How Physicists Have Been Able To Detect The Orbital Angular Momentum Of Light Using A Compact Setup

Diagram showing one set up - using U-shaped electrodes and  a 200 nm thick wafer of tungsen ditelluride  to detect the orbital angular momentum of light.


The  phenomenon of orbital angular momentum applied to light waves has only come into physicists' purview in the last 3 decades.   This is because before that interval there was little or no ability to impart orbital angular momentum.  Let's first back up a bit to state that light does often have spin angular momentum but we refer to it as left or right circular polarization.

To refresh readers' (non-physicists') memories, electromagnetic waves (like light)  are  polarized when their E- field  components are preferentially oriented in a particular direction

Examples of polarization include:

Linearly or horizontally polarized: I.e. the E- vector is confined to one  (horizontal) plane:

---------à E

Vertically polarized: I.e. the E- vector is confined to one  (vertical) plane

^ E

!
!
!

Circular: The E-vector rotates through 360 degrees. 

Right circular polarization occurs when the electric field rotates in a right hand sense in respect to the direction of propagation.   Left circular polarization occurs when the electric field rotates in a left hand (screw-) sense in the direction of propagation.  E.g.





In the case of orbital angular momentum for light, we note that instead of the usual flat wavefront we instead have a helical wavefront that turns like a cork screw (see top graphic)   In the lower right of the graphic, the values for m (= 0, + 1)  denote a quantum number describing how tight the corkscrew motion is.  So m = +1 would mean a full clockwise rotation over one wavelength. m = - 1  means a full counterclockwise rotation, and m = 0 denotes no rotation.

Enter now Ritesh Agarwal of the Univ. of Pennsylvania and colleagues. They have found (cf. Physics Today, July, p. 18)  a compact and convenient method to detect light's orbital angular momentum which hitherto required a literal roomful of bulky optics. 

The key breakthrough enabling the transition was the identification of what are called "Weyl semimetals"  for the first time in 2015.  Before then CCDs and solid state sensors were needed. and limited to detecting only one m mode at a time.  Worse, phase information was usually  lost when intensity (I) was converted to a photocurrent. 

In a critical design step, Agarwal and his (Ph.D.) graduate student Zhu-run Ji, prepared a 50 - 200 nm thick layer of the Weyl semimetal tungsten ditelluride to form a 'wafer' (black rectangle) bearing inverted -U shaped electrodes. This was specifically to probe the photocurrent of the light with orbital angular momentum.  (The U-shape was found to be the best at measuring a radial current and canceling out other, unwanted effects.)

The electrode shape was the key to distinguishing the current arising from the orbital angular momentum from current due to intensity gradients and other effects.  Agarwal et al discovered that the photocurrent in Weyl semimetals was (fortuitously) sensitive to the intensity gradients  - not to the light intensity.  (In most materials, photocurrents depend on light's local intensity)

In the setup shown in the top graphic light with orbital angular momentum excites photo-currents in a wafer (black rectangle) with U-shaped electrodes (yellow).  Agarwal's team then measured the photo-current with the ammeter (A) to determine the orbital angular momentum quantum number m.

Before reaching the Weyl Te detector, a beam with orbital angular momentum traverses a rotating quarter wave plate.  The function of the plate is to cycle the polarization of the beam from 0^o  linear to left circular to 90^o  linear, to right circular and back to 0^o  linear.

The team found that the measured (radial) photo-current for beams with m =  + 1 displayed three components:

-  One depending on the linear polarization state

- One depending on  ℓ _c   the circular polarization state

- One independent of polarization (but which includes thermal currents)

It will be interesting to see further research, especially from independent teams,  further establishing the relationship between intensity gradients and phase gradients.  One would also like to see other researchers - using a similar compact Weyl- Te setup-  detecting comparable beams with +  m yielding opposite helicities but with the same intensity profiles.


A related paper on the orbital angular momentum of light can be found here:

https://www.ijert.org/the-orbital-angular-momentum-of-light-oam