Light Modulation Frequency Research Articles

Human-centred physical neuromorphics with visual brain-computer interfaces

Steady-state visual evoked potentials (SSVEPs) are widely used for brain-computer interfaces (BCIs) as they provide a stable and efficient means to connect the computer to the brain with a simple flickering light. Previous studies focused on low-density frequency division multiplexing techniques, i.e. typically employing one or two light-modulation frequencies during a single flickering light stimulation. Here we show that it is possible to encode information in SSVEPs excited by high-density frequency division multiplexing, involving hundreds of frequencies. We then demonstrate the ability to transmit entire images from the computer to the brain/EEG read-out in relatively short times. High-density frequency multiplexing also allows to implement a photonic neural network utilizing SSVEPs, that is applied to simple classification tasks and exhibits promising scalability properties by connecting multiple brains in series. Our findings open up new possibilities for the field of neural interfaces, holding potential for various applications, including assistive technologies and cognitive enhancements, to further improve human-machine interactions.

Temporal light modulation: A phantom array visibility measure

At temporal light modulation (TLM) frequencies between 80 Hz and 20 000 Hz observers may perceive a series of repeated images called the phantom array effect (PAE) when they move their eyes in large saccades across a modulating light source or across a scene lit by the modulating light source. To date, there is no well-established measure for quantifying PAE visibility, but there is growing awareness of the need for one among design professionals and sensitive populations. This paper documents a new measure, the phantom array visibility measure (PAVM), which is based on the results of recent human factors experiments. The measure follows the mathematical underpinning used by the flicker visibility measure and the stroboscopic visibility measure, where the time-domain TLM waveform is converted into its Fourier frequency components; each component is evaluated through a threshold curve of modulation depth, then summed through an equation employing a Minkowski exponent. This scales the PAVM so that a value of 1 indicates a waveform at a threshold visibility in the conditions of the underlying experiment.

Characterization of an uncooled mid-infrared Schottky photodetector based on a 2D Au/PtSi/p-Si nanohole array at a higher light modulation frequency.

In this study, an uncooled 2D nanohole array PtSi/p-Si Schottky mid-infrared (MIR) photodetector, which is essential for on-chip Si-based low-barrier MIR detectors, is presented. Room temperature operation introduces susceptibility to thermal noise and can impact stability. Through modulation frequency and reverse bias optimization, the stability improved by 7 times at 170Hz and -3.5V, respectively. The effective light detection and stability were confirmed through ON/OFF response measurements over a longer time. The wavelength-dependent responsivity, measured with a tunable MIR laser, confirmed the responsiveness of the device in the MIR region of 2.5µm to 4.0µm, with a maximum specific detectivity (D*) of 2.0×103 c m H z 1/2 W -1 at 3.0µm; this result shows its potential applicability for noninvasive human lipid monitoring. Overall, this study focuses on the crucial role of signal analysis optimization in enhancing the performance of MIR photodetectors at room temperature.

Wavelength and frequency optimization in spatial frequency domain imaging for two-layer tissue

Spatial frequency domain imaging is a non-contact, wide-field, fast-diffusion optical imaging technique, which in principle uses steady-state spatially modulated light to irradiate biological tissue, reconstruct two-dimensional or three-dimensional tissue optical characteristic map through optical transmission model, and further quantify the spatial distribution of tissue physiological parameters by multispectral imaging technique. The selection of light source wavelength and light field spatial modulation frequency is directly related to the accuracy of tissue optical properties and tissue physiological parameters extraction. For improvement of the measurement accuracy of optical properties and physiological parameters in the two-layer tissue, a multispectral spatial frequency domain imaging system is built based on liquid crystal tunable filter, and a data mapping table of spatially resolved diffuse reflectance and optical properties of two-layer tissue is established based on scaling Monte Carlo method. Combined with the dispersion effect and window effect of light-tissue interaction, the study applies numerical simulation to optimize the wavelength in the 650-850 nm range with spectral resolution of 10 nm. In order to minimize the uncertainty of the optical properties, Cramér-Rao bound is used to optimize the optical field spatial modulation frequency by transmitting the uncertainty of optical properties. The results showed that in order to realize the detection of melanin, oxyhemoglobin, deoxyhemoglobin, water and other physiological parameters in two-layer tissue, the best wavelength combination was determined as 720, 730, 760 and 810 nm according to the condition number. The findings of the Cramér-Rao bound analysis reveal that the uncertainty of optical characteristics for the frequency combinations [0, 0.3] mm-1, [0, 0.2] mm-1, and [0, 0.1] mm-1 increases successively. Under the optimal combination of wavelength and frequency, the diffuse reflectance of the gradient gray-scale plate measured by the multi-spectral spatial frequency domain imaging system is linearly correlated with the calibration value. The error between the measured liquid phantom absorption coefficient and the collimation projection system based on colorimetric dish is less than 2%. The experimental results of human brachial artery occlusion indicate that under the optimal wavelength combination, the change of the second layer absorption coefficient captured by the three frequency combinations decreases in turn, so as the change of oxygen saturation.

All-optical Ti3C2Tx modulator based on a sandwich structure.

All-optical modulators based on a MXene-Ti3C2Tx have recently garnered much attention due to their broadband light-matter interactions and ultrafast carrier dynamics. To investigate the modulation characteristics of pump intensity and pump light modulation frequency, we establish an all-optical modulator with a sandwich structure based on MXene-Ti3C2Tx/PVA (polyvinyl alcohol) film. The result shows that this modulator can achieve a high modulation depth of 12.55 dB and a modulation frequency of 50 kHz corresponding to a response time at the microsecond scale. The successful preparation of the modulator is attributed to the saturable absorption characteristics of the MXene-Ti3C2Tx. This modulator has great potential in all-optical communications and ultrafast optical signal processing.

Frequency shifting model for diffuse optical tomography

The Diffuse Optical Tomography (DOT) imaging modality is an interesting research field for researchers since it has uncertainties in the solution space. DOT imaging modality is an unsolved scientific problem. Inverse problem solution and image reconstruction have never been of their best quality. Reconstructed images have low spatial resolution. The scattering nature of low-energized diffusive light is the obscuring effect for DOT modality. DOT has 3 functional sub-branches which are Continuous Wave, Time-Resolved, and Frequency-Domain. In this work, one new approach to Frequency Domain Diffuse Optical Tomography (FDDOT) biomedical optic imaging modality is presented to the readers. Frequency shifting data were added to the forward model problem which has source-detector couplings and several imaging voxels. 100 MHz center core light modulation frequency was selected as modulation frequency. 169 source-detector matches were used on back-reflected imaging geometry. Absorption coefficient µa was selected 0.1 cm−1. Scattering coefficient µs was selected 100 cm−1. 1-µm x, y, z cartesian grid coordinates were used in each direction for imaging tissue-like simulation media. A total of 100 frequency shifts were added to the forward model problem which has 5 Hz frequency step. 2 inclusion objects were embedded inside the imaging simulation phantom to be reconstructed. Their optical absorption coefficient µa = 0.7 cm−1. Two inclusion images were successfully reconstructed with the low contrast to noise ratio (CNR) and position error. The frequency shifting technique is first applied for FDDOT here in this work. This technique increased the total number of equations in the forward model problem; hence it is helping to solve the inverse problem more accurately. In this work, the positive effect of using frequency shifting methodology was observed. Differentiation of 2 embedded inclusions was completed and illustrated in this work. Two different scenarios were compared with each other. In the first scenario, only 100 MHz center core frequency was used for one source-detector match. In the second scenario, 100 additional shifted frequencies over 100 MHz center core frequency were added. It was seen that the second scenario which has more frequency shifts is superior to the general concept FDDOT methodology. This work shows that the frequency shift technique might be used for future FDDOT devices.

Modulated-light induced photocurrent through the liquid-silicon-liquid interfaces

The silicon-based photoelectric conversion devices with P-N junctions are designed to trap and convert photons into electric current. In this work, we report the photocurrent generated at the liquid-silicon-liquid system under high-frequency intensity modulated light illumination. The system exhibits stable and repeatable characteristics with quick response to light, and the photocurrent amplitude dependents on solution concentration, light power and light modulation frequency. Additionally, possible photocurrent generation mechanisms are also discussed that the ionic photocurrent is attributed to modulated light to SPV and electrochemical reaction.

Colloidal HgTe Quantum Dot/Graphene Phototransistor with a Spectral Sensitivity Beyond 3µm.

Infrared light detection enables diverse technologies ranging from night vision to gas analysis. Emerging technologies such as low‐cost cameras for self‐driving cars require highly sensitive, low‐cost photodetector cameras with spectral sensitivities up to wavelengths of 10 µm. For this purpose, colloidal quantum dot (QD) graphene phototransistors offer a viable alternative to traditional technologies owing to inexpensive synthesis and processing of QDs. However, the spectral range of QD/graphene phototransistors is thus far limited to 1.6 µm. Here, HgTe QD/graphene phototransistors with spectral sensitivity up to 3 µm are presented, with specific detectivities of 6 × 108 Jones at a wavelength of 2.5 µm and a temperature of 80 K. Even at kHz light modulation frequencies, specific detectivities exceed 108 Jones making them suitable for fast video imaging. The simple device architecture and QD film patterning in combination with a broad spectral sensitivity manifest an important step toward low‐cost, multi‐color infrared cameras.

Theoretical analysis of injection driven thermal light emitters based on graphene encapsulated by hexagonal boron nitride

We develop the device model for the proposed injection (electrically) driven thermal light emitters (IDLEs) based on the vertical hexagonal boron nitride layer/graphene layer/ hexagonal boron nitride layer (hBNL/GL/hBNL) heterostructures and analyze their dynamic response. The operation of the IDLEs is associated with the light emission of the hot two-dimensional electron-hole plasma (2DEHP) generated in the GL by both the lateral injection from the side contacts and the vertical injection through the hBNL (combined injection) heating the 2DEHP. The temporal variation of the injection current results in the variation of the carrier effective temperature and their density in the GL leading to the modulation of the output light. We determine the mechanisms limiting the IDLE efficiency and the maximum light modulation frequency. A large difference between the carrier and lattice temperatures of the IDLEs with an effective heat removal enables a fairly large modulation depth at the modulation frequencies about dozen of GHz in contrast to the standard incandescent lamps. We compare the IDLEs with the combined injection under consideration and IDLEs using the carrier Joule heating by lateral current. The obtained results can be used for the IDLE optimization.

Light-ellipticity and polarization-angle dependence of magnetic resonances in rubidium vapor using amplitude-modulated light: Theoretical and experimental investigations

We report on experimental and theoretical investigations of the polarization dependence of magnetic resonance generated by synchronous optical pumping. Magnetic resonances with narrow linewidth are generated experimentally using a rubidium vapor cell with octade-cyltrichlorosilane (OTS) antirelaxation coating on inner walls. We studied the effect of light ellipticity on the amplitudes and widths of magnetic resonances by matching the light modulation frequency with 2 {\Omega}_L (alignment) and {\Omega}_L(orientation) in a Bell-Bloom interaction geometry, where {\Omega}_L corresponds to the Larmor frequency. Both 2 {\Omega}_L and \OmegaL resonance amplitudes showed a strong dependence on the light ellipticity. In addition, we showed that the duty cycle of light modulation changes the slope of amplitude variations in 2{\Omega}_L and {\Omega}_L resonances with light ellipticity. As a potential application, we showed that the difference between 2{\Omega}_Land {\Omega}_Lresonance amplitudes can be used for in situ measurement of light ellipticity. We also studied the dependence of 2{\Omega}_L and {\Omega}_L resonance amplitudes on the polarization angle of linearly polarized light. These amplitudes oscillate periodically with the polarization angle. We found this oscillatory behavior to be sensitive to the tilt in magnetic field direction from the polarization plane. Such a property could be used to realize a vector magnetometer. A density matrix based theoretical model is developed to simulate the magnetic resonance spectrum for different light polarizations. Our theoretical model accurately reproduces the above mentioned experimental observations.

ELECTRO-OPTICAL SWITCHING OF THE MAIN OPTICAL AXIS OF A FERROELECTRIC LIQUID CRYSTAL SPIRAL NANOSTRUCTUREIN A PLANAR-ORIENTED DISPLAY CELL

In a known display cell with the nematic liquid crystal (NLC) and interdigital electrodes on one of the glass substrates, the “In-Plane Switching” (IPS) mode is implemented, in which the NLC main optical axis reorients in a plane parallel to substrates, providing the most correct color reproduction at different angles view, up to 178 ° horizontally and vertically. Unfortunately, the creation of interdigital metal electrodes complicates and increases the technological process cost and causes a decrease in image contrast. At the same time, experimental results and calculations based on classical electro-optics of crystals indicate that electrooptical switching in the IPS mode is a natural and intrinsic feature of a conventional (with continuous electrodes) display cell with a planar-oriented layer of the ferroelectric liquid crystal (FLC), in which the effect of the deformed (by the electric field) helix FLC nanostructure is realized (DHF effect). In such a cell, the reorientation of the main optical axis under the influence of a weak electric field also occurs in the substrate plane if the FLC has a small pitch (about 100 nm or less) and a large tilt angle of molecules in the layer (about 38 ° or more). The dependences of the FLC cell light transmittance measured in this work, confirmed the achievement of the IPS electro-optical mode in the DHF FLC cell; moreover, the light modulation frequency was 1 kHz. Thus, while maintaining all the advantages of the IPS mode known in NLC, its implementation in FLC allows additionally obtaining technological advantages and multiple increase in modulation frequency.

Focus-Induced Photoresponse Technique-Based NIR Photodetectors Containing Dimeric Polymethine Dyes

A group of near infrared (NIR)-absorbing dimeric polymethine dyes have been developed and successfully applied in photodetectors based on focus-induced photoresponse technology. The devices are based upon simple and inexpensive solid-state dye-sensitized solar cell architecture. The investigated molecules, designed and synthesized specifically for focus-induced photoresponse application, demonstrate relatively narrow and intense absorption in the NIR region of the spectrum up to 900 nm. Detectors containing the investigated dyes function in the near-infrared region of the spectrum, and suitable photocurrent and signal-to-noise ratio can be obtained at both 730 nm and 850 nm wavelengths. Additionally, these photodetectors can also be used in applications requiring simultaneous tracking of multiple light spots, as devices maintain appropriate characteristics at light modulation frequencies between 1 Hz and 1000 Hz.

Multistacked Detectors with Transparency-Controlled Polymer:Nonfullerene Bulk Heterojunction Sensing Layers for Visible Light Communications.

Here, we report multistacked organic photodiodes (OPDs) for potential light-fidelity (Li-Fi) detectors in visible light communications. The multistacked detectors were fabricated by integrating semitransparent inverted-type OPDs with polymer:nonfullerene bulk heterojunction (BHJ) layers. The thickness of BHJ sensing layers in the inverted-type OPDs was controlled to 30, 60, and 100 nm for top, middle, and bottom cells in the multistacked detectors, respectively, while 30 nm thick metal (silver) electrodes were used for securing both electrical conductivity and optical transparency. Results showed that each OPD cell in the multistacked detectors could deliver exact current signals upon illumination with a modulated white light from inorganic light-emitting diode lamp. The current signal from each OPD cell was linearly decreased as the modulation frequency of white light increased, supporting a stable operation of the multistacked detectors. Finally, the present multistacked detectors were able to exactly detect the programmed Li-Fi signals with a particular digitized information.

A Low-Frequency Photoacoustic Spectrometer with an RGB Light-Emitting Diode for Evaluating Photosynthetic Activity in Plant Leaves

A photoacoustic spectrometer based on a three-colored light-emitting diode (with peak emission wavelengths of 465, 525, and 640 nm) designed to determine the intensity of photosynthesis in different deep layers of plant leaves is described. The physical properties of the photoacoustic signal were studied at different wavelengths and light modulation frequencies. It was shown that the proposed spectrometer can be employed for quantitative evaluation of heat dissipation and photochemical assimilation of the absorbed light energy in this medium.

In situ measurement of light polarization with ellipticity-induced nonlinear magneto-optical rotation

A precise, accurate, and relatively straightforward in-situ method to measure and control the ellipticity of light resonantly interacting with an atomic vapor is described. The technique can be used to minimize vector light shifts. The method involves measurement of ellipticity-induced resonances in the magnetic-field dependence of nonlinear magneto-optical rotation of frequency-modulated light. The light propagation direction is orthogonal to the applied magnetic field $\textbf{B}$ and the major axis of the light polarization ellipse is along $\textbf{B}$. When the light modulation frequency matches the Larmor frequency, elliptically polarized light produces precessing atomic spin orientation transverse to $\textbf{B}$ via synchronous optical pumping. The precessing spin orientation causes optical rotation oscillating at the Larmor frequency by modulating the atomic vapor's circular birefringence. Based on this technique's precision, in-situ nature (which avoids systematic errors arising from optical interfaces), and independent control of the most important systematic errors, it is shown that the accuracy of light ellipticity measurements achievable with this technique can exceed that of existing methods by orders of magnitude.

Fast bistable intensive light scattering in helix-free ferroelectric liquid crystals.

A new type of ferroelectric liquid crystal (FLC) is considered, where the reorientation of the director (main optical axes) at the interaction of an electric field with the FLC's spontaneous polarization is due to the movement of spatially localized waves with a stationary profile: solitons arise at the transition due to the Maxwellian mechanism of energy dissipation. Under certain conditions, the appearance of such waves leads to the formation of a structure of transient domains, and as a consequence, to the scattering of light. The Maxwellian mechanism of energy dissipation allows one to reduce the electric field strength at which the maximum efficiency of light scattering is achieved down to 2-3 V/μm and to increase the frequency of light modulation up to 3-5 kHz. Intensive bistable light scattering in an electro-optical cell filled with a specially designed helix-free FLC was studied, and a stable scattering state can be switched on and off for a few tens of microseconds and memorized for a few tens of seconds.

Hadamard-transform fluorescence-lifetime imaging.

We discuss a Hadamard-transform-based fluorescence-lifetime-imaging (HT-FLI) technique for fluorescence-lifetime-imaging microscopy (FLIM). The HT-FLI uses a Fourier-transform phase-modulation fluorometer (FT-PMF) for fluorescence-lifetime measurements, where the modulation frequency of the excitation light is swept linearly in frequency from zero to a specific maximum during a fixed duration of time. Thereafter, fluorescence lifetimes are derived through Fourier transforms for the fluorescence and reference waveforms. The FT-PMF enables the analysis of multi-component samples simultaneously. HT imaging uses electronic exchange of HT illumination mask patterns, and a high-speed, high-sensitivity photomultiplier, to eliminate frame-rate issues that accompany two-dimensional image detectors.

A New GaAs Metal-Semiconductor-Metal Photodetector Based on Hybrid Plasmonic Structure to Improve the Optical and Electrical Responses

In this article, a new hybrid plasmonic based metal-semiconductor-metal photodetector (MSM-PD) is proposed. A subwavelength slit, the metal nanoscale gratings, and the metal pads which are extended into the absorption layer are used in a basic hybrid plasmonic structure to enhance the absorption coefficient. The finite-difference time-domain (FDTD) method is used to simulate the new structure. The absorption coefficient of the hybrid plasmonic MSM-PD becomes 42 times greater than that of the conventional plasmonic MSM-PD made of only subwavelength slit, which is known as the reference structure. This result is equivalently about 1.5 times greater than that of a recently reported structure. It is also demonstrated that the quantum efficiency of the proposed structure is 10 times more, if compared with the reference one. Moreover, considering the incident light modulation frequency, the frequency response of the hybrid plasmonic MSM-PD is improved, where the cutoff frequency is increased 22 times greater than that of the reference MSM-PD.

Depth Profiles in Maize (Zea mays L.) Seeds Studied by Photoacoustic Spectroscopy

Photoacoustic spectroscopy (PAS) has been used to analyze agricultural seeds and can be applied to the study of seed depth profiles of these complex samples composed of different structures. The sample depth profile can be obtained through the photoacoustic (PA) signal, amplitude, and phase at different light modulation frequencies. The PA signal phase is more sensitive to changes of thermal properties in layered samples than the PA signal amplitude. Hence, the PA signal phase can also be used to characterize layers at different depths. Thus, the objective of the present study was to obtain the optical absorption spectra of maize seeds (Zea mays L.) by means of PAS at different light modulation frequencies (17 Hz, 30 Hz, and 50 Hz) and comparing these spectra with the ones obtained from the phase-resolved method in order to separate the optical absorption spectra of seed pericarp and endosperm. The results suggest the possibility of using the phase-resolved method to obtain optical absorption spectra of different seed structures, at different depths, without damaging the seed. Thus, PAS could be a nondestructive method for characterization of agricultural seeds and thus improve quality control in the food industry.

Thermal Images of Seeds Obtained at Different Depths by Photoacoustic Microscopy (PAM)

The objective of the present study was to obtain thermal images of a broccoli seed (Brassica oleracea) by photoacoustic microscopy, at different modulation frequencies of the incident light beam ((0.5, 1, 5, and 20) Hz). The thermal images obtained in the amplitude of the photoacoustic signal vary with each applied frequency. In the lowest light frequency modulation, there is greater thermal wave penetration in the sample. Likewise, the photoacoustic signal is modified according to the structural characteristics of the sample and the modulation frequency of the incident light. Different structural components could be seen by photothermal techniques, as shown in the present study.