1 \subsection{Highly damped case}
3 For highly damped systems, the inertial term becomes insignificant
5 This model is commonly used for optically trapped beads\citep{TODO}.
6 Because it is simpler and solutions are more easily available%
7 \citep{grossman05,TODO},
8 it will server to outline the general approach before we dive into the
11 Fourier transforming \cref{eq:DHO} with $m=0$ and applying
12 \cref{eq:four-deriv} we have
13 % ODHO stands for very Over Damped Harmonic oscillator
15 (i \gamma \omega + k) x(\omega) &= F(\omega) \label{eq:ODHO-freq} \\
16 |x(\omega)|^2 &= \frac{|F(\omega)|^2}{k^2 + \gamma^2 \omega^2} \;.
19 \index{Damped harmonic oscillator!extremely overdamped}
20 We compute the \PSD\ by plugging \cref{eq:ODHO-xmag} into
24 = \normLimT \frac{2\magSq{F(\omega)}}{k^2 + \gamma^2\omega^2} \;.
29 Because thermal noise is white (not autocorrelated + Wiener-Khinchin Theorem),
30 we can denote the one sided thermal power spectral density per unit time by
33 = \normLimT 2 \magSq{F(\omega)} \;. \label{eq:GOdef} % label O != zero
36 Plugging \cref{eq:GOdef} into \cref{eq:ODHO-psd} we have
38 \PSD(x, \omega) = \frac{G_0}{k^2 + \gamma^2\omega^2} \;.
40 This is the formula we would use to fit our measured \PSD, but let us go a
41 bit farther to find the expected \PSD\ and thermal noise
42 given $m$, $\gamma$ and $k$.
44 Integrating over positive $\omega$ to find the total power per unit time yields
46 \iOInf{\omega}{\PSD(x, \omega)}
47 = \iOInf{\omega}{\frac{G_0}{k^2 + \gamma^2\omega^2}}
48 = \frac{G_0}{\gamma}\iOInf{z}{\frac{1}{k^2 + z^2}}
49 = \frac{G_0 \pi}{2 \gamma k} \;,
51 where the integral is solved in \cref{sec:integrals}.
53 Plugging into our corollary to Parseval's theorem (\cref{eq:parseval-var}),
55 \avg{x(t)^2} = \frac{G_0 \pi}{2 \gamma k} \;. \label{eq:ODHO-var}
58 Plugging \cref{eq:ODHO-var} into \cref{eq:equipart} we have
60 k \frac{G_0 \pi}{2 \gamma k} &= k_BT \\
61 G_0 &= \frac{2 \gamma k_BT}{\pi} \;.
64 So we expect $x(t)$ to have a power spectral density per unit time given by
66 \PSD(x, \omega) = \frac{2}{\pi}
68 \frac{\gamma k_BT}{k^2 + \gamma^2\omega^2} \;.