Photonic probing of radio waves for k-space tomography.

We harness coherent optical processing to simultaneously sense the angle of arrival and the frequency of radio waves. Signals captured by a distributed antenna array are up-converted to optical domain using electro-optic modulators coupled to individual antennas. Employing a common laser source to feed all the modulators ensures spatially coherent up-conversion of radio-frequency (RF) waves to optical beams carried by optical fibers. Fiber-length dispersion extends the spatial aperture of the distributed antenna array into the temporal dimension. The interference of beams emanating from the fibers is captured by a CCD and used to computationally reconstruct RF waves in k-space.

Measured comparison of contrast and crossover periods for passive millimeter-wave polarimetric imagery.

Several targets are set-up outside and imaged by a passive millimeter-wave sensor over a 24 hour period. The sensor is capable of measuring two linear polarization states simultaneously and the contrasts of the targets are compared for the different polarizations. The choice of polarization is shown to have an impact on the contrast of different targets throughout the day. In an extreme case the contrast of a target experiences a crossover event and disappears for one polarization while it presents a strong contrast (9 K) with the other polarization. Experimental results are shown along with a simulation of the scene using a ray tracing program.

Full spectrum millimeter-wave modulation.

In recent years, the development of new lithium niobate electro-optic modulator designs and material processing techniques have contributed to support the increasing need for faster optical networks by considerably extending the operational bandwidth of modulators. In an effort to provide higher bandwidths for future generations of networks, we have developed a lithium niobate electro-optic phase modulator based on a coplanar waveguide ridged structure that operates up to 300 GHz. By thinning the lithium niobate substrate down to less than 39 µm, we are able to eliminate substrate modes and observe optical sidebands over the full millimeter-wave spectrum.

Passive 77 GHz millimeter-wave sensor based on optical upconversion.

A passive millimeter-wave (mmW) sensor operating at a frequency of 77 GHz is built and characterized. The sensor is a single pixel sensor that raster scans to create an image. Optical upconversion is used to convert the incident mmW signal into an optical signal for detection. Components were picked to be representative of a single element in a distributed aperture system. The performance of the system is analyzed, and the noise equivalent temperature difference is found to be 0.5 K (for a 1 s integration time) with a diffraction limited resolution of ~8 mrad. Representative images are shown that demonstrate the phenomenology associated with this spectrum.

RF imaging, architectures, and applications: introduction to the feature issue.

This Applied Optics feature issue spotlights developments in the emerging areas of millimeter, submillimeter, and terahertz imaging.

Experimental demonstration of self-collimation inside a three-dimensional photonic crystal.

We present our experimental demonstration of self-collimation inside a three-dimensional (3D) simple cubic photonic crystal at microwave frequencies. The photonic crystal was designed with unique dispersion property and fabricated by a high precision computer-controlled machine. The self-collimation modes were excited by a grounded waveguide feeding and detected by a scanning monopole. Self-collimation of electromagnetic waves in the 3D photonic crystal was demonstrated by measuring the 3D field distribution, which was shown as a narrow collimated beam inside the 3D photonic crystal but a diverged beam in the absence of the photonic crystal.

Perfect lens makes a perfect trap.

In this work, we present for the first time a new and realistic application of the "perfect lens", namely, electromagnetic traps (or tweezers). We combined two recently developed techniques, 3D negative refraction flat lenses (3DNRFLs) and optical tweezers, and experimentally demonstrated the very unique advantages of using 3DNRFLs for electromagnetic traps. Super-resolution and short focal distance of the flat lens result in a highly focused and strongly convergent beam, which is a key requirement for a stable and accurate electromagnetic trap. The translation symmetry of 3DNRFL provides translation-invariance for imaging, which allows an electromagnetic trap to be translated without moving the lens, and permits a trap array by using multiple sources with a single lens. Electromagnetic trapping was demonstrated using polystyrene particles in suspension, and subsequent to being trapped to a single point, they were then accurately manipulated over a large distance by simple movement of a 3DNRFL-imaged microwave monopole source.

Three-dimensional subwavelength imaging by a photonic-crystal flat lens using negative refraction at microwave frequencies.

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.