DE1541842C3 - Magneto-optical detection method - Google Patents

Description

The invention relates to a method for the magneto-optical detection of a magnetic field or a magnetization with the help of the Faraday effect, in particular for imaging normal conducting and superconducting areas and / or magnetic fields in superconductors.

From »Journal Optical Soc. America ", Vol. 37 (1947), p. 451, and Vol. 38 (1948), p. 483, are reflective filters known, which due to their layer structure have zero points of the value of the reflection.

The magneto-optical imaging method of areas and fields in superconductors is a well-known one Tools for studying important properties of superconductors.

The basis of the figure is the effect, named after Faraday, of the magnetic rotation of the plane of polarization. The effect occurs when linearly polarized light is parallel to the direction of magnetization passes into a translucent body that is in a magnetic field. The size of the Faraday rotation depends on the specific rotation, the magnetizing field and the Light penetrated layer thickness. The specific rotation is given by the so-called Verdet constantc characterized. The magneto-optical resolution of areas is the better, the thinner the area of light interspersed layer compared to the thickness of the superconductor. However, the Faraday rotation grows proportionally to the layer thickness and decreases very sharply with thin layers.

The object of the present invention is to provide a method for the magneto-optical detection of a magnetic field or a magnetization with

ίο help locate the Faraday effect in which one Reinforcement of the Faraday effect is achieved.

This object is achieved with a method as stated at the beginning, which is characterized according to the invention in that to amplify the Faraday rotation of the plane of polarization of the linearly polarized incident light, which is caused in the presence of a magnetic field, the incident light is split into at least two parts, one of which is through rotated the Faraday effect (F) and with the. other non-Faraday-rotated component (N) is brought to interference out of phase and that the amplitude of the non-Faraday-rotated component (N) is less than twice the amplitude of the component capable of interference (F ₀ cos «) of the Faraday-rotated component (F ^, in particular the amplitude of this component (F ₀ cos a) is approximately the same.

The invention is based on the idea, with the help of an interference method, the rotation of the To amplify the plane of polarization beyond the value given by Faraday's law.

Further explanations and preferred exemplary embodiments of the invention are provided below Hand of the figures described.

Linearly polarized monochromatic light of amplitude E ₀ coming from a medium with refractive index H ₁ falls perpendicularly onto a dielectric, magneto-optical layer 1 with refractive index n ₂ and thickness d, which is located on an optically and magnetically transparent carrier plate 2 and which is magnetized by a magnetic field H , then a part of the vibration upon impact on the layer surface is essentially unaffected by the magnetic field in the usual manner according to the reflection coefficient of the Fresnel formula for perpendicular incidence

/ ιι -, - ηΛ ^{2 1} V "2 + " J

reflected. (See Fig. 1. Rays are for the better Overview drawn diagonally instead of vertically. 4 indicates the sample to be examined.) This one The proportion is called normal oscillation

N ₀ =

The portion ^ (1 - r _t ) F ^ experiences when passing through the magnetized layer 1 with the refractive index n ₂

a rotation of the plane of oscillation by the angle ^.

If the magneto-optical detection in the method according to the invention takes place in reflection, then the portion f (\ - r,) E ₀ after the reflection at the layer 3 provided for this purpose with the refractive index n ₃ according to 7 "ΐζ"

and another layer pass another rotation

of the same amount and direction of rotation. It generates the Faraday oscillation of the amplitude

F ₀ = (I-

e '"E ₀ ,

^il ""-'i°>

2n, The Faraday rotation ι / increased by interference is:

whose plane of oscillation is rotated by the angle α with respect to the normal oscillation. (Multiple reflections are disregarded.)
For paramagnetic substances z. B .:

IO

α = Αγ2ά

for homogeneous fields that are not too high and temperatures that are too low. - = ■ = Verdet constant.

Oscillations of the same oscillation planes and constant phase differences are susceptible to interference. Each oscillation is divided into two oscillations with any, z. B. vertically standing swirls ·; transmission levels can be broken down, the Faraday oscillation F z. B. into a component in the oscillation plane of the normal oscillation N and a perpendicular to it, as shown in FIG. 2 is shown.

If the incident oscillation E is of the form

25th

E = E ₀ e '

so the time-independent phase of the normal oscillation is N at the interface Ti ₁ - ^ n ₂

ΨΙΤ— C- " ⁷ + <fo) i

because of the phase jump π at the interface M ₁ - ■ ♦ M ₂ for "2>« 1 · The phase of the Faraday oscillation is after double passage through the layer with the refractive index H ₂ and a single phase jump π at the interface M ₂ -> M ₃ for M ₃ > U ₁

.

'fo - 2.-7

If in the method according to the invention the imaging takes place in reflection, then the optical light path in the layer with the refractive index M ₂

to In ₂ (I = (2m + 1) j, m = 0, 1, 2,3 ... selected so that the interfering oscillations are out of phase and weaken ... '■, ■ .-. ..'. ^ · .. ·,;. '.,; ^: In Fig. 2 the normal oscillation N ₀ and the Faraday oscillation F _{0 are} shown... ...

..The planes of both oscillations are rotated against each other by the angle α. It can be seen that the rotation of the plane of the Faraday oscillation can be increased to α ' _{by interference of the amplitudes of normal oscillation JV 0} and the corresponding component of Faraday oscillation F _{0 *.} . . ■ .. '..' ■, ..-._.,; ... ■ ... '. ■■■:. ■ ... ,,: ■ <-. ■■ ~, ■■■ · ■: ■ '■■■. . After the interference, the amplitude is the normal oscillation. JV ₀ "same.;., .. '.. ·

NO = N ₀ - F ₀ cos η = [ItJ "- (1 - ^)] Jr ₂ cos«] E ₀ ■ and that of the Faraday oscillation F _{0 is} equal to /.

sin 11

tga

* (1

COS 11 -

\ _ '° IF ₀ 1 cos a

The reinforcement V

1 -

1/2

(1.— T ₁ )

-1

IF ₀ 1 cos a '■

is therefore generally still dependent on the Faraday rotation α itself. . In the approximation of thin layers and fields that are not too large (cos «-» 1), the gain is independent of the layer thickness and is a maximum for! ATI = If] (amplitude condition). The phase condition allows only discrete values of the layer thickness of the magneto-optical layer, in which ^ the phase jumps, due to reflection from JV and F. must be taken into account. The amplitude condition provides relationships between the indices of refraction of the media that produce the reflections required by the method for the interference.

According to a further development of the invention, a method for magneto-optical detection of a magnetic field or magnetization with the aid of the Faraday effect is proposed, in which transmitted Faraday-rotated components ^ and deflected, non-Faraday-rotated components N or transmitted, deflected Faraday-rotated components F and non-Faraday rotated components N are brought to interference out of phase.

In Fi g. 3 shows the magnetic material 1 to be examined, which is arranged between two dielectric dividing plates T ₁ and T ₂ . A perpendicular to the plane of incidence linearly polarized wave E is divided before the Magnetikum \ .by a dielectric divider plate T ₁ in E ₁ and E. ₂ A partial wave passes through the magnetic 1 and arrives as a Faraday oscillation F with the _{normal oscillation JV deflected to the mirrors Sp 1} and Sp ₂ , behind the second divider plate T _2, to interference. In the same way, transmitted, deflected Faraday-rotated components and non-Faraday-rotated components can also be made to interfere in opposite phase. \ _; / i, ;;. ':,;. _; ; · ■■ -,. The mode of action of the interference is the same as it has been described in .reflections for the detection. The phase condition can be changed here by varying the, optical. Path difference taking into account the phase jumps of the reflected oscillation, the amplitude condition. by weakening the larger amplitude of the two oscillations JV * or F, e.g. by means of a polarizer "P".

According to another development of the invention, a method for magneto-optical detection of a magnetic field or magnetization with the aid of the Faraday effect is proposed, in which several reflected Faraday-rotated components F and several reflected non-Faraday-rotated components N are in phase opposition to the interference to be brought.

From the preceding description it can be seen that in the case of two-beam interference and

90 ° rotation the field-dependent Faraday component F = F ■ sin a = (1 - Γι) fr ^ £ sin «

Since T ₁ <: 1 and | e | = 1 can be set is

| F | = ^ sin «.

From this relationship, it is apparent that increase in consideration of the interference principle, the intensity of the Faraday rotated fraction F except as by the Faraday rotation "by a larger reflection IABT ₁ Faraday oscillation F, and therefore normal mode N are in-phase partial waves caused by reflections of dielectric

τ layers are generated on equal amplitudes

Size ^ reinforced. The resulting Faraday oscillation Fb is brought into interference in antiphase with the resulting normal oscillation N _R. Under these conditions, the increased Faraday rotation is 90 ° here as well. The amplification of the Faraday intensity depends on the number of reflective interfaces. By designing the layers as dielectric mirror systems, there is also a significant increase in the Faraday rotation due to multiple reflections.

4 shows the structure of a side view Multi-layer arrangement. The sample to be examined is indicated by 4. On a glass plate 2 with a low refractive index that is anti-reflective 5

can is a sequence of alternating ^ layers

high 6 and low 7 refractive index, so-called alternating layers. It closes the magneto-optical layer 1. Their optical thickness is an integral multiple of a ^ layer.

Another system of j alternating shifts follows,

that starts with the last layer of the first system. The last layer of the system consists of a weakly absorbing layer 3 with a low refractive index, which absorbs the Faraday wave passing through. In the case of a strongly absorbing layer, the phase jump of the reflected wave in the layer thickness of the preceding layer must be taken into account. Both systems have the same number of interfaces which are formed by the same alternating layers (6, 7). The reflection on the first layer system generates a series of in-phase normal oscillations fi? _v , the reflection on the second layer system is a series of in-phase Faraday oscillations F _v . The resulting normal and Faraday oscillation are out of phase. Because of the equality of the two oscillation systems with regard to reflection, their amplitudes are the same.

F i g. 5 explains the. Magneto-optical mode of operation of the multilayer arrangement without taking multiple reflections into account. The arrangement is in the magnetic field. The amplitudes of the normal oscillations JV _V are in phase and add up algebraically, as are the Faraday oscillations F, whose planes of oscillation are rotated by the same angle in the magneto-optical layer. The angle of rotation has. therefore the same amount as in a layer 1 of the same thickness without alternating layers.

The reflected Faraday intensity and so too

Multiple reflection becomes maximum when the degree of reflection of the two alternating layer systems approaches 100% goes. This degree of reflection can be achieved with a small number of alternating layers.

In a variant of the multi-layer arrangement described, in addition to the middle layer,

half of the lower system of the ^ alternating layers is made up of magneto-optical layers. As a result, the individual, successive Faraday oscillations F _v receive an increasing rotation u _r. The oscillations are then no longer added algebraically, as in FIG. 5, but rather vectorially. The resulting angle of rotation a _R is larger than «. This increase in rotation through additional magneto-optical layers is essential if the middle layer is for technological reasons

can only be a simple j layer.

According to a further embodiment of the invention, the interference of vibrations that occur metallic layers are used to amplify Faraday rotation and intensity will.

If the conditions are met, that Faraday and normal oscillation are out of phase vibrate and that the amplitudes of these two vibrations are almost equal, then the one in question Layer arrangement in the absence of a magnetic field or magnetization almost anti-reflective. These conditions can be met with any interference filter that has a minimum of reflection for the wavelength / of the light used for magneto-optical detection, if the dielectric spacer layer is made of a magneto-optic substance.

The dielectric, magneto-optical layer is preferably between a homogeneous, partially transparent layer Mirror and a full mirror embedded. The partial mirror in this case has one

", ..,.,, 377 a / cm ² ^"
Surface resistance of about - ::. The system

"Carrier

is reflection-free if the optical thickness of the dielectric, magneto-optical layer is an odd multiple of j . ■ ^r

. : Further, the magneto-optical layer can be used as Spacer layer between two partially permeable metallic layers of the type known per se Perot-Fabry interferometer be arranged. This system has a minimum of reflection when the optical Thickness of the magneto-optical layer is an integral multiple of ^.

This freedom of reflection applies as long as only normal vibrations, i.e. H. Oscillations with the same, non-rotated Interference levels of vibration. Once the dielectric, magneto-optical layer of a If the magnetic field is magnetized, the reflection takes place which is additionally reinforced by multiple reflections Faraday component.

The proposed method for magneto-optical detection of a magnetic field or a Magnetization with the help of the Faraday effect, in particular for imaging normal conducting and

Claims (4)

Patent claims: 1. A method for the magneto-optical detection of a magnetic field or a magnetization using the Faraday effect, in particular for mapping normal conducting and superconducting areas and / or magnetic fields in superconductors, characterized in that for amplifying the Faraday rotation caused in the presence of a magnetic field the plane of polarization of the linearly polarized incident light, the incident light is split into at least two parts, one of which is rotated by the Faraday effect (F) and is brought to interference with the other non-Faraday-rotated part (N) and that the ^ The amplitude of the non-Faraday-rotated portion (N) is less than twice the amplitude of the interference-capable component (F ₀ cos α) of the Faraday-rotated portion (F), in particular the amplitude of this component (F ₀ cos α) is approximately the same is. 2. The method according to claim 1, characterized in that reflected Faraday rotated Components (F) and reflected non-Faraday rotated components (JV) brought to interference out of phase (Fig. 1). 3. The method according to claim 1, characterized in that transmitted Faraday rotated portions (F) and deflected non-Faraday rotated portions (N) or deflected, transmitted Faraday rotated portions (F) and non-Faraday rotated portions (N ) are brought to interference in antiphase (FIG. 3). 4. The method according to claim 1 or 2, characterized in that a plurality of reflected Faraday rotated Components (F) and several reflected non-Faraday rotated components (N) in phase opposition to Interference are brought (Fig. 4).