Name: experimental methods for spin- and angle-resolved photoemission spectroscopy combined with polarization-variable laser
Uploaded: 2018-06-28T13:00:00.000+00:00
Duration: 9 min
Description: Watch this Scientific Journal Video about Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser at JoVE.com

We thank M. Nakayama, S. Toyohisa, A. Fukushima and Y. Ishida for supports to the experimental setup. We gratefully acknowledge funding from the JSPS Grantin-Aid for Scientific Research (B) through Project No. 26287061 and for Young Scientists (B) through Project No. 15K17675. This work was also supported by MEXT of Japan (Innovative Area &#34;Topological Materials Science,&#34; Grant No. 16H00979) and JSPS KAKENHI (Grant No. 16H02209)

1. Sample Mount and Installation
<ol>
	<li>Cut single-crystal samples of Bi2Se313 in an approximate size of 1 &#215; 1 &#215; 0.5 mm3 and use sliver-based epoxy to glue the sample to the sample holder.</li>
	<li>Paste the scotch tape on the sample surface. 
	NOTE: The scotch tape is used to cleave the sample in ultrahigh vacuum (UHV) chamber to obtain an atomically clean surface.</li>
	<li>Install the sample into the sample magazine in the load lock, and start the pump until the pressure of the load lock is lower than 1&#215;10-5 Pa.</li>
</ol>
2. Sample Cleaving
<ol>
	<li>Open the UHV valve between the load lock and the UHV preparation chamber.</li>
	<li>Move the sample magazine from the load rock to the preparation chamber by using the linear/rotary feedthrough which is attached to the load lock chamber.</li>
	<li>Pick up the sample from the sample magazine by the transfer rod attached to the preparation chamber.</li>
	<li>Put back the sample magazine into the load lock and close the UHV valve.</li>
	<li>Wait until the pressure of the preparation chamber is below 5&#215;10-7 Pa.</li>
	<li>Peel the scotch tape by using wobble stick in the preparation chamber and cleave the sample under the UHV condition.</li>
</ol>
3. Sample Transfer to The Measurement Position
<ol>
	<li>Transfer the sample to the UHV measurement chamber, and fix the sample to the main gonio-stage by the screw driver equipped with the measurement chamber.</li>
	<li>Move the gonio-stage to the measurement position and use the micrometer stage to precisely move the sample position onto the focus of the spectrometer.</li>
</ol>
4. 7eV-laser Setup
<ol>
	<li>Turn on the Nd:YVO4 laser. 
	NOTE: The laser generates 355 nm laser light with a high repetition rate of 120 MHz.</li>
	<li>Open the laser beam shutter, and make sure that the laser passes through the KBBF crystal and a second-harmonic wave of 177 nm (6.994 eV) is generated.</li>
	<li>Optimize the power of the 7eV-laser by changing the power of the 355-nm laser with the variable attenuator.</li>
</ol>
5. ARPES Data Acquisition
<ol>
	<li>Open the analyzer control software on the desktop computer. 
	NOTE: We use &#34;SES software&#34; which is a general program for controlling ScientaOmicron analyzer with an electron deflector.</li>
	<li>Select Setup&#8230; below Sequence on the menu bar (Figure 2, step i.2-1).</li>
	<li>Choose ARPES configuration (Figure 3, step i.3-1) and ARPES Mapping in the list (Figure 3, step i.3-2) to perform Fermi surface mapping with the photoelectron deflector.</li>
	<li>Click Edit (Figure 3, step i.3-3) and configure Fermi surface mapping ranging from -12&#176; to 12&#176; of an emission angle &#952;y with step size of 0.5&#176; (Figure 3, step i.3-4). 
	NOTE: The hemispherical analyzer with an electron deflector enables us to map the Fermi surface without the sample rotations.</li>
	<li>Click Run (Figure 2, step i.2-3).</li>
</ol>
6. SARPES Data Acquisition
<ol>
	<li>Manually change the machine set-up for SARPES measurement including the analyzer entrance slit and the aperture size (Figure 1).</li>
	<li>Select Setup&#8230; below Sequence on the menu bar (Figure 2, step i.2-2).</li>
	<li>Choose Spin configuration (Figure 4, step i.4-1) and Normal in the list (Figure 4, step i.4-2), and click OK (Figure 4, step i.4-3).</li>
	<li>Select DA30 (Figure 5, step i.5-1) on the menu bar and Control Theta&#8230; (Figure 5, step i.5-2) to open the setting panel for the DA30 angle (&#952;x, &#952;y) configuration.</li>
	<li>Choose the emission angle (&#952;x, &#952;y) = (-6&#176;, 0&#176;) to take SARPES spectra (Figure 5, step i.5-3).</li>
	<li>Apply magnetic field by controlling the bipolar condenser bank to magnetize the VLEED target in positive direction along the particular axis (&#945;: x, y, or z). 
	NOTE: In our system, this process can be done through command prompt [Figure 6 (a)].</li>
	<li>Click Run to take intensity spectrum (Figure 2, step i.2-3).</li>
	<li>Apply magnetic field to magnetize the VLEED target in negative direction along &#945; and start scan to take intensity spectrum.</li>
	<li>Calculate the spin-polarization and the spin-resolved spectra.</li>
</ol>
7. Scanning the light polarization dependence
<ol>
	<li>Change the angle of the &#955;/2-waveplate precisely controlled by the stepping motor to tune the light polarization of the 7 eV-laser. 
	NOTE: In our system, this process can be done through command prompt [Figure 6 (b)].</li>
	<li>Take the spin-resolved spectra for x, y and z axes.</li>
	<li>Scan the spin-resolved spectra as a function of the light polarization with varying the half waveplate angle from 0&#176; to 102&#176; with step size of 3&#176;.</li>
</ol>

<ol>
	<li>Damascelli, A., Hussain, Z., Shen, Z. -. X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Angle-resolved+photoemission+studies+of+the+cuprate+superconductors.">Angle-resolved photoemission studies of the cuprate superconductors.</a> Rev. Mod. Phys. 75 (2), 473-541 (2003).</li><li>Qiao, S., Kimura, A., Harasawa, A., Sawada, M., Chung, J. -. G., Kakizaki, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+compact+electron+spin+polarimeter+with+a+high+efficiency.">A new compact electron spin polarimeter with a high efficiency.</a> Rev. Sci. Instrum. 68 (12), 4390-4395 (1997).</li><li>Dil, J. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin+and+angle+resolved+photoemission+on+non-magnetic+low-dimensional+systems.">Spin and angle resolved photoemission on non-magnetic low-dimensional systems.</a> J. Phys. Condens. Matter. 21 (40), 403001 (2009).</li><li>Hoesch, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin-polarized+Fermi+surface+mapping.">Spin-polarized Fermi surface mapping.</a> J. Electron Spectrosc. 124 (2), 263-279 (2002).</li><li>Souma, S., Takayama, S., Sugawara, K., Sato, T., Takahashi, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrahigh-resolution+spin-resolved+photoemission+spectrometer+with+a+mini+Mott+detector.">Ultrahigh-resolution spin-resolved photoemission spectrometer with a mini Mott detector.</a> Rev. Sci. Instrum. 81 (9), 096101 (2010).</li><li>Okuda, T., Kimura, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin-+and+angle-resolved+photoemission+of+strongly+spin-orbit+coupled+systems.">Spin- and angle-resolved photoemission of strongly spin-orbit coupled systems.</a> J. Phys. Soc. Jpn. 82 (2), 021002 (2013).</li><li>Okuda, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+spin-polarized+photoemission+spectrometer+with+very+high+efficiency+and+energy+resolution.">A new spin-polarized photoemission spectrometer with very high efficiency and energy resolution.</a> Rev. Sci. Instrum. 79 (12), 123117 (2008).</li><li>Jozwiak, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+high-efficiency+spin-resolved+photoemission+spectrometer+combining+time-of-flight+spectroscopy+with+exchange-scattering+polarimetry.">A high-efficiency spin-resolved photoemission spectrometer combining time-of-flight spectroscopy with exchange-scattering polarimetry.</a> Rev. Sci. Instrum. 81 (5), 053904 (2010).</li><li>Okuda, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+spin+resolved+spectroscopy+observation+machine+at+Hiroshima+Synchrotron+Radiation+Center.">Efficient spin resolved spectroscopy observation machine at Hiroshima Synchrotron Radiation Center.</a> Rev. Sci. Instrum. 82 (10), 103302 (2011).</li><li>Yaji, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-resolution+three-dimensional+spin-+and+angle-resolved+photoelectron+spectrometer+using+vacuum+ultraviolet+laser+light.">High-resolution three-dimensional spin- and angle-resolved photoelectron spectrometer using vacuum ultraviolet laser light.</a> Rev. Sci. Instrum. 87 (5), 053111 (2016).</li><li>Shimojima, T., Okazaki, K., Shin, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Low-temperature+and+high-energy-resolution+laser+photoemission+spectroscopy.">Low-temperature and high-energy-resolution laser photoemission spectroscopy.</a> J. Phys. Soc. Jpn. 84 (7), 072001 (2015).</li><li>Augustine, S., Mathai, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth,+morphology,+and+microindentation+analysis+of+Bi2Se3,+Bi1.8In0.2Se3,+and+Bi2Se2.8Te0.2+single+crystals.">Growth, morphology, and microindentation analysis of Bi2Se3, Bi1.8In0.2Se3, and Bi2Se2.8Te0.2 single crystals.</a> Mater. Res. Bull. 36 (13), 2251-2261 (2001).</li><li>Hasan, M. Z., Kane, C. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Colloquium:+Topological+insulators.">Colloquium: Topological insulators.</a> Rev. Mod. Phys. 82 (4), 3045-3067 (2010).</li><li>Ando, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Topological+insulator+materials.">Topological insulator materials.</a> J. Phys. Soc. Jpn. 82 (10), 102011 (2013).</li><li>Zhang, H., Liu, C. -. X., Qi, X. -. L., Dai, X., Fang, Z., Zhang, S. -. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Topological+insulators+in+Bi2Se3,+Bi2Te3+and+Sb2Te3+with+a+single+Dirac+cone+on+the+surface.">Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface.</a> Nature Phys. 5 (6), 438-442 (2009).</li><li>Cao, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mapping+the+orbital+wavefunction+of+the+surface+states+in+three-dimensional+topological+insulators.">Mapping the orbital wavefunction of the surface states in three-dimensional topological insulators.</a> Nature Phys. 9 (8), 499-504 (2013).</li><li>Zhang, H., Liu, C. -. X., Zhang, S. -. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin-orbital+texture+in+topological+insulators.">Spin-orbital texture in topological insulators.</a> Phys. Rev. Lett. 111 (6), 066801 (2013).</li><li>Kuroda, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coherent+control+over+three-dimensional+spin-polarization+for+the+spin-orbit+coupled+surface+state+of+Bi2Se3.">Coherent control over three-dimensional spin-polarization for the spin-orbit coupled surface state of Bi2Se3.</a> Phys. Rev. B. 94 (16), 165162 (2016).</li><li>Meier, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interference+of+spin+states+in+photoemission+from+Sb/Ag(111)+surface+alloys.">Interference of spin states in photoemission from Sb/Ag(111) surface alloys.</a> J Phys-Condens Mat. 23 (7), 072207 (2011).</li><li>Dil, J. H., Meier, F., Osterwalder, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rashba-type+spin+splitting+and+spin+interference+of+the+Cu(111)+surface+state+at+room+temperature.">Rashba-type spin splitting and spin interference of the Cu(111) surface state at room temperature.</a> J Electron Spectrosc. 201, 42-46 (2015).</li><li>Yaji, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spin-dependent+quantum+interference+in+photoemission+process+from+spin-orbit+coupled+states.">Spin-dependent quantum interference in photoemission process from spin-orbit coupled states.</a> Nature Commun. 8, 14588 (2017).</li><li>Noguchi, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+mapping+of+spin+and+orbital+entangled+wave+functions+under+interband+spin-orbit+coupling+of+giant+Rashba+spin-split+surface+states.">Direct mapping of spin and orbital entangled wave functions under interband spin-orbit coupling of giant Rashba spin-split surface states.</a> Phys. Rev. B. 95 (6), 04111(R) (2017).</li></ol>

The authors declare that they have no competing financial interests.

ARPES and SARPES techniques have been commonly used for studying electronic band structures through the band mapping and spin-detection1,2. In addition to these general advantages shown above, laser-SARPES based on orbital selection rule in optical dipole excitation can be employed as a novel technique for visualizing the spin-orbital coupling effect in the wavefunction and quantum spin interference. As demonstrated in Figure 9 and 10, the polarization of laser can be easily manipulated just by waveplates to tilted linear polarizations between p- and s-polarizations19. In principle, circular and even elliptical polarized lights can be obtained and used in laser-SAPRES. This variety of tunable polarization is hardly obtained in conventional light source such as noble-gas-discharge lamp and synchrotron radiation. Therefore, a combination of polarization-variable laser and SARPES with 3D spin-resolution dramatically increases a capability of photoemission technique.
To perform laser-SARPES under the best condition, one must be always careful about space-charge effect12, which generally broadens the energy of the photoelectrons due to Coulomb repulsion in a dense electron packet emitted by the high intensity laser. If this issue appears, one needs optimize the power of the 7-eV laser (step 4.3). Second, if the photoelectron intensity from samples is poor, the analyzer entrance slit, and the aperture should be open (step 6.1), however, the energy resolution is sacrificed in this case. Therefore, one needs carefully select the experimental set-up to fit laser-SARPES experiments.
The main disadvantage of laser-SARPES compared to standard photoemission technique with synchrotron radiation is that in laser-SAPRES the photon energy of laser is generally not tunable. In photoemission technique, the tunable photon is necessary to probe kz dispersion, and identifies 3D bulk band structures and two-dimensional surface states1. Moreover, the photon energy of 7 eV used in this paper can scan small k area compared to that with higher photon energy. Therefore, laser-SARPES is likely restricted into the investigations of two-dimensional surface states around Brillouin zone center.
However, it should be noted that the power of laser-SARPES technique can be widely applied for spin-orbit coupled states. Recently, by using the protocol described within this paper, we have further revealed strong spin-orbit coupling effect and its significant k-dependence in Bi thin film22 and BiAg2/Ag(111) surface alloy23. Also, it is worth noting that high-efficient SARPES technique is just beginning to develop and gradually becomes a standard experimental technique. The protocol is intended to help researchers use SARPES and understand the produced data.

Angle-resolved photoemission spectroscopy (ARPES) technique has developed into one of the most powerful tool to investigate quasiparticle band structures in solid states1. The most of attractive feature of ARPES is the capability for band mapping to characterize electronic states in energy and momentum space. Spin-resolved ARPES (SARPES), which is here equipped with spin-detectors, e.g. Mott detector2,3, further enables us to resolve the spin character of the observed band structures4. Since the Mott detector can measure the spin with two axes (x and z, or y and z), the combination of the two Mott detectors further allows one to obtain the spin orientation in three dimension4,5. For several decades, however, the SARPES experiments were suffered from their low efficiency (typically 1/10000 compared to that for spin-integrated ARPES measurement)3,4,5,6,7, which had limited the energy and angular-resolutions. Recently, the energy resolution of SARPES has been increased with a high-efficient spin detector based on exchange scattering, the so-called very-low-energy electron-diffraction (VLEED) detector7,8,9,10. With this detector, the data quality has been significantly improved and the data acquisition time has been shortened. Recently, SARPES has succeeded greatly to address spin-polarized electronic states and particularly spin-orbit coupling effect resulting in the spin texture of the surface bands7.
Here, we employ SARPES measurements with a polarization-variable vacuum ultraviolet laser light (laser-SARPES) and demonstrate the great advantages of this combined technique. Through the investigation on the spin-orbit coupled surface states in Bi2Se3, we present two capabilities of laser-SARPES. Firstly, due to the orbital selection rule of linearly polarized lasers in dipole transition regime, p- and s-polarized lights selectively excite a part of eigen-wavefunctions with different orbital symmetry. Such an orbital selective excitation is thereby available in SARPES, namely, orbital-selective SARPES. Secondly, three-dimensional (3D) spin-detection in SARPES shows the direction of the spin quantum axis and directly displays full information of the light-polarization dependence. In this protocol, we briefly describe a methodology to perform this state-of-the-art laser-SARPES technique to study the strong spin-orbit coupling effects.
Our laser-SARPES system is located at The Institute for Solid State Physics, The University of Tokyo11. The schematic drawing of our laser-SAPRES machine is shown in Figure 1. The polarization-variable 7-eV laser light12 illuminates the sample surface and the photoelectrons are emitted from the sample. The polarization of laser is automatically controlled by MgF2-based &#955;/2- and &#955;/4-waveplates to selectively use linear and circular polarizations. A hemispherical electron analyzer corrects the photoelectrons, and analyzes their kinetic energy (Ekin) and emission angle (&#952;x and &#952;y). The photoelectron intensities are mapped on the Ekin-&#952;x screen monitored by a CCD camera. This image is directly transformed into the energy band structure in reciprocal space.
For SARPES measurement, the photoelectrons with a specific emission angle and kinetic energy analyzed by the electron analyzer are&#160;guided to two VLEED-type spin detectors with a 90-degree photoelectron deflector and the photoelectron beams are focused onto two different targets of Fe(001)-p(1&#215;1) films terminated by oxygen. The photoelectrons reflected by the targets are detected in single channel detection by using a channeltron placed in each spin detector. The VLEED targets can be magnetized with Helmholtz-type electric coils which are arranged with orthogonal geometry with respect to each other. The magnetization direction is controlled by the bipolar condenser bank. The double VLEED spin detectors thereby enable us to analyze the spin-polarization vector of the photoelectron in three dimensions.

<table><tbody><tr><td>DA30-L hemispherical analyzer</td><td>ScientaOmicron</td><td></td><td>http://www.scientaomicron.com/en/products/353/1170</td></tr><tr><td>Silver-based epoxy</td><td>Epoxy Technology</td><td>H20E</td><td></td></tr><tr><td>Sctoch tape</td><td>3M</td><td>801-1-18C</td><td></td></tr><tr><td>UHV valve</td><td>VAT</td><td>01034-KE01</td><td></td></tr><tr><td>linear/rotary feedthrough</td><td>Ferrovac</td><td>MD40</td><td></td></tr><tr><td>transfer rod</td><td>UHV design</td><td>PP series</td><td></td></tr><tr><td>wobble stick</td><td>Ferrovac</td><td>WM40</td><td></td></tr><tr><td>Paladin compact 355</td><td>Coherent</td><td></td><td></td></tr><tr><td>half waveplate</td><td>Kogakugiken</td><td></td><td>order made</td></tr><tr><td>Bipolar condenser bank</td><td>Tsuji electronics</td><td></td><td></td></tr></tbody></table>

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.

Before starting SARPES experiments, k positions need to be accurately determined for taking spin-resolved spectrum by using high statistic spin-integrated ARPES results with high energy- and angular-resolutions (protocol 5.1-5.5). This is demonstrated in Figure 7 where the ARPES results for a Bi2Se3 single crystal are presented. This material is known as a prototypical topological insulator with a spin-polarized surface states14,15. The ARPES band map clearly resolves the very steep Dirac-cone-like energy dispersion of the two-dimensional surface state16. The ARPES results confirm high quality of the cleaved surface and the sample orientation. From the energy and momentum information of the band map and the Fermi surface mapping, one can now choose the specific emission angle for SARPES experiment.
Figure 8(a) represents the energy distribution curves (EDCs) for different magnetization direction (+My and -My) taken at (&#952;x, &#952;y)=(-6&#176;, 0&#176;) across -kF of the surface band, corresponding to the cut along the dashed line shown in Figure 7. From the data, the spin-resolved EDCs can be obtained in the following. At first, the spin polarization (Py) is estimated by using this relation:
<img alt="Equation 1" src="/files/ftp_upload/57090/57090eq1.jpg" />
where &#945; is the resolved axis (x, y and z), and I+M &#945; (I-M &#945;) is the photoelectron intensity for +M &#945; (-M &#945;), and Seff is the Sherman function which is typically 0.311. The obtained Py curve is shown in Figure 8 (b). The spin-resolved spectra for spin-up (<img alt="Equation 2" src="/files/ftp_upload/57090/57090eq2.jpg" />) and spin-down (<img alt="Equation 3" src="/files/ftp_upload/57090/57090eq3.jpg" />) are then obtained by:
<img alt="Equation 4" src="/files/ftp_upload/57090/57090eq4.jpg" />
The resulting spin-resolved spectra are shown in Figure 8(c).
The double VLEED spin detectors allow us to obtain 3D spin-resolution along x, y and z axes. This is demonstrated in Figure 9(a) where the spin-resolved spectra by using p-polarization set-up and the corresponding spin-polarization (Px, Py and Pz) are shown. The clear peak near the Fermi energy is assigned to the surface state of Bi2Se3. The data represents that Py is fully spin-polarized ~100 % while the other components, Px and Pz, are negligibly small. The 3D spin-resolved spectra thus illustrate the spin quantum axis of the surface state fixed along y, which is consistent with band calculations16,17,18.
Then, we focus on the orbital selective excitation of p- and s-polarized laser. In general, under strong spin-orbit coupling, different orbital symmetry is mixed with opposite spinor in a single eigenfunction17,18. In our experimental geometry, p-polarized (s-polarized) light is sensitive to px and pz (py) orbital components in the spin-orbit coupled wavefunction (inset in Figure 9). Thereby, through the spin-orbital coupling, orbital-selective laser-SARPES should detect opposite spin-polarization for p- and s-polarization. Indeed, this is demonstrated in Figure 9(a) and 9(b). We clearly observe significant light polarization dependence of Py directly displaying spin-orbit coupling effect in the surface state17,18.
In addition, laser-SARPES affords to further investigate the linear-polarization evolution of Px, Py and Pz even with tilted light polarization between p- and s-polarizations19. As shown in Figure 10(a), laser-SARPES with 3D spin detection displays Px, Py and Pz at 0.025 eV of the binding energy as a function of the linear-polarizations. Here, the result contains 102 data points, which was acquired within 6 h. The polarization dependence of Py is easily explained by a fact that the positive and negative Py of the photoelectrons excited by p- and s-components of the applied laser cancel out. However, this cannot explain the evolution of Px and Pz. To completely describe this result, it is necessary to consider coherent spin process in photoemission as summarized in Figure 10(b). If the linear polarization simultaneously excites spin-up and spin-down states, these two quantum-spin bases are coherently superposed in a photoelectron state, resulting in the spin rotation. In fact, the displayed polarization dependence is well reproduced by the model calculation with a consideration of coherent interference between spin-up and spin-down excited by p- and s-polarizations19. The similar spin effect has been alternatively observed by 3D SARPES with synchrotron radiation20,21.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/57090/57090fig1.jpg" /> 
Figure 1: Schematic drawing of the detection systems of laser-SARPES. Two VLEED spin-detectors arranged with orthogonal geometry are connected to the hemispherical photoelectron analyzer. This figure has been modified from Yaji, K. et al.11. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/57090/57090fig2.jpg" /> 
Figure 2: Screenshots of the analyzer control software. Steps i.2-1 to i.2-3 show how to start for selecting detection mode (ARPES or SARPES) and taking data. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/57090/57090fig3.jpg" /> 
Figure 3: Screenshots of the control panel for choosing detection-mode. Steps i.3-1 to i.3-4 show how to start Fermi surface mapping. If the button Edit is pressed (step i.3-3), the new panel will pop up to define the properties of the mapping (step i.3-4).
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57090/57090fig4.jpg" /> 
Figure 4: Screenshots of the control panel for choosing detection-mode. Steps i.4-1 to i.4-3 show how to start SARPES mode. If the region of spin is selected (step i.4-1) and the bottom OK is pressed (step i.4-3), the panel will close and the whole analyzer setup will turn to be the SARPES mode.
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57090/57090fig5.jpg" /> 
Figure 5: Screenshots of the electron deflector control-panel. Steps i.5-1 to i.5-3 show how to control the photoelectron deflector. If the region control theta&#8230; is pressed (step i.5-2), the new panel will pop up to define the properties of the photoelectron deflector (step i.5-3). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/57090/57090fig6.jpg" /> 
Figure 6: Screenshots of the panels for magnetic field of spin targets and light polarization. These properties are controlled by command lines in our system. (a) the command to control the magnetic field for spin targets: &#34;spin_coil.exe + X&#34; corresponds to &#34;the name of the application file&#34;, &#34;the direction of the field, + or -&#34; and &#34;the axis, x, y or z&#34;. (B) the command to control the light polarization: &#34;wave_plate.exe 180&#34; corresponds to &#34;the name of the application file&#34; and &#34;the angle of the &#955;/2-waveplate&#34;.
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/57090/57090fig7.jpg" /> 
Figure 7: Fermi surface mapping and E-k band mapping of Bi2Se3 surface state by using ARPES. The dash line indicates k position for taking spin-resolved spectra shown in Figure 8 and Figure 9. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/57090/57090fig8.jpg" /> 
Figure 8: Spin- and angle-resolved surface band spectra of Bi2Se3. (a) Energy distribution curves (EDCs) measured for different magnetization directions +My and -My at a fixed emission angle corresponding to the dashed line cut in Figure 7. (b) Spin polarizations as a function of binding energy obtained from the spin-resolved analysis. (c) The resulting EDCs for spin-up (red triangles) and spin-down (blue triangles) channels. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/57090/57090fig9.jpg" /> 
Figure 9: p- and s-polarization dependence of photoelectron spin from the surface state of Bi2Se3. (a) and (b) 3D spin-resolved spectra forx, y and z axes, and the corresponding spin polarizations (Px, Py and Pz) as a function of binding energy obtained by p- and s-polarizations at a fixed emission angle corresponding to the dashed line cut in Figure 7. In the inset, experimental configurations for p- and s-polarizations are shown. This figure has been modified from Kuroda K. et al.19. The p- (s-) polarization selectively excites px and pz (py) like orbital wavefunction. The px and pz (py) states are coupled to +y spin (&#8722;y spin) in the spin-orbital coupled surface state16,17. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 10" class="xfigimg" src="/files/ftp_upload/57090/57090fig10.jpg" /> 
Figure 10: Three-dimensional spin polarization effect induced by linearly polarized light. (a)The plots of Px,y,z of the Bi2Se3 surface state with respect to the applied linearly polarized laser. In the inset, the applied electric field of the laser projected onto the x-z plane is shown. The overall data points were taken within 6 h. This figure has been modified from Kuroda, K. et al.19. (b) Visualized 3D spin rotation due to the interference of spin-up and spin-down spin. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57090/57090fig10large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser at JoVE.com

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

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 ...

experimental-methods-for-spin-angle-resolved-photoemission

Nanyang Technological University

Research

JoVE Journal

Engineering

9.8K Views.  University of Tokyo. 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.

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

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 ...

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.

A protocol to detect and automate mode locking in a pre-adjusted nonlinear polarization rotation fiber laser is presented. The detection of a sudden change in the output polarization state when mode locking occurs is used to command the alignment of an intra-cavity polarization controller in order to find mode-locking conditions.

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a ...

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 ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Chemistry

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 ...

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

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.

We present a high-performance source of unconditional polarization-entangled photons that have a high-emission rate, a broadband distribution, are ...

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.

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 ...

In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum environment, the stress-induced birefringence effect of the vacuum windows will affect the polarization states of laser light inside the vacuum system, and it is very difficult to measure and optimize the polarization states of the laser light in situ. The purpose of this protocol is to demonstrate how to optimize the polarization states of the laser light based on the fluorescence of ions in the vacuum system, and how to calculate the birefringence of vacuum windows based on azimuthal angles of external wave plates with Mueller matrix. The fluorescence of 25Mg+ ions induced by laser light that is resonant with the transition of |32P3/2,F = 4, mF&#160;= 4<img align="center" alt="Equation 100" src="/files/ftp_upload/61175/61175img1.jpg" style="color: rgb(68, 0, 68); font-size: 18px; font-weight: 700;" />&#160;&#8594;&#160;|32S1/2,F = 3, mF&#160;= 3<img align="center" alt="Equation 100" src="/files/ftp_upload/61175/61175img1.jpg" style="color: rgb(68, 0, 68); font-size: 18px; font-weight: 700;" />&#160;is sensitive to the polarization state of the laser light, and maximum fluorescence will be observed with pure circularly polarized light. A combination of half-wave plate (HWP) and quarter-wave plate (QWP) can achieve arbitrary phase retardation and is used for compensating the birefringence of the vacuum window. In this experiment, the polarization state of the laser light is optimized based on the fluorescence of 25Mg+ ion with a pair of HWP and QWP outside the vacuum chamber. By adjusting the azimuthal angles of the HWP and QWP to obtain maximum ion fluorescence, one can obtain a pure circularly polarized light inside the vacuum chamber. With the information on the azimuthal angles of the external HWP and QWP, the birefringence of the vacuum window can be determined.

In Situ Measurement of Vacuum Window Birefringence using 25Mg+ Fluorescence

Presented here is a method to measure the birefringence of vacuum windows by maximizing the fluorescence counts emitted by Doppler cooled 25Mg+ ions in an ion trap. The birefringence of vacuum windows will change the polarization states of the laser, which can be compensated by changing the azimuthal angles of external wave plates.

Accurate control of the polarization states of laser light is important in precision measurement experiments. In experiments involving the use of a vacuum ...

Characterizing Spherulites with Polarization-Sensitive Two-Photon Microscopy

Polarization-Sensitive Two-Photon Microscopy for a Label-Free Amyloid Structural Characterization

Compared to its one-photon counterpart, two-photon excitation is beneficial for bioimaging experiments because of its lower phototoxicity, deeper tissue penetration, efficient operation in densely packed systems, and reduced angular photoselection of fluorophores. Thus, the introduction of polarization analysis in two-photon fluorescence microscopy (2PFM) provides a more precise determination of molecular organization in a sample compared to standard imaging methods based on linear optical processes. In this work, we focus on polarization-sensitive 2PFM (ps-2PFM) and its application in the determination of molecular ordering within complex bio-structures-amyloid spherulites. Neurodegenerative diseases such as Alzheimer's or Parkinson's are often diagnosed through the detection of amyloids-protein aggregates formed due to an impaired protein misfolding process. Exploring their structure leads to a better understanding of their creation pathway and consequently, to developing more sensitive diagnostic methods. This paper presents the ps-2PFM adapted for the determination of local fibril ordering inside the bovine insulin spherulites and spherical amyloidogenic protein aggregates. Moreover, we prove that the proposed technique can resolve the three-dimensional organization of fibrils inside the spherulite.

Author Spotlight: Non-Invasive Imaging of Complex Bio-Structures Using Polarization-Sensitive Two-Photon Microscopy 

This paper describes how polarization-sensitive two-photon microscopy could be applied to characterize the local organization within label-free amyloid superstructures-spherulites. It also describes how to prepare and measure the sample, assemble the required setup, and analyze the data to obtain information about the local organization of amyloid fibrils.

Compared to its one-photon counterpart, two-photon excitation is beneficial for bioimaging experiments because of its lower phototoxicity, deeper tissue ...

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

Summary

Explore More Videos

Chapters in this video