning method of the wavelength for the etalon has been demonstrated, scanning the beam acceleration voltage and keeping a beam current constant during a single shot. The peak wavelength of the etalon is adjusted in the direction to both blue- and redshifts by changing the angle of incidence and increasing the temperature, respectively. Time evolution of the edge current density profile has been determined for the current ramp experiment in the Ohmically heated discharges. In addition, the edge current density profile with the local peak of j(ped) approximately 0.15-0.25 MA/m(2) at r/a approximately 0.9 has been identified in the H-mode plasma, which is correlated with large pressure gradient in the pedestal region.

Improved Resolution and Stability in a Dispersion Interferometer Using a Modulation Amplitude Ratio

On steady-state fusion reactors, reliable electron density measurement is required for fueling control. However, the signal-to-noise ratio (S/N) of conventional interferometers is affected by mechanical vibrations. In addition, fringe jump errors are possible at high densities. One possible approach to these problems is the use of a dispersion interferometer (DI) [1]. The DI is immune to mechanical vibrations, and hence does not need a vibration isolator. A two-color system is also not needed even if short-wavelength lasers (a CO2 laser and a YAG laser, for example) are used. The fringe jump errors are attributed to 2π ambiguity in the phase shift. If short-wavelength laser light with a phase shift smaller than 2π is selected, the interferometer becomes free from fringe jump errors in principle. However, the S/N is significantly degraded because the phase shift due to vibrations (∝ 1/λ, λ: wavelength) increases and that due to a plasma (∝ λ) decreases for conventional interferometers. In contrast, the DI has no issue even if the plasma term is smaller than 2π owing to its immunity to vibrations. Thus, the DI becomes free from fringe jump errors if an appropriate wavelength is selected. Techniques that improve the resolution of DIs are currently being introduced [2–5]. This paper reports improved resolution and long-time stability in a DI that adopts phase modulation [2] with a photoelastic modulator (PEM) and a new signal processing method that uses a modulation amplitude ratio [4, 5]. Figure 1 shows a schematic view of the present optical and electrical systems used for bench testing. The light source is a CO2 laser with an output power and wavelength of 8 W and 10.6 μm, respectively. The focused laser light is injected into a nonlinear crystal of AgGaSe2 to generate the frequency-doubled component (FD1). A mixed beam consisting of the fundamental and frequency-doubled components, whose polarizations are orthogonal, passes through a PEM with a drive frequency

Near-Field Structural Studies of Lipid Bilayers

We use a Near-field Scanning Optical Microscope (NSOM) in conjunction with a Photo Elastic Modulator (PEM) to conduct birefringence (ne-no) measurements with a spatial resolution of ∼80nm. With our current setup we are able to distinguish changes in retardance S on the order of 10-4 radians. Simultaneously while gathering information about S we extract information about the samples optical orientation θ referenced to the system's axis, with an accuracy of ∼7.24x10-4 radians. We use our system on 1,2-dipalmitoylphosphatidylchorline (DPPC) bilayers, which at room temperature are in the gel state, (i.e.: their acyl chains have a ∼32o degree azimulthal tilt with respect to the membranes normal). Modeling the membrane as a uniaxial crystal we are able determine the position of the acyl chains by measuring the birefringence and optical orientation. By controlling the temperature of our sample we hope to better study the structural changes that occur during phase transitions from gel to liquid states. The investigation of other lipid mixtures and the transformations they undergo during different phases will also be discussed.

Transient vibrational circular dichroism spectrometer: technical development

We recently reported the first measurements of transient VCD in the C-H-stretch region following visible excitation of cobalt(-)-spartein complex (Co(sp)Cl2) with picosecond time resolution [1]. This poster presents the detailed description of the setup based on the synchronization of a femtosecond laser system with a photo elastic modulator [2]. A very precise control of the probe pulse polarization is a requirement to avoid linear dichroism artefacts. This is particularly important in crossed polarizer “quasi-null” technique which can be used to significantly enhance chiral signals [3].

Measuring circular dichroism in a capillary cell using the b23 synchrotron radiation CD beamline at diamond light source

Synchrotron radiation circular dichroism (SRCD) is a well-established method in structural biology. The first UV-VIS beamline dedicated to circular dichroism at Diamond Light Source, a third generation synchrotron facility in South Oxfordshire, has recently become operational and it is now available for the user community. Herein we present an important application of SRCD: the CD measurement of protein solutions in fused silica rectangular capillary cells. This was achieved without the use of any lens between the photoelastic modulator and the photomultiplier tube detectors by exploiting the high photon flux of the collimated beam that can be as little as half a millimeter squared. Measures to minimize or eliminate vacuum-UV protein denaturation effects are discussed. The CD spectra measured in capillaries is a proof of principle to address CD measurements in microdevice systems using the new B23 SRCD beamline.

Conceptual Design of a Dispersion Interferometer Using a Ratio of Modulation Amplitudes

Since a dispersion interferometer is free from mechanical vibrations, it does not need a vibration compensation system even for a probe beam with a short wavelength. This paper describes a new signal processing of the dispersion interferometer using a ratio of modulation amplitudes with a photoelastic modulator. The proposed method is immune to changes in detected signal intensities, thus making the signal processing system simple. Designs of the optical system of the dispersion interferometer for proof of principle, especially specification of a nonlinear optical crystal, are also shown.

In situ PM-IRRAS study of powder catalyst: Dynamic evolutions of species on catalyst and in gas phase during NO x storage-reduction

The potential of polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) for in situ and time-resolved studies using powder samples is demonstrated using as an example the NO x storage-reduction behaviour of a Pt-Ba/Al 2O 3 powder catalyst. The unique feature of PM-IRRAS, the simultaneous but separate detection of gas phase species and species on the catalyst surface, is illustrated. Bands due to the powder prominently appeared in the gas phase spectra due to transmittance and absorption of IR-light by the solid. The difference in the appearance of powder bands in the gas phase and surface spectra was explained by the distinct local sensitivity in the gas phase and surface spectra for powder samples and also by optoelectronic effects caused by the photoelastic modulator. The study shows that PM-IRRAS is not only suitable for in situ mechanistic studies on idealized model film catalysts, but also for the investigation of technically more relevant powder catalysts.

Polarization-modulated infrared reflection difference microspectroscopy: Experiment and model

Polarization-modulated infrared reflection difference microspectroscopy is a sensitive tool to spatially and spectrally resolve optical anisotropy of micrometric crystalline domains. However, intrinsic artifacts are associated with polarization modulation experiments, in particular due to the wavenumber-dependent efficiency of the photoelastic modulator. Here an account addressing this problem in our experimental approach is given. Comparing data from measurements on Ca 1.4Sr 0.6RuO 4 and Bi 0.3Ca 0.7MnO 3+ δ with model calculations, it is shown that a simple model, which is based on reports in the literature, can be used to describe the experimental results.

Vibrational circular dichroism signal enhancement using self-heterodyning with elliptically polarized laser pulses

Vibrational circular dichroism (VCD) spectra were recorded using elliptically polarized ultrashort laser pulses, produced with the help of a photoelastic modulator. The short polarization axis of the elliptical light acts as a phase-locked local oscillator field, heterodyning the chiral signal generated by the field along the long polarization axis. This leads to VCD signals that increase linearly with the ellipticity of the probe pulses and enhanced signal to noise, which is expected to improve recently reported transient VCD scans. An analogous scheme allows for vibrational optical rotary dispersion measurements. The techniques are compared with similar approaches using both a linear response picture and the Jones matrix calculus.

Optical confirmation of biaxial nematic (Nb) phase in a bent-core mesogen

A bent-core mesogen with different end groups has been studied for different surface conditions in both planar and homeotropic cells using techniques for measuring biaxiality and optical switching. Biaxial nematic phase observed in between the uniaxial nematic and smectic phases is evidenced by a sharp increase in the biaxiality in a homeotropic cell measured using a photoelastic modulator. The material in this phase is switchable through the minor director with an in-plane electric field. In a planar cell, a step in the difference in the refractive indices resulting from the uniaxial to biaxial transition is also observed.

Cross‐Polarization Detection Enables Fast Measurement of Vibrational Circular Dichroism

of the total absorption), and as conventional optical elements may be used, the construction of the spectrometers is fairly straightforward. It was soon realized that utilizing a similar phenomenon in the vibrational region could provide more detailed and reliable stereochemical information, because the vibrational spectrum contains many more bands sensitive to minute features of the molecular structure. [7, 8] The vibrational region was successfully tackled in 1973 when the circular-polarization dependence of vibrational Raman scattering was first measured with visible laser light. [9] In 1974 this success was followed by a report on VCD. [10] Ever since then, VOA has been applied to a broad scale of problems ranging from small chiral molecules to proteins, nucleic acids, and viruses. [11–17] Nevertheless, the technique remained confined to specialized laboratories, predominantly in the United States, for a long time. As the light wavelength is much larger than the molecular dimensions, the VOA signal is very weak, typically mere 10 � 4 –10 � 5 of the total scattering or absorption. Accurate measurements required advanced instrumentation, high stability of the optics and light sources, and special detectors coupled with dedicated electronics, such as lockin amplifiers. First commercial attempts to sell the VCD instruments failed in the eighties. The instrumentation problems, however, were eventually solved and this vibrational optical activity was re-introduced to the market around 1997, this time successfully. A few years later, an ROA spectrometer became commercially available as well. Similar to conventional spectrometers, the new detection technique utilizes the familiar phenomenon of linearly polarized light being rotated and partially depolarized by an optically active medium, as shown diagrammatically in Figure 1. In a traditional VCD measurement, a piezoelectric photoelastic modulator is used to periodically yield the left- and rightcircularly polarized (LCP, RCP) light. This produces a varying signal at the detector, which is electronically weighted against the total (non-polarized) absorption. [18]

Submicrometer resolution far field high sensitivity Kerr microscopy for in-plane magnetization detection

We report that a far field magneto-optical system designed for detecting the off-plane magnetization in polar configuration can also detect the in-plane magnetization, while preserving the high sensitivity by using symmetric illumination and photoelastic modulation, even though the optical axis of the system is vertical to the sample surface. The spatial resolution can reach 500 nm at 514 nm in wavelength. The in-plane sensitivity can be eliminated by using an aperture to remove the off-axis obliquely incident light generated by the high numerical aperture objective lens that focuses the laser beam onto the sample surface.

Performance evaluation of short-wavelength FIR laser polarimeter with dual silicon photoelastic modulators

Measurement of an internal magnetic field distribution in magnetically confined fusion devices is indispensable for both understanding the plasma physics and controlling plasmas. A polari-interferometer based on the Faraday effect has been used for such a purpose. This paper describes performance evaluations of a part of the polarimeter with photoelastic modulators (PEMs) of the short-wavelength far-infrared (FIR) laser polari-interferometer. The wavelengths of the light source are 57.2 and 47.7 μ m, which are suitable for a high-density operation and large fusion devices. The PEM for the FIR region is newly developed with high-resistive silicon in which absorption of the FIR laser is small. The polarization angle is successfully measured and an angle resolution of 0.01° with a time resolution of 1 ms is achieved. A drift of the baseline of about 0.1° for 1000 s is observed and is found to be caused by changes in the room temperature.

Nonlinear Optical Stokes Ellipsometry. 1. Theoretical Framework

The theoretical framework for nonlinear optical Stokes ellipsometry (NOSE) is developed as a faster alternative to previous nonlinear optical ellipsometry (NOE) techniques. NOE is the determination of the complex-valued elements within the Jones χ(n) tensor, from which all experimental observables in a given configuration can be predicted. By replacing the moving optics with a photoelastic modulator (PEM) operating at 50 kHz and employing a high repetition rate laser, full ellipsometric detection of the χ(2) tensor can in theory be performed in as little as 20 μs. Two complementary models were developed to analyze the proposed instrumental setup for NOSE. The first more general approach is based on a regression analysis method, which was compared to a second explicit analytical model valid for a particular experimental configuration. Additionally, the rigor of the regression analysis method against noise was investigated.

Nonlinear Optical Stokes Ellipsometry. 2. Experimental Demonstration

Second harmonic generation (SHG) has developed into a powerful tool for characterizing oriented thin films, surfaces, and interfaces. Furthermore, the nonlinear optical nature of wave-mixing processes typically results in the generation of a coherent signal beam with a well-defined polarization state. This coherence offers unique opportunities for the extraction of detailed molecular and surface properties from polarization analysis. In previous studies, nonlinear optical ellipsometry (NOE) has been developed as a means to retain sign and phase information between the different nonzero χ(2) tensor elements present in a given sample. However, those previous methods and related approaches for polarization analysis have all relied on the physical movement of optical elements to perform the analysis. The time required to physically move the appropriate optical elements ultimately dictates the fastest analysis time possible in a given technique. Such long acquisition times have limited NOE analyses to systems exhibiting excellent photostability and have precluded the feasibility of NOE imaging. Development of nonlinear optical Stokes ellipsometry (NOSE) was shown to address many of these problems. By increasing the repetition rate of the laser system and replacing previously slow rotating polarization optics with a rapid photoelastic modulator, the acquisition time with full polarization analysis was reduced from several hours to less than a second. This technique was validated against established NOE techniques using z-cut quartz as a reference sample and then demonstrated on a dye thin film. Additionally, an orientation analysis of the thin film was performed. These studies resulted in an order of magnitude improvement in precision relative to previous NOE techniques, while simultaneously accompanied by a reduction in acquisition time of more than four orders of magnitude.

A New Method for Performing Polarization Modulation Infrared Reflection-Adsorption Spectroscopy of Surfaces

A new and relatively simple polarization modulation technique is presented and tested that enables the whole spectral range to be detected between 400 and 4000 cm(-1). This experiment is conventionally carried out using a photoelastic modulator that modulates incident plane polarized light through 90 degrees . This suffers from the drawback that it enables spectra to be collected only over a relatively narrow spectral range. As an alternative, a polarizer is placed in the beam and oriented at 45 degrees to the sample normal. This produces incident radiation fluxes with identical intensities for both s- and p-polarized light. A second polarizer is then modulated through 90 degrees and the surface spectrum is then extracted in the usual manner from the difference between these signals, normalized to their sum. The method is demonstrated for a self-assembled monolayer of 11-mercapto-undecanoicacid (11-MUA) on gold on mica, and it is shown that, while the resulting spectra are extremely sensitive to optical alignment, the method yields spectra that are in excellent agreement with published data.

Polarization-sensitive surface plasmon enhanced ellipsometry biosensor using the photoelastic modulation technique

A surface plasmon enhanced ellipsometry (SPEE) biosensor scheme based on the use of a photoelastic modulator (PEM) is reported. We show that the polarization parameters of a laser beam, tan ψ, cos Δ and ellipse orientation angle ϕ, can be directly measured by detecting the modulation signals at the first and second harmonics of the modulated frequency under a certain birefringence geometry. This leads to accurate measurement of refractive index variations within the evanescent field region close to the gold sensor surface, thereby enabling biosensing applications. Our experimental results confirm that the new scheme offers a respectable detection limit of 6 × 10 −7 refractive index unit (RIU) or 15 ng/ml of biomolecule solute concentration without any compromise in dynamic range. In addition, PEM offers the possibility of single-beam phase-sensitive SPR detection that drastically reduces the complexity of the optical system, thus readily making it possible for SPR to be adopted in label-free micro-array biochips.