Higher-order fractal transverse modes observed in microlasers

Area of Science:

• Physics
• Optics
• Laser Science

Background:

• Microlasers can support complex spatial eigenmodes.
• Fractal structures exhibit self-similarity and scale invariance.
• Previous research predicted fractal modes in optical systems.

Purpose of the Study:

• To computationally predict and experimentally observe higher-order, fractal spatial eigenmodes in microlasers.
• To investigate the role of microsphere arrays in generating fractal laser modes.
• To analyze the properties and fractal dimension of these modes.

Main Methods:

• Utilizing a Fabry-Pérot resonator with a close-packed array of microspheres acting as a refractive slit array.
• Employing edge diffraction from the slit array to generate high spatial frequencies.
• Observing and analyzing laser modes using quasi-rectangular (4-microsphere) and triangular (3-microsphere) apertures.

Main Results:

• Observed two classes of higher-order, fractal spatial eigenmodes computationally and experimentally.
• Identified specific higher-order modes (m=2,4,5 for 4-sphere, m=1,2 for 3-sphere apertures).
• Calculated fundamental and 2nd-order modes for the 4-sphere aperture show similar intensity profiles and near-degenerate frequencies with different parity.
• Fractal dimension (D) rapidly increases beyond the aperture and varies between 2.2 and 2.5 along the resonator axis.

Conclusions:

• Microsphere arrays within microlasers can generate and support fractal laser modes.
• The observed fractal modes possess unique spatial characteristics, including a variable fractal dimension.
• Generating fractal laser modes in optical waveguides could lead to novel optical devices and imaging protocols.