Search

Optical fiber tapers have been widely proposed and demonstrated as reliable optical fiber structures for sensing, lasers, and supercontinuum generation applications. This paper proposes an innovative approach to fabricating optical fiber tapers using plasma as the heat source. From our literature review, and to the best of our knowledge, this is the first time that plasma has been used as the heat source for producing optical fiber tapers. The system is not intricate and simple to replicate. Moreover, the elements involved make this machine attractive to research groups devoted to optical fibers. The setup consistently generates robust biconical optical fiber tapers. A typical waist of ∼8 μm and taper lengths ranging from 3 to 15 mm are achieved. Our results showed tapers with interference fringes up to 12 dB, from 1465 nm to 1599 nm. Furthermore, the statistical evaluation presented demonstrates a good level of reproducibility in our tapering process., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Author(s).)

Miniature free-space optical beams, originating from on-chip microstructures, are usually measured and quoted without reference to a particular polarization state. We develop an automated platform to characterize tightly focused free-space optical beams in three dimensions. We present a detailed description of each subsystem including the calibration and test procedure. We demonstrate how amplitude and phase are measured at sub-wavelength resolution using a cleaved fiber with a heterodyne reference. Further analysis provides information about the phase and intensity profile of the beam with regards to its polarization content and spatial confinement. We perform a proof-of-concept experiment for a custom waveguide-coupled micro-mirror. The work opens new possibilities for rapid analysis of micro-mirrors in prototyping and optimization of integrated optical systems.

Quasi-two-dimensional (2D) metal halide perovskites (MHPs) are promising photovoltaic (PV) materials because of their impressive optical and optoelectronic properties and improved stability compared to their 3D counterparts. The presence of domains with varying numbers of inorganic layers between the organic spacers ( n -phases), each with different bandgaps, makes the photoinduced carrier dynamics in films of these materials complex and intriguing. Existing interpretations of the ultrafast femto- or picosecond spectroscopy data have been inconsistent, most of them focusing either on exciton/charge transfer from low- n to high- n phases or on hot carrier cooling, but not combined. Here, we present a comprehensive study of the carrier dynamics in the Dion-Jacobson type (PDMA)(MA) ( n -1) Pb n I (3 n +1) (PDMA = 1,4-phenylenedimethylammonium, MA = methylammonium) perovskite, stoichiometrically prepared as ⟨ n ⟩ = 5. Within the film, a coexistence of various n -phases is observed instead of solely the n = 5 phase, resulting in an interesting energy landscape for the motion of excitons and charge carriers. We disentangle hot carrier cooling from exciton transfer between low- n and high- n phases using ultrafast time-resolved photoluminescence and transient absorption spectroscopy. Photophysical modeling by target analysis shows that carrier cooling occurring on a subpicosecond time scale is followed by exciton transfer from low- n into high- n phases in ca. 35 ps when the film is excited by 532 or 490 nm light. Carriers in the high- n phase are much longer lived and decay in a ns time window. Overall, our results provide a comprehensive understanding of the photophysics of this material, which helps to optimize quasi-2D MHP materials for a new generation of PV devices., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

This article provides a careful comparison between the electric and mechanical excitation of a tuning fork for shear force feedback in scanning probe microscopy, an analysis not found in present literature. A setup is designed and demonstrated for robust signal and noise measurements at comparable levels of physical movement of the probe. Two different signal amplification methods, combined with two excitation ways provide three possible configurations. For each method a quantitative analysis, supported by analytical elaboration and numerical simulations, is provided. Finally, it is shown that in practical circumstances electric excitation followed by detection with a transimpedance amplifier provides the best result., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier B.V. All rights reserved.)

Dye-sensitized photoelectrochemical (DSPEC) water splitting is an attractive approach to convert and store solar energy into chemical bonds. However, the solar conversion efficiency of a DSPEC cell is typically low due to a poor performance of the photocathode. Here, we demonstrate that Cu-doping improves the performance of a functionalized NiO-based photocathode significantly. Femtosecond transient absorption experiments show longer-lived photoinduced charge separation for the Cu:NiO-based photocathode relative to the undoped analogue. We present a photophysical model that distinguishes between surface and bulk charge recombination, with the first process (∼10 ps) occurring more than 1 order of magnitude faster than the latter. The longer-lived photoinduced charge separation in the Cu:NiO-based photocathode likely originates from less dominant surface recombination and an increased probability for holes to escape into the bulk and to be transported to the electrical contact of the photocathode. Cu-doping of NiO shows promise to suppress detrimental surface charge recombination and to realize more efficient photocathodes., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Recent studies on hydrogen-generating supramolecular bimetallic photocatalysts indicate a more important role of the peripheral ligands than expected, motivating us to design a Ru/Pt complex with 4,7-diphenyl-1,10-phenanthroline peripheral ligands. Photoinduced intra- and inter-ligand internal conversion processes have been investigated using transient absorption spectroscopy, spanning the femto- to nanosecond timescale. After photoexcitation and ultrafast intersystem crossing, triplet states localised on either the peripheral ligands or on the bridging ligand/catalytic unit are populated in a non-equilibrated way. Time-resolved photoluminescence demonstrates that the lifetime for the Ru/Pt dinuclear species (795±8 ns) is significantly less than that of the mononuclear analogue (1375±20 ns). The photocatalytic studies show modest hydrogen turnover numbers, which is possibly caused by the absence of an excited state equilibrium. Finally, we identify challenges that must be overcome to further develop this class of photocatalysts and propose directions for future research., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The effect of the catalytic moiety on the early-time photodynamics of Ru/M (M=Pt or Pd) bimetallic photocatalysts is studied by ultrafast transient absorption spectroscopy. In comparison to the Ru/Pd photocatalyst described earlier, the Ru/Pt analogue shows complex excited-state dynamics with three distinct kinetic components ranging from sub-ps to 10(2)  ps, requiring a more sophisticated photophysical model than that developed earlier for the Ru/Pd complex. In the Pu/Pt photocatalyst, an additional lower-lying excited state is proposed to quench the hot higher-lying triplet metal-to-ligand charge-transfer states. Furthermore, a strong excitation wavelength dependence on the population of excited states is observed for both the Ru/Pt and Ru/Pd complexes, indicating a non-equilibrated distribution even on the 10(2)  ps timescale. These insights shed light on the significant impact of the catalytic moiety on the fundamental early-time photophysics of Ru-based photocatalysts., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Small metal structures sustaining plasmon resonances in the optical regime are of great interest due to their large scattering cross sections and ability to concentrate light to subwavelength volumes. In this paper, we study the dipolar plasmon resonances of optical antennas with a constant volume and a sinusoidal modulation in width. We experimentally show that by changing the phase of the width-modulation, with a small 10 nm modulation amplitude, the resonance shifts over 160 nm. Using simulations we show how this simple design can create resonance shifts greater than 600 nm. The versatility of this design is further shown by creating asymmetric structures with two different modulation amplitudes, which we experimentally and numerically show to give rise to two resonances. Our results on both the symmetric and asymmetric antennas show the capability to control the localization of the fields outside the antenna, while still maintaining the freedom to change the antenna resonance wavelength. The antenna design we tested combines a large spectral tunability with a small footprint: all the antenna dimensions are factor 7 to 13 smaller than the wavelength, and hold potential as a design element in meta-surfaces for beam shaping.

The contrasting early-time photodynamics of two related Ru/Pt photocatalysts with very different photocatalytic H2 generation capabilities are reported. Ultrafast equilibration (535 ± 17 fs) creates an electron reservoir on the peripheral ligands of the ester substituted complex, allowing a dramatic increase in photocatalytic performance. This insight opens the way towards a novel design strategy for H2 generating molecular photocatalysts.

Surface plasmon polaritons are electromagnetic waves that propagate tightly bound to metal surfaces. The concentration of the electromagnetic field at the surface as well as the short wavelength of surface plasmons enable sensitive detection methods and miniaturization of optics. We present an optical frequency plasmonic analog to the phased antenna array as it is well known in radar technology and radio astronomy. Individual holes in a thick gold film act as dipolar emitters of surface plasmon polaritons whose phase is controlled individually using a digital spatial light modulator. We show experimentally, using a phase sensitive near-field microscope, that this optical system allows accurate directional emission of surface waves. This compact and flexible method allows for dynamically shaping the propagation of plasmons and holds promise for nanophotonic applications employing propagating surface plasmons.

We demonstrate a system for the phase-resolved epi-detection of coherent anti-Stokes Raman scattering (CARS) signals in highly scattering and/or thick samples. With this setup, we measure the complex vibrational responses of multiple components in a thick, highly-scattering pharmaceutical tablet in real time and verify that the epi- and forward-detected information are in very good agreement.

Nature has developed many pathways to produce medicinal products of extraordinary potency and specificity with significantly higher efficiencies than current synthetic methods can achieve. Identification of these mechanisms and their precise locations within plants could substantially increase the yield of a number of natural pharmaceutics. We report label-free imaging of Δ⁹-tetrahydrocannabinolic acid (THCa) in Cannabis sativa L. using coherent anti-Stokes Raman scattering microscopy. In line with previous observations we find high concentrations of THCa in pistillate flowering bodies and relatively low amounts within flowering bracts. Surprisingly, we find differences in the local morphologies of the THCa-containing bodies: organelles within bracts are large, diffuse, and spheroidal, whereas in pistillate flowers they are generally compact, dense, and have heterogeneous structures. We have also identified two distinct vibrational signatures associated with THCa, both in pure crystalline form and within Cannabis plants; at present the exact natures of these spectra remain an open question.

We demonstrate a method for performing nonlinear microspectroscopy that provides an intuitive and unified description of the various signal contributions, and allows the direct extraction of the vibrational response. Three optical fields create a pair of Stokes Raman pathways that interfere in the same vibrational state. Frequency modulating one of the fields leads to amplitude modulations on all of the fields. This vibrational molecular interferometry technique allows imaging at high speed free of nonresonant background, and is able to distinguish between electronic and vibrational contributions to the total signal., (© 2011 American Physical Society)

This article demonstrates a quick and easy way of quantifying the performance of a vibration-isolated platform. We measure the vibration transfer from floor to table using background noise excitation from the floor. As no excitation device is needed, our setup only requires two identical sensors (in our case, low noise accelerometers), a data acquisition system, and processing software. Background noise excitation from the floor has the additional advantage that any non-linearity in the suspension system relevant to the actual vibration amplitudes will be taken into account. Measurement time is typically a few minutes, depending on the amount of background noise. The (coherent) transfer of the vibrations in the floor to the platform, as well as the (non-coherent) acoustical noise pick-up by the platform are measured. Since we use calibrated sensors, the absolute value of the vibration levels is established and can be expressed in vibration criterion curves. Transfer measurements are shown and discussed for two pneumatic isolated optical tables, a spring suspension system, and a simple foam suspension system., (© 2011 American Institute of Physics)

In coherent anti-Stokes Raman scattering (CARS), the emitted signal carries both amplitude and phase information of the molecules in the focal volume. Most CARS experiments ignore the phase component, but its detection allows for two advantages over intensity-only CARS. First, the pure resonant response can be determined, and the nonresonant background rejected, by extracting the imaginary component of the complex response, enhancing the sensitivity of CARS measurements. Second, selectivity is increased via determination of the phase and amplitude, allowing separation of individual molecular components of a sample even when their vibrational bands overlap. Here, using vibrational phase contrast CARS (VPC-CARS), we demonstrate enhanced sensitivity in quantitative measurements of ethanol/methanol mixtures and increased selectivity in a heterogeneous mixture of plastics and water. This powerful technique opens a wide range of possibilities for studies of complicated systems where overlapping resonances limit standard methodologies.

In biological samples the resonant coherent anti-Stokes Raman scattering signal of less abundant constituents can be overwhelmed by the nonresonant background, preventing detection of those molecules. We demonstrate a method to obtain the phase of the oscillators in the focal volume that allows discrimination of those hidden molecules. The phase is measured with respect to the local excitation fields using a cascaded phase-preserving chain. It is measured point-by-point and takes into account refractive index changes in the sample, phase curvature over the field-of-view, and interferometric instabilities. The detection of the phase of the vibrational motion can be regarded as a vibrational extension of the linear (refractive index) phase contrast microscopy introduced by Zernike around 1933.

We present a novel atomic force microscope (AFM) system, operational in liquid at variable gravity, dedicated to image cell shape changes of cells in vitro under hypergravity conditions. The hypergravity AFM is realized by mounting a stand-alone AFM into a large-diameter centrifuge. The balance between mechanical forces, both intra- and extracellular, determines both cell shape and integrity. Gravity seems to be an insignificant force at the level of a single cell, in contrast to the effect of gravity on a complete (multicellular) organism, where for instance bones and muscles are highly unloaded under near weightless (microgravity) conditions. However, past space flights and ground based cell biological studies, under both hypogravity and hypergravity conditions have shown changes in cell behaviour (signal transduction), cell architecture (cytoskeleton) and proliferation. Thus the role of direct or indirect gravity effects at the level of cells has remained unclear. Here we aim to address the role of gravity on cell shape. We concentrate on the validation of the novel AFM for use under hypergravity conditions. We find indications that a single cell exposed to 2 to 3 x g reduces some 30-50% in average height, as monitored with AFM. Indeed, in situ measurements of the effects of changing gravitational load on cell shape are well feasible by means of AFM in liquid. The combination provides a promising technique to measure, online, the temporal characteristics of the cellular mechano-response during exposure to inertial forces.

In this article we show that heterodyne CARS, based on a controlled and stable phase-preserving chain, can be used to measure amplitude and phase information of molecular vibration modes. The technique is validated by a comparison of the imaginary part of the heterodyne CARS spectrum to the spontaneous Raman spectrum of polyethylene. The detection of the phase allows for rejection of the non-resonant background from the data. The resulting improvement of the signal to noise ratio is shown by measurements on a sample containing lipid.

By spectral phase shaping of both the pump and probe pulses in coherent anti-Stokes Raman scattering (CARS) spectroscopy we demonstrate the extraction of the frequencies, bandwidths and relative cross sections of vibrational lines. We employ a tunable broadband Ti:Sapphire laser synchronized to a ps-Nd:YVO mode locked laser. A high resolution spectral phase shaper allows for spectroscopy with a precision better than 1 cm(-1) in the high frequency region around 3000 cm(-1). We also demonstrate how new spectral phase shaping strategies can amplify the resonant features of isolated vibrations to such an extent that spectroscopy and microscopy can be done at high resolution, on the integrated spectral response without the need for a spectrograph.

We report the near-field observation of the phase shifts associated with total internal reflection on a glass-air interface and surface plasmon resonance on a glass-gold-air system. The phase of the evanescent waves on glass and gold surfaces, as a function of incident angle, is measured using a phase-sensitive Photon Scanning Tunneling Microscope (PSTM) and shows a good agreement with theory.

We describe the realization of a phase-sensitive and ultrafast near-field microscope, optimized for investigation of surface plasmon polariton propagation. The apparatus consists of a homebuilt near-field microscope that is incorporated in Mach-Zehnder-type interferometer which enables heterodyne detection. We show that this microscope is able to measure dynamical properties of both photonic and plasmonic systems with phase sensitivity.

We demonstrate heterodyne detection of CARS signals using a cascaded phase-preserving chain to generate the CARS input wavelengths and a coherent local oscillator. The heterodyne amplification by the local oscillator reveals a window for shot noise limited detection before the signal-to-noise is limited by amplitude fluctuations. We demonstrate an improvement in sensitivity by more than 3 orders of magnitude for detection using a photodiode. This will enable CARS microscopy to reveal concentrations below the current mMolar range.

DC-SIGN, a C-type lectin exclusively expressed on dendritic cells (DCs), plays an important role in pathogen recognition by binding with high affinity to a large variety of microorganisms. Recent experimental evidence points to a direct relation between the function of DC-SIGN as a viral receptor and its spatial arrangement on the plasma membrane. We have investigated the nanoscale organization of fluorescently labeled DC-SIGN on intact isolated DCs by means of near-field scanning optical microscopy (NSOM) combined with single-molecule detection. Fluorescence spots of different intensity and size have been directly visualized by optical means with a spatial resolution of less than 100 nm. Intensity- and size-distribution histograms of the DC-SIGN fluorescent spots confirm that approximately 80 % of the receptors are organized in nanosized domains randomly distributed on the cell membrane. Intensity-size correlation analysis revealed remarkable heterogeneity in the molecular packing density of the domains. Furthermore, we have mapped the intermolecular organization within a dense cluster by means of sequential NSOM imaging combined with discrete single-molecule photobleaching. In this way we have determined the spatial coordinates of 13 different individual dyes, with a localization accuracy of 6 nm. Our experimental observations are all consistent with an arrangement of DC-SIGN designed to maximize its chances of binding to a wide range of microorganisms. Our data also illustrate the potential of NSOM as an ultrasensitive, high-resolution technique to probe nanometer-scale organization of molecules on the cell membrane.

We have studied the dispersion of ultrafast pulses in a photonic crystal waveguide as a function of optical frequency, in both experiment and theory. With phase-sensitive and time-resolved near-field microscopy, the light was probed inside the waveguide in a non-invasive manner. The effect of dispersion on the shape of the pulses was determined. As the optical frequency decreased, the group velocity decreased. Simultaneously, the measured pulses were broadened during propagation, due to an increase in group velocity dispersion. On top of that, the pulses exhibited a strong asymmetric distortion as the propagation distance increased. The asymmetry increased as the group velocity decreased. The asymmetry of the pulses is caused by a strong increase of higher order dispersion. As the group velocity was reduced to 0.116(9) .c, we found group velocity dispersion of -1.1(3) .10(6) ps(2)/km and third order dispersion of up to 1.1(4) .10(5) ps(3)/km. We have modelled our interferometric measurements and included the full dispersion of the photonic crystal waveguide. Our mathematical model and the experimental findings showed a good correspondence. Our findings show that if the most commonly used slow light regime in photonic crystals is to be exploited, great care has to be taken about higher-order dispersion.

We report on the concept, generation, and first observations of focused surface plasmons on shaped gratings. The grating patterns are designed to realize focusing and directing through noncollinear phasematching. The plasmons are generated on patterned gold surfaces, and the plasmon propagation is observed using phase-sensitive photon scanning tunneling microscopy (PSTM) to extract the propagation pattern, direction, and wavelength.

The eigenfield distribution and the band structure of a photonic crystal waveguide have been measured with a phase-sensitive near-field scanning optical microscope. Bloch modes, which consist of more than one spatial frequency, are visualized in the waveguide. In the band structure, multiple Brillouin zones due to zone folding are observed, in which positive and negative dispersion is seen. The negative slopes are shown to correspond to a negative phase velocity but a positive group velocity. The lateral mode profile for modes separated by one reciprocal lattice vector is found to be different.

We show the real-space observation of fast and slow pulses propagating inside a photonic crystal waveguide by time-resolved near-field scanning optical microscopy. Local phase and group velocities of modes are measured. For a specific optical frequency we observe a localized pattern associated with a flat band in the dispersion diagram. During at least 3 ps, movement of this field is hardly discernible: its group velocity would be at most c/1000. The huge trapping times without the use of a cavity reveal new perspectives for dispersion and time control within photonic crystals.

The amplitude and phase evolution of ultrashort pulses in a bimodal waveguide structure has been studied with a time-resolved photon scanning tunneling microscope (PSTM). When waveguide modes overlap in time intriguing phase patterns are observed. Phase singularities, arising from interference between different modes, are normally expected at equidistant intervals determined by the difference in effective index for the two modes. However, in the pulsed experiments the distance between individual singularities is found to change not only within one measurement frame, but even depends strongly on the reference time. To understand this observation it is necessary to take into account that the actual pulses generating the interference signal change shape upon propagation through a dispersive medium. This implies that the spatial distribution of phase singularities contains direct information on local dispersion characteristics. At the same time also the mode profiles, wave vectors, pulse lengths, and group velocities of all excited modes in the waveguide are directly measured. The combination of these parameters with an analytical model for the time-resolved PSTM measurements shows that the unique spatial phase information indeed gives a direct measure for the group velocity dispersion of individual modes. As a result interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping become accessible in a local measurement.

A noninvasive pulse-tracking technique has been exploited to observe the time-resolved motion of an ultrashort light pulse within an integrated optical microresonator. We follow a pulse as it completes several round trips in the resonator, directly mapping the resonator modes in space and time. Our time-dependent and phase-sensitive measurement provides direct access to the angular group and phase velocity of the modes in the resonator. From the measurement the coupling constants between the access waveguides and the resonator are retrieved while at the same time the loss mechanisms throughout the structure are directly visualized.

We report on the direct visualization of a femtosecond pulse propagating through a dispersive waveguide at a telecom wavelength. The position of a propagating pulse is pinpointed at a particular point in space and time using a scanning probe based measurement. The actual propagation of the pulse is visualized by changing the reference time. Our phase-sensitive and time-resolved measurement provides local information on all properties of the light pulse as it propagates, in particular its phase and group velocity. Here, we show that the group velocity dispersion can be retrieved from our measurement by developing an analytical model for the measurements performed with a time-resolved photon scanning tunneling microscope. As a result, interesting and useful effects, such as pulse compression, pulse spreading, and pulse reshaping, become accessible in the local measurement.

We show that the propagation of a femtosecond laser pulse inside a photonic structure can be directly visualized and tracked as it propagates using a time-resolved photon scanning tunneling microscope. From the time-dependent and phase-sensitive measurements, both the group velocity and the phase velocity are unambiguously and simultaneously determined. It is expected that this technique will find applications in the investigation of the local dynamic behavior of photonic crystals and integrated optical circuits.

For the first time the local optical phase evolution in and around a small, one-dimensional photonic crystal has been visualized with a heterodyne interferometric photon scanning tunnelling microscope. The measurements show an exponential decay of the optical intensity inside the crystal, which consists of a periodic array of subwavelength air rods fabricated in a conventional ridge waveguide. In addition it is found that the introduction of the air rods has a counterintuitive effect on the phase development inside the structure. The heterodyne detection scheme allows the detection of low-intensity scattered waves. In the vicinity of the scattering air rods phase singularities are found with a topological charge of plus or minus one.

The phase evolution of light in an optical waveguide structure has for the first time been visualized with subwavelength resolution using a novel heterodyne interferometric photon scanning tunneling microscope. Phase singularities in the optical field of the waveguide have been observed. The phase singularities of charge one appear at locations where the modal field amplitude vanishes, due to the interference of various modes in the waveguide. Excellent agreement of the data with calculations has been obtained.

The simultaneous detection of TE- as well as TM-polarized light with a photon scanning tunneling microscope leads to a quasi-interference pattern of these mutually perpendicular polarized fields. This interference pattern has been observed in the optical field distribution as a function of both position and wavelength. Comparison of experimental data with simulations confirms the interference of mutually orthogonal fields. This quasi interference is caused by conversion of the linearly polarized light of both modes into elliptically polarized light by a fiber probe.

Whispering gallery modes in cylindrical integrated optics microcavities have, for what is to our knowledge the first time, been mapped with a photon scanning tunneling microscope. Optical images were obtained with a spatial resolution of 50 nm. By combination of information on the spatial optical distributions with wavelength-dependent measurements, an unexpectedly rich variety of intracavity phenomena, such as polarization conversion and interference of copropagating and counterpropagating modes, could be directly observed. A quantitative comparison of the experimental data with computer simulations results in a comprehensive understanding of the various whispering gallery modes inside the microcavity.