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In optics, polarized light can be described using the Jones calculus, discovered by R. C. Jones in 1941. Polarized light is represented by a Jones vector, and linear optical elements are represented by Jones matrices. When light crosses an optical element the resulting polarization of the emerging light is found by taking the product of the Jones matrix of the optical element and the Jones vector of the incident light.

Note that Jones calculus is only applicable to light that is already fully polarized. Light which is randomly polarized, partially polarized, or incoherent must be treated using Mueller calculus.

The Jones vector

The Jones vector describes the polarization of light in free space or another homogeneous isotropic non-attenuating medium, where the light can be properly described as transverse waves. Suppose that a monochromatic plane wave of light is travelling in the positive z-direction, with angular frequency ω and wavevector k = (0,0,k), where the wavenumber k = ω/c. Then the electric and magnetic fields E and H are orthogonal to k at each point; they both lie in the plane "transverse" to the direction of motion. Furthermore, H is determined from E by 90-degree rotation and a fixed multiplier depending on the wave impedance of the medium. So the polarization of the light can be determined by studying E. The complex amplitude of E is written

Note that the physical E field is the real part of this vector; the complex multiplier serves up the phase information. Here

is the imaginary unit with

Thus, the Jones vector represents the amplitude and phase of the electric field in the x and y directions.

The sum of the squares of the absolute values of the two components of Jones vectors is proportional to the intensity of light. It is common to normalize it to 1 at the starting point of calculation for simplification. It is also common to constrain the first component of the Jones vectors to be a real number. This discards the overall phase information that would be needed for calculation of interference with other beams.

Note that all Jones vectors and matrices on this article employ the convention that the phase of the light wave is given by

, a convention used by Hecht. Under this convention, increase in

(or

) indicates retardation (delay) in phase, while decrease indicates advance in phase. For example, a Jones vectors component of

(

) indicates retardation by

(or 90 degree) compared to 1 (

). Circular polarisation described under Jones' convention is called : "From the point of view of the receiver". Collett uses the opposite definition for the phase (

). Circular polarisation described under Collett's convention is called : "From the point of view of the source". The reader should be wary of the choice of convention when consulting references on the Jones calculus.

A general vector that points to any place on the surface is written as a ket

. When employing the Poincaré sphere (also known as the Bloch sphere), the basis kets (

and

) must be assigned to opposing (antipodal) pairs of the kets listed above. For example, one might assign

and

. These assignments are arbitrary. Opposing pairs are

The polarization of any point not equal to

and not on the circle that passes through

is known as elliptical polarization.

Jones matrices

The Jones matrices are operators that act on the Jones vectors defined above. These matrices are implemented by various optical elements such as lenses, beam splitters, mirrors, etc. Each matrix represents projection onto a one-dimensional complex subspace of the Jones vectors. The following table gives examples of Jones matrices for polarizers:

Phase retarders

Phase retarders introduce a phase shift between the vertical and horizontal component of the field and thus change the polarization of the beam. Phase retarders are usually made out of birefringent uniaxial crystals such as calcite, MgF₂ or quartz. Uniaxial crystals have one crystal axis that is different from the other two crystal axes (i.e., n_i ≠ n_j = n_k). This unique axis is called the extraordinary axis and is also referred to as the optic axis. An optic axis can be the fast or the slow axis for the crystal depending on the crystal at hand. Light travels with a higher phase velocity along an axis that has the smallest refractive index and this axis is called the fast axis. Similarly, an axis which has the largest refractive index is called a slow axis since the phase velocity of light is the lowest along this axis. "Negative" uniaxial crystals (e.g., calcite CaCO₃, sapphire Al₂O₃) have n_e < n_o so for these crystals, the extraordinary axis (optic axis) is the fast axis, whereas for "positive" uniaxial crystals (e.g., quartz SiO₂, magnesium fluoride MgF₂, rutile TiO₂), n_e > n _o and thus the extraordinary axis (optic axis) is the slow axis.

Any phase retarder with fast axis equal to the x- or y-axis has zero off-diagonal terms and thus can be conveniently expressed as

where

and

are the phase offsets of the electric fields in

and

directions respectively. In the phase convention

, define the relative phase between the two waves as

. Then a positive

(i.e.

) means that

doesn't attain the same value as

until a later time, i.e.

leads

. Similarly, if

, then

leads

For example, if the fast axis of a quarter wave plate is horizontal, then the phase velocity along the horizontal direction is ahead of the vertical direction i.e.,

leads

. Thus,

which for a quarter wave plate yields

In the opposite convention

, define the relative phase as

. Then

means that

doesn't attain the same value as

until a later time, i.e.

leads

The special expressions for the phase retarders can be obtained by taking suitable parameter values in the general expression for a birefringent material. In the general expression:

Note that for linear retarders,

= 0 and for circular retarders,

= ±

/2,

/4. In general for elliptical retarders,

takes on values between -

/2 and

/2.

Axially rotated elements

Assume an optical element has its optic axis{{clarify|reason=Does this mean (1) the "optic axis" of a (presumably uniaxial) birefringent material, or (2) the "optic axis" (also known as optical axis) of a rotationally symmetric lens system?|date=May 2015}} perpendicular to the surface vector for the plane of incidence{{clarify|reason=What is the "surface vector for the plane of incidence"? Is it the normal vector? This would then be tangent to the surface of the refracting material, right?|date=May 2015}} and is rotated about this surface vector by angle θ/2 (i.e., the principal plane,{{clarify|reason="Principal plane" is not defined in this article nor in the article on polarized light. Is it defined for lens systems?|date=May 2015}} through which the optic axis passes,{{clarify|reason=What is the geometric relation between a vector and a plane expressed by "passes through"?|date=May 2015}} makes angle θ/2 with respect to the plane of polarization of the electric field{{clarify|reason=What is "the plane of polarization" of the electric field? I thought polarization was expressed by a vector. Does it mean the plane orthogonal to the direction of propagation, in which E can take its values?|date=May 2015}} of the incident TE wave). Recall that a half-wave plate rotates polarization as twice the angle between incident polarization and optic axis (principal plane). Therefore, the Jones matrix for the rotated polarization state, M(θ), is

This agrees with the expression for a half-wave plate in the table above. These rotations are identical to beam unitary splitter transformation in optical physics given by

where the primed and unprimed coefficients represent beams incident from opposite sides of the beam splitter. The reflected and transmitted components acquire a phase θ_r and θ_t, respectively. The requirements for a valid representation of the element are ^[5]

Arbitrarily rotated elements

This would involve a three-dimensional rotation matrix. See Russell A. Chipman^[6] and Garam Yun for work done on this.^[7]^[8]^[9]

The Jones vector

Jones matrices

Phase retarders

Axially rotated elements

Arbitrarily rotated elements

See also

Notes

References

Further reading

External links