Phase-sensitive Detection Scheme Research Articles

Multi-wavelength lock-in spectroscopy for extracting perturbed spectral responses: molecular signatures in nanocavities.

Detecting small changes in spectral fingerprints at multiple wavelength bands simultaneously is challenging for many spectroscopic techniques. Because power variations, drift, and thermal fluctuations can affect such measurements on different timescales, high speed lock-in detection is the preferred method, however this is typically a single channel (wavelength) technique. Here, a way to achieve multichannel (multi-wavelength) lock-in vibrational spectroscopy is reported, using acousto-optic modulators to convert nanosecond periodic temporal perturbations into spatially distinct spectra. This simultaneously resolves perturbed and reference spectra, by projecting them onto different locations of the spectrometer image. As an example, we apply this multichannel time-resolved methodology to detect molecular frequency upconversion in plasmonic nanocavities from the perturbed Raman scattering at different wavelengths. Our phase-sensitive detection scheme can be applied to any spectroscopy throughout the visible and near-infrared wavelength ranges. Extracting perturbed spectra for measurements on nanosecond timescales allows for capturing many processes, such as semiconductor optoelectronics, high-speed spectro-electrochemistry, catalysis, redox chemistry, molecular electronics, or atomic diffusion across materials.

A Table-Top Pilot Experiment for Narrow Mass Range Light Cold Dark Matter Particle Searches

This report presents the detection framework and a proposal for a pilot table-top experiment (supported by simulations and preliminary test results) for adoption into narrow mass range light Cold Dark Matter (CDM) searches, specifically for axions or Axion-Like Particles (ALPs) in a resonant cavity-based scheme. The novelty of this proposal lies in an attempt to concentrate searches corresponding to specific axion masses of interest (coinciding with recent proposals), using multiple cavities in a symmetric scheme, instead of using noisy and complicated tuning mechanisms, and in reduction of associated hardware by employing simpler underlying instrumentation instead of heterodyne mode of detection, by means of a low-noise ac amplification and dc phase-sensitive detection scheme, in order to make a viable and compact table-top experiment possible. These simplifications could possibly be valuable in substantially reducing detection hardware, experiment complexities (and associated noise) and long run-times, while maintaining low noise similar to conventional axion searches. The feasibility of proposed scheme and the experiment design are demonstrated with some calculations, simulations and preliminary tests with artificial axion signals injected into the cavities. The technique and ideas reported here have significant potential to be developed into a small-scale table-top, narrow-range, dark matter axion/ALP spectroscopy experiment, in addition to aiding in the on-going resonant cavity-based and broadband experiments.

Resolution enhancement in magnetic particle imaging via phase-weighting

Spatial resolution is a key metric for characterizing any imaging modality. Here we describe magnetic particle imaging (MPI) experiments and complementary magnetic particle spectroscopy (MPS) experiments that reveal and probe significant variations in MPI spatial resolution that are intrinsically coupled to the magnetization dynamics of superparamagnetic nanoparticles. In particular, employing a phase-sensitive MPI detection scheme, we observe a marked spatial sharpening effect as the phase of the demodulation reference is advanced. This increase in spatial resolution is accompanied by degradation of image contrast, to the point where reconstructed images effectively vanish when the demodulation phase is advanced by approximately 90 degrees relative to the phase that yields images with the highest signal-to-noise ratio (SNR). Relaxation- or phase-weighted magnetic particle images with apparent spatial resolutions of order a few hundred microns or better, and with SNRs of order 10 dB or more, are presented, and a qualitative framework for interpreting the role of phase as it pertains to spatial resolution in MPI is introduced.

8-port homodyne detection of optical fields using IQ demodulation

A phase-sensitive detection scheme for optical fields is described, which allows for the simultaneous measurement of orthogonal field quadratures. The recent achievement of -radian cross-phase shifts between a single photon and a second light field has opened the door to quantum computing protocols which utilize photonic quantum bits. Optical phase measurements play an integral role in these schemes. The detection scheme presented here was designed to measure such transient phase shifts of a laser field as it interacts with another light field through a cloud of laser-cooled Rb atoms. In place of conventional spatial interferometry, which is very sensitive to mechanical instabilities, the scheme utilizes beat-note interferometry to improve on the robustness of the measurement. Beat-note interferometry serves to map a signal from a THz-frequency carrier onto a radio-frequency carrier. A variant of 8-port optical homodyne detection, the scheme then uses IQ demodulation, a well-established technique in communications engineering, to extract amplitude information for both the in-phase (I) and quadrature-phase (Q) components of the radio-frequency signal. This IQ data can then be used to extract any amplitude variation and/or phase shifts of the original THz-frequency optical field. The technique is illustrated here by simultaneously measuring the absorption and dispersion experienced by an optical beam as it passes through a cloud of laser-cooled atoms, which have been prepared in a state of electromagnetically-induced transparency. A thorough characterization of the measurement is presented, which identifies the dominant noise sources and illustrates the utility of IQ demodulation in eliminating noise. This scheme has been used to measure rad phase shifts on nanosecond time scales.

Macroscale fluorescence imaging against autofluorescence under ambient light

Macroscale fluorescence imaging is increasingly used to observe biological samples. However, it may suffer from spectral interferences that originate from ambient light or autofluorescence of the sample or its support. In this manuscript, we built a simple and inexpensive fluorescence macroscope, which has been used to evaluate the performance of Speed OPIOM (Out of Phase Imaging after Optical Modulation), which is a reference-free dynamic contrast protocol, to selectively image reversibly photoswitchable fluorophores as labels against detrimental autofluorescence and ambient light. By tuning the intensity and radial frequency of the modulated illumination to the Speed OPIOM resonance and adopting a phase-sensitive detection scheme that ensures noise rejection, we enhanced the sensitivity and the signal-to-noise ratio for fluorescence detection in blot assays by factors of 50 and 10, respectively, over direct fluorescence observation under constant illumination. Then, we overcame the strong autofluorescence of growth media that are currently used in microbiology and realized multiplexed fluorescence observation of colonies of spectrally similar fluorescent bacteria with a unique configuration of excitation and emission wavelengths. Finally, we easily discriminated fluorescent labels from the autofluorescent and reflective background in labeled leaves, even under the interference of incident light at intensities that are comparable to sunlight. The proposed approach is expected to find multiple applications, from biological assays to outdoor observations, in fluorescence macroimaging.

Development of a scanning electron microscopy with polarization analysis system for magnetic imaging with ns time resolution and phase-sensitive detection.

Scanning electron microscopy with polarization analysis is a powerful lab-based magnetic imaging technique offering simultaneous imaging of multiple magnetization components and a very high spatial resolution. However, one drawback of the technique is the long required acquisition time resulting from the low inherent efficiency of spin detection, which has limited the applicability of the technique to certain quasi-static measurement schemes and materials with high magnetic contrast. Here we demonstrate the ability to improve the signal-to-noise ratio for particular classes of measurements involving periodic excitation of the magnetic structure via the implementation of a digital phase-sensitive detection scheme facilitated by the integration of a time-to-digital converter to the system. The modified setup provides dynamic imaging capabilities using selected time windows and finally full time-resolved imaging with a demonstrated time resolution of better than 2 ns.

Coherent perfect absorption of path entangled single photons

We examine the question of coherent perfect absorption (CPA) of single photons, and more generally, of the quantum fields by a macroscopic medium. We show the CPA of path entangled single photons in a Fabry-Perot interferometer containing an absorptive medium. The frequency of perfect absorption can be controlled by changing the interferometer parameters like the reflectivity and the complex dielectric constant of the material. We exhibit similar results for path entangled photons in micro-ring resonators. For entangled fields like the ones produced by a down converter the CPA aspect is evident in phase sensitive detection schemes such as in measurements of the squeezing spectrum.

Fast vibrational imaging of single cells and tissues by stimulated Raman scattering microscopy.

ConspectusTraditionally, molecules are analyzed in a test tube. Taking biochemistry as an example, the majority of our knowledge about cellular content comes from analysis of fixed cells or tissue homogenates using tools such as immunoblotting and liquid chromatography–mass spectrometry. These tools can indicate the presence of molecules but do not provide information on their location or interaction with each other in real time, restricting our understanding of the functions of the molecule under study. For real-time imaging of labeled molecules in live cells, fluorescence microscopy is the tool of choice. Fluorescent labels, however, are too bulky for small molecules such as fatty acids, amino acids, and cholesterol. These challenges highlight a critical need for development of chemical imaging platforms that allow in situ or in vivo analysis of molecules. Vibrational spectroscopy based on spontaneous Raman scattering is widely used for label-free analysis of chemical content in cells and tissues. However, the Raman process is a weak effect, limiting its application for fast chemical imaging of a living system.With high imaging speed and 3D spatial resolution, coherent Raman scattering microscopy is enabling a new approach for real-time vibrational imaging of single cells in a living system. In most experiments, coherent Raman processes involve two excitation fields denoted as pump at ωp and Stokes at ωs. When the beating frequency between the pump and Stokes fields (ωp – ωs) is resonant with a Raman-active molecular vibration, four major coherent Raman scattering processes occur simultaneously, namely, coherent anti-Stokes Raman scattering (CARS) at (ωp – ωs) + ωp, coherent Stokes Raman scattering (CSRS) at ωs – (ωp – ωs), stimulated Raman gain (SRG) at ωs, and stimulated Raman loss (SRL) at ωp. In SRG, the Stokes beam experiences a gain in intensity, whereas in SRL, the pump beam experiences a loss. Both SRG and SRL belong to stimulated Raman scattering (SRS), in which the energy difference between the pump and Stokes fields is transferred to the molecule for vibrational excitation. The SRS signal appears at the same wavelengths as the excitation fields and is commonly extracted through a phase-sensitive detection scheme. The detected intensity change because of a Raman transition is proportional to Im[χ(3)]IpIs, where χ(3) represents the third-order nonlinear susceptibility, Ip and Is stand for the intensity of the pump and Stokes fields.In this Account, we discuss the most recent advances in the technical development and enabling applications of SRS microscopy. Compared to CARS, the SRS contrast is free of nonresonant background. Moreover, the SRS intensity is linearly proportional to the density of target molecules in focus. For single-frequency imaging, an SRS microscope offers a speed that is ∼1000 times faster than a line-scan Raman microscope and 10 000 times faster than a point-scan Raman microscope. It is important to emphasize that SRS and spontaneous Raman scattering are complementary to each other. Spontaneous Raman spectroscopy covers the entire window of molecular vibrations, which allows extraction of subtleties via multivariate analysis. SRS offers the speed advantage by focusing on either a single Raman band or a defined spectral window of target molecules. Integrating single-frequency SRS imaging and spontaneous Raman spectroscopy on a single platform allows quantitative compositional analysis of objects inside single live cells.

Phase-sensitive detection for unconventional Bose-Einstein condensation

We propose a phase-sensitive detection scheme to identify the unconventional ${p}_{x}\ifmmode\pm\else\textpm\fi{}i{p}_{y}$ symmetry of the condensate wave functions of bosons, which have already been proposed and realized in high bands in optical lattices. Using the impulsive Raman operation combined with time-of-flight imaging, the off-diagonal correlation functions in momentum space give rise to the relative phase information between different components of condensate wave functions. This scheme is robust against the interaction and interband effects, and provides evidence of unconventional Bose-Einstein condensations with nontrivial condensation symmetries.

Phase-Coherent Heterodyne Detection in the Terahertz Regime With a Photomixer

A low-temperature-grown GaAs photomixer with a spiral antenna has a quadratic response to the optical fields and an intrinsic nonlinear IV characteristic that allow the detection of THz radiation. A transmission line coupled to the antenna provides the photomixer with direct-current access in the microwave domain. The non-ohmic response of the photomixer is probed by rectification. The optical beat between two continuous-wave lasers illuminating the photomixer serves as a local oscillator with an adjustable difference-frequency in the THz domain. The beat between a THz field modulated at a radiofrequency rate with a radiofrequency-modulated optical beat is exploited in a phase-sensitive detection scheme.

Measuring temperatures in a high concentration solar simulator – Demonstration of the principle

A new single color pyrometric method for samples heated radiatively via an artificial light source is presented. The method allows separation of the external radiation reflected by a sample from its thermal emission as the intensity of arc lamps, commonly used in high concentration solar simulators, can be electrically modulated at frequencies well above the thermal response (ω≫1/τth) of the sample. Thus, the external radiation reflected off the sample can be measured separately using a phase sensitive detection scheme at the modulation frequency of the light source. In addition, a composite signal consisting of the thermal emission with the reflected external radiation superimposed is detected. This signal is obtained by observing the sample with a mechanical chopper operating at a different frequency placed in the observation path. The composite signal is measured simultaneously with identical optics and detector. The thermal emission of the sample can then be extracted as the difference of the two measured signals after suitable calibration. The working principle of the method and its calibration is explained and the performance of the method is demonstrated using its implementation on a lab scale.

Nanoparticle detection using dual-phase interferometry

The detection and identification of nanoparticles is of growing interest in atmospheric monitoring, medicine, and semiconductor manufacturing. While elastic light scattering with interferometric detection provides good sensitivity to single particles, active optical components prevent scalability of realistic sizes for deployment in the field or clinic. Here, we report on a simple phase-sensitive nanoparticle detection scheme with no active optical elements. Two measurements are taken simultaneously, allowing the amplitude and phase to be decoupled. We demonstrate the detection of 25 nm Au particles in liquid in Δt ∼ 1 ms with a signal-to-noise ratio of ∼ 37. Such performance makes it possible to detect nanoscale contaminants or larger proteins in real time without the need of artificial labeling.

CPT Magnetometer with Atomic Energy Level Modulation

We propose and experimentally investigate a coherent population trapping state based magnetometer prototype with 87Rb atoms. Through modulating Zeeman sublevels with an ac magnetic field, not only a phase sensitive detection scheme suitable for miniature magnetometer is realized, but also the detection resolution of magnetic field intensity could be improved by a factor of two. Our study result indicates that it is a promising low power consumption miniature sensitive low magnetic field sensor offering spatially resolved measurement at the sub-millimetre level.

Geometric Effects Due to Quadrature Phase-Sensitive Detection: Artifact Minimization and Removal Schemes for Spectroscopic Lineshape Measurement

An integral operator is defined to simulate the quadrature phase-sensitive detection (PSD) of a time-independent characteristic function (e.g., spectroscopic lineshapes in magnetic resonance), with its dependent variable modulated and scanned during the detection. This integral operation is equivalent to a convolution, with its kernel determined by the modulation function and the demodulation order. The convolution kernel results from the corresponding PSD as applied to the Dirac delta function. The convolution form not only facilitates our understanding of the geometric effects due to the quadrature and the nonquadrature PSD but also indicates hardware schemes that are capable of accurately approximating or directly measuring the functional form that is being probed. These schemes involve no a priori assumptions such as those required in conventional post-PSD data manipulation to approximate true lineshapes. In the case of sinusoidal modulation, the significance of simultaneous multichannel demodulations at all even or all odd harmonics within the frequency bandwidth is presented. Unconventional PSD schemes using square-wave and sawtooth-wave modulations are also discussed. As an aside for mathematics, the PSD convolution kernel corresponds to a transform between two compact-support functions; it generates an infinite set of basis-function sets, with the set of the Chebyshev polynomials being a special case.

Automatic matching control circuit based on phase-sensitive detection for in vivo CW-EPR spectroscopy

This note describes an automatic matching control (AMC) circuit for use in in vivo continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy. This AMC circuit is based on a phase-sensitive detection scheme, and can automatically stabilize the input impedance of a 1.1 GHz tunable resonator, even if the subject moves during data acquisition. The AMC circuit was designed for measuring small animals such as mice or rats. Noise suppression with the AMC circuit was investigated with a motion phantom and a live mouse. Experimental findings support the notion that the feedback loop in the developed AMC circuit is stable and provides good noise suppression. Moreover, AMC can improve the stability of the baselines in EPR spectra with an animal subject.

A systematic study of ion and cluster ion formation in continuous supersonic planar plasma

A systematic mass spectrometric survey has been performed to study ionic species that are formed in continuous supersonic planar plasma. The plasma is generated by electron impact ionization of gas that is expanded supersonically through a long and narrow slit. A quadrupole mass spectrometer is used to sample the adiabatically cooled plasma downstream for different precursor mixtures. The number of different ions and cluster ions formed in the plasma is remarkably large. Densities are sufficiently high to monitor rovibrational transitions in direct absorption by tunable diode laser spectroscopy, using concentration modulation and a phase sensitive detection scheme.

Phase-sensitive lock-in detection of semiconductor waveguide intensity profiles

A phase-sensitive lock-in detection scheme is employed in the measurement of transverse intensity distributions in semiconductor waveguide structures. A modulated (chopped) sampling beam is scanned across the waveguide, and the photocurrent signal from a 1550-nm signal beam in the waveguide is monitored through phase-sensitive lock-in detection (referenced to the sampling beam chopping frequency). It is determined that the photoinjected free-carrier perturbation by the scanning beam can be successfully mapped onto the local signal intensity, and the transverse intensity profile of the signal beam in the waveguide can be extracted. For the 3-μm-wide waveguide and the 2-μm sampling spot size employed in this investigation, the spatial resolution was limited mainly by the 1.7-μm carrier diffusion length in the semiconductor.

Design of high-performance photodiode receivers for optical tomography

The design of instrumentation hardware for tomographic systems must take careful account of measurement noise. This is especially true in near-infrared absorption tomography, where the signal of interest is typically only a few percent of the total signal at the detector, and the available optical power may have to be shared among many measurement channels. In this paper, the monitoring of photodiodes in near-IR absorption tomography is examined in detail, but much of the material is applicable at wavelengths ranging from the UV to beyond 2.5 /spl mu/m. The authors' application involves the frequency region 50 kHz to 2 MHz, which lies above that utilized in the majority of radiometric sensing systems, yet substantially below telecoms bit rates. The problem is further distinguished by the use of phase-sensitive detection schemes, which make local noise density more relevant than wideband noise performance and relax the requirement for dc precision. Alternative transimpedance circuit configurations, including both single-ended and differential topologies, are analyzed with a view to optimization of the signal-to-noise ratio. Typical values of photodiode capacitance and shunt resistance are shown to result in significant noise gain, greatly increasing the importance of amplifier voltage noise relative to other intrinsic noise sources. It is shown that for applications of this type, viable alternatives to the traditionally dominant FET amplifier do exist. The relative susceptibility to coupled interference is also considered. The results of practical tests, involving class-leading operational amplifiers, are presented to support the analyses. These results also underline the need for careful circuit layout and shielding if the capabilities of these devices are to be fully exploited.

High-resolution cavity enhanced absorption spectroscopy using phase-sensitive detection

High-resolution cavity enhanced absorption spectroscopy (CEAS) using a fast power-modulated laser with a phase-sensitive detection scheme has been adopted. A detailed investigation into the effects of laser chopping frequency and cavity modulation frequency (via a piezoelectric translator) on the sensitivity will be presented. The observed detection sensitivity has been improved by more than an order of magnitude after the implementation of phase sensitive detection. The observed single-scan sensitivity of the present setup is about 4.4 ppm Hz −1/2 in fractional absorption using cavity mirrors with moderate reflectivity of ∼0.9997 and a ringdown cavity cell of 175 cm in length.

In Situ Infrared Spectroscopy of Catalytic Solid−Liquid Interfaces Using Phase-Sensitive Detection: Enantioselective Hydrogenation of a Pyrone over Pd/TiO2

The potential of modulation excitation spectroscopy and phase-sensitive detection in combination with attenuated total reflection (ATR) for in situ infrared spectroscopy of catalytic solid−liquid interfaces is demonstrated. The method is based on the periodic variation of an external parameter such as reactant concentration. The periodically varying signals are subsequently demodulated using a phase-sensitive detection scheme. In this way, the small periodically varying signals are separated from the large static ones, yielding high quality difference spectra. Species, which have different response to the excitation, i.e., species with different kinetics, can easily be separated in the spectra. The method is applied to the enantioselective hydrogenation of 4-methoxy-6-methyl-2-pyrone over a 5 wt % Pd/TiO2 powder catalyst modified by cinchonidine. Upon modulation of the reactant concentration, the ATR spectra exhibit varying signals from dissolved reactant, product as well as from adsorbed species. Part of the signals are associated with carboxylates adsorbed on the TiO2. The kinetics of these species are distinctly different from the one of the primary hydrogenation product. The carboxylates are formed from alcoholysis of the lactone, which is obtained by a second hydrogenation step. The enantiomeric excess was also measured phase sensitive. Its time dependence indicates a negative influence of the carboxylates on enantioselection.