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Proposal:

Nonthermal Opto-Magnetic Memristors and Characterization

     by Ultrafast Pump and Probe Polarimetric Microsc...
Proposal:

Nonthermal Opto-Magnetic Memristors and Characterization

     by Ultrafast Pump and Probe Polarimetric Microsc...
Aim: The purpose of this project was to devise methods to make an optical
memristor, which will be an optical equivalent o...
light wave(s) with the material that the device is made of. The result of the interaction

may be steered (or influenced) ...
o The changes made to the device’s behavior should be reversible.

       o The device’s setting should not be affected by...
o The response time of the device should be small enough, as compared the

            existing electrical memristor, to j...
Optical Memristor: The most promising route towards the realization of
optical memristance (after discarding photochromic ...
Figure 1: Shallow hysteresis curve for memristor, that does not change abruptly at either extreme [1]




A material meant...
demagnetization being an ultrafast process “writing” a new magnetization vector into a

material can be done faster than i...
Mechanism of Nonthermal Optical Control of

Magnetism:               The magneto-optical Faraday Effect [14] says that the...
Figure 4: Ultrafast spin-flip via the process of the stimulated Raman scattering [7].




Mechanism for Maximizing Coheren...
Figure 5: Pulse shaping for reduce competition from photons of unwanted energy [7]




Controlling Magnetization Vector Pr...
Figure 6: Taken from [7]




If the electrons are hit with a second pulse while they are precessing then there will either...
through. So, for an optomagnetic memristor the Faraday rotation of linearly polarized

probe pulses will act as the ‘resis...
Figure 8: Taken from [8]




Optomemristor materials can be chosen for their precession speed, their spin-orbit

coupling ...
or by measuring the MOKE (Magneto-Optic Kerr Effect) [19], which is the rotation of

the polarization of linearly polarize...
semiconductors, such as CdSe [20]) held in position by a polymer matrix, perhaps with

carbon nanotubes in the matrix, so ...
Figure 11: Typical MOKE Setup [13]




                  Figure 12: Typical Setup for Measuring Faraday Rotation [7]




D...
also rotation resulting from various depths of the sample. While MOKE systems will not

allow probing very deep into the m...
Microscopy Requirements: An instrument suitable for
characterizing opto-magnetic memristors will need the following featur...
It must be noted that regardless of the whether the probing is done by measurements of

the Faraday Effect or MOKE the opt...
The following are the tasks that a microscope that could be used towards the development

of optomagnetic memristor materi...
clear pictures of what the individual magnetic domains look like (size, shape,

      position, density, and magnetization...
one wants to detect (using surface plasmon coupling) and send a signal to the

      detection circuit directly. A further...
take advantage of the greater than 430 GHz precession rates of the magnetization

      vector of optomagnetic materials a...
will magnetize in accordance to the external field immediately following

               the laser pulse that heated it to...
Figure 15: The basic longitudinal Kerr (MOKE) effect [29]




The detection mechanism for the Kerr rotation can be a rotat...
Figure 16: Kerr rotation detection mechanism [26]




The Faraday rotation detection mechanism is similar, though the anal...
Figure 17: Kerr (MOKE) setup [29]




The difference in time between when the pump hits the sample and when the probe hits...
Figure 18: Entire setup for magneto-optic measurements, including laser sources [10]




When the Inverse-Faraday rotation...
memristance change) one pulse starts the precession of the magnetization vector

        and the other pulse comes and sto...
Figure 19: Optical magnetization can occur at the periphery of a Gaussian beam that excites a

                           ...
demagnetization (which can happen at the periphery of beams that heat a materialto its

Curie temperature, as reported in ...
Figure 22: Image of individual magnetic domains. This picture was not taken with a confocal

                             ...
References:
   [1] Dmitri B. Strukov, Gregory S. Snider, Duncan R. Stewart & R. Stanley

      Williams, The missing memri...
[9] J.-Y. Bigot , M. Vomir, L.H.F. Andrade, E. Beaurepaire, Ultrafast magnetization

      dynamics in ferromagnetic cobal...
[18]R. Wilks, R. J. Kicken, M. Ali, B. J. Hickey, J.D. R. Buchanan, A. T. G. Pym,

      and B. K. Tanner, J. Appl. Phys. ...
[25]C. D. Stanciu, F. Hansteen, A.V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and

      Th. Rasing, All-Optical Magneti...
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Nonthermal Opto-Magnetic Memristors and Characterization by Ultrafast Pump and Probe Polarimetric Microscopy

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Proposal for concepts for opto memristor and proposed microscopy techniques for testing the said optical memristor materials.

Apparently, Slideshare cannot have the page numbers (from MS Word documents) starting from zero, and it cannot display Word's automatically generated table of contents. So, the table of contents shown here is a graphic with the numbers displaced by one -- if the page number is said to be '1' then the material is on a page which is numbered as '2.' Sorry for the inconvenience, but this is a little out of my hands.

This is a follow up to:
http://www.slideshare.net/faissal.bd/draft-proposal-for-concepts-for-opto-memristor-and-proposed-microscope-design-for-testing-the-said-optical-memristor-materials

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Nonthermal Opto-Magnetic Memristors and Characterization by Ultrafast Pump and Probe Polarimetric Microscopy

  1. 1. Proposal: Nonthermal Opto-Magnetic Memristors and Characterization by Ultrafast Pump and Probe Polarimetric Microscopy M. Faisal Halim City College of New York (CUNY) Posted: Friday, 11th June, 2010 Contents and Page Numbers ABSTRACT: ________________________________________________________________________ 1 AIM: _______________________________________________________________________________ 2 INTRODUCTION: ___________________________________________________________________ 2 OPTICAL MEMRISTOR: _____________________________________________________________ 6 MECHANISM OF NONTHERMAL OPTICAL CONTROL OF MAGNETISM: _______________ 9 MECHANISM FOR MAXIMIZING COHERENT QUANTUM CONTROL OF SPINS: ________ 10 CONTROLLING MAGNETIZATION VECTOR P RECESSION – THE DOUBLE PULSE METHOD: _________________________________________________________________________ 11 OPTOMEMRISTANCE MEASUREMENTS: ___________________________________________ 14 OPTOMEMRISTOR ARCHITEC TURES: ______________________________________________ 15 OPTOMEMRISTOR DETECTION SCHEMES: _________________________________________ 16 MICROSCOPY REQUIREMENTS: ___________________________________________________ 19 MICROSCOPY, TECHNIQUES, AND IMPLICATIONS: _________________________________ 25 CONCLUSION: ____________________________________________________________________ 33 REFERENCES:_____________________________________________________________________ 34
  2. 2. Proposal: Nonthermal Opto-Magnetic Memristors and Characterization by Ultrafast Pump and Probe Polarimetric Microscopy M. Faisal Halim City College of New York (CUNY) Posted: Friday, 11th June, 2010 Course Name: Advanced Optical Microscopy Course Professor: Gilchrist, Chemical Engineering, City College of New York (CUNY) Departmental Advisors: Dorsinville and Walser, Electrical Engineering Department, City College of New York (CUNY) Abstract: In order to realize an optical memristor the Inverse-Faraday Effect has been investigated (through literature search) as a means for dynamically altering the magnetization vector of materials, thus altering their Magneto-Optic Kerr Rotation response and their Faraday Effect response faster than any other measurable change in response that can be engineered and measured, and microscopy methods have been investigated (through literature search) that can be used to characterize these responses from microscopic samples (since practical devices made from these materials will need to have microscopic components) at the speeds at which the optical memristor devices are intended to operate. M. Faisal Halim Opto-Magnetic Memristor 2
  3. 3. Aim: The purpose of this project was to devise methods to make an optical memristor, which will be an optical equivalent of HP’s electrical memristor [1], originally proposed by Leon Chua [2], and to devise methods to characterize the materials and devices, once they have been fabricated, in the microscopic size and ultrafast time regimes in which they are intended to be used. A possible application is all- optical neural networking. Introduction: Light propagates faster than electrons and holes and so the optical responses of materials occur at much shorter time scales than they occur for electrical responses. As a result there has been a push in the industry, for many years, to make optical equivalents of electrical devices. Out of the four fundamental passive electrical components possible [2] the memristor is one that brings neural networking closer to reality, thus enabling a whole host of applications, like simulating the complexity of a small animal’s brain, as was done by Jo, et al. [3]. At this junction lies the optical memristor: a (possible) enabling technology that will also have a speed advantage from being all-optical. For a device to be all optical, and thus have all the advantages of using light in place of electrons or holes, the signals that it receives have to be optical, the signals that it sends have to be optical, and the processing (which may be enhancement, attenuation, or change of polarization of the incoming light, or the addition or subtraction of two or more signals, etc.) has to occur due to the interaction of the electric or magnetic field of the M. Faisal Halim Opto-Magnetic Memristor 3
  4. 4. light wave(s) with the material that the device is made of. The result of the interaction may be steered (or influenced) by the presence of a constant field (electric or magnetic) that does not need to be manipulated by non-optical means (which would negate the speed advantages of a fully optical device). To realize a practical all-optical memristor, or optical memristor, one would need a device such that: • Exhibits the following behavior: o It should be possible to modify one aspect of its optical response quickly and dynamically (i.e., the same material/device will have one of its optical properties changed as and when needed, under optical excitations, and at time scales that will not create speed disadvantages). To make an analogy, think of a memristor as a variable resistor where one is using current to very quickly vary the resistance, rather than using a hand operated knob (which is relatively slow). An optical memristor, following this line of reasoning, is then something whose optical response can be changed (very quickly) using light. So, for example, a high intensity light pulse can be used to set the optical memristor’s “resistance” value, and a low intensity light pulse can then be used to take the desired reading. o The change should be thermally irreversible, and should occur only when exposed to light of sufficient intensity (lower intensity light pulses can then be used to take the reading that was intended). As a consequence of this the device’s setting is non-volatile – it does not change if one turns the device off. M. Faisal Halim Opto-Magnetic Memristor 4
  5. 5. o The changes made to the device’s behavior should be reversible. o The device’s setting should not be affected by taking a reading. o The contrast between the two extremes of the device’s output should be consistently unambiguous when measured. o The optical response of the device should not vary very sharply at either extreme of its parameters, but should rather have a smooth gradient so that its intermediate responses are easily accessible/measurable. This will make it possible to subdivide the range of parameters that the device can be operated with into smaller steps. Ideally, the device’s response should be a straight line through the origin (the gradient would have to be small enough so that the output readings can be taken unambiguously), where the abscissa denotes the state that the device has been set to and the ordinate denotes the resulting response to a signal that comes in to get processed/modified. o The device should be durable (i.e., it should have high fatigue resistance). Organic photochromophores, for examples, have the desired reversibility, but they break down after a few thousand transitions [4] – that is not desirable in a device which might go through those transitions within a fraction of a second. A practical optical memristor should not degrade with use from having its setting rewritten. • Fits the following requirements: o The physical dimensions of the device should be small, so that a large number of devices can be put on a small chip. M. Faisal Halim Opto-Magnetic Memristor 5
  6. 6. o The response time of the device should be small enough, as compared the existing electrical memristor, to justify the expense of development and implementation. • Does not depend too much on technology that has not yet been invented: o The materials and procedures for fabricating the device should ideally rely on mature technologies, so as to allow widespread use. o The methods required for characterization of the materials and devices should ideally not require too many innovations in characterization technology or paradigm so as to reduce the risk of creating a characterization method that may not work for the items being tested. It should be noted that while the memristors created by HP had variable resistances (though they did not exactly fit all of the mathematical criteria that Chua put forth [2]) one does not need to dogmatically stick to Chua’s definitions to make a practical device. In fact, one can choose any particular parameter one wants to (i.e., any one that one can) modify dynamically in order to make a passive device whose behavior can be modified to suit a dynamically changing requirement. Such a device would not be an optical memristor in the literal sense of the word, but it would certainly fit the bill in the figurative sense. So, one can make an optical memristor with a dynamically variable absorption coefficient, or a dynamically variable refractive index, or a dynamically variable polarization, etc. M. Faisal Halim Opto-Magnetic Memristor 6
  7. 7. Optical Memristor: The most promising route towards the realization of optical memristance (after discarding photochromic switching materials, for fatigue problems, and after discarding optically induced hysteresis in liquid crystals, for their lack of thermal stability) led towards optically induced changes in a material’s dipole moment and optical hysteresis: since magnetic materials have hysteresis curves and since light is an electromagnetic wave it should be possible to alter the magnetic state of a magnetic material using photonic excitation, and since magnetic states (in tapes, discs, etc.) are thermally stable the optically induced magnetization changes should also be thermally stable. It was found that Hansteen [5] and Stanciu [6] had already thought of this idea and successfully tested it, with Rasing, Kimel, Kirilyuk, Hunderi, et al [7-8], for the purpose of improving magneto-optical disc drives, which currently use a laser to irreversibly erase data (through demagnetization) and use an externally applied magnetic field to write new data. The primary difference between the kind of materials that [5-8] used and what an optical memristor would best use would be that an optical memristor’s memristive material (“memresponsive” would perhaps be more appropriate) would need to have the kind of shallow hysteresis curves (which do not change rapidly towards either extreme of memristance) that HP’s memristors had (the looping is not very important): M. Faisal Halim Opto-Magnetic Memristor 7
  8. 8. Figure 1: Shallow hysteresis curve for memristor, that does not change abruptly at either extreme [1] A material meant for recording binary information, on the other hand, requires a hysteresis curve that is steep, and sharp at the edges [9]: Figure 2: Steep hysteresis curve for recording binary bits – changes very abruptly [9] Optical memristance will also need to be very fast (to compete with the electrical memristor). Since it will have to be fast now, and it will need to be competitive in the future it cannot use an externally applied magnetic field to generate a magnetization vector within the optical memristor’s material (after it has been erased by an ultrafast laser that heats the material to its Curie temperature [10]) because despite M. Faisal Halim Opto-Magnetic Memristor 8
  9. 9. demagnetization being an ultrafast process “writing” a new magnetization vector into a material can be done faster than is possible by the application of a magnetic field to a hot magnetic material (the re-write frequency is hampered by the cooling time, and this method requires the material to dissipate heat quickly for the next re-write, thus introducing thermal management problems [11]). A faster process would be a nonthermal optical control of magnetism [7]: Figure 3: Time Scales for Magnetic and Optical Processes [7] Other advantages of nonthermal control are that deterministic magnetization cannot be achieved if an applied magnetic field pulse (no matter how strong) is shorter than 2 ps [12], and that spontaneous magnetization reversal does not occur under a nonthermal regime, which happens with magnetization of a material after thermalization to the Curie temperature, as was found during single-pulse magneto-optic microscopy experiments [13]. M. Faisal Halim Opto-Magnetic Memristor 9
  10. 10. Mechanism of Nonthermal Optical Control of Magnetism: The magneto-optical Faraday Effect [14] says that the magnetization vector of a material will affect light propagating through that material (for example, linearly polarized light passing through a magnetized material will act as if it went through a polarizer). The (opto-magnetic) Inverse-Faraday Effect, predicted by Pitaevskii [15], predicted that nonthermal optical control of magnetism would be possible (i.e., light could change the magnetization of a magnetic material that it was passing through, if it was intense enough), but there used to be doubts about that since producing the effect experimentally was challenging [16-18]. With the advent of ultrafast laser pulses, however, one can generate extremely strong fields, and this has now enabled the experimental observation of the Inverse Faraday Effect [7-8]. The Inverse Faraday Effect is essentially a photon-direction and spin preserving stimulated Raman Effect. Essentially, an electron in a non-degenerate state absorbs a higher energy photon – hω1 – (which has less energy than the material’s band gap, thus eliminating the possibility of electronic transitions and thus, large thermal effects) from a spectrally broad ultrashort pulse and goes through a spin flip in the ground state (which takes up energy hω1- hω2) before going up into a virtual state (so it is as if the electron was excited by a photon of lesser energy than the energy of the photon that it absorbed – hω2); then this electron undergoes stimulated emission upon being hit by another photon of energy hω2 and returns to the ground state. So, the number of photons is conserved, but one photon has given up part of its energy in flipping an electron’s spin. The hω1- hω2 value depends on the material and its temperature conditions. M. Faisal Halim Opto-Magnetic Memristor 10
  11. 11. Figure 4: Ultrafast spin-flip via the process of the stimulated Raman scattering [7]. Mechanism for Maximizing Coherent Quantum Control of Spins: A system will be set up to shape the beam that will be incident on the opto-memristor material so that the beam will have two peaks in the Fourier domain: one at hω1 and one at hω2. This will reduce the number of photons with unwanted energy values that compete for interactions with electrons. M. Faisal Halim Opto-Magnetic Memristor 11
  12. 12. Figure 5: Pulse shaping for reduce competition from photons of unwanted energy [7] Controlling Magnetization Vector Precession – the Double Pulse Method: A single pulse of circularly polarized light, in the absence of an external magnetic field will start the magnetization vector of the optomemristor material precessing, and it will keep precessing (with a speed dependent on the material properties, excitation parameters, and the beam propagation direction relative to the crystallographic axes) until the electrons go back to the ground state. M. Faisal Halim Opto-Magnetic Memristor 12
  13. 13. Figure 6: Taken from [7] If the electrons are hit with a second pulse while they are precessing then there will either be constructive or destructive interference of the spin precessions (if there is constructive interference then the precession still eventually dies, but the second pulse just adds its own amplitude and lifetime to the existing precession). If the second pulse’s amplitude and timing (delay) is just right then it should be possible to stop the precession of the magnetization vector at any desired direction, and this new direction will decide what angle of Faraday rotation the material will now put linearly polarized probe pulses M. Faisal Halim Opto-Magnetic Memristor 13
  14. 14. through. So, for an optomagnetic memristor the Faraday rotation of linearly polarized probe pulses will act as the ‘resistance’ (to form an analogy with the electronic memristor) and the Inverse Faraday Effect will be the means to modify this ‘resistance.’ Figure 7: Taken from [7] M. Faisal Halim Opto-Magnetic Memristor 14
  15. 15. Figure 8: Taken from [8] Optomemristor materials can be chosen for their precession speed, their spin-orbit coupling resonant frequency, or their bandgap. Considering that the Radboud University Nijmegen group [7-8] were able to use materials with different properties for their pulsed experiments it should be possible to fabricate optomemristors with various sets of advantages. Optomemristance Measurements: Optomemristance, as described above, can possibly be measured using the Faraday Effect [14], i.e., by measuring the rotation of linearly polarized light that has just passed through the sample, M. Faisal Halim Opto-Magnetic Memristor 15
  16. 16. or by measuring the MOKE (Magneto-Optic Kerr Effect) [19], which is the rotation of the polarization of linearly polarized light that has just been reflected from the sample. Optomemristor Architectures: For devices that will utilize MOKE the material can be a ferromagnetic or an antiferromagnetic, or a maybe a ferrimagnetic garnet, or (poly)crystalline thin film. Figure 9: Possible configuration for optomemristor device utilizing MOKE for probing For devices that will incorporate the Faraday Effect for taking readings garnets or orthoferrites [7,8] can be used, as well as films with quantum dots (metallic, as well as M. Faisal Halim Opto-Magnetic Memristor 16
  17. 17. semiconductors, such as CdSe [20]) held in position by a polymer matrix, perhaps with carbon nanotubes in the matrix, so as to enhance the Faraday rotation, since the carbon nanotubes will take on the surrounding magnetic field [21]. Figure 10: Possible configuration for optomemristor device utilizing Faraday Rotation for probing Optomemristor Detection Schemes: The probe readings that will be taken from the optomemristor/optomagnetic memristor proposed here will be in the form of MOKE rotation (Fig. 11) or Faraday Effect (Fig. 12) rotation. Therefore, the detector system will involve an analyzer that will block out scattered and transmitted light that has not undergone rotation, so that the rotated light (linearly polarized probe beam light whose polarization got rotated upon interaction with the sample) can excite photomultiplier tubes (PMTs) [22] or APDs. M. Faisal Halim Opto-Magnetic Memristor 17
  18. 18. Figure 11: Typical MOKE Setup [13] Figure 12: Typical Setup for Measuring Faraday Rotation [7] During development stages for a new process (i.e., when the method is still in its early stages) it may be necessary to probe not just the overall polarization rotation result, but M. Faisal Halim Opto-Magnetic Memristor 18
  19. 19. also rotation resulting from various depths of the sample. While MOKE systems will not allow probing very deep into the material they can still be probed layer by layer, so to speak. Such a system will be extremely useful for the development of an optical memristor, especially since such a device has never really been tried, and the uniformity of performance of material at the different depths may be important for ensuring that no part of an optomemristor material undergoes undue stress (which would degrade device longevity). Not only that, but it will be important to monitor the individual magnetic domains (size, shape, density – which will be especially important for composite materials consisting of nanomaterials embedded in polymer or some such matrix) within any optomemristor material. A confocal MOKE microscope that could be used for such a purpose can be found in [23]: Figure 13: Confocal MOKE Microscope [23] M. Faisal Halim Opto-Magnetic Memristor 19
  20. 20. Microscopy Requirements: An instrument suitable for characterizing opto-magnetic memristors will need the following features: 1. Ultrafast optical pump(s), available in the full range of polarizations, from varied orientations relative to probing 2. Ultrafast optical probes 3. Confocal excitation (short focal length optics, to ensure excitation within a voxel) 4. Confocal probing (independent of depth of region excited within sample) 5. High spatial resolution (so as to monitor individual magnetic domains) 6. High Polarimetric resolution (so that distinguishable outputs may be obtained for varied inputs to a sample) 7. Variable resolution (so that material films can be probed for device performance measurements as well as performance at the level of individual magnetic domains). 8. Faraday and MOKE configurations 9. Orientable electromagnets for supplying constant magnetic fields (for use if work is pursued into photomagnetic memristors) 10. Variable numerical apertures, for light delivery and collection (including solid immersion lens [24]), for investigating various approaches to thickness and magnetic domain densities (for the development of composites with magnetic material embedded in them, so, for example, magnetic quantum dots embedded in an organic matrix can be excited using two-photon absorption, leaving the matrix unharmed as its material can be chosen to not interact with the wavelength used for two-photon absorption) M. Faisal Halim Opto-Magnetic Memristor 20
  21. 21. It must be noted that regardless of the whether the probing is done by measurements of the Faraday Effect or MOKE the opto-magnetic pumping of the material (in order to ‘write’ information onto it) is still the same: the circularly polarized ultrafast light pulses are propagated through the optomagnetic memristor material (be it an orthoferrite, or a dielectric garnet, granular magnetic media, polycrystalline film, or a magnetic alloy [25]) and in the case of a crystalline material (orthoferrite or dielectric garnet) at an angle to one of the crystallographic axes [7,8]. Figure 14 Optical pulse propagation relative to crystallographic axes [8] The techniques for MOKE microscopy are quite mature, as they have been employed for a long time in the continued development of optical magnetic recording discs (magneto- optic discs, going by how the discs are read from and written to), along with scanning laser microscopy techniques (scanning laser microscopy is used in the development of technologies like the compact disc and the DVD). M. Faisal Halim Opto-Magnetic Memristor 21
  22. 22. The following are the tasks that a microscope that could be used towards the development of optomagnetic memristor materials will need to accomplish: 1. Generating ultrashort (400nm to 800nm wavelength femtosecond pulses now, and attosecond pulses in the future) laser pulses (at high repetition rates, to test how quickly the memristance value can be changed) of: a. High power for pumping, featuring: i. Circular polarization (right and left handed) – for use in the double pulse method of effecting changes to the optomagnetic memristance. ii. Linear polarization – for possible future photomagnetic memristance studies b. Low power for probing (i.e., probing of material performance), featuring: i. Linear polarization – so that the polarization rotation of the light can be used as a measure of memristance. NOTE: Since polarization rotation of linearly polarized light will change the intensity of light passing through a polarizer the intensity of light detected after the probe pulse passes through the polarizer can also be used as the measure of memristance. 2. Generating longer pulses for use with a laser coherence removal mechanism, like a rotating glass disc and glass fiber arrangement [26], for imaging individual magnetic domains in the reflective (MOKE) mode. This probe beam will still need to be polarized. The decoherence will prevent the formation of speckle patterns that result from interference of reflected laser radiation and thus give M. Faisal Halim Opto-Magnetic Memristor 22
  23. 23. clear pictures of what the individual magnetic domains look like (size, shape, position, density, and magnetization). Imaging of individual magnetic domains need not be done by single pulses, as this will only be used to measure the quality/ yield of the optomagnetization process, rather than gauge overall device performance directly; instead, this reading can be taken by integrating over multiple readings. 3. Guiding the pump pulse to the optomagnetic memristance material. 4. Guiding the probe pulse to the optomagnetic material from different position (i.e., the pump and probe pulses cannot be collinear) so that there is no risk that the high intensity pump pulse does not hit the detection optics and electronics – this will protect the detection optics and electronics from large doses of potentially damaging high intensity radiation. This may not be necessary if the said optics’ and detectors’ damage thresholds are above the intensity of the pump pulse. 5. Collecting light in the probe pulse that is reflected off the sample (for MOKE), or light in the probe pulse that passes through the sample (Faraday rotation), using an objective lens, and analyzing the collected light for polarization rotation – using either a polarizer-detector combination or a polarization beam splitter with a detector for both polarizations. NOTE: In the future the objective lens-polarizer-detector combination could be replaced by polarization sensitive films containing ordered nets of nanostructures that only absorb light of a particular polarization. So, instead of a polarizer simply absorbing light of one polarization and letting the other pass through to the detector head the polarizer will actually absorb the waves of the polarization that M. Faisal Halim Opto-Magnetic Memristor 23
  24. 24. one wants to detect (using surface plasmon coupling) and send a signal to the detection circuit directly. A further enhancement would be a single detector (which would act as a router for polarizations) that will use surface plasmon (SP) coupling to channel the signal from one polarization to one circuit and the signal of the other polarization to the other circuit [27]. 6. Exciting optomagnetic changes to a memristor material at various depths (using a confocal arrangement) , so as to measure how effectively the optomagnetic changes are effected (and if the material being tested end up having its magnetization vector in one direction at one depth and another direction at a different depth [26]), and how deep into a material optomagnetic changes can be made (this is especially important in metallic materials that will be used in the MOKE mode, as light does not penetrate very deeply into metals). 7. Reading the direction of the magnetization vector at various depths within the optomagnetic material – so an independent confocal magneto-optic (not optomagnetic, which is used to ‘write’ the direction of the magnetization vector of the material) reading setup will be required, and this reading setup should be able to work alongside the setup for writing the magnetization vector so that the speed at which memristance values can be written can be measured. 8. Providing multiple laser sources for pumps and probes so that memristance values can be ‘written’ to a device/material at GigaHertz speeds. Modern laser sources can provide pulse repetition rates in excess of 100 GHz [28], but higher speeds will be required in order to do multiple double pulse magnetization writings that M. Faisal Halim Opto-Magnetic Memristor 24
  25. 25. take advantage of the greater than 430 GHz precession rates of the magnetization vector of optomagnetic materials at room temperature [8]. 9. Low temperature readings (cryogenic conditions) for proof of concept work, because at this temperature while the magnetization precession rates of optomagnetic materials is lower (and hence easier to detect) the amplitude of the polarization rotations of the probe beam is also higher (also making a signal easier to detect). 10. Providing in-plane (to the sample) and out-of-plane electromagnetic field: a. For the suppression of magnetic precession – for possible use in future photomagnetic memristance studies. b. For finding the pump beam fluence threshold for heating any proposed memristor material to its Curie temperature – at this temperature the material heats up and looses its magnetization to spin scattering processes, through thermalization [10]. Optomagnetic (and photomagnetic) memristors will need to be written to by laser fluences that do not heat the material to its Curie temperature; though certain methods [25] will require getting very close to the Curie temperature of the material (albeit in the absence of an external magnetic field). The laser fluence that heats any given material to its Curie temperature could be found by taking magneto-optical readings after pumping with linearly polarized laser pulses of increasing fluence in the presence and absence of the external magnetic field. If this is done then that material M. Faisal Halim Opto-Magnetic Memristor 25
  26. 26. will magnetize in accordance to the external field immediately following the laser pulse that heated it to its Curie temperature. NOTE: A system for testing potential optomagnetic materials will thus involve two microscopes in one: one for pumping the sample, and one for probing it. Microscopy, Techniques, and Implications: Microscopy for the study of optomemristors will consist of studying materials like CoPt3 (polycrystalline) [22], GdFeCo (alloy) [25], Co-doped iron garnets [8], etc. in the opto- magnetic (or photomagnetic), and magneto-optic regimes, using beam spots of 300nm to 600nm diameter while maintaining the polarization state of the pump and probe light pulses (polarization state is deteriorated by reflective optics) and minimizing the group velocity dispersion of the pump and probe pulses (group velocity is dispersed during passage through transparent refractive media, like lenses) [22]. The basic longitudinal Kerr (MOKE) effect is shown in [29]. It clearly shows the possibility of differing magnetization directions at different depths within the material. M. Faisal Halim Opto-Magnetic Memristor 26
  27. 27. Figure 15: The basic longitudinal Kerr (MOKE) effect [29] The detection mechanism for the Kerr rotation can be a rotatable polarizer with a detector behind, or a polarizing beam splitter with a detector at each output port. M. Faisal Halim Opto-Magnetic Memristor 27
  28. 28. Figure 16: Kerr rotation detection mechanism [26] The Faraday rotation detection mechanism is similar, though the analyzer lies behind the sample. For microscopy using the Kerr effect it is important that the sample is illuminated obliquely, whereas for Faraday rotation the sample has to be illuminated near the normal. M. Faisal Halim Opto-Magnetic Memristor 28
  29. 29. Figure 17: Kerr (MOKE) setup [29] The difference in time between when the pump hits the sample and when the probe hits the sample is generated by making the pump and probe pulses go through slightly different path lengths (using a system of delay lines). M. Faisal Halim Opto-Magnetic Memristor 29
  30. 30. Figure 18: Entire setup for magneto-optic measurements, including laser sources [10] When the Inverse-Faraday rotation is to be used to opto-magnetically write a memristance value to the sample, of course, a setup like Figure 18: Entire setup for magneto-optic measurements, including laser sources [10] will need to be modified in that: 1. The beam/pulse coming out of the laser oscillator-amplifier system will need to be linearly polarized (this will be convenient). 2. The pump beam will need to be split evenly, with one beam going through a quarter wave plate to become right circularly polarized and the other beam going through another quarter wave plate to become left circularly polarized. 3. A slight path length difference will need to be introduced between the right and left circularly polarized pulses so that when they hit the sample (for double pulse M. Faisal Halim Opto-Magnetic Memristor 30
  31. 31. memristance change) one pulse starts the precession of the magnetization vector and the other pulse comes and stops the precession just as the magnetization vector has gone through the desired rotation (see Figure 8). 4. The probe pulse will have to go through a delay line that accounts for the probe pulses slowing down when they go through the quarter wave plates, and for what portion of the precession the probe pulse is going to be used to measure. 5. It must also be noted that experiments in writing values to the device (i.e., double pulsing multiple times) will require more copies of the laser oscillator-amplifier combination, all timed and coming in collinearly to the bottom third of Figure 18: Entire setup for magneto-optic measurements, including laser sources [10]. These changes will be required of the setup shown in Figure 18: Entire setup for magneto-optic measurements, including laser sources [10] regardless of whether MOKE or Faraday rotation will be used to change the direction of the magnetization vector of the sample. In a real world device the probe pulse rotation, which will depend on the opto-magnetic memristor’s magnetization direction (and with the probe pulse being the input), the output of the opto-magnetic memristor device can be the rotation of probe pulse. Alternatively, the output can be the intensity of light detected past a polarizer that sits after the opto-magnetic memristor’s optical material component: since the degree of probe beam polarization rotation will dictate what intensity of light gets past the polarizer. Of course, the readings will have to account for the fact that the opto-magnetic memristor material will give light that is slightly elliptically polarized, not just light that has had its polarization axis rotated. M. Faisal Halim Opto-Magnetic Memristor 31
  32. 32. Figure 19: Optical magnetization can occur at the periphery of a Gaussian beam that excites a sample to its Curie temperature [25] Figure 20: Illumination by coherent light produces speckle patterns, not domain images [26] In order to probe the response of regions around that which is being excited the most (i.e., where the beam intensity is highest) a confocal microscopy system can be used. Such a system can be used to find how far before and after the focal point, along the optical axis, a pump pulse was able to cause magnetization precession, as well as which parts of the exposed region underwent magnetization precession and which parts underwent M. Faisal Halim Opto-Magnetic Memristor 32
  33. 33. demagnetization (which can happen at the periphery of beams that heat a materialto its Curie temperature, as reported in [25]). A non-confocal image, in contrast, can only give information along the x-y axes, but not along the z-axis (the z-axis is the axis along which light propagates, and the x-y planes are all perpendicular to this axis) [30]. Advantage of Confocal Microscopy Detector only Probe receives Pulse light from point of interest Aperture Sample Figure 21: Advantage of Confocal Microscopy Confocal microscopy, when combined with a spatial resolution that can go down to 300 nm (for example, with a wavelength of 600nm and a beam spot diameter of 300nm, as in [22]) can be used to examine the individual magnetic domains of a material. M. Faisal Halim Opto-Magnetic Memristor 33
  34. 34. Figure 22: Image of individual magnetic domains. This picture was not taken with a confocal microscope. [8] Conclusion: Citing optically triggered and optically controlled precession of the magnetization vector in a photon field (following the Landau-Lifshitz equation in the nondissipative approximation [31]) an all optical memristor has been proposed and microscopy techniques have been suggested for characterization purposes during the development of the proposed device. The first such devices may be single element units, but future devices could be arrays of memristor materials (Figure 9, 10) that are pumped and probed synchronously by laser pulses (from multiple sources) that are distributed to the memristor elements much like a computer processor’s clock signals, thus helping towards applications like high speed neural networks. M. Faisal Halim Opto-Magnetic Memristor 34
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