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

A Method to Fabricate Disconnected Silver Nanostructures in 3D

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

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 ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Using Microwave and Macroscopic Samples of Dielectric Solids to Study the Photonic Properties of Disordered Photonic Bandgap Materials

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). In this article we will describe the methods for constructing and characterizing macroscopic disordered photonic structures using microwaves. The microwave regime offers the most convenient experimental sample size to build and test PBG media. Easily manipulated dielectric lattice components extend flexibility in building various 2D structures on top of pre-printed plastic templates. Once built, the structures could be quickly modified with point and line defects to make freeform waveguides and filters. Testing is done using a widely available Vector Network Analyzer and pairs of microwave horn antennas. Due to the scale invariance property of electromagnetic fields, the results we obtained in the microwave region can be directly applied to infrared and optical regions. Our approach is simple but delivers exciting new insight into the nature of light and disordered matter interaction.

Our representative results include the first experimental demonstration of the existence of a complete and isotropic PBG in a two-dimensional (2D) hyperuniform disordered dielectric structure. Additionally we demonstrate experimentally the ability of this novel photonic structure to guide electromagnetic waves (EM) through freeform waveguides of arbitrary shape.

Disordered structures offer new mechanisms for forming photonic bandgaps and unprecedented freedom in functional-defect designs. To circumvent the computational challenges of disordered systems, we construct modular macroscopic samples of the new class of PBG materials and use microwaves to characterize their scale-invariant photonic properties, in an easy and inexpensive manner.

Recently, disordered photonic materials have been suggested as an alternative to periodic crystals for the formation of a complete photonic bandgap (PBG). ...

Writing and Low-Temperature Characterization of Oxide Nanostructures

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions. Oxide interfaces exhibit a wide range of properties that can be controlled include conduction, piezoelectric behavior, ferromagnetism, superconductivity and nonlinear optical properties. Recently, methods for controlling these properties at extreme nanoscale dimensions have been discovered and developed. Here are described explicit step-by-step procedures for creating LaAlO3/SrTiO3 nanostructures using a reversible conductive atomic force microscopy technique. The processing steps for creating electrical contacts to the LaAlO3/SrTiO3 interface are first described. Conductive nanostructures are created by applying voltages to a conductive atomic force microscope tip and locally switching the LaAlO3/SrTiO3 interface to a conductive state. A versatile nanolithography toolkit has been developed expressly for the purpose of controlling the atomic force microscope (AFM) tip path and voltage. Then, these nanostructures are placed in a cryostat and transport measurements are performed. The procedures described here should be useful to others wishing to conduct research in oxide nanoelectronics.

Oxide nanostructures provide new opportunities for science and technology. The interfacial conductivity between LaAlO3 and SrTiO3 can be controlled with near-atomic precision using a conductive atomic force microscopy technique. The protocol for creating and measuring conductive nanostructures at LaAlO3/SrTiO3 interfaces is demonstrated.

Oxide nanoelectronics is a rapidly growing field which seeks to develop novel materials with multifunctional behavior at nanoscale dimensions.  Oxide ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

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 ...

A Photonic System for Generating Unconditional Polarization-Entangled Photons Based on Multiple Quantum Interference

We present a high-performance source of unconditional polarization-entangled photons that have a high-emission rate, a broadband distribution, are degenerated and postselection free. The property of the source is based on the multiple quantum interference effect with a round-trip configuration of a Sagnac interferometer. The quantum interference effects make it possible to use the high generation efficiency of the polarization-entangled photons to process parametric down-conversion, and separate degenerated photon pairs into different optical modes without a postselection requirement. The principle of the optical system was described and experimentally used to measure the fidelity and Bell parameters, and also to characterize the generated polarization-entangled photons from a minimum of six combinations of polarization correlated data. The experimentally obtained fidelity and Bell parameters exceeded the classical local correlation limit and are clear evidence of the generation of unconditional polarization-entangled photons.

We describe an optical system for the generation of unconditional polarization-entangled photons based on multiple quantum interference effects with a detection scheme to estimate the experimental fidelity of generated entangled photons.