[Physics] Unpolarized light vs. randomly rotating polarized light

electromagnetismopticspolarization

I am confused with physical picture about unpolarized light.

Is unpolarized light very fast rotating polarized light? or co-existing state of two orthogonal polarization? (or something else?)

If there is a linear polarizer which rotates very very fast and randomly (the polarizer in imagine), the output light is same to unpolarized light? I don't think so but I am not sure.

—

or, instead of linear polarizer, a Faraday rotator with magnetic field whose amplitude is randomly chnaged can be considered, I think.

Best Answer

Unpolarized light can be thought of as a superposition of wave trains of a finite duration of order $0<\tau<\infty$, each of which has a certain pure polarization, which may be elliptical, with a random direction. The phases of the pulses and their start and end times are also random.

What this means in practice is that any unpolarized light source has a coherence time $\tau$. If you look at the polarization with higher temporal resolution than this, you will see a pure polarization (per spectral component! If the light source is not monochromatic the picture is more complicated). If you measure with a lower resolution, the randomly rotating polarization will average out and you will observe no polarization effects.

To put things in scale, the coherence length ($=c\tau$) of sunlight is about $0.6\,\mu\text m$ (doi). In practice this means that any polarization-dependent interferometry must involve path differences shorter than that, or you will be seeing the (lack of) interference between two different pulse trains with random relative polarizations and phases.

Related Solutions

[Physics] How to treat partially polarized light with Jones vectors

The Fresnel transmission coefficients at the Brewster angle between two media of $n=1$ and $n=1.5$ are not 0.86 and 1.

I haven't checked the Maths, but looking at this: https://www.geogebratube.org/student/m325541 I would say the transmission coefficients are $t_s=0.6$ and $t_p=0.66$. The reflection coefficients $r_s=-0.4$ and $r_p=0$.

The transmission coefficients expressed in terms of power are about 0.86 and 1. Recall that the Transmittance, is $$ T_p = \frac{n_2}{n_1}\frac{\cos\theta_2}{\cos\theta_1} t_p^{2}$$

It's hard to follow what you are asking in the rest of the question. Using these transmission coefficients and the fact that unpolarised light can be treated as 50% p-polarized and 50% s-polarised, then the transmitted light will be p-polarised as $$ \frac{E_s}{E_{total}} = \frac{0.5 \times t_s}{0.5\times t_p + 0.5\times t_s} \simeq 47.6\%$$.

Or, in terms of power $$\frac{I_{s}}{I_{total}}= \frac{0.5 \times t_s^{2}}{0.5 \times t_s^{2} + 0.5 \times t_{p}^{2}} \simeq 45.2\%,$$ because the magnification factors are the same for both polarizations.

This of course only deals with the transmission through the first interface. You must then tackle the glass/air interface in a similar way if you wish to see what happens to light transmitted all the way through a block.

EDIT: As an aside; completely unpolarised light can be treated as equal amounts of light (i.e. equal E-field amplitudes) in two perpendicular directions in the plane perpendicular to the wave motion. The two components must not have any fixed phase relationship (otherwise you get elliptically polarised light). You can choose the two perpendicular directions to suit the problem. In this case, choosing the directions parallel (p-) and perpendicular (s-) to the plane of incidence is sensible!

Optics – Polarized Slits in Young’s Double-Slit Experiment

I am going to assume a broad audience with some who have seen complex plane-wave analysis, and many who have not. Anyone who can do the ordinary Young's double-slit experiment should have no trouble doing the polarized-mixed version. Little or no math is needed, but there is a little art in making a polarized double-slit.

(See me showing the fringes going through all four outcomes just by rotating the mixer here.)

(A note about references: As much publicly-funded information is lately behind paywalls, I also include freely-available sources – some tertiary. I do not believe the paid sources are really needed to understand this answer, but they are included for completeness and lineage.)

The experiment really can be done in one's kitchen with no math needed. I make the slits by hand with roughly 1mm width, and spaced ~1.5mm apart. I use electrical tape to mask off the boundary between two abutted sheet polarizers whose transmission axes are perpendicular, and then cut and peal away the slit windows. I may include more detail on the art of making a polarized double-slit, if people request it. Laser pointers are usually polarized, but sometimes unstable. To be sure of the source's polarization, I use a polarizer right after the laser which I call a "prefilter". I find it easier to rotate the mixer between D and AD than to rotate the laser/prefilter assembly. The mixer can even be placed right against the observation screen and rotated (or removed) at that point. It is fun to watch the fringes shift phase and disappear altogether as I spin the mixer.

From the comments, I think people are catching on that diagonal and anti-diagonal (D and AD) inputs produce fringing output and that H and V input cannot make fringes. What takes maybe all of us by surprise (certainly me) is that AD makes a dark central (zeroeth) fringe. This means that when the mixer and input polarization angles are mutually perpendicular (the meaning of AD), the fringes look like a photo-negative of the familiar ordinary Young's fringes.

I start with cartoons of the qualitative output of the following setups: single-slit$^{(7, 8)}$, double-slit$^{(9, 10)}$, polarized double-slit, and polarized double-slit with mixer (Figures 0, 1). For a reminiscent layout, see the quantum eraser of Chiao, R., P. Kwiat and Aephraim M. Steinberg$^{(11,12)}$ (see Figs. 9b, 9c of v1 of the preprint, appearing at the last pages due to a LaTeX error).

I then show photographs of my experimental arrangement and real output (Figures 6-10). I finish with Table 0 summarizing the experimental results and a following complex plane-wave analysis (with accompanying Figure 5). The analysis harmonizes with experiment for ordinary, polarized, and polarized-mixed configurations. Figure 0 reviews the output characteristics of ordinary single- and double-slits, and builds to show the polarized double-slit, and mixed polarized double-slit. Figure 1 completes the results for the mixed polarized double-slit. Historically, I initially work out the plane-wave solution to make sure that I understand the problem. Having trouble with the answer I finally realize that the solution is correct, but the answer predicts a dark fringe for AD input. I then look back at my photographic data and realize the central fringe is dark. (Previously, I deliberately make the mixer AD to the input for some reason.) I then make the mixer D to the input and the bright central fringe returns.

Now for an overview of the math: I frame the problem using a complex plane wave allowing for insertion or removal of the polarizers (for example, removing the mixer is equivalent to using the identity matrix in its place which does not change the wavefunction at that point). I define the problem in Figure 5. I find some sense of confidence in getting a mathematical answer aligning with my experimental journey. The following complex plane-wave analysis gives the same results for fringe existence and phase I see for experiment.

Input Polarization	Ordinary Double-Slit	Polarized Double-Slit	Polarized Double-Slit with Mixer
D (Aligned with mixer)	yes(bright central fringe)	no	yes (bright central fringe)
AD (Orthogonal to mixer)	yes(bright central fringe)	no	yes (dark central fringe)
H	yes(bright central fringe)	no	no
V	yes(bright central fringe)	no	no

TABLE 0 (All basic experimental cases are summarized here and proved in the complex analysis.)

I use the Jones Calculus$^{(1, 2, 6)}$. My approach is to make the general case include the ordinary Young's for confirmation that I am solving the problem correctly. I also simplify by keeping the specific geometry implicit and looking for a multiplicative sinusoidal term which indicates fringing. The equations still look a little scary, but they would be far worse if I were to try for a profile of the intensity versus diffraction angle. Anyway, I am only studying the existence and phase of the fringes for each input of H, V, D, and AD. The Jones calculus simply treats each optical element as a linear transformation of the complex plane wave, so the J.C. is a compact way of writing out the equations that one normally encounters when writing out the polarization components and projections as vector terms. Finally, the Born rule ("squaring" = the complex result dotted with its complex conjugate) tells us the observed intensity. The results match experiment for every input polarization for the naked double-slit, the polarized double-slit, and the polarized double-slit with mixer. I model each physical setup from the general equation, reduce algebraically, and then "square" separately, after which I apply the specific input polarization vectors. (See below the unique matrix arising for each case just before applying the Born rule.)

I write an incident plane wave$^{(1,3)}$, $$\Psi_0 = \vec E_0 e^{i(\vec k \cdot \vec x-\omega t)}\tag{0}\label{eq0}$$ where, $\vec k$ is the wavevector, $ \vec E_0= \left[\begin{matrix} E_{0y} \\ E_{0z} \\ \end{matrix} \right ] $ (and $E_{0y}, E_{0z} \in \Re$), $\vec k \parallel \vec x$, and $t$ is averaged over many cycles at any given $x$, so that only relative phases are important. Therefore, I treat the exponential in $\eqref{eq0}$ as a complex constant of arbitrary phase and choose to set it to 1. I do not care about absolute intensity. I only care about the existence of output fringes, so I look for a surviving sinusoidal term, and use proportionalities liberally. The slits happen to the incident wave simultaneously, so the result is a sum. The slit illumination factors are the same while their relative complex phases differ by diffraction$^{(7, 8)}$ angle, $\theta$, slit separation$^{(9, 10)}$, $d$, and slit filter (polarizer). Removing an optical element is handled by the identity matrix$^{(1, 2, 6)}$. The final function, $\Psi_3$, is the product of the mixer$^{(1, 2, 6)}$ (if any) and the sum of the slit functions$^{(1, 2, 6)}$, $\Psi_1$ and $\Psi_2$. So, $\Psi_3 \propto M_{(identity \lor mixer)}(\Psi_1+\Psi_2)$, where $M_{(identity \lor mixer)}$ is the identity matrix (if no mixer is present) or the Jones matrix for a diagonal polarizer$^{(1, 2, 6)}$. The slit functions are $\Psi_0$ modulated by relative phase due to geometry ($\theta$ and $d$) and absolutely transformed by any polarizing filters. Expanding, $$\Psi_3 \propto M_{(identity \lor mixer)}(M_{(identity \lor H)}\Psi_1e^{i\phi_1}+M_{(identity \lor V)}\Psi_2e^{i\phi_2})\tag{1}\label{eq1}$$ where $M_{H \lor V}$ are the Jones matrices for horizontal and vertical polarizers, respectively, and $\phi_1$, $\phi_2 \equiv \phi_1(\theta, +d), \phi_2(\theta, -d)$. Any surviving sinusoidal dependence on $\phi_{1,2}$ indicates fringing, so $\theta$ and $d$ remain implicit from here on out. For slits only $\eqref{eq1}$ reduces to, $$\Psi_3 \propto I(I\vec E_0e^{i\phi_1}+I\vec E_0e^{i\phi_2})$$, where $I$ is the identity matrix. Therefore, $$\Psi_3 \propto \vec E_0(e^{i\phi_1}+ e^{i\phi_2})$$ and taking the complex vector dot product$^{(4,5)}$ (aka “squaring”) gives, $$\lvert\Psi_3\rvert^2 \propto \lvert E_0\rvert^2(e^{i\phi_1}+ e^{i\phi_2})(e^{-i\phi_1}+ e^{-i\phi_2})$$ which reduces to, $$\lvert\Psi_3\rvert^2 \propto \lvert E_0\rvert^2(1+\cos(\phi_2 - \phi_1))\tag{2}\label{eq2}$$ The sinusoidal term indicates fringing. Note that the basis components of $\vec E_0$ are unimportant, so any input polarization gives fringes and the whole “ordinary double-slit” first column of Table 0 is proven. Now, for polarized slits,

$$ \Psi_3 \propto I \bigl ( \left(\begin{matrix} 1 & 0 \\ 0 & 0 \\ \end{matrix} \right ) \vec E_0 e^{i \phi_1} + \left(\begin{matrix} 0 & 0 \\ 0 & 1 \\ \end{matrix} \right ) \vec E_0 e^{i \phi_2} \bigr) $$

$$ \Psi_3 \propto \left( \begin{matrix} e^{i \phi_1} & 0 \\ 0 & e^{i \phi_2} \\ \end{matrix} \right ) \left[\begin{matrix} E_{0y} \\ E_{0z} \\ \end{matrix} \right ] $$ $$ \Psi_3 \propto \left [ \begin{matrix} E_{0y}e^{i\phi_1} \\ E_{0z}e^{i\phi_2} \\ \end{matrix} \right ] $$ $$ \lvert\Psi_3\rvert^2 \propto \left [ \begin{matrix} E_{0y}e^{-i\phi_1} & E_{0z}e^{-i\phi_2} \\ \end{matrix} \right ] \left [ \begin{matrix} E_{0y}e^{i\phi_1} \\ E_{0z}e^{i\phi_2} \\ \end{matrix} \right ] $$ $$ \lvert\Psi_3\rvert^2 \propto \lvert\vec E_{0y}\rvert^2 + \lvert\vec E_{0z}\rvert^2 = \lvert\vec E_{0}\rvert^2 \tag{3}\label{eq3} $$ So that any input does not give fringing, and the second “polarized Young’s” column of Table 0 is proven. Including the mixer, $\eqref{eq1}$ becomes $$ \Psi_3 \propto \left(\begin{matrix} 1 & 1 \\ 1 & 1 \\ \end{matrix} \right ) \bigl ( \left(\begin{matrix} 1 & 0 \\ 0 & 0 \\ \end{matrix} \right ) \vec E_0 e^{i \phi_1} + \left(\begin{matrix} 0 & 0 \\ 0 & 1 \\ \end{matrix} \right ) \vec E_0 e^{i \phi_2} \bigr) $$ $$ \Psi_3 \propto \left(\begin{matrix} 1 & 1 \\ 1 & 1 \\ \end{matrix} \right ) \left ( \begin{matrix} e^{i \phi_1} & 0 \\ 0 & e^{i \phi_2} \\ \end{matrix} \right ) \vec E_0 $$ $$ \Psi_3 \propto \left (\begin{matrix} e^{i \phi_1} & e^{i \phi_2} \\ e^{i \phi_1} & e^{i \phi_2} \\ \end{matrix} \right ) \vec E_0 $$ $$ \Psi_3 \propto \left [ \begin{matrix} E_{0y}e^{i\phi_1}+E_{0z}e^{i\phi_2} \\ E_{0y}e^{i\phi_1}+E_{0z}e^{i\phi_2} \\ \end{matrix} \right ] $$ $$ \lvert\Psi_3\rvert^2 \propto \left [ \begin{matrix} E_{0y}e^{-i\phi_1}+E_{0z}e^{-i\phi_2} & E_{0y}e^{-i\phi_1}+E_{0z}e^{-i\phi_2} \end{matrix} \\ \right ] \left [ \begin{matrix} E_{0y}e^{i\phi_1}+E_{0z}e^{i\phi_2} \\ E_{0y}e^{i\phi_1}+E_{0z}e^{i\phi_2} \\ \end{matrix} \right ] $$ $$ \lvert\Psi_3\rvert^2 \propto \left [ \begin{matrix} E_{0y}e^{-i\phi_1}+E_{0z}e^{-i\phi_2} & E_{0y}e^{-i\phi_1}+E_{0z}e^{-i\phi_2} \end{matrix} \\ \right ] \left [ \begin{matrix} E_{0y}e^{i\phi_1}+E_{0z}e^{i\phi_2} \\ E_{0y}e^{i\phi_1}+E_{0z}e^{i\phi_2} \\ \end{matrix} \right ] $$ $$ \lvert\Psi_3\rvert^2 \propto \lvert\vec E_0\rvert^2+2 E_{0y} E_{0z}\cos(\phi_2-\phi_1) \tag{4}\label{eq4} $$

Now, $\lvert\vec E_0\rvert^2 = \lvert\vec E_{0y}\rvert^2 + \lvert\vec E_{0z}\rvert^2 = E_{0y}^2 + E_{0z}^2 $ , and for D, AD, H, and V input, $\vec E_0= \left[\begin{matrix} \pm1 \\ \pm1 \\ \end{matrix} \right ] $, $ \left[\begin{matrix} \pm1 \\ \mp1 \\ \end{matrix} \right ] $, $ \left[\begin{matrix} 0 \\ \pm1 \\ \end{matrix} \right ] $, $ \left[\begin{matrix} \pm1 \\ 0 \\ \end{matrix} \right ] $, respectively. Plugging in to $\eqref{eq4}$ shows that the sinusoidal fringing disappears for H and V. For D and AD, the sinusoid remains, but is dark for AD wherever D is bright due to the sign inversion against the cosine term. So, the last column of Table 0 is proven.

So, I guess my theory skills are "correct" because I get the "right answer" for all cases, but that might be a long way from answering why this effect happens. In English, I would say that the complex phase of D is 180 degrees away from that for AD, so all else being equal (along the central fringe), the input amplitude has zero transmission through the mixer. That statement seems to make sense to me, if I imagine that the light vector components really are rotating in and out of imaginary space. For anyone not liking that image, I am not sure what I can do for you.

Acknowledgements
My gratitude to those who have supported and inspired my enthusiasm for science; there is not space here to mention them all. Zeroth, I thank Being One. I acknowledge myself for resolving to initiate this line of research at my personal expense which is not an immediate outgrowth of previous projects I have been blessed to work on. Of those not known to me in the flesh, Socrates who reminds me that I am allowed to start from zero, Huygens who proves more than he guesses, Tesla who trusts in the ether, B. Franklin who shows the way, J. C. Bose who sees life, Heaviside with his dogged quest for understanding, application, and proper credit, Newton and Kepler for sweeping away epicycles, Euler showing unity of cycle, Gauss empowering in surfaces and distributions,… Of those known personally: the late Robert Lin, Davin Larson, and Janet Luhmann who watched over my undergraduate scholarship at Space Sciences Lab, and my then other co-authors S. W. Kahler and N. U. Crooker; Jerry Edelstein, leader and mentor on SPEAR (Spectroscopy of Plasma Evolution from Astrophysical Radiation) and T-EDI (TripleSpec Exoplanet Discovery Instrument) projects, David Erskine, inventor and pioneer of EDI and X-EDI (Ordinary and Crossfading Externally Dispersed Interferometry); Julia Kregenow, Dae-Hee Lee, Mario Marckwordt, Matthew Ward Muterspaugh, Kaori Nishikida, Kwangsun Ryu, Michael Scholl, Kwang-Il Seon, Martin Sirk, the late C. H. Townes, Barry Welsh, and Ed Wishnow for many meaningful interactions. My late parents Esther and Bill Feuerstein, as well as being my teachers, always supported me in my sense of purpose.

1. Fowles, G. (1989). Introduction to Modern Optics (2nd ed.). Dover. p. 35.
2. Wikipedia contributors, "Jones calculus," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Jones_calculus&oldid=1020573977 (accessed August 22, 2021). 
3. Wikipedia contributors, "Plane wave," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Plane_wave&oldid=1031803393 (accessed August 22, 2021). 
4. Born, M. Zur Quantenmechanik der Stoßvorgänge. Z. Physik 37, 863–867 (1926). https://doi.org/10.1007/BF01397477
5. Wikipedia contributors, "Born rule," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Born_rule&oldid=1032140227 (accessed August 23, 2021).
6. H. Hurwitz and R. Jones, "A New Calculus for the Treatment of Optical SystemsII. Proof of Three General Equivalence Theorems," J. Opt. Soc. Am.  31, 493-499 (1941).
7. Chr. Huygens, Traité de la Lumière (drafted 1678; published in Leyden by Van der Aa, 1690), translated by Silvanus P. Thompson as Treatise on Light (London: Macmillan, 1912; Project Gutenberg edition, 2005), p.19.
8. Wikipedia contributors, "Huygens–Fresnel principle," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Huygens%E2%80%93Fresnel_principle&oldid=1040360110 (accessed August 24, 2021). 
9. Wikipedia contributors, "Double-slit experiment," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Double-slit_experiment&oldid=1034409853 (accessed August 24, 2021). 
10. Young, Thomas. "The Bakerian lecture. Experiments and calculation relative to physical optics". Retrieved 14 July 2021.
11. Chiao, R., P. Kwiat and Aephraim M. Steinberg. “Quantum non-locality in two-photon experiments at Berkeley.” Quantum and Semiclassical Optics: Journal of The European Optical Society Part B 7 (1995): 259-278.
12. arXiv:quant-ph/9501016v1

W. Michael Feuerstein is an independent researcher investigating orthogonal projection interferometry and interferometric basis discrimination. He has professional research experience in space physics (particularly the relationship between the interplanetary magnetic field and electron pitch-angle distributions), space-born extreme and far ultraviolet astronomy, and externally-dispersed interferometry for Doppler planet-finding.

Best Answer

Related Solutions

[Physics] How to treat partially polarized light with Jones vectors

Optics – Polarized Slits in Young’s Double-Slit Experiment

Related Question