Name: spectral and angle-resolved magneto-optical characterization of photonic nanostructures
Uploaded: 2019-11-21T12:30:00.000+00:00
Duration: 8 min 1 s
Description: Watch this Scientific Journal Video about Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures at JoVE.com

We acknowledge financial support by the Spanish Ministerio de Econom&#237;a y Competitividad through projects MAT2017-85232-R (AEI/FEDER,UE), Severo, Ochoa (SEV-2015-0496) and by the Generalitat de Catalunya (2017, SGR 1377), by CNPq &#8211; Brazil, and by the European Comission (Marie Sk&#322;odowska-Curie IF EMPHASIS - DLV-748429).

1. Mounting the setup
<ol>
	<li>Optics 
	NOTE: Build the setup as depicted in Figure 3A on an optical table with sufficient vibration isolation. To avoid spherical and other aberrations, center all the optical components (lenses, pinholes etc.) with respect to the beam. The optical arrangement is shown in Figure 2 with the distances between components indicated.
	<ol>
		<li>Guide the light from the white light source to a monochromator to obtain a monochromatic light beam. See the Table of Materials for details of the setup used in this work. Set the monochromator to a wavelength that has a good intensity and visibility, e.g., 550 nm. A wavelength from the visible part of the spectrum makes it easier to position the optical elements.</li>
		<li>Using a coupling lens, couple the light to a fiber and collimate it with an objective at the fiber termination. Depending on the light source used, this step may be omitted.</li>
		<li>Place a polarizer 250 mm from the collimating lens to linearly polarize the beam and place a beam splitter 100 mm from the polarizer to guide the light to the microscope objective lens. 
		NOTE: Due to the collimated beam, the indicated positions of the above-mentioned components do not affect the optics of the measurement setup and are given just for guidance.</li>
		<li>Place the sample on the sample holder equipped with a x-y-z translation stage and a rotation stage that enables a 360-degree sample rotation around the z-axis, i.e., the axis of light impinging on the sample.</li>
		<li>Mount the objective lens on a translation stage that enables movement in three directions. The most crucial of these is the z-axis that is needed for focusing onto the sample. 
		NOTE: The required equipment for sample translation depends on the samples used. Large, homogenous samples may be positioned manually while samples with small useful area will require more careful positioning, especially when using a pinhole to limit the imaged area (step 1.1.7.). The optics of the beam emerging from the sample is schematically depicted in Figure 2. The infinity corrected objective lens directs wave fronts emerging from each point of the sample into collinear beams.</li>
		<li>Place a collector lens with f = 200 mm (tube lens), 330 mm from the objective to re-focus the beams to form an image at the image plane. Due to the collinear propagation of the light emanating from the sample, the collector lens can be placed at any distance from the objective lens. 
		NOTE: As before, the light emerging from the objective lens is collimated. However, the tube lens should be placed after the beam splitter.</li>
		<li>Place a pinhole into the image plane at 200 mm from the collector lens to limit the imaged region to the patterned area. Place the pinhole at the center of the beam. If using a pinhole, use the real space image of the sample to position it. For samples where the patterned area is larger than the area illuminated by the light beam, this is not necessary.</li>
		<li>Place a second lens with f = 75 mm (Bertrand lens), 120 mm after the image plane to create a Fourier transform of the angular components of the image. The transform is created at the focus of the second lens and imaged with a scientific sCMOS camera which is placed 75 mm from the Bertrand lens.</li>
		<li>For LMOKE measurements only, insert an additional polarizer with an angle with respect to the first polarizer between the beam splitter and the collector lens.</li>
	</ol>
	</li>
	<li>Magnet
	<ol>
		<li>Connect the magnet to a power supply and mount it so that the magnetic field can be applied on the sample. Choose whether the magnetic field is applied in the longitudinal, transversal or polar direction (Figure 1).</li>
	</ol>
	</li>
	<li>Sample preparation
	<ol>
		<li>Mechanically dismantle a commercial DVD disk; subsequently the exposed grating surface can be easily identified due its diffractive proprieties. Use an adhesive tape to peel the previous coatings. Clean the surface, soak it in ethanol for 10 min. The grating is now ready to receive a magneto-plasmonic coating. 
		NOTE: Different commercial optical disks as Blu-ray and CDs, may need a different preparation protocol.</li>
		<li>Deposit the metal film on the exposed grating by electron-beam evaporation. To ensure low roughness, use evaporation rates smaller than 5 &#197;/s.</li>
		<li>Starting with a 4 nm Cr adhesive layer, deposit alternating gold and cobalt layers, finishing with a gold capping layer to ensure protection from oxidation. 
		NOTE: We used the following number of layers and thicknesses: Cr (4&#8239;nm)/Au (16&#8239;nm)/[Co (14&#8239;nm)/Au (16&#8239;nm)]&#8239;&#215;&#8239;4/Co (14&#8239;nm)/Au (7&#8239;nm).</li>
		<li>Perform optical or electron microscopy (Figure 4A) to verify the sample surface conditions, in case of homogeneity and low defects proceed with the measurement.</li>
	</ol>
	</li>
</ol>
2. Measurement procedure
<ol>
	<li>Sample positioning 
	NOTE: As an illustrative sample, we will measure a DVD grating covered with magnetoplasmonic Au/Co/Au film. Due to the periodic corrugation of the grating, SPPs can be excited at certain angles of incidence based on the wavefront wavelength.
	<ol>
		<li>Mount the sample on the sample holder using a small drop of silver paint. Let the silver paint dry for 10 min.</li>
		<li>Insert a flip mirror after the image plane to enable real space imaging of the sample. Insert a lens L1 with f = 125 mm so that the image plane is in focus and place L2 with f = 250 mm at 135 mm distance from L1.</li>
		<li>Finally place a charge-coupled device (CCD) camera 210 mm from L2 to capture a magnified image of the image plane. Move lenses L1 and L2 until the pinhole placed in image plane in good focus on the CCD camera.</li>
		<li>Move the objective lens towards the sample until the sample is in good focus in the CCD camera.</li>
	</ol>
	</li>
	<li>Optical reflectivity measurement
	<ol>
		<li>Using the real space image of the sample, position the light spot over a reflective (unpatterned) part of the sample. Flip the flip mirror to see the BFP of the microscope. 
		NOTE: Here, for the DVD-grating we use the continuous metallic film at edge of the DVD disk.</li>
		<li>Select the area of the back focal plane that corresponds to the desired polarization state. The relationship between polarization and position in the back focal plane is shown in Figure 3B. Select an area of interest (AOI) as a rectangular cross section of the objective back focal plane (blue rectangle in Figure 3C) along the axis that corresponds to TM-polarization. 
		NOTE: In the instrumentation software used in this manuscript, this is achieved by selecting the AOI using the cursor selectors. The software then averages the intensities along the short dimension of the rectangle and treats the resulting spectrum as a 1D array of data where each data point corresponds to a different emission angle of the sample. In plasmonic gratings, only TM-polarized light, i.e., EM radiation with electric field perpendicular to the grating grooves, can excite the plasmon resonances. Thus, depending on the grating orientation, it is necessary to select the correct polarization state by either choosing a vertical or horizontal slice of the BFP.</li>
		<li>Measure the spectrum of the light source by clicking Measure normalization spectrum, which will be used later to normalize the measured reflectivity data. As each wavelength yields a 1D set of data points, the full spectrum of the light source is saved as a 2D tensor where each data point represents a combination of wavelength and angle.</li>
		<li>Using again the real space image of the sample, position the light source over the photonic crystal of interest. When switching back to BFP, ensure that the plasmon modes are visible as dark lines crossing the back focal plane. The lines move as the wavelength of the incident light is modified.</li>
		<li>Using the same AOI and measurement settings (i.e., exposure times, number of averages), measure the reflection spectrum of the photonic crystal by clicking Measure reflection spectrum.</li>
		<li>To account for the spectral variation in light source intensity, normalize the obtained spectrum by the spectrum of the light source. This will yield a 2D array of numbers from 0 to 1 where 1 corresponds to fully reflective and 0 to fully absorptive conditions.</li>
	</ol>
	</li>
	<li>Magneto-optical measurement
	<ol>
		<li>Start the magneto-optical measurement by measuring a hysteresis loop using an angle and wavelength that are known to correspond to a good magneto-optical response, usually these conditions can be found close to the SPP excitations. To do that, choose a small AOI near the SPP excitations and measure a single loop. 
		NOTE: The data analysis needed to quantify the magneto-optical activity depends on the type of magnetism that the sample exhibits. Here, we assume a ferromagnetic response and treat the results accordingly. Dia- or paramagnetic response is essentially linear to applied magnetic field and can be quantified as change in optical properties per applied magnetic field unit. Ferromagnetic materials exhibit a non-linear permittivity that requires additional consideration when defining the magneto-optical response (See Figure 3D). The TMOKE is defined as change in reflected intensity as the function of applied magnetic field, i.e., <img alt="Equation 2" src="/files/ftp_upload/60094/60094equ04.jpg" />, where I(M) is the intensity reflected by the sample at magnetization state M.</li>
		<li>Using the hysteresis loop measured in 2.3.1., choose the range of magnetic fields to loop. For ferromagnetic samples, loop the fields from a fully saturated state to an oppositely saturated state, extending the range comfortably over the saturation field. Later, use the points measured in the saturated state to analyze and remove any dia- or paramagnetic contributions that can be verified by their linear contribution.</li>
		<li>Finally, measure the intensity reflected by the sample at each magnetic field point defined, repeating over multiple loops if desired. Each wavelength and magnetization point yield a single 1D array of numerical data (i.e., measured light intensity) where each point of the array corresponds to a particular angle.</li>
	</ol>
	</li>
</ol>
3. Data analysis
<ol>
	<li>Using the hysteresis loop of the sample measured at step 2.3.1, assign each frame measured at step 2.3.3. to either of the saturated states or to the intermediate state (Figure 3C).</li>
	<li>Discard the intermediate frames and calculate the magneto-optical activity from the measured intensities by <img alt="Equation 2" src="/files/ftp_upload/60094/60094equ03v2.jpg" />, where the operations are carried out separately for each angular and wavelength data point. 
	NOTE: As TMOKE is expressed as a relative intensity change, the results do not have to be normalized to the lamp spectrum.</li>
	<li>If the sample presents large paramagnetic (or more rarely, diamagnetic) activity that needs to be subtracted for a reliable comparison between the saturated magnetic states, subtract the linear contribution arising from para- or diamagnetic activity by fitting a line (again, pixelwise separately for each angle and wavelength point) on the points measured at saturation and remove the linear contribution.</li>
</ol>

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The authors have nothing to disclose.

We have introduced a measurement setup and protocol to obtain angular resolved magneto-optical spectra of optical crystals. In particular, the case of ferromagnetic materials, that requires additional data analysis to account for the nonlinear permeability of the material, has been laid out. Angular resolved magneto-optical spectroscopy presents an additional advantage over non-angular resolved methods that the confined modes can be more readily identified as they appear as clearly defined bands in both optical and magne...

Confined electromagnetic modes in photonic crystals can arise from a variety of different origins, such as plasmon resonances around metal/dielectric interfaces or Mie resonances in high refractive index dielectric nanostructures1,2,3, and can be designed to appear at specifically defined frequencies4,5. Their presence gives rise to many fascinating phenomena such as photonic band gaps6,7,8, strong photon localization9, slow light10 and Dirac cones11. Fourier plane microscopy and spectroscopy are basic tools for characterization of photonic nanostructures as they enable capturing many essential properties of confined modes occurring in them. In Fourier space microscopy, as opposed to conventional real plane imaging, the information is presented as the function of angular coordinates12,13. It is alternatively known as back focal plane (BFP) imaging as the angular decomposition of the light emanating from the sample is recorded from the back focal plane of the microscope objective. The angular spectrum, i.e., the far field emission pattern of the sample is related to the momentum of light emanating from it (&#295;k). In particular, it represents its in-plane momentum (kx,ky) distribution14.
In magneto-optically active samples, the presence of confined photonic excitations has been shown to result in considerable enhancement of the magneto-optical response15,16,17,18,19. Magneto-optical effects depend on the mutual geometry of the magnetic field and the incident electromagnetic radiation. Most commonly encountered magneto-optical geometries for linearly polarized light and their nomenclature are depicted in Figure 1. Here, we demonstrate a setup that can be used to explore two magneto-optical effects that are observed in reflection: transverse and longitudinal magneto-optical Kerr effects, abbreviated, respectively, as TMOKE and LMOKE. TMOKE is an intensity effect, where the reflectivities of the opposing magnetization states are different while LMOKE manifests as a rotation of the reflected light polarization axis. The effects are distinguished by the orientation of the magnetization with respect to the light incidence, where for LMOKE, the magnetization is oriented parallel to the in plane component of the wave vector of the light while for TMOKE it is transverse to it. For normally incident light, both in-plane components of the momentum of light are null (kx = ky = 0) and, consequently, both effects are zero. Configurations where both effects are present can be easily conceived. However, to simplify the data analysis, in this demonstration we limit ourselves to situations where only one of the effects is present, namely TMOKE.
Several optical configurations can be used to measure the angular distribution of light emitted from magnetophotonic crystals. For example, in Kalish et al.20 and Borovkova et al.21, such a setup was successfully used in transmission geometry to unveil plasmon influence on magneto-optical phenomena. As an illustration, in Kurvits et al.22, some possible configurations are presented for a microscope that uses an infinity corrected objective lens. In our configuration, depicted in Figure 2A, we use an infinity corrected lens where the light coming from a given point in the sample is directed by the objective lens into collinear beams. In Figure 2A, beams emerging from the top (dashed lines) and the bottom (solid lines) of the sample are schematically depicted. Then, a collecting lens is used to refocus these beams to form an image at the image plane (IP). A second lens, also known as Bertrand lens, is then placed after the image plane to separate the incoming light at its focal plane into angular components, depicted in Figure 2A in red, blue and black. From this back focal plane, the angular distribution of the light emitted by the sample can be measured with a camera. Effectively, the Bertrand lens performs a Fourier transform on the light beam arriving at it. The spatial intensity distribution at the BFP corresponds to the angular distribution of the incident radiation. A full reciprocal space reflectance map of the sample can be established by illuminating the sample with the same objective that is used to collect the response of the sample. The incoming and out going beams are separated using a beam splitter. The complete setup is depicted in Figure 3A. To obtain a spectrum, a tunable light source or a monochromator is needed. The measurement can then be repeated over different wavelengths, keeping in mind that due to the spectrum of standard light sources, the results need to be normalized to the reflectivity of a control sample. For this purpose, one can use a mirror or a part of the sample that has been purposefully left unpatterned to allow for a high reflectivity. To assist in positioning, we show how to integrate the setup with an additional optical system that enables real-space imaging of the sample, shown in Figure 2B.
We now proceed to establish a method for measuring the angular resolved magneto-optical spectrum of a photonic crystal, using as a representative sample, a DVD grating covered with an Au/Co/Au film where the presence of ferromagnetic cobalt gives rise to considerable magneto-optical activity23. The periodic corrugation of the DVD grating enables surface plasmon polariton (SPP) resonances at distinct wavelength-angle combinations that are given by 
<img alt="Equation 1" src="/files/ftp_upload/60094/60094equ01.jpg" /> 
where n is the refractive index of the surrounding environment, k0 the wave vector of light in free space, &#952;0 the incidence angle, d the periodicity of the grating and m is an integer denoting the order of the SPP. The SPP wave vector is given by <img alt="Equation 2" src="/files/ftp_upload/60094/60094equ02.jpg" /> where &#949;1 and &#949;2 are the permittivities of the metallic layer and the surrounding dielectric environment. Due to the thickness of the gold/cobalt multilayer film, we can assume that SPPs are only excited on top of the multilayer film.

<table><tbody><tr><td>Beam splitter</td><td>Thorlabs</td><td>BSW27</td><td></td></tr><tr><td>Bertrand lens</td><td>Thorlabs</td><td>LA1608</td><td>f = 75 mm</td></tr><tr><td>CCD Camera</td><td>Thorlabs</td><td>1500M-GE-TE</td><td>Camera for real space imaging</td></tr><tr><td>Collecting lens</td><td>Thorlabs</td><td>ITL200</td><td>f = 200 mm</td></tr><tr><td>Collimating lens</td><td>Zeiss</td><td>420640-9800</td><td>Magnification 10x NA 0.3</td></tr><tr><td>Flip mirror</td><td>Thorlabs</td><td>CCM1-P01/M</td><td></td></tr><tr><td>Flip mirror mount</td><td>Thorlabs</td><td>FM90/M</td><td></td></tr><tr><td>L1-lens</td><td>Thorlabs</td><td>LA1986</td><td>f = 125 mm</td></tr><tr><td>L2-lens</td><td>Thorlabs</td><td>LA1461</td><td>f = 250 mm</td></tr><tr><td>Objective lens</td><td>Nikon</td><td>MUE10500</td><td>Magnification 50x NA 0.8</td></tr><tr><td>Pinhole</td><td>Thorlabs</td><td>ID8/M</td><td></td></tr><tr><td>Polarizer</td><td>Thorlabs</td><td>GTH10M</td><td>For LMOKE measurements, two polarizers are needed</td></tr><tr><td>sCMOS camera</td><td>Andor</td><td>ZYLA-4.2P-USB3</td><td></td></tr></tbody></table>

spectral and angle-resolved magneto-optical characterization of photonic nanostructures

Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by local enhancement of electric field intensity that strengthens light-matter interactions, enabling applications such as surface-enhanced Raman scattering (SERS) and surface plasmon enhanced sensing. In the presence of magneto-optically active materials, the local field enhancement gives rise to anomalous magneto-optical activity. Typically, the confined modes of a given photonic crystal depend strongly on the wavelength and incidence angle of the incident electromagnetic radiation. Thus, spectral and angular-resolved measurements are needed to fully identify them as well as to establish their relationship with the magneto-optical activity of the crystal. In this article, we describe how to use a Fourier-plane (back focal plane) microscope to characterize magneto-optically active samples. As a model system, here we use a plasmonic grating built out of magneto-optically active Au/Co/Au multilayer. In the experiments, we apply a magnetic field on the grating in situ and measure its reciprocal space response, obtaining the magneto-optical response of the grating over a range of wavelengths and incident angles. This information enables us to build a complete map of the plasmonic band structure of the grating and the angle and wavelength dependent magneto-optical activity. These two images allow us to pinpoint the effect that the plasmon resonances have on the magneto-optical response of the grating. The relatively small magnitude of magneto-optical effects requires a careful treatment of the acquired optical signals. To this end, an image processing protocol for obtaining magneto-optical response from the acquired raw data is laid out.

Figure 4A shows a scanning electron microscope (SEM) micrograph of a commercial DVD grating covered with Au/Co/Au multilayer that was used a demonstration sample in our experiments. Its optical and magneto-optical spectra are shown in Figure 4B,C respectively. Details on sample fabrication are presented elsewhere23. Black lines in Figure 4A,B show the plasmon...

Watch this Scientific Journal Video about Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures at JoVE.com

Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures

Photonic band structure enables understanding how confined electromagnetic modes propagate within a photonic crystal. In photonic crystals that incorporate magnetic elements, such confined and resonant optical modes are accompanied by enhanced and modified magneto-optical activity. We describe a measurement procedure to extract the magneto-optical band structure by Fourier space microscopy.

Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by ...

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7.0K Views.  Institut de Ciència de Materials de Barcelona. Photonic band structure enables understanding how confined electromagnetic modes propagate within a photonic crystal. In photonic crystals that incorporate magnetic elements, such confined and resonant optical modes are accompanied by enhanced and modified magneto-optical activity. We describe a measurement procedure to extract the magneto-optical band structure by Fourier space microscopy.

Video: Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus have potential applications in nonlinear optics, which requires extraordinary optical intensity. One of the most studied nonlinearities in plasmonics is nonlinear absorption, including saturation and reverse saturation behaviors. Although scattering and absorption in nanoparticles are closely correlated by the Mie theory, there has been no report of nonlinearities in plasmonic scattering until very recently. 
Last year, not only saturation, but also reverse saturation of scattering in an isolated plasmonic particle was demonstrated for the first time. The results showed that saturable scattering exhibits clear wavelength dependence, which seems to be directly linked to the localized surface plasmon resonance (LSPR). Combined with the intensity-dependent measurements, the results suggest the possibility of a common mechanism underlying the nonlinear behaviors of scattering and absorption. These nonlinearities of scattering from a single gold nanosphere (GNS) are widely applicable, including in super-resolution microscopy and optical switches. 
In this paper, it is described in detail how to measure nonlinearity of scattering in a single GNP and how to employ the super-resolution technique to enhance the optical imaging resolution based on saturable scattering. This discovery features the first super-resolution microscopy based on nonlinear scattering, which is a novel non-bleaching contrast method that can achieve a resolution as low as l/8 and will potentially be useful in biomedicine and material studies.

Saturable and reverse saturable scattering were discovered in isolated plasmonic particles and adopted as a novel non-bleaching contrast method in super-resolution microscopy. Here the experimental procedures of detecting and extracting nonlinear scattering are explained in detail, as well as how to enhance resolution with the aid of saturated excitation microscopy.

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus ...

High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon&#39;s energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

The manuscript describes a method of phonon-assisted quasi-resonant fluorescence spectroscopy that incorporates both laser-limited resolution and photoluminescence (PL) spectroscopy. This method utilizes optical phonons to provide linewidth-limited resolution spectra of atom-like semiconductor structures in the energy domain. The method is also easily realized with a single spectrometer optical spectroscopy setup.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...

Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser (laser-SARPES), and demonstrate a power of this technique for studying solid state physics. Laser-SARPES achieves two great capabilities. Firstly, by examining orbital selection rule of linearly polarized lasers, orbital selective excitation can be carried out in SAPRES experiment. Secondly, the technique can show full information of a variation of the spin quantum axis as a function of the light polarization. To demonstrate the power of the collaboration of these capabilities in laser-SARPES, we apply this technique for the investigations of spin-orbit coupled surface states of Bi2Se3. This technique affords to decompose spin and orbital components from the spin-orbit coupled wavefunctions. Moreover, as a representative advantage of using the direct spin detection collaborated with the polarization-variable laser, the technique unambiguously visualizes the light polarization dependence of the spin quantum axis in three-dimension. Laser-SARPES dramatically increases a capability of photoemission technique.

Here, we combine polarization-variable 7-eV laser with spin- and angle-resolved photoemission technique to visualize the spin-orbital coupling effect in solid states.

The goal of this protocol is to present how to perform spin- and angle-resolved photoemission spectroscopy combined with polarization-variable 7-eV laser ...

Construction and Operation of a Light-driven Gold Nanorod Rotary Motor System

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and artificial nanomotors and may provide new routes towards single cell analysis, studies of non-equilibrium thermodynamics, and mechanical actuation of nanoscale systems. A facile way to drive rotation is to use focused circularly polarized laser light in optical tweezers. Using this approach, metallic nanoparticles can be operated as highly efficient scattering-driven rotary motors spinning at unprecedented rotation frequencies in water.
In this protocol, we outline the construction and operation of circularly-polarized optical tweezers for nanoparticle rotation and describe the instrumentation needed for recording the Brownian dynamics and Rayleigh scattering of the trapped particle. The rotational motion and the scattering spectra provides independent information on the properties of the nanoparticle and its immediate environment. The experimental platform has proven useful as a nanoscopic gauge of viscosity and local temperature, for tracking morphological changes of nanorods and molecular coatings, and as a transducer and probe of photothermal and thermodynamic processes.

Plasmonic gold nanorods can be trapped in liquids and rotated at kHz frequencies using circularly-polarized optical tweezers. Introducing tools for Brownian dynamics analysis and light scatteringspectroscopy leads to a powerful system for research and application in numerous fields of science.

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and ...

Performing Spectroscopy on Plasmonic Nanoparticles with Transmission-Based Nomarski-Type Differential Interference Contrast Microscopy

Differential interference contrast (DIC) microscopy is a powerful imaging tool that is most commonly employed for imaging microscale objects using visible-range light. The purpose of this protocol is to detail a proven method for preparing plasmonic nanoparticle samples and performing single particle spectroscopy on them with DIC microscopy. Several important steps must be followed carefully in order to perform repeatable spectroscopy experiments. First, landmarks can be etched into the sample substrate, which aids in locating the sample surface and in tracking the region of interest during experiments. Next, the substrate must be properly cleaned of debris and contaminants that can otherwise hinder or obscure examination of the sample. Once a sample is properly prepared, the optical path of the microscope must be aligned, using Kohler Illumination. With a standard Nomarski style DIC microscope, rotation of the sample may be necessary, particularly when the plasmonic nanoparticles exhibit orientation-dependent optical properties. Because DIC microscopy has two inherent orthogonal polarization fields, the wavelength-dependent DIC contrast pattern reveals the orientation of rod-shaped plasmonic nanoparticles. Finally, data acquisition and data analyses must be carefully performed. It is common to represent DIC-based spectroscopy data as a contrast value, but it is also possible to present it as intensity data. In this demonstration of DIC for single particle spectroscopy, the focus is on spherical and rod-shaped gold nanoparticles.

The goal of this protocol is to detail a proven approach for the preparation of plasmonic nanoparticle samples and for performing single particle spectroscopy on them with differential interference contrast (DIC) microscopy.

Differential interference contrast (DIC) microscopy is a powerful imaging tool that is most commonly employed for imaging microscale objects using ...

Chemistry

Magnetic Field Mapping using Off-Axis Electron Holography in the Transmission Electron Microscope

Off-axis electron holography is a powerful technique that involves the formation of an interference pattern in a transmission electron microscope (TEM) by overlapping two parts of an electron wave, one of which has passed through a region of interest on a specimen and the other is a reference wave. The resulting off-axis electron hologram can be analyzed digitally to recover the phase difference between the two parts of the electron wave, which can then be interpreted to provide quantitative information about local variations in electrostatic potential and magnetic induction within and around the specimen. Off-axis electron holograms can be recorded while a specimen is subjected to external stimuli such as elevated or reduced temperature, voltage, or light. The protocol that is presented here describes the practical steps that are required to record, analyze, and interpret off-axis electron holograms, with a primary focus on the measurement of magnetic fields within and around nanoscale materials and devices. Presented here are the steps involved in the recording, analysis, and processing of off-axis electron holograms, as well as the reconstruction and interpretation of phase images and visualization of the results. Also discussed are the need for optimization of the specimen geometry, the electron optical configuration of the microscope, and the electron hologram acquisition parameters, as well as the need for the use of information from multiple holograms to extract the desired magnetic contributions from the recorded signal. The steps are illustrated through a study of specimens of B20-type FeGe, which contain magnetic skyrmions and were prepared with focused ion beams (FIBs). Prospects for the future development of the technique are discussed.

Guidelines are presented for recording and interpreting off-axis electron holograms to provide quantitative images of magnetic fields in nanoscale materials and devices in the transmission electron microscope.

Off-axis electron holography is a powerful technique that involves the formation of an interference pattern in a transmission electron microscope (TEM) by ...

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with &lt;0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and &lt;100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.

Magnetic force microscopy (MFM) employs a vertically magnetized atomic force microscopy probe to measure sample topography and local magnetic field strength with nanoscale resolution. Optimizing MFM spatial resolution and sensitivity requires balancing decreasing lift height against increasing drive (oscillation) amplitude, and benefits from operating in an inert atmosphere glovebox.

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force ...

Trapping of Micro Particles in Nanoplasmonic Optical Lattice

The plasmonic optical tweezer has been developed to overcome the diffraction limits of the conventional far field optical tweezer. Plasmonic optical lattice consists of an array of nanostructures, which exhibit a variety of trapping and transport behaviors. We report the experimental procedures to trap micro-particles in a simple square nanoplasmonic optical lattice. We also describe the optical setup and the nanofabrication of a nanoplasmonic array. The optical potential is created by illuminating an array of gold nanodiscs with a Gaussian beam of 980 nm wavelength, and exciting plasmon resonance. The motion of particles is monitored by fluorescence imaging. A scheme to suppress photothermal convection is also described to increase usable optical power for optimal trapping. Suppression of convection is achieved by cooling the sample to a low temperature, and utilizing the near-zero thermal expansion coefficient of a water medium. Both single particle transport and multiple particle trapping are reported here.

We describe a procedure to optically trap micro-particles in nanoplasmonic optical lattice.

The plasmonic optical tweezer has been developed to overcome the diffraction limits of the conventional far field optical tweezer. Plasmonic optical ...