CA2341817C - Method and apparatus for generating and controlling spin propagation using multiple coherent light beams - Google Patents
Method and apparatus for generating and controlling spin propagation using multiple coherent light beams Download PDFInfo
- Publication number
- CA2341817C CA2341817C CA 2341817 CA2341817A CA2341817C CA 2341817 C CA2341817 C CA 2341817C CA 2341817 CA2341817 CA 2341817 CA 2341817 A CA2341817 A CA 2341817A CA 2341817 C CA2341817 C CA 2341817C
- Authority
- CA
- Canada
- Prior art keywords
- coherent light
- spin
- light beams
- photoconductor
- beams
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
- 230000001427 coherent effect Effects 0.000 title claims abstract description 49
- 238000000034 method Methods 0.000 title claims abstract description 37
- 230000010287 polarization Effects 0.000 claims abstract description 20
- 239000000463 material Substances 0.000 claims abstract description 16
- 230000001678 irradiating effect Effects 0.000 claims abstract description 5
- 239000004065 semiconductor Substances 0.000 claims description 8
- 239000002086 nanomaterial Substances 0.000 claims description 3
- 238000010521 absorption reaction Methods 0.000 abstract description 2
- 239000003086 colorant Substances 0.000 abstract 1
- 239000000969 carrier Substances 0.000 description 18
- 230000003287 optical effect Effects 0.000 description 5
- 230000000694 effects Effects 0.000 description 4
- 238000004020 luminiscence type Methods 0.000 description 4
- 239000013598 vector Substances 0.000 description 4
- 239000013078 crystal Substances 0.000 description 2
- 229910001218 Gallium arsenide Inorganic materials 0.000 description 1
- 239000002800 charge carrier Substances 0.000 description 1
- 230000005493 condensed matter Effects 0.000 description 1
- 238000013500 data storage Methods 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000005294 ferromagnetic effect Effects 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 238000002347 injection Methods 0.000 description 1
- 239000007924 injection Substances 0.000 description 1
- 230000005291 magnetic effect Effects 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000003672 processing method Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/0126—Opto-optical modulation, i.e. control of one light beam by another light beam, not otherwise provided for in this subclass
Landscapes
- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- Electromagnetism (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Computer Hardware Design (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Power Engineering (AREA)
- Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
Abstract
The present invention uses quantum interference of one and two photon absorption from a multiple color fields to optically inject ballistic spin currents in unbiased photoconductors. The spin currents can be generated with and without an accompanying electrical current and can be controlled using the relative phase of the colors. In one aspect of the there is provided a method of generating spin currents in a photoconductor material comprising producing a first coherent light beam having a first frequency .omega.1 and a second coherent light beam having a frequency twice the first frequency 2.omega.1, polarizing the first and second coherent light beams to have a preselected polarization with respect to each other, and simultaneously irradiating a selected region of the photoconductor material with the first coherent light beam and the second coherent light beam to generate a spin current in the photoconductor.
Description
FIELD OF THE INVENTION
This invention relates to a method and apparatus for the generation and control of spin currents, comprising spin polarized charge carriers, in photoconductors. More particularly the present invention provides a method of using the polarization properties of multiple coherent light beams, and phase differences between multiple coherent light beams, to control the magnitude and direction of spin currents in a photoconductor.
BACKGROUND OF THE INVENTION
The control of electronic spin in semiconductors is important for the study of spin dynamics in many-body systems and crucial for the development of new data storage and processing methods based on the spin degree of freedom of charged particles. This will be essential as a first step towards a solid state implementation of a quantum computer; see, e.g. D. D. Awschalom and J.M.
Kikkawa, Phys. Today 52, No. 6, 33 (1999).
There has been considerable work on achieving spin-polarized currents in semiconductors using transport in the presence of magnetic impurities, see M.
Oestreich et al. Appl. Phys. Lett. 74, 1251 (1999), R. Fiederling et al., Nature (London) 402, 787, (1999) and Y. Ohno et al., Nature (London) 402, 790, (1999), or using injection of carriers from a ferromagnetic contact, see P.R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999), and S. Gardelis et al., Phys. Rev. B 60, (1999). In these cases a voltage applied across the semiconductor drives the spin current.
It is known that spin-polarized carriers can be optically injected into a semiconductor using circularly polarized light, see United States Patent No.
3,968,376, and M. I. Dyakonov and V.I. Perel, in Optical Orientation, edited by F.
Meier and B. P. Zakharchenya, Modern Problems in Condensed Matter Sciences, Vol. 8 (North-Holland, Amsterdam, 1984), Chapter 2. A spin current may be generated from these spin-polarized carriers by applying a voltage across the semiconductor, see D. Hagele et al., Appl. Phys. Lett. 73, 1580 (1980), and J.M. Kikkawa and D. D. Awschalom, Nature (London) 397, 139 (1999).
All of the above methods use a voltage difference to move the carriers (electrons and holes), and hence there is always an electrical current as well as a spin current. As well, the spin currents can only be modulated as fast as the voltage difference can be modulated.
United States Patent No. 5,790,296 discloses a method for generating and controlling an electrical current in a semiconductor using the interference between multiple laser beams. This patent is restricted to the ways in which multiple light beams can be used to generate and control electrical currents, and does not discuss how to generate and control spin-polarized currents.
It would therefore be very advantageous to provide a method of generating polarized spin currents in photoconductors that can be modulated on ultrafast timescales without the need for a bias voltage to be applied.
This invention relates to a method and apparatus for the generation and control of spin currents, comprising spin polarized charge carriers, in photoconductors. More particularly the present invention provides a method of using the polarization properties of multiple coherent light beams, and phase differences between multiple coherent light beams, to control the magnitude and direction of spin currents in a photoconductor.
BACKGROUND OF THE INVENTION
The control of electronic spin in semiconductors is important for the study of spin dynamics in many-body systems and crucial for the development of new data storage and processing methods based on the spin degree of freedom of charged particles. This will be essential as a first step towards a solid state implementation of a quantum computer; see, e.g. D. D. Awschalom and J.M.
Kikkawa, Phys. Today 52, No. 6, 33 (1999).
There has been considerable work on achieving spin-polarized currents in semiconductors using transport in the presence of magnetic impurities, see M.
Oestreich et al. Appl. Phys. Lett. 74, 1251 (1999), R. Fiederling et al., Nature (London) 402, 787, (1999) and Y. Ohno et al., Nature (London) 402, 790, (1999), or using injection of carriers from a ferromagnetic contact, see P.R. Hammar et al., Phys. Rev. Lett. 83, 203 (1999), and S. Gardelis et al., Phys. Rev. B 60, (1999). In these cases a voltage applied across the semiconductor drives the spin current.
It is known that spin-polarized carriers can be optically injected into a semiconductor using circularly polarized light, see United States Patent No.
3,968,376, and M. I. Dyakonov and V.I. Perel, in Optical Orientation, edited by F.
Meier and B. P. Zakharchenya, Modern Problems in Condensed Matter Sciences, Vol. 8 (North-Holland, Amsterdam, 1984), Chapter 2. A spin current may be generated from these spin-polarized carriers by applying a voltage across the semiconductor, see D. Hagele et al., Appl. Phys. Lett. 73, 1580 (1980), and J.M. Kikkawa and D. D. Awschalom, Nature (London) 397, 139 (1999).
All of the above methods use a voltage difference to move the carriers (electrons and holes), and hence there is always an electrical current as well as a spin current. As well, the spin currents can only be modulated as fast as the voltage difference can be modulated.
United States Patent No. 5,790,296 discloses a method for generating and controlling an electrical current in a semiconductor using the interference between multiple laser beams. This patent is restricted to the ways in which multiple light beams can be used to generate and control electrical currents, and does not discuss how to generate and control spin-polarized currents.
It would therefore be very advantageous to provide a method of generating polarized spin currents in photoconductors that can be modulated on ultrafast timescales without the need for a bias voltage to be applied.
SUMMARY OF THE INVENTION
The present invention provides a method of generating in photoconductors polarized spin currents that can be modulated on ultrafast timescales without the need for a bias voltage to be applied.
S As used herein, the term spin current means a current of charges (such as electrons and holes) which are spin polarized. The method utilizes the quantum interference of one-and two-photon absorption processes in a light field produced preferably by multiple laser beams. Spin currents can be produced with or without accompanying net electrical currents depending on the polarization of the beams with respect to each other. The magnitude and direction of the spin currents are determined by the phase difference between multiple laser beams, and the polarization of the beams.
In one aspect of the invention there is provided a method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing a first coherent light beam having a first frequency c~~ and a second coherent light beam having a frequency twice the first frequency 2c~~, polarizing said first and second coherent light beams to have a preselected polarization with respect to each other, and simultaneously irradiating a selected region of the photoconductor material with said first coherent light beam and said second coherent light beam to generate a spin current in said photoconductor.
The present invention is not restricted to a requirement for two coherent light beams. Thus, in anther aspect fo the invention there is provided a method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing at least three coherent light beams of frequencies c~~, w2, and w3, such that UJ~=OJ2~'GJ3, polarizing each of said at least three coherent light beams to have a preselected polarization with respect to the other coherent light beams, and simultaneously irradiating a selected region of the photoconductor material with said at least three coherent light beams to generate a spin current in said photoconductor.
In the above aspects of the invention the method may include adjusting a phase relationship between the coherent light beams to change the direction of the spin current generated in the photoconductor.
While using multiple laser beams is a preferred embodiment, the method of the present invention may also be achieved using single optical pulses that contain within the pulses the multiple frequency components required to give the 1 S same effect achieved using multiple laser beams.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:
Figure 1 shows a block diagram of an apparatus used to produce spin currents using two coherent light beams in accordance with the present invention;
The present invention provides a method of generating in photoconductors polarized spin currents that can be modulated on ultrafast timescales without the need for a bias voltage to be applied.
S As used herein, the term spin current means a current of charges (such as electrons and holes) which are spin polarized. The method utilizes the quantum interference of one-and two-photon absorption processes in a light field produced preferably by multiple laser beams. Spin currents can be produced with or without accompanying net electrical currents depending on the polarization of the beams with respect to each other. The magnitude and direction of the spin currents are determined by the phase difference between multiple laser beams, and the polarization of the beams.
In one aspect of the invention there is provided a method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing a first coherent light beam having a first frequency c~~ and a second coherent light beam having a frequency twice the first frequency 2c~~, polarizing said first and second coherent light beams to have a preselected polarization with respect to each other, and simultaneously irradiating a selected region of the photoconductor material with said first coherent light beam and said second coherent light beam to generate a spin current in said photoconductor.
The present invention is not restricted to a requirement for two coherent light beams. Thus, in anther aspect fo the invention there is provided a method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing at least three coherent light beams of frequencies c~~, w2, and w3, such that UJ~=OJ2~'GJ3, polarizing each of said at least three coherent light beams to have a preselected polarization with respect to the other coherent light beams, and simultaneously irradiating a selected region of the photoconductor material with said at least three coherent light beams to generate a spin current in said photoconductor.
In the above aspects of the invention the method may include adjusting a phase relationship between the coherent light beams to change the direction of the spin current generated in the photoconductor.
While using multiple laser beams is a preferred embodiment, the method of the present invention may also be achieved using single optical pulses that contain within the pulses the multiple frequency components required to give the 1 S same effect achieved using multiple laser beams.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:
Figure 1 shows a block diagram of an apparatus used to produce spin currents using two coherent light beams in accordance with the present invention;
Figure 2 shows an alternative embodiment of an apparatus used to produce spin currents using two coherent light beams; and Figure 3 shows schematic illustrations of the net electron motion for (a) the case with both beams right circularly polarized using the apparatus of Figure 2, and (b) the case with the coherent light beams cross-polarized.
DETAILED DESCRIPTION OF THE INVENTION
The spin current is denoted by the second rank pseudotensor Kab=<Sa~'>
where <Sa~'> denotes the average of the product of carrier spin and velocity, the vector S is the carrier spin, and the vector v is the carrier velocity.
Superscript letters denote Cartesian components of vectors or tensors, and can take on one of the values x, y, or z. Without limiting the invention, we expect the spin current satisfies K ab = ~ abcde (E(o ~ ~ ~, ~~2 Ew 3 + C.C. ~ K ab /T ~1~
IS
where E~,~, E~,2, and Ew3 are the (complex) field vector amplitudes of the beams, K
is the time rate of change of the spin current pseudotensor, ~ is a fifth rank material response pseudotensor, and T is a phenomenological relaxation time.
For materials with cubic or isotropic symmetry, ~ can be written in terms of only four parameters C~_4 as abcde = C,f ~ ad ~ bce + ~ ae~ bcd ~ C,Z ~ bd E ace + b 6e~ acd ~ C,3S deE abc + C4 ~ cd E abe + ~ ceE abd where 8ab IS a Kronecker delta, and sabc is the Levi-Cevita tensor.
There are two cases of interest for these materials. We will assume the beams are co-propagating along the z-axis. The first case is where the beams cocircularly polarized all having the same circular polarization. In this case, there is a net current from the interference of the beams as described in United States Patent No. 5,790,296. The electrical current is in the plane perpendicular to the beam propagation direction, and its direction in that plane depends on the relative phase of the beams. Calling that direction m, we have m = x sin~0~ ~~ y cos~0~
where the top sign is for right-circularly polarized beams and the bottom sign is for left-circularly polarized beams, and ~~ -'Yw2 +'1'm3 ~wl The optically injected carriers have a net spin along the axis of propagation of the coherent light beams, thus the current is spin polarized.
Even though the carriers should only be 50% spin polarized, the current can have a higher degree of spin polarization: 57% for GaAs, see R.D.R. Bhat and J.E.
Sipe Phys. Rev. Lett. 85, 5432 (2000). There will also be a spin current such that the spin component along m of carriers with a component of motion along positive z will be opposite to the spin component along m of carriers with a component of motion along negative z. This is a pure spin current, since there is no electrical current along the z direction. Figure 3(a) shows a schematic illustration of the net electron motion for with both beams right circularly polarized using the apparatus of Figure 2 discussed hereinafter.
The second case of interest is when the beams have crossed linear polarization, such that for example, the c~~ beam is polarized along y, while the other two beams are polarized along x. In this case, there is no net spin polarization of the carriers, but there are spin currents nonetheless. The electrical current as described in United States Patent No. 5,790,296 (which is incorporated herein in its entirety) is in the direction of the polarization of the c~, beam and its magnitude depends sinusoidally on ~~. In the present invention there are pure spin currents perpendicular to the electrical current. Carriers with a component of motion along positive x will have their spin along z opposite to carriers with a component of motion along negative x. Also carriers with a component of motion along positive z will have their spin along x opposite to carriers with a component of motion along negative z. Both of these pure spin currents will have a magnitude which depends on the cosine of o~. Figure 3(b) shows a schematic illustration of the net electron motion obtained with the cross-polarized coherent light beams using the apparatus of Figure 1 discussed hereinafter.
The two cases described above for cubic or isotropic materials are illustrative important examples. In general the beams need not be co-propagating, and other polarization combinations may be used. Further, the method disclosed herein for producing spin currents does not rely on any specific crystal symmetry so that materials of different symmetry could be used. In the general case, the spin current is contemplated by the inventors to still be well described by equation (1 ). Further, it is contemplated that spin currents can be produced in materials having nanostructure geometries using the method disclosed herein.
Even more generally, one could replace any one of the beams by one or more beams of lower frequency but higher intensity such that the role of each photon from the original beam is taken on by an odd number of photons from the new beams. For example, a beam of frequency c~2 may be replaced by two beams of frequencies G)A=c~2/3 and wB=2w2/3, so that the role of each photon of frequency w2 is replaced by two photons of frequency c~A and one of frequency wB.
An apparatus 10 for producing spin current in a two color field using two coherent light beams of frequency c~9 and 2~g is shown in Figure 1. A light source 12 produces a coherent light beam 14, such as a laser beam. An example source 12 may be an actively mode locked picosecond Ti:sapphire laser operating at 800 nm with a corresponding frequency w9. The first beam wg pumps an optical parametric generator 16. A lens L1 having a focal length of f=5 cm focuses the light beam wg passing through the chopper 20 onto a 1 mm thick ~3-barium borate (BBO) crystal 30 using type I phase matching which generates a second beam (hereinafter 2wg) as the second harmonic of the first beam w9.
The two beams w9 and 2w9 are focused by curved mirror MC1 to the flat mirror MF3, which directs the two beams to the planar dichroic mirror D set at an angle of 45 degrees with respect to the direction of the beams. The wg beam is transmitted by dichroic plate D, whereupon it is back-reflected by flat mirror that can be translated by a piezoelectric transducer (PZT) to control the relative phase of the two beams. The 2wg beam is simply reflected by D and then back-s reflected by flat mirror MFS. The two beams are reflected off (2wg), and transmitted through (w9), dichroic mirror D towards planar mirror MC2 which reflects both beams onto curved mirror MC2 which in turn focuses both beams onto a selected area on photoconductor 26. The two beams, after being back-reflected from MF5 and MF4 off, and through D, will have crossed linear polarizations with respect to each other.
The two cross-polarized beams wg and 2wg are focused onto the surface close to one side of the photoconductor 26, and a polarizer 32 is placed in front of the photoconductor 26 to analyze the polarization of the luminescence emitted from the photoconductor. The luminescence is collected by a photodetector 34 that is connected to the lock-in amplifier 22. Apparatus 10 includes a chopper connected to a lock-in amplifier 22. In combination the chopper 20 and lock-in amplifier 22 average the signals produced by the coherent light beams.
Detection of the polarization of the luminescence will in effect measure the spin current because the carriers scattering off the edge 27 of the photoconductor 26 will have their spins randomized. Those spin-polarized carriers moving in the opposite direction in photoconductor 26 away from edge 27 will not have their spins randomized as quickly, and thus if the spins moving in the opposite direction have opposite spins, the result will be a net spin polarization of the carriers that will be seen through the degree of circular polarization of the luminescence emitted from the photoconductor.
Other polarization combinations can also be realized. For example, referring to Figure 2, co-circularly polarized beams are produced by modifying the apparatus of Figure 1 to give apparatus 40 by the addition of a 7~/8 waveplate 42 into the optical circuit between dichroic mirror D and mirror MF4 through which the w9 beam is transmitted twice and a ~,/8 waveplate 44 between dichroic mirror D and mirror MF5 through which the 2w9 beam is transmitted twice.
The embodiments of the apparatus shown in Figures 1 and 2 for producing spin currents used only two beams, such that one has twice the frequency of the other. However, the method of producing spin currents in accordance with the present invention may be implemented in general with three beams of frequencies c~~, c~2, and c~3, such that Co~=G)2+w3. The magnitude of the frequencies should be such that the beam with largest frequency has a photon energy that is large enough to excite carriers across the bandgap of the photoconductor. If the photon energy is too large such that it can excite carriers from the spin-orbit split-off band, the magnitude of the effect will be decreased.
While using multiple coherent light beams such as laser beams is a preferred embodiment, the method of the present invention may also be achieved using single optical pulses short enough to give the required bandwidth that contain within the pulses the multiple frequency components required to give the same effect achieved using multiple laser beams.
The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.
DETAILED DESCRIPTION OF THE INVENTION
The spin current is denoted by the second rank pseudotensor Kab=<Sa~'>
where <Sa~'> denotes the average of the product of carrier spin and velocity, the vector S is the carrier spin, and the vector v is the carrier velocity.
Superscript letters denote Cartesian components of vectors or tensors, and can take on one of the values x, y, or z. Without limiting the invention, we expect the spin current satisfies K ab = ~ abcde (E(o ~ ~ ~, ~~2 Ew 3 + C.C. ~ K ab /T ~1~
IS
where E~,~, E~,2, and Ew3 are the (complex) field vector amplitudes of the beams, K
is the time rate of change of the spin current pseudotensor, ~ is a fifth rank material response pseudotensor, and T is a phenomenological relaxation time.
For materials with cubic or isotropic symmetry, ~ can be written in terms of only four parameters C~_4 as abcde = C,f ~ ad ~ bce + ~ ae~ bcd ~ C,Z ~ bd E ace + b 6e~ acd ~ C,3S deE abc + C4 ~ cd E abe + ~ ceE abd where 8ab IS a Kronecker delta, and sabc is the Levi-Cevita tensor.
There are two cases of interest for these materials. We will assume the beams are co-propagating along the z-axis. The first case is where the beams cocircularly polarized all having the same circular polarization. In this case, there is a net current from the interference of the beams as described in United States Patent No. 5,790,296. The electrical current is in the plane perpendicular to the beam propagation direction, and its direction in that plane depends on the relative phase of the beams. Calling that direction m, we have m = x sin~0~ ~~ y cos~0~
where the top sign is for right-circularly polarized beams and the bottom sign is for left-circularly polarized beams, and ~~ -'Yw2 +'1'm3 ~wl The optically injected carriers have a net spin along the axis of propagation of the coherent light beams, thus the current is spin polarized.
Even though the carriers should only be 50% spin polarized, the current can have a higher degree of spin polarization: 57% for GaAs, see R.D.R. Bhat and J.E.
Sipe Phys. Rev. Lett. 85, 5432 (2000). There will also be a spin current such that the spin component along m of carriers with a component of motion along positive z will be opposite to the spin component along m of carriers with a component of motion along negative z. This is a pure spin current, since there is no electrical current along the z direction. Figure 3(a) shows a schematic illustration of the net electron motion for with both beams right circularly polarized using the apparatus of Figure 2 discussed hereinafter.
The second case of interest is when the beams have crossed linear polarization, such that for example, the c~~ beam is polarized along y, while the other two beams are polarized along x. In this case, there is no net spin polarization of the carriers, but there are spin currents nonetheless. The electrical current as described in United States Patent No. 5,790,296 (which is incorporated herein in its entirety) is in the direction of the polarization of the c~, beam and its magnitude depends sinusoidally on ~~. In the present invention there are pure spin currents perpendicular to the electrical current. Carriers with a component of motion along positive x will have their spin along z opposite to carriers with a component of motion along negative x. Also carriers with a component of motion along positive z will have their spin along x opposite to carriers with a component of motion along negative z. Both of these pure spin currents will have a magnitude which depends on the cosine of o~. Figure 3(b) shows a schematic illustration of the net electron motion obtained with the cross-polarized coherent light beams using the apparatus of Figure 1 discussed hereinafter.
The two cases described above for cubic or isotropic materials are illustrative important examples. In general the beams need not be co-propagating, and other polarization combinations may be used. Further, the method disclosed herein for producing spin currents does not rely on any specific crystal symmetry so that materials of different symmetry could be used. In the general case, the spin current is contemplated by the inventors to still be well described by equation (1 ). Further, it is contemplated that spin currents can be produced in materials having nanostructure geometries using the method disclosed herein.
Even more generally, one could replace any one of the beams by one or more beams of lower frequency but higher intensity such that the role of each photon from the original beam is taken on by an odd number of photons from the new beams. For example, a beam of frequency c~2 may be replaced by two beams of frequencies G)A=c~2/3 and wB=2w2/3, so that the role of each photon of frequency w2 is replaced by two photons of frequency c~A and one of frequency wB.
An apparatus 10 for producing spin current in a two color field using two coherent light beams of frequency c~9 and 2~g is shown in Figure 1. A light source 12 produces a coherent light beam 14, such as a laser beam. An example source 12 may be an actively mode locked picosecond Ti:sapphire laser operating at 800 nm with a corresponding frequency w9. The first beam wg pumps an optical parametric generator 16. A lens L1 having a focal length of f=5 cm focuses the light beam wg passing through the chopper 20 onto a 1 mm thick ~3-barium borate (BBO) crystal 30 using type I phase matching which generates a second beam (hereinafter 2wg) as the second harmonic of the first beam w9.
The two beams w9 and 2w9 are focused by curved mirror MC1 to the flat mirror MF3, which directs the two beams to the planar dichroic mirror D set at an angle of 45 degrees with respect to the direction of the beams. The wg beam is transmitted by dichroic plate D, whereupon it is back-reflected by flat mirror that can be translated by a piezoelectric transducer (PZT) to control the relative phase of the two beams. The 2wg beam is simply reflected by D and then back-s reflected by flat mirror MFS. The two beams are reflected off (2wg), and transmitted through (w9), dichroic mirror D towards planar mirror MC2 which reflects both beams onto curved mirror MC2 which in turn focuses both beams onto a selected area on photoconductor 26. The two beams, after being back-reflected from MF5 and MF4 off, and through D, will have crossed linear polarizations with respect to each other.
The two cross-polarized beams wg and 2wg are focused onto the surface close to one side of the photoconductor 26, and a polarizer 32 is placed in front of the photoconductor 26 to analyze the polarization of the luminescence emitted from the photoconductor. The luminescence is collected by a photodetector 34 that is connected to the lock-in amplifier 22. Apparatus 10 includes a chopper connected to a lock-in amplifier 22. In combination the chopper 20 and lock-in amplifier 22 average the signals produced by the coherent light beams.
Detection of the polarization of the luminescence will in effect measure the spin current because the carriers scattering off the edge 27 of the photoconductor 26 will have their spins randomized. Those spin-polarized carriers moving in the opposite direction in photoconductor 26 away from edge 27 will not have their spins randomized as quickly, and thus if the spins moving in the opposite direction have opposite spins, the result will be a net spin polarization of the carriers that will be seen through the degree of circular polarization of the luminescence emitted from the photoconductor.
Other polarization combinations can also be realized. For example, referring to Figure 2, co-circularly polarized beams are produced by modifying the apparatus of Figure 1 to give apparatus 40 by the addition of a 7~/8 waveplate 42 into the optical circuit between dichroic mirror D and mirror MF4 through which the w9 beam is transmitted twice and a ~,/8 waveplate 44 between dichroic mirror D and mirror MF5 through which the 2w9 beam is transmitted twice.
The embodiments of the apparatus shown in Figures 1 and 2 for producing spin currents used only two beams, such that one has twice the frequency of the other. However, the method of producing spin currents in accordance with the present invention may be implemented in general with three beams of frequencies c~~, c~2, and c~3, such that Co~=G)2+w3. The magnitude of the frequencies should be such that the beam with largest frequency has a photon energy that is large enough to excite carriers across the bandgap of the photoconductor. If the photon energy is too large such that it can excite carriers from the spin-orbit split-off band, the magnitude of the effect will be decreased.
While using multiple coherent light beams such as laser beams is a preferred embodiment, the method of the present invention may also be achieved using single optical pulses short enough to give the required bandwidth that contain within the pulses the multiple frequency components required to give the same effect achieved using multiple laser beams.
The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.
Claims (15)
1. A method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing a first coherent light beam having a first frequency .omega.1 and a second coherent light beam having a frequency twice the first frequency
producing a first coherent light beam having a first frequency .omega.1 and a second coherent light beam having a frequency twice the first frequency
2.omega.1, polarizing said first and second coherent light beams to have a preselected polarization with respect to each other, and simultaneously irradiating a selected region of the photoconductor material with said first coherent light beam and said second coherent light beam to generate a spin current in said photoconductor.
2. The method according to claim 1 including adjusting a phase relationship between the first and second coherent light beams to change the direction of the spin current generated in said photoconductor.
2. The method according to claim 1 including adjusting a phase relationship between the first and second coherent light beams to change the direction of the spin current generated in said photoconductor.
3. The method according to claims 1 or 2 wherein said first coherent light beam and said second coherent light beam are copropagating.
4. The method according to claim 3 wherein said copropagating first and second coherent light beams are linearly cross-polarized, and wherein said spin current is a pure spin current in which spin is transported but not electrical current.
5. The method according to claim 3 wherein said copropagating first and second coherent light beams are circularly polarized in the same direction, and wherein said spin current includes a pure spin current in the direction of propagation of the coherent light beams and a spin polarized electrical current in which both spin and electrical charge are transported in a direction perpendicular to the direction of propagation of the coherent light beams.
6. The method according to claim 5 wherein said first and second circularly polarized coherent light beams are right circularly polarized.
7. The method according to claim 5 wherein said first and second circularly polarized coherent light beams are left circularly polarized.
8. The method according to claim 3 wherein said copropagating first and second coherent light beams are colinearly polarized.
9. The method according to claims 1, 2, 3, 4, 5, 6, 7 or 8 wherein said photoconductor is a semiconductor.
10. The method according to claims 1, 2, 3, 4, 5, 6, 7, 8 or 9 wherein said photoconductor has a nanostructure geometry.
11. A method of generating spin currents in a photoconductor material, the method comprising the steps of:
producing at least three coherent light beams of frequencies .omega.1, .omega.2, and .omega.3, such that .omega.1=.omega.2+.omega.3, polarizing each of said at least three coherent light beams to have a preselected polarization with respect to the other coherent light beams, and simultaneously irradiating a selected region of the photoconductor material with said at least three coherent light beams to generate a spin current in said photoconductor.
producing at least three coherent light beams of frequencies .omega.1, .omega.2, and .omega.3, such that .omega.1=.omega.2+.omega.3, polarizing each of said at least three coherent light beams to have a preselected polarization with respect to the other coherent light beams, and simultaneously irradiating a selected region of the photoconductor material with said at least three coherent light beams to generate a spin current in said photoconductor.
12. The method according to claim 11 including adjusting a phase relationship between said at least three coherent light beams to change the direction of the spin current generated in said photoconductor.
13. The method according to claims 11 or 12 wherein said at least three coherent light beams are copropagating.
14. The method according to claims 11, 12 or 13 wherein said photoconductor is a semiconductor.
15. The method according to claims 11, 12, 13 or 14 wherein said photoconductor has a nanostructure geometry.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2341817 CA2341817C (en) | 2001-03-22 | 2001-03-22 | Method and apparatus for generating and controlling spin propagation using multiple coherent light beams |
| CA002378560A CA2378560A1 (en) | 2001-03-22 | 2002-03-22 | Method and apparatus for generating and controlling spin propagation using one or more coherent light beams |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2341817 CA2341817C (en) | 2001-03-22 | 2001-03-22 | Method and apparatus for generating and controlling spin propagation using multiple coherent light beams |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2341817A1 CA2341817A1 (en) | 2002-09-22 |
| CA2341817C true CA2341817C (en) | 2011-01-25 |
Family
ID=4168666
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2341817 Expired - Fee Related CA2341817C (en) | 2001-03-22 | 2001-03-22 | Method and apparatus for generating and controlling spin propagation using multiple coherent light beams |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2341817C (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP3472657A1 (en) * | 2016-06-17 | 2019-04-24 | Sorbonne Université | Device for illuminating an object with a controlled light intensity and associated method |
-
2001
- 2001-03-22 CA CA 2341817 patent/CA2341817C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2341817A1 (en) | 2002-09-22 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| Neshev et al. | Observation of discrete vortex solitons in optically induced photonic lattices | |
| Stevens et al. | Quantum interference control of ballistic pure spin currents in semiconductors | |
| Stevens et al. | Coherent control of an optically injected ballistic spin-polarized current in bulk GaAs | |
| Bartels et al. | Identification of higher-order electronic coherences in semiconductors by their signature in four-wave-mixing signals | |
| Heberle et al. | Coherent control of exciton density and spin | |
| KR20080044261A (en) | Method and device for polarization conversion using quantum dots | |
| Peshkov et al. | Rayleigh scattering of twisted light by hydrogenlike ions | |
| Stepanov et al. | Nonstationary holographic recording for efficient amplification and phase conjugation | |
| Weilnau et al. | Spatial optical (2+ 1)‐dimensional scalar‐and vector‐solitons in saturable nonlinear media | |
| Papp et al. | Lens design for computing with anisotropic spin waves | |
| Zhukov et al. | Spin coherence of electrons and holes in ZnSe‐based quantum wells studied by pump–probe Kerr rotation | |
| Kinoshita et al. | Terahertz radiation by subpicosecond spin-polarized photocurrent originating from Dirac electrons in a Rashba-type polar semiconductor | |
| CA2341817C (en) | Method and apparatus for generating and controlling spin propagation using multiple coherent light beams | |
| Mamaev et al. | Pattern formation in a linear photorefractive oscillator | |
| US6643047B2 (en) | Method and apparatus for generating and controlling spin propagation using one or more coherent light beams | |
| RU2636808C1 (en) | Method and device of polarization-entangled photon source with maximum possible degree of entanglement | |
| CA2378560A1 (en) | Method and apparatus for generating and controlling spin propagation using one or more coherent light beams | |
| US4740984A (en) | Magnetically induced laser | |
| Lee et al. | Nonclassicality and wave-particle duality in quantum interferometer | |
| Dmitriev et al. | Quantum-interference resonant photocurrent | |
| Bieler et al. | Generation of injection currents in (110)-oriented GaAs quantum wells: Experimental observation and development of a microscopic theory | |
| Xu | Coherent optical spectroscopy of a single semi-conductor quantum dot | |
| Nelson | Coherent Nonlinear Optical Spectroscopy of InGaN Disks in GaN Nanowires | |
| Brida et al. | Tailoring polarization entanglement in anisotropy-compensated spontaneous parametric down-conversion | |
| SHEVATE | Saturation Aided Non-Linear Absorption in a Double Λ System of EIT |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| MKLA | Lapsed |
Effective date: 20130322 |