JPH03185339A - Apparatus for measuring spin polarization - Google Patents

Abstract

PURPOSE:To make it possible to measure the spin polarization of a carrier at high time resolution by projection exciting light on a material to be measured, forming spin polarization, thereafter projecting probe light on the material to be measured, and measuring the spin polarization with a photodetector. CONSTITUTION:The very-short-pulse light from a short-pulse dye laser light source 60 is made to be the circularly polarized very-short-pulse laser light with a circular-polarization converting device 62. The light beam is split into exciting light 24 and probe light 22. The probe light 22 is guided to an optical delaying device 10. A cube corner is finely moved at an equal speed by the driving of a step motor. The probe light 22 is optically delayed in a minute time from the exciting light 24. Therefore, up-spinning electrons are excited in a semiconductor 64 by the exciting light 24. The absorptivity of the semicon ductor 64 with the probe light 22 reaching the semiconductor 64 after the elapse of time DELTAt is changed at the exciting rate of the up-spinning. Therefore, the delay in light emitting time of the probe light 22 with respect to the exciting light 24 is finely shifted, and measurement is repeated. Thus the time change in absorptivity can be measured, and the time change in spin polarization can be measured.

Description

[Detailed Description of the Invention] [Summary] This invention relates to a spin polarization measuring device that measures the spin polarization of carriers such as electrons in a device to be measured such as a semiconductor with high time resolution. The purpose of the present invention is to provide a spin polarization measuring device that can measure polarization with high time resolution and that can easily measure near room temperature. circularly polarized light conversion means for converting into probe light and irradiating it onto the object to be measured; optical time delay means for generating an optical time delay in the probe light; and measuring the intensity of the probe light that has passed through the object to be measured. irradiating the object to be measured with the excitation light to form spin polarization, then irradiating the object to be measured with the probe light, and transmitting the probe light measured by the light receiving element. It is configured to measure spin polarization based on intensity.

[Industrial Application Field] The present invention relates to a spin polarization measurement device that measures spin polarization of carriers such as electrons in a device to be measured such as a semiconductor.

[Prior Art] Generally, when a semiconductor is irradiated with light, electrons in the valence band are excited and transition to the conduction band. The child has two spin directions: up spin and down spin. When a semiconductor is irradiated with unpolarized light, both spin-up and spin-down electrons are excited by the same amount.

However, when circularly polarized light is used as the irradiating light, the transition mode differs depending on whether it is right-handed or left-handed circularly polarized light.For example, when right-handed circularly polarized light is irradiated onto a semiconductor, it usually causes excitation of up-spin and down-spin electrons. The ratio is 3:1, and heavy holes and light holes are also generated in the valence band at a 3:1 ratio.

When left in this state, up-spin electrons gradually change to down-spin electrons, and after a certain period of time, the ratio of up-spin electrons and down-spin electrons becomes 50% each. Changes in the ratio of up-spin electrons to down-spin electrons over time are changes in spin polarization over time.

At this time, when up-spin electrons recombine with holes, the luminescence light becomes right-handed circularly polarized light, and when down-spin electrons recombine with holes, the luminescence light becomes left-handed circularly polarized light. By observing the polarization of luminescent light and measuring how much right-handed circularly polarized light there is, it is possible to measure the proportion of up-spin and down-spin electrons in the conduction band.

The conventional spin polarization measurement apparatus shown in FIG. 5 measures spin polarization using this principle.

A conventional spin polarization measurement device has a polarizer that converts the laser beam into linearly polarized light and a quarter-wave polarizer that converts the linearly polarized light into circularly polarized light on the optical axis of a short-pulse laser light source 60 that generates short-pulse laser light. A semiconductor 64, which is an object to be measured, is installed to measure spin polarization of electrons via a circular polarization conversion device 62 having a wavelength plate.

A quarter-wave plate 66 that converts the circularly polarized luminescence light 80 emitted by the semiconductor 64 into linearly polarized light and a polarizer 68 are arranged in a straight line on an optical axis different from the optical axis of the short pulse laser light source 60. Linearly polarized luminescent light 80 generated from one of left and right circularly polarized luminescent light 80
A streak camera 70 is installed to record temporal intensity changes.

The streak camera 70 is a camera that records high-speed temporal changes in light, and has a photocathode 72 and an anode 74 that convert an optical image into an electron beam image, and a sweep voltage type f#176 for sweeping the electron beam at high speed. Furthermore, it has a channel plate 78 that performs image amplification.

Next, the operation will be explained.

The short pulse light generated by the short pulse laser light source 60 is
The light is converted into, for example, right-handed circularly polarized light by the circularly polarized light converter 62, and is irradiated onto the semiconductor 64 as excitation light 79 for spin polarization of electrons. The spin polarization of electrons in the semiconductor 64 is generated by the irradiated circularly polarized light, and the luminescence light 80 generated by the spin polarization of the semiconductor 64 also has a circularly polarized component that depends on the spin polarization. By measuring the time change in luminescence intensity, the time change in spin polarization in the semiconductor 64 can be measured.

Circularly polarized luminescence light 80 generated from the semiconductor 64
is passed through a quarter-wave plate 66 and converted into two linearly polarized luminescent lights 80 with polarization planes different by 90 degrees, and further passed through a polarizer 68 to convert linearly polarized light according to the right-handed circularly polarized component, for example. Only the luminescent light 80 is made incident on the photocathode 72 of the streak camera 70.

The optical image incident on the photocathode 72 is converted into an electronic image, which is quickly swept by a sweep voltage type [i76, forming a streak image on the channel plate 78, which is converted into an electrical signal and analyzed (Fig. (not shown).

[Problems to be solved by the invention] However, the temporal resolution of streak cameras is currently 10 ps.
Furthermore, since photoluminescence has poor luminous efficiency at room temperature, it is difficult to measure near room temperature.

The present invention has been made in consideration of the above circumstances, and provides a spin polarization measuring device that can measure the spin polarization of carriers such as electrons in a semiconductor or other measured object with high time resolution and that can easily perform measurements near room temperature. The purpose is to provide.

[Means for Solving the Problem] The above object is to provide a pulsed laser light source and a circularly polarized light conversion means for converting the pulsed light emitted from the pulsed laser light source into circularly polarized excitation light and probe light and irradiating the object to be measured with the circularly polarized excitation light and probe light. and an optical time delay unit that generates an optical time delay in the probe light, and a light receiving element that measures the intensity of the probe light that has passed through the object to be measured, and directs the excitation light to the object to be measured. A spin polarization measuring device characterized in that after irradiating the object to form spin polarization, the object to be measured is irradiated with the probe light, and the spin polarization is measured based on the transmitted intensity of the probe light measured by the light receiving element. achieved by

The above object is achieved in the spin polarization measurement apparatus according to claim 1, wherein the circularly polarized light conversion means generates excitation light and probe light of the same circularly polarized light from the pulsed light emitted from the pulsed laser light source. This is accomplished by a spin polarization measurement device.

The above object is achieved by providing the spin polarization measurement apparatus according to claim 1, wherein the circularly polarized light conversion means generates excitation light and probe light of mutually opposite circular polarization from the pulsed light emitted from the pulsed laser light source. This is achieved using a spin polarization measuring device.

[Function] Since the present invention is configured as described above, the spin polarization of carriers such as electrons in a material such as a semiconductor can be measured with high time resolution, and measurement can be easily performed near room temperature.

[Example] The spin polarization measuring device according to the first example of the present invention was
This will be explained using figures.

A circular polarization conversion device that has a light filter that converts laser light into linearly polarized light and a quarter-wave plate that converts linearly polarized light into circularly polarized light on the optical axis of a short-pulse dye laser light source 60 that generates short-pulse laser light. A device 62 is installed, and a beam splitter 14 is installed ahead of it. The beam splitter 14 splits the light into two parts, one of which is sent to an optical time delay device 210.
The other side heads toward the reflecting mirror 17.

The optical time delay device 10 changes the optical path length by moving a cube corner by driving, for example, a step motor. The light emitted from the optical time delay device 10 is incident on the semiconductor 64 which is the object to be measured. semiconductor 64
The light that has passed through is received by the light receiving element 20.

On the other hand, the optical axis of the light traveling to the reflecting mirror 17 is reflected on the semiconductor 64 which is the object to be measured! Although it coincides with the optical axis from i16, it is adjusted so that it does not enter the light receiving element 20.

The semiconductor 64 has a superlattice structure and is formed to have a thickness of about 800 nm so that the irradiated light can pass through.
It is glued onto a glass plate with a thickness of about mm using a transparent adhesive.

Next, the operation will be explained.

Generally, when a semiconductor is irradiated with circularly polarized light, the ratio of up-spin or down-spin electrons to be excited is 3:1.
Heavy holes and light holes are generated in the valence band. In a semiconductor superlattice, the energy level of heavy holes in the valence band is slightly lower than that of light holes, and among the exciton co-III@ levels in the semiconductor superlattice, for example, the energy level of heavy holes in the valence band is When circularly polarized light is irradiated, for example, only up-spin electrons are excited, and when circularly polarized light is irradiated with the same wavelength as the electron-lighthole transition, only down-spin electrons are excited.

In this practical example, a circularly polarized light pulse with a wavelength corresponding to the electron-heavy hole transition is generated and irradiated onto a semiconductor having a superlattice structure, thereby exciting only up-spin electrons.

Here, circularly polarized light that excites up-spin electrons is called excitation light, and light that is the same circularly polarized light as this excitation light and reaches the semiconductor with a delay of Δ seconds from the excitation light is called probe light.

Up-spin electrons excited by irradiation with excitation light relax over time. A portion of the probe light, which arrives with a delay of Δ seconds, excites up-spin electrons depending on the degree of spin relaxation, but the rest passes through the semiconductor as is. Therefore, by examining the intensity of the transmitted light of the probe light, the spin polarization Δ can be determined as the proportion of up-spin electrons in the sand diameter.The present embodiment measures spin polarization based on this principle.

A pulse @ from a short-pulse dye laser light source 60 that generates a circularly polarized pulse of wavelength corresponding to the electron-heavy hole transition.
Ultrashort pulsed light of lps is generated at a pulse frequency of 82 MH2, converted into circularly polarized ultrashort pulsed light by a circular polarization conversion device 62, and split into excitation light 24 and probe light 22 by a beam splitter 14.

The probe light 22 is fed to the optical time delay device 10, and by driving the step motor to slightly move the cube corner at a constant velocity, the probe light 22 is caused to undergo a minute optical time delay compared to the excitation light 24 (second figure).

Therefore, the excitation light 24 first reaches the semiconductor 64, and the up-spin electrons in the semiconductor 64 are excited by the excitation light 24. After the time Δt has elapsed, the absorption rate of the semiconductor 64 due to the probe light 22 that has reached the semiconductor 64 changes depending on the rate of up-spin excitation. Therefore, probe light 2
The transmitted light of No. 2 also changes according to the rate of excitation of up-spin electrons, and the intensity of the transmitted light that reaches the light receiving element 20 changes.

By repeatedly measuring while slightly shifting the delay in the emission time of the probe light 22 relative to the excitation light 24, the change in absorption rate over time can be measured. In other words, changes in spin polarization over time can be measured.

The spin polarization measuring device according to the second embodiment of the present invention is
This will be explained using figures.

Components that are the same as those of the spin polarization measuring device shown in FIG. 1 are given the same reference numerals, and description thereof will be omitted or simplified.

The spin polarization measurement device of this embodiment is the same as the spin polarization measurement device of the first embodiment, except that a half-wave plate 3o is installed between the beam splitter 14 and the optical time delay device 10.

When the circularly polarized directions of the excitation light 24 and the probe light 22 are reversed, the intensity of the probe light 22 transmitted through the semiconductor 64 changes in accordance with the time change in the ratio of down-spin electrons, so that the excitation light 24 excites the probe light 22. It becomes possible to measure the process in which up-spin electrons relax into down-spin electrons.

First, a short pulse dye laser light source 60 that generates a circularly polarized light pulse with a wavelength corresponding to the electron-heavy hole transition is used to generate short pulse light with a pulse width of 1 ps at a pulse frequency of 82 MHz, and a circularly polarized light converter 62 generates a short pulse light with a pulse frequency of 82 MHz. The polarized ultra-short pulse light is split into excitation light 24 and probe light 22 by a beam splitter 14.

The probe light 22 passes through the half-wave plate 3o, becomes left-handed circularly polarized light, and is guided to the optical time delay device 1o. The cube corner is slightly moved at a constant speed by driving the step motor, causing a slight optical time delay in the probe light 22 compared to the excitation light 24 (FIG. 2).

Therefore, the excitation light 24 first reaches the semiconductor 64, and the up-spin electrons in the semiconductor 64 are excited by the excitation light 24. After a period of time Δt has elapsed, the absorption rate of the semiconductor 64 due to the probe light 22 that has reached the semiconductor 64 changes depending on the rate of excitation of down-spin electrons. Therefore, the transmitted light of the probe light 22 also changes according to the proportion of down-spin electrons, and the intensity of the transmitted light that reaches the light receiving element 20 changes.

By repeatedly measuring the delay in the emission time of the probe light 22 with respect to the excitation light 24 while slightly shifting the time, changes in the absorption rate over time can be determined. That is, it becomes possible to observe the process in which up-spin electrons relax into down-spin electrons, and it is possible to measure changes in spin polarization over time.

The spin polarization measuring device according to the third embodiment of the present invention is
This will be explained using figures.

Components that are the same as those of the spin polarization measurement apparatus shown in FIG. 1 or 3 are given the same reference numerals, and explanations thereof will be omitted or simplified.

The spin polarization measuring device of this embodiment splits ultrashort pulse light emitted from a short pulse dye laser light source 60 into an excitation light 24 and a probe light 22 by a beam splitter 14, and then connects each optical path to a circular polarization converter 62. .63 was installed.

The circularly polarized light converter 62.6 is configured such that the circularly polarized light converter 62 and the circularly polarized light converter 63 generate circularly polarized light with opposite directions.
The polarizer and quarter-wave plate inside 3 are set so that the vibration directions of light are tilted 90 degrees to each other.

In the manner described above, measurements similar to those in the second embodiment can be made.

In this embodiment, since the time resolution of the measurement system is determined in principle by the time resolution of the optical pulse, the time resolution can be reduced to, for example, the currently available order of O, lps. It also has the excellent feature of being able to easily perform measurements at room temperature because it does not use photoluminescence light.

The present invention is not limited to the above-mentioned embodiments, and various modifications are possible.

For example, in this example, a short pulse dye laser was used as the short pulse laser light source, but other lasers such as semiconductor laser, YAG
A solid laser such as a laser, or a gas laser such as an Ar laser may be used.

Further, in this embodiment, a polarizer and a quarter-wave plate are used as the circularly polarized light conversion device, but a polarized laser may be used as a light source and a quarter-wave plate may be used.

Further, in this embodiment, an optical time delay is given to the probe light, but any other device may be used as long as it can give a relatively more time delay to the probe light than to the excitation light.

Therefore, the optical path length of the excitation light may be relatively shorter than the optical path length of the probe light.

[Effects of the Invention] As described above, according to the present invention, spin polarization of carriers such as electrons in a material such as a semiconductor can be measured with high time resolution, and measurement can be easily performed near room temperature.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a spin polarization measuring device according to a first embodiment of the present invention, FIG. 2 is a diagram showing a time delay time chart of the first embodiment, and FIG. FIG. 4 is a diagram showing a spin polarization measurement device according to the second embodiment; FIG. 4 is a diagram showing a spin polarization measurement device according to a third real-plane saturation example of the present invention; FIG. FIG. In the figure, 10... optical time delay device 14... beam splitter 17... reflecting mirror 20... light receiving element 22... probe light 24... excitation light 30... half wavelength Plate 60... Short pulse laser light source 62... Circular polarization conversion device 63... Circular polarization conversion device 64... Semiconductor 66... Quarter wavelength plate 68... Polarizer 70... Streak camera 72...Photocathode 74...Anode 76...Sweep voltage electrode 78...Channel plate 79...Excitation light 80...Luminescence light

Claims (1)

[Scope of Claims] 1. A pulsed laser light source; a circularly polarized light conversion means for converting the pulsed light emitted from the pulsed laser light source into circularly polarized excitation light and probe light and irradiating the object to be measured; and the probe light. an optical time delay unit that generates an optical time delay in the object to be measured, and a light receiving element that measures the intensity of the probe light that has passed through the object to be measured, and irradiates the object to be measured with the excitation light to generate spin polarization. 2. A spin polarization measurement apparatus, characterized in that after forming a probe light, the object to be measured is irradiated with the probe light, and spin polarization is measured based on the transmitted intensity of the probe light measured by the light receiving element. 2. The spin polarization measuring device according to claim 1, wherein the circularly polarized light conversion means generates excitation light and probe light of the same circularly polarized light from the pulsed light emitted from the pulsed laser light source. measuring device. 3. The spin polarization measuring device according to claim 1, wherein the circularly polarized light conversion means generates excitation light and probe light of mutually opposite circular polarization from the pulsed light emitted from the pulsed laser light source. Spin polarization measurement device.