The October 24, 2012 on-line edition of Light Matters explores a new solid state light emitting optical device which utilizes OAM (Optical Orbital Momentum) to impart phase change (or twist) on emitted beams of light.
What does this mean and why is it significant? Some twisted history follows:
Years before LED (Light Emitting Diodes) and advanced laser optics, phase shifting technologies were utilized in radio and TV broadcast systems to impart information on radio signals utilizing FSK (Frequency Shift Keying). It was found that a radio carrier wave could be momentarily shifted in frequency (and phase with relation to the receiving radio) and manipulated in order to form encoded characters comprising the alphabet and later ASCII characters (American Standard Code for Information Interchange). This technology became known as radio teletype (RTTY) and was utilized by news wire services for their world wide distribution. Later, as early television broadcast technology advanced, efforts were made to improve the quality of TV reception by improving the design of antennas. Remember TV antennas? Metropolitan area TV reception was often subject to “ghosting” which occurs when the original TV signal and the identical reflected signal (off of a nearby building or structure) arrive at a TV receiver a split second apart (phase shifted), creating a double “ghosted” image on the TV screen. A technique utilized by broadcasters to eliminate this problem employs circular (or twisted) polarization. In this technique the TV signal is transmitted with an antenna that simultaneously imparts both horizontal and vertical polarity to the signal which travels through the air like a cork screw. A properly designed receive antenna can capture the cork screw shaped signal in its original phase and “null out” phase shifted reflections, eliminating “ghosts” on your TV screen. TV and radio signals are actually radio frequency energy, a lower wavelength of light invisible to our eyes. At these lower wavelengths the physical antenna structures required to transmit “invisible light waves” are enormous (an AM radio station broadcast tower is a good example). Until recently, phase shifting RF (Radio Frequency) energy and optical wavelength light required big antennas or bulky optical assemblies.
The importance of the analogy:
Circularly polarized antennas for microwave and VHF frequencies range in size from meters to millimeters. OAM is important as it enables the ability to manipulate the transmission and phase of light with a solid state device comprised of structures only 8 microns in size. The evolution of advanced optics and lasers resulted from the ability to precisely shape materials in the exact dimensions required to control optical wave fronts, polarity and other parameters. Solid state lasers are now common place (your laser pointer) and microwave antennas are now etched on silicon or gallium nitride wafers and fit inside your smart phone. Chip manufacturers anticipate the interconnection of data pathways utilizing beams of light instead of copper conductors, increasing device speed and data throughput. Nano structures and plasmon technologies are being developed to build nano scale lasers for on-chip data transmission at the speed of light. The ability to create an 8 micron sized OAM (Optical Orbital Momentum) device enabling the phase shift of emitted light enables a wide range of applications in both metrology and communications. We are witnessing an evolution and convergence of nanoscale technologies at an ever accelerating pace. OAM devices could potentially enhance current FTIR (Fourier Transform Infrared) metrology, scatterometry and other techniques currently used in semiconductor quality control. For communication applications, OAM has potential for use in wafer scale photonic networking and the secure cryptographic transmission of data. Check out NIST (National Institute of Standards and Technology) for new metrology and signal processing research.
OAM device technology puts a new twist on light and will brighten our future.
Thomas D. Jay
Semiconductor Industry Consultant