First Observation of Photons Carrying Orbital Angular Momentum in Undulator Radiation

Photon beams of 99 eV energy carrying orbital angular momentum (OAM) have been observed in the 2nd harmonic off-axis radiation of a helical undulator at the 3rd generation synchrotron radiation light source BESSY II. For detection, the OAM carrying photon beam was superimposed with a reference beam without OAM. The interference pattern, a spiral intensity distribution, was recorded in a plane perpendicular to the propagation direction. The orientation of the observed spiral structure is related to the helicity of the undulator radiation. Excellent agreement between measurements and simulations has been found.

Figure 1

Surfaces of equal phase for beams with various orbital angular momenta.

Figure 2

Setup of the experiment with two undulators and a phase shifter in between (not shown), a movable pinhole in front of the monochromator and the detector behind the refocusing optics.

Figure 3

Measured (symbols) and simulated (solid lines) intensity cuts of the individual undulators: vertical cuts of the linear undulator (dots and solid line) and diagonal cuts of the helical undulator (diamonds and solid line) are plotted. The unused undulator is set to a magnetic gap of 100 mm. Inset: Measured color-coded intensity profiles from the linear (upper) and the helical (lower) undulator, respectively.

Figure 4

Measurement [(a) and (d)] and simulation [(b) and (c)] for the case of 10° [(a) and (b)] and 0° [(d) and (c)] setting of the phase shifter. In the numeric simulations the spiral orientation is not a fitted parameter and the agreement with measurements demonstrates a realistic modeling of the undulators and the phase shifter. The dashed spirals in the upper plots are evaluated from the analytic equation Eq. (5) where the constant phase term is set to zero to match the observed orientation of the spiral.

Figure 5

Measured intensity distribution at 0° setting of the phase shifter and negative helicity. The spiral sense of rotation is reversed as compared to the results for positive helicity (Fig. 4).

Figure 6

3D plots of simulations at 0° setting of the phase shifter and positive helicity of the upstream undulator [Fig. 4c] calculated for the actual (left) and zero (right) emittance, respectively.