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The Kramers–Kronig relations are mathematical properties, connecting the real and imaginary parts of any complex function which is analytic in the upper half plane. These relations are often used to relate the real and imaginary parts of response functions in physical systems because causality implies the analyticity condition is satisfied, and conversely, analyticity implies causality of the corresponding physical system.^[1] The relation is named in honor of Ralph Kronig^[2] and Hendrik Anthony Kramers.^[3]

[edit] Definition

For a complex function $χ(ω) = χ 1 (ω) + i χ 2 (ω)$ of the complex variable $ω$ , analytic in the upper half plane of $ω$ and which vanishes faster than $1 / | ω |$ as $|\omega| \rightarrow \infty$ , the Kramers–Kronig relations are given by

$\chi_1(\omega) = {1 \over \pi} \mathcal{P} \int \limits_{-\infty}^{\infty} {\chi_2(\omega') \over \omega' - \omega}\,d\omega'$

and

$\chi_2(\omega) = -{1 \over \pi} \mathcal{P} \int \limits_{-\infty}^{\infty} {\chi_1(\omega') \over \omega' - \omega}\,d\omega',$

where $\mathcal{P}$ denotes the Cauchy principal value. We see that the real and imaginary parts of such a function are not independent, so that the full function can be reconstructed given just one of its parts.

[edit] Derivation

The proof begins with an application of Cauchy's residue theorem for complex integration. Given any analytic function $χ(ω')$ in the upper half plane, the function $χ(ω') / (ω' - ω)$ where $ω$ is real will also be analytic in the upper half of the plane. The residue theorem consequently states that

$\oint {\chi(\omega') \over \omega'-\omega}\,d\omega' = 0$

Integral contour for deriving Kramers-Kronig relations.

for any contour within this region. We choose the contour to trace the real axis, a hump over the pole (complex analysis) at $ω = ω'$ , and a semicircle in the upper half plane at infinity. We then decompose the integral into its contributions along each of these three contour segments. The length of the segment at infinity increases proportionally to $| ω |$ , but its integral component vanishes as long as $χ(ω)$ vanishes faster than $1 / | ω |$ . We are left with the segment along the real axis and the half-circle around the pole:

$\oint {\chi(\omega') \over \omega'-\omega}\,d\omega' = \mathcal{P} \int \limits_{-\infty}^\infty {\chi(\omega') \over \omega'-\omega}\,d\omega' - i \pi \chi(\omega) = 0.$

The second term in the middle expression is obtained using the theory of residues.^[4] Rearranging, we arrive at the compact form of the Kramers–Kronig relations,

$\chi(\omega) = {1 \over i \pi} \mathcal{P} \int \limits_{-\infty}^\infty {\chi(\omega') \over \omega'-\omega}\,d\omega'.$

The single $i$ in the denominator hints at the connection between the real and imaginary components. Finally, split $χ(ω)$ and the equation into their real and imaginary parts to obtain the forms quoted above.

[edit] Physical interpretation and alternate form

We can apply the Kramers–Kronig formalism to response functions. In physics, the response function $\chi(t-t')\!$ describes how some property $P(t)\!$ of a physical system responds to a small applied force $F(t')\!$ . For example, $P(t)\!$ could be the angle of a pendulum and $F (t)$ the applied force of a motor driving the pendulum motion. The response $χ(t - t')$ must be zero for $t<t'\!$ since a system cannot respond to a force before it is applied. It can be shown that this causality condition implies the Fourier transform $\chi(\omega)\!$ is analytic in the upper half plane.^[5] Additionally, if we subject the system to an oscillatory force with a frequency much higher than its highest resonant frequency, there will be no time for the system to respond before the forcing has switched direction, and so $\chi(\omega)\!$ vanishes as $\omega\!$ becomes very large. From these physical considerations, we see that $\chi(\omega)\!$ satisfies conditions needed for the Kramers–Kronig relations to apply.

The imaginary part of a response function describes how a system dissipates energy, since it is out of phase with the driving force. The Kramers–Kronig relations imply that observing the dissipative response of a system is sufficient to determine its in-phase (reactive) response, and vice versa.

The formulas above are not useful for reconstructing physical responses, as the integrals run from $-\infty$ to $\infty$ , implying we know the response at negative frequencies. Fortunately, in most systems, the positive frequency-response determines the negative-frequency response because $χ(ω)$ is the Fourier transform of a real quantity $χ(t - t')$ , so $χ( - ω) = χ * (ω)$ . This means $χ 1 (ω)$ is an even function of frequency and $χ 2 (ω)$ is odd.

Using these properties, we can collapse the integration ranges to $[0,\infty)$ . Consider the first relation giving the real part $χ 1 (ω)$ . Transform the integral into one of definite parity by multiplying the numerator and denominator of the integrand by $ω' + ω$ and separating:

$\chi_1(\omega) = {1 \over \pi} \mathcal{P} \int \limits_{-\infty}^\infty {\omega' \chi_2(\omega') \over \omega'^2 - \omega^2}\, d\omega' + {\omega \over \pi} \mathcal{P} \int \limits_{-\infty}^\infty {\chi_2(\omega') \over \omega'^2 - \omega^2}\,d\omega'.$

Since $χ 2 (ω)$ is odd, the second integral vanishes, and we are left with

$\chi_1(\omega) = {2 \over \pi} \mathcal{P} \int \limits_{0}^{\infty} {\omega' \chi_2(\omega') \over \omega'^2 - \omega^2}\,d\omega'.$

The same derivation for the imaginary part gives

$\chi_2(\omega) = -{2 \over \pi} \mathcal{P} \int \limits_{0}^{\infty} {\omega \chi_1(\omega') \over \omega'^2 - \omega^2}\,d\omega' = -{2 \omega \over \pi} \mathcal{P} \int \limits_{0}^{\infty} {\chi_1(\omega') \over \omega'^2 - \omega^2}\,d\omega'.$

These are the Kramers–Kronig relations useful for physical response functions.

[edit] See also

[edit] References

[edit] Inline

^ John S. Toll, Causality and the Dispersion Relation: Logical Foundations, Physical Review, vol. 104, pp. 1760 - 1770 (1956).
^ R. de L. Kronig, On the theory of the dispersion of X-rays, J. Opt. Soc. Am., vol. 12, pp. 547-557 (1926).
^ H.A. Kramers, La diffusion de la lumiere par les atomes, Atti Cong. Intern. Fisica, (Transactions of Volta Centenary Congress) Como, vol. 2, p. 545-557 (1927) .
^ Mathematical Methods for Physicists G. Arfken (Academic Press, Orlando 1985)
^ John David Jackson (1999). Classical Electrodynamics. Wiley. pp. 332-333. ISBN 0-471-43132-X.

[edit] General

Mansoor Sheik-Bahae: Nonlinear Optics Basics. Kramers–Kronig Relations in Nonlinear Optics, in: Robert D. Guenther (Ed.): Encyclopedia of Modern Optics, Academic Press, Amsterdam 2005, ISBN 0-12-227600-0
Valerio Lucarini, Jarkko J. Saarinen, Kai-Erik Peiponen, and Erik M. Vartiainen : Kramers-Kronig relations in Optical Materials Research, Springer, Heidelberg, 2005, ISBN 3-540-23673-2
J. D. Jackson, Classical Electrodynamics, 2nd edition, Wiley, New York (1975), Sec. 7.10, ISBN 0-471-43132-X.

[0] John S. Toll, Causality and the Dispersion Relation: Logical Foundations, Physical Review, vol. 104, pp. 1760 - 1770 (1956).

[1] R. de L. Kronig, On the theory of the dispersion of X-rays, J. Opt. Soc. Am., vol. 12, pp. 547-557 (1926).

[2] H.A. Kramers, La diffusion de la lumiere par les atomes, Atti Cong. Intern. Fisica, (Transactions of Volta Centenary Congress) Como, vol. 2, p. 545-557 (1927) .

[3] Mathematical Methods for Physicists G. Arfken (Academic Press, Orlando 1985)

[4] John David Jackson (1999). Classical Electrodynamics. Wiley. pp. 332-333. ISBN 0-471-43132-X.

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