Synthesis of sub-diffraction quasi-non-diffracting beams by angular spectrum compression.

Area of Science:

• Optics and Photonics
• Beam Physics

Background:

• Quasi-non-diffracting beams are crucial for applications like optical manipulation and super-resolution microscopy.
• Generating super-long beams with sub-diffraction and sub-wavelength sizes presents a significant challenge.

Purpose of the Study:

• To develop a practical method for designing and generating super-long quasi-non-diffracting beams with sub-wavelength transverse dimensions.
• To provide a clear physical understanding of the formation of these specialized beams.

Main Methods:

• A novel method based on compressing a normalized angular spectrum was developed.
• A lens was designed and fabricated using concepts of local grating and super-oscillation for a wavelength of 632.8 nm.
• Numerical simulations and experimental studies were conducted to validate the method.

Main Results:

• The normalized angular spectrum compression method successfully enabled the design of quasi-non-diffracting beams with super-oscillatory sub-wavelength transverse size.
• An optical hollow needle exceeding 100 wavelengths in length was experimentally demonstrated.
• A sub-diffraction and sub-wavelength optical hollow needle was observed within a non-diffracting propagation distance of 94 wavelengths.

Conclusions:

• The normalized angular spectrum compression technique is a viable and practical approach for creating advanced optical beams.
• The experimental demonstration confirms the potential for generating ultra-long optical hollow needles with sub-wavelength features.
• Further improvements in lens design (larger radius, shorter effective wavelength) are expected to yield even longer non-diffracting propagation distances.