Donut Beam Research Articles

Transverse magnetic supermodes in plasmonic optical fibers excited by radially polarized light

The overlap integrals method, with a fully vectorial formulation, is used to model the selective excitation of the TM01 mode in a few-mode optical fiber with a radially polarized donut beam, and its coupling to guided modes having a plasmonic character (supermodes). The analyses were performed on a waveguide formed as a step-index few-mode optical fiber coated with a thin gold film, at an operating wavelength of 1310 nm. The waveguide was found to support modes having optical fiber, circular metallic waveguide, and surface plasmon characteristics, depending on geometrical and material parameters. Three purely bound transverse magnetic (radially polarized) supermodes were identified: Two symmetric, labeled sTM01 and sTM02 modes, and one asymmetric, labeled ab mode, where symmetry pertains to the transverse electric field distribution over the gold film. The effective mode indices of the supermodes were studied as a function of the thickness of the gold film and its proximity to the fiber core. Considerations for the selective excitation of the sTM01 mode are discussed along with its possible applications. The transmittance of the supermodes is found to be robust even at sharp waveguide transitions. The results predict that effective excitation of TM supermodes with strong plasmonic character, without significant coupling losses, can be achieved by exciting the fiber with a radially polarized donut beam.

Quantifying protein-protein interactions using confined photoconversion and single-particle tracking.

Single particle tracking (SPT) has become a powerful tool to investigate the dynamics of single molecules in live cells. Typical SPT measurement require sparsely labeled molecules in order to identify and localize the individual molecules. This low density limits the rate at which diffusion-limited reactions can be observed. We are developing a novel technique that increases the rate of observable interactions by two orders of magnitude. We call this method SPT-REversible Saturable OpticaL Fluorescence Transitions (SPT-RESOLFT) because it uses a concept from the RESOLFT super-resolution method. The concept relies on saturated labeling and selectively imaging proteins of interest within roughly 100 nm spot. Fluorophores are activated using diffraction-limited Gaussian beam and deactivated outside of 100 nm by a Laguerre-Gauss donut beam of (p, l) = (0, 2) mode which corresponds to topological charge of 2. We experimentally demonstrate the concept of SPT-RESOLFT using EGFR-mEos4b, which form dimers with ∼1 s lifetimes, and with the gamma subunit of FcεRI, gamma-mEos4b, which forms stable dimers.

MINSTED nanoscopy enters the Ångström localization range

Super-resolution techniques have achieved localization precisions in the nanometer regime. Here we report all-optical, room temperature localization of fluorophores with precision in the Ångström range. We built on the concept of MINSTED nanoscopy where precision is increased by encircling the fluorophore with the low-intensity central region of a stimulated emission depletion (STED) donut beam while constantly increasing the absolute donut power. By blue-shifting the STED beam and separating fluorophores by on/off switching, individual fluorophores bound to a DNA strand are localized with σ = 4.7 Å, corresponding to a fraction of the fluorophore size, with only 2,000 detected photons. MINSTED fluorescence nanoscopy with single-digit nanometer resolution is exemplified by imaging nuclear pore complexes and the distribution of nuclear lamin in mammalian cells labeled by transient DNA hybridization. Because our experiments yield a localization precision σ = 2.3 Å, estimated for 10,000 detected photons, we anticipate that MINSTED will open up new areas of application in the study of macromolecular complexes in cells.

Effect of time‐gating on stimulated emission depletion microscopy using sub‐nanosecond pulses

Stimulated emission depletion (STED) microscopy is a super-resolution microscopy technique that overcomes the diffraction limit using a donut-shaped beam switching off fluorescence through stimulated emission. Because the resolution of STED microscopy is directly affected by beam intensity, a light source for the donut beam is very important. Because of their output power, cost, and convenience, sub-nanosecond fiber lasers have become popular. However, their pulses (approximately 500 ps) are somewhat long, which can degrade the resolution. In this study, we used time-gating to improve the resolution of an STED microscope built with a sub-nanosecond fiber laser. Quantitative analysis of single DNA origami bead images revealed that time-gating increased the resolution and signal-to-background ratio by 15% and twofold, respectively. A similar effect was observed in cell imaging. This result suggests that time-gating would be beneficial for STED microscopes using slightly long pulses.

>50 MW peak power, high brightness Nd:YAG/Cr4+:YAG microchip laser with unstable resonator.

We demonstrated a flat-convex unstable cavity Nd:YAG/Cr4+:YAG ceramic air-cooled microchip laser (MCL) generating a record 37.6 and 59.2 MW peak power pulses with an energy of 17.0 and 24.1 mJ and a width of 452 and 407 ps at 20 Hz by using a uniform power square and hexagon pump, respectively. For hexagon pump, the near field hexagon donut beam was changed in to a Bessel-like beam in far field, whose beam quality was estimated as 2nd moment M2 of 7.67. The brightness scale of unstable resonator MCL was achieved up to 88.9 TW/(sr·cm2) in contrast with flat-flat cavity MCL. However, the high intense center part of Bessel-like beam increased its brightness effectively more than 8 times, up to 736 TW/(sr·cm2).

Fast dual-beam alignment method for stimulated emission depletion microscopy using aggregation-induced emission dye resin

A stimulated emission depletion is capable of breaking the diffraction limit by exciting fluorescent molecules with a solid Gaussian beam and quenching the excited molecules with another donut beam through stimulated emission. The coincidence degree of these two beams in three dimensions will significantly influence the spatial resolution of the microscope. However, the conventional alignment approach based on raster scanning of gold nanoparticles by the two laser beams separately suffers from a mismatch between fluorescence and scattering modes. To circumvent the above problems, we demonstrate a fast alignment design by scanning the second beam over the fabricated sample, which is made of aggregation-induced emission (AIE) dye resin. The relative positions of solid and donut laser beams can be represented by the fluorescent AIE from the labeled spots in the dye resin. This design achieves ultra-high resolutions of 22 nm in the x/y relative displacement and 27 nm in the z relative displacement for fast spatial matching of the two laser beams. This study has potential applications in scenarios that require the spatial matching of multiple laser beams, and the field of views of different objectives, for example, in a microscope with high precision.

Influence of laser beam profile on the selective laser melting process of AlSi10Mg

Selective laser melting (SLM) offers great potential to manufacture customized and complex metallic parts. Major drawbacks that limit its industrial application are the high cost of the process that is related to low process speeds and issues with reproducibility. One important process parameter that has the potential to increase the reproducibility and speed of the process is the laser beam intensity profile. Since its influence has not been sufficiently investigated, the goal of this study is to analyze the effect of the beam profile on the SLM process of AlSi10Mg. Single tracks and density cubes are manufactured with different process parameters and two beam profiles (standard Gaussian and Donut beam profiles) and analyzed with respect to appearance, the size of melt tracks, porosity, and the types of defect. The results reveal several advantages of the Donut beam profile such as fewer defects and a significantly broader process window that promises a more robust process.

A mode generator and multiplexer at visible wavelength based on all-fiber mode selective coupler

Abstract Using an all-fiber mode selective coupler (MSC) at the visible band, here we experimentally demonstrate a generating and wavelength multiplexing scheme for the cylindrical vector (CV) and vortex beams (VBs). The proposed MSCs act as efficient mode converters to produce spectrally insensitive high-order modes (HOMs) at the wavelength ranging from 450 to 980 nm, which have broad operation bandwidth (more than 7 nm), high mode conversion efficiency (94%), and purity (98%), and low insert loss (below 0.5 dB). By adjusting the polarization state and the phase shift of linear polarization (LP)11 mode respectively, the donut-shaped CVs and circular-polarization VBs are achieved. The focused intensity distribution of the donut beam on the cross- and axial-sections is monitored by using a confocal system. The all-fiber solution of producing and multiplexing HOMs opens a new route for stimulated emission depletion microscopy applications.

Inherent property of signal from nanoparticle affects measured donut profile in stimulated emission depletion microscopy

Stimulated emission depletion (STED) microscopy is one of the highly explored far field microscopic techniques capable of providing resolution down to few tenth of nanometers even in in situ environments. Optically engineered donut beam is the main element of a STED microscope. To accurately predict the resolution of a STED microscope, donut beam spatial profile needs to be properly measured at the focal plane of high numerical aperture objective. In this work, we measured donut profile by collecting Raman scattering from polymeric nanoparticles, and one photon luminescence (1PL), pure scattering, and up-conversion luminescence from gold nanoparticles. We found that donut profile obtained from Raman scattering, 1PL, and pure scattering can be fitted with a x2 function (normal profile); whereas donut obtained from up-conversion luminescence can be fitted with x4 function (distorted profile). Calculation of STED point spread function for continuous wave excitation using normal donut profile shows full width half maximum width (i.e. resolution) of 60 nm but distorted donut profile, in comparison to excitation width of 250 nm, barely shows any improvement. This study shows that the measured focal plane intensity profile of donut beam is severely affected by inherent property of optical signal collected from nanoparticle.

Synergic Combination of Stimulated Emission Depletion Microscopy with Stimulated Raman Scattering

Stimulated Raman scattering (SRS) is suitable for combination with superresolution microscopy techniques such as Stimulation Depletion Emission (STED) microscopy to explore nanoscale biological processes. Whereas SRS may be employed for label-free imaging of lipids and proteins, STED microscopy allows for super-resolution of small labelled-vesicles such as exosomes, which play a key role in cell-to-cell communication. The combination of these two imaging techniques allows for more comprehensive study of biological samples under investigation. Herein, we implemented STED microscopy onto an existing custom SRS setup. Herein, the same stoke and pump beam used for SRS were employed for excitation and depletion beam on STED microscopy. The pulse widths of the picosecond lasers were independently adjusted for efficient SRS and STED imaging. The point spread function was engineered to alternate between a conventional Gaussian and Laguerre-Gaussian donut beam for sequential SRS and STED imaging, respectively. The similarity of the techniques facilitated their combination.

Accounting for polarization in the calibration of a donut beam axial optical tweezers.

Advances in light shaping techniques are leading to new tools for optical trapping and micromanipulation. For example, optical tweezers made from Laguerre-Gaussian or donut beams display an increased axial trap strength and can impart angular momentum to rotate a specimen. However, the application of donut beam optical tweezers to precision, biophysical measurements remains limited due to a lack of methods for calibrating such devices sufficiently. For instance, one notable complication, not present when trapping with a Gaussian beam, is that the polarization of the trap light can significantly affect the tweezers’ strength as well as the location of the trap. In this article, we show how to precisely calibrate the axial trap strength as a function of height above the coverslip surface while accounting for focal shifts in the trap position arising from radiation pressure, mismatches in the index of refraction, and polarization induced intensity variations. This provides a foundation for implementing a donut beam optical tweezers capable of applying precise axial forces.

Improving emission uniformity and linearizing band dispersion in nanowire arrays using quasi-aperiodicity

We experimentally investigate a new class of quasi-aperiodic structures for improving the emission pattern in nanowire arrays. Efficient normal emission, as well as lasing, can be obtained from III-nitride photonic crystal (PhC) nanowire arrays that utilize slow group velocity modes near the Γ-point in reciprocal space. However, due to symmetry considerations, the emitted far-field pattern of such modes are often ‘donut’-like. Many applications, including lighting for displays or lasers, require a more uniform beam profile in the far-field. Previous work has improved far-field beam uniformity of uncoupled modes by changing the shape of the emitting structure. However, in nanowire systems, the shape of nanowires cannot always be arbitrarily changed due to growth or etch considerations. Here, we investigate breaking symmetry by instead changing the position of emitters. Using a quasi-aperiodic geometry, which changes the emitter position within a photonic crystal supercell (2x2), we are able to linearize the photonic bandstructure near the Γ-point and greatly improve emitted far-field uniformity. We realize the III-nitride nanowires structures using a top-down fabrication procedure that produces nanowires with smooth, vertical sidewalls. Comparison of room-temperature micro-photoluminescence (µ-PL) measurements between periodic and quasi-aperiodic nanowire arrays reveal resonances in each structure, with the simple periodic structure producing a donut beam in the emitted far-field and the quasi-aperiodic structure producing a uniform Gaussian-like beam. We investigate the input pump power vs. output intensity in both systems and observe the simple periodic array exhibiting a non-linear relationship, indicative of lasing. We believe that the quasi-aperiodic approach studied here provides an alternate and promising strategy for shaping the emission pattern of nanoemitter systems.

Improved Axial Optical Trapping

Conventional optical tweezers suffer from several complications when applying axial forces to surface-tethered molecules. Aberrations from the refractive-index mismatch between an oil-immersion objective's medium and the aqueous trapping environment degrade the trapping strength with focal depth. Furthermore, interference effects from back-scattered light make it difficult to use back-focal-plane interferometry for high-bandwidth position detection. Holographic optical tweezers were employed to correct for aberrations to achieve a constant axial stiffness and modulate artifacts from backscattered light. By discarding light from the centre of the trapping beam, interference from back-reflections could be reduced, but not eliminated without significant degradation to the trapping strength. However, careful calibration of the detector sensitivity allows for the application and measurement of axial forces on small surface-tethered molecules. The use of a donut beam to reduce the enhanced photobleaching of fluorophores by the trapping light was also investigated with the goal of combining our holographic axial tweezers with single-molecule fluorescence.

Q-plate enabled spectrally diverse orbital-angular-momentum conversion for stimulated emission depletion microscopy

Spin to orbital angular momentum (OAM) conversion using a device known as a q-plate has gained recent attention as a convenient means of creating OAM beams. We show that the dispersive properties of a q=1/2 plate, specifically its group index difference Δng for ordinary and extraordinary polarization light, can be tuned for achieving single-aperture, alignment-tolerant stimulated emission depletion (STED) nanoscopy with versatile control over the color combinations as well as laser bandwidths. Point spread function measurements reveal the ability to achieve single-aperture STED illumination systems with high throughput (transmission >89%) and purity (donut beam extinction ratios as high as |−18.75| dB, i.e., ∼1% residual light in the dark center of the donut beam) for a variety of color combinations covering the entire visible spectrum, hence addressing several of the fluorescent dyes of interest in STED microscopy. In addition, we demonstrate dual-color STED illumination that would enable multiplexed imaging modalities as well as schemes that could use wide bandwidths up to 19 nm (and hence ultrashort pulses down to ∼50 fs). Switching between any of these color settings only involves changing the bias of the q-plate that does not alter the alignment of the system, hence potentially facilitating alignment-free, spectrally diverse multiplexed nanoscale imaging.

Directed dewetting of amorphous silicon film by a donut-shaped laser pulse

Irradiation of a thin film with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the film into nanoscale structures. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidification, the ultrathin amorphous silicon film is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding film material by isotropic silicon etching. These results suggest that laser-based dewetting of thin films can be an effective way for scalable manufacturing of patterned nanostructures.

Dipole‐like backscatter stimulated Raman scattering for in vivo imaging

Stimulated Raman scattering (SRS) scanning microscopy has the potential to enable label‐free in vivo imaging for research and clinical medicine. Volume SRS from focus occurs in the forward scattered direction. Therefore, multiple scattering events are required to direct the light out of the tissue, reducing imaging depth and resolution. Here, a method called Stokes interference SRS (SISRS) is introduced that operates by the addition to the standard pump and stimulated emission probe beams a third beam called the donut beam. The donut is close in wavelength to the probe beam and, after passage through a π phase plate, forms an annular beam in the focal plane with bright nodes above and below focus. The donut beats with the probe beam, and when they destructively interfere with each other, the microscope's 3‐D stimulated emission focal spot is reduced to subwavelength dimensions. A subwavelength focal volume emits a dipole pattern of SRS with forward and backscatter lobes, enabling high‐resolution single‐backscatter imaging from deep within tissues. The reduction of the focal volume also increases the resolution of the scanning image creating imaging beyond the diffraction limit. SISRS imaging may provide in vivo label‐free Raman images comparable with that achieved in stained in vitro tissues in all planes of section. Copyright © 2014 John Wiley & Sons, Ltd.

Tuning donut profile for spatial resolution in stimulated emission depletion microscopy

In stimulated emission depletion (STED)-based or up-conversion depletion-based super-resolution optical microscopy, the donut-shaped depletion beam profile is of critical importance to its resolution. In this study, we investigate the transformation of the donut-shaped depletion beam focused by a high numerical aperture (NA) microscope objective, and model STED point spread function (PSF) as a function of donut beam profile. We show experimentally that the intensity profile of the dark kernel of the donut can be approximated as a parabolic function, whose slope is determined by the donut beam size before the objective back aperture, or the effective NA. Based on this, we derive the mathematical expression for continuous wave (CW) STED PSF as a function of focal plane donut and excitation beam profiles, as well as dye properties. We find that the effective NA and the residual intensity at the center are critical factors for STED imaging quality and the resolution. The effective NA is critical for STED resolution in that it not only determines the donut shape but also the area the depletion laser power is dispersed. An improperly expanded depletion beam will have negligible improvement in resolution. The polarization of the depletion beam also plays an important role as it affects the residual intensity in the center of the donut. Finally, we construct a CW STED microscope operating at 488 nm excitation and 592 nm depletion with a resolution of 70 nm. Our study provides detailed insight to the property of donut beam, and parameters that are important for the optimal performance of STED microscopes. This paper will provide a useful guide for the construction and future development of STED microscopes.

Multicolor Nanoscopy using a New Class of Low-Power Probes

STED/RESOLFT approaches provide the most promising technology for real-time video nanoscopy. Unfortunately, STED has sometimes been hobbled by two practical problems: large laser powers that (especially in the visible) damage cells and Stokes' shift similarities among dyes that inhibit multicolor imaging. The former occurs because emission must be separated from the powerful STED beam, pushing deactivation into a redder portion of the emission where the cross section is reduced. Our new probes, in contrast, all have very strong cross sections, and they all deactivate at the same NIR wavelength. Hence, we are able to obtain simultaneous superresolved multicolor imaging at the convenient (and less damaging) wavelength of 780nm.We constructed a home-built microscope to test our probes.Our system generates a green excitation pulse with Gaussian profile in space of just 4-8μW and a more powerful “donut” beam at ∼780nm, using several picosecond wide pulses at 80MHz repetition.We achieve resolutions of ∼50nm when coating 20nm aliphatic-amine beads with NHS-ester versions of our dyes. In fixed cells, microtubules coated with a secondary dye-anti mouse antibody bound to a primary mouse antibody specific to α-tubulin demonstrated ∼90nm resolution with only a few mW applied to the donut beam. We also directly labeled microtubules with orange and red versions of our dye; in this case, we achieve ∼100nm resolution with a single depleting beam.These dyes are essentially normal fluorophores modified to contain “quenching antennae” at 780nm. Unlike STED, these antennae are best with many ps to hundreds of ps wide laser pulses; thus, inexpensive ns-pulsed 780nm diodes may be helpful in disseminating the technology. We are currently pursuing alternative antennae and additional dye colors appropriate to in vivo nanoscopy.

Development of a laser-induced plasma probe to measure gas phase plasma signals at high pressures and temperatures

The ability of laser induced breakdown spectroscopy (LIBS) technique for on line simultaneous measurement of elemental concentrations has led to its application in a wide number of processes. The simplicity of the technique allows its application to harsh environments such as present in boilers, furnaces and gasifiers. This paper presents the design of a probe using a custom optic which transforms a round beam into a ring (Donut) beam, which is used for forming a plasma in an atmosphere of nitrogen at high pressure (20bar) and temperature (200°C). The LIBS experiments were performed using a high pressure cell to characterize and test the effectiveness of the donut beam transmitted through the LIBS probe and collect plasma signal in back scatter mode. The first tests used the second harmonic of a Nd:YAG laser, pulse width 7ns, to form a plasma in nitrogen gas at five different pressures (1, 5, 10, 15 and 20bar) and three different gas temperatures (25, 100 and 200°C). The uniqueness of this probe is the custom made optic used for reshaping the round laser beam into a ring (Donut) shaped laser beam, which is fed into the probe and focused to form a plasma at the measurement point. The plasma signal is collected and collimated using the laser focusing lens and is reflected from the laser beam axis onto an achromatic lens by a high reflection mirror mounted in the center section of the donut laser beam. The effect of gas pressure and temperature on N(I) lines in the high pressure cell experiment shows that the line intensity decreases with pressure and increases with temperature. Mean plasma temperature was calculated using the ratios of N(I) line intensities ranging from 7400K to 8900K at 1bar and 2400K to 3200K at 20bar for the three different gas temperatures. The results show that as a proof of principle the donut beam optics in combination with the LIBS probe can be used for performing extensive LIBS measurements in well controlled laboratory environment as well as harsh and demanding environments of practical devices at both high pressures and temperatures.