Draft proposal for concepts for opto memristor and proposed microscope design for testing the said optical memristor materials. The follow up to this is here: http://www.slideshare.net/faissal.bd/nonthermal-optomagnetic-memristors-and-characterization-by-ultrafast-pump-and-probe-polarimetric-microscopy/v1

1. Proposal:
Microscope Design for Ultrafast Pump and Probe Polarimetric
Characterization of Proposed Novel Materials and Structures Intended
For Use As Optical Memristors Working In Nonthermal Opto-Magnetic
Regime
Faisal Halim, City College of New York (CUNY)
Posted: Monday, 31st May, 2010
Course Professor: Gilchrist, Chemical Engineering, City College of New York (CUNY)
NOTE: This is a draft – I have not yet completed the microscopy aspect of the project,
and I may have to make a few refinements to the optomemristor concept. The
optomemristor concept is my own, and relies on prior work from [5,6,7].
As I have no way to test my ideas they remain what I would describe as “crazy ideas,”
but if anyone find my work useful, then I would appreciate being cited. Thanks.
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

2. have microscopic components) at the speeds at which the optical memristor devices are
intended to operate.
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.
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

3. signals, etc.) has to occur due to the interaction of the electric or magnetic field of the
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

4. 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.
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:

5. o The physical dimensions of the device should be small, so that a large number
of devices can be put on a small chip.
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.

6. 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):

7. 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

8. 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].

9. 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.

10. 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.

11. 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.

12. 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).

13. Figure 7: Taken from [7]

14. Figure 8: Taken from [7]
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.

15. Optomemristance Measurements: Optomemristance, as described above,
can be measured using the Faraday Effect [14], i.e., by measuring the rotation of linearly
polarized light that has just passed through the sample, 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 Material Schemes: For devices that will utilize MOKE the material can
be a ferromagnetic or an antiferromagnetic, or a maybe a ferrimagnetic garnet, or
crystalline thin film.
Figure 9: Possible configuration for optomemristor device utilizing MOKE for probing

16. For devices that will incorporate the Faraday Effect for taking readings garnets or
crystalline films can be used, as well as films with quantum dots (metallic, as well as
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) Faraday Effect (Fig. 12) rotations. 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

17. polarization got rotated upon interaction with the sample) can excite photomultiplier
tubes (PMTs) [22] or APDs.
Figure 11: Typical MOKE Setup [13]
Figure 12: Typical Setup for Measuring Faraday Rotation [7]

18. 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
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). A confocal MOKE microscope that could be used for such a purpose can be
found in [23]:
Figure 13: Confocal MOKE Microscope [23]
Microscopy Requirements: Given that the each mem

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