Terahertz Polaritonics and Technique Development

The Polaritonics Platform: THz Generation, Control and Detection on a Chip

Terahertz (THz) radiation is the part of the electromagnetic spectrum that lies between the infrared and microwave, and it is typically associated with frequencies between 0.1 - 10 THz (wavelength: 30 - 3000 um, energy: 3 - 300 cm-1 = 0.4 - 40 meV). THz radiation is an important tool in basic science because it can be used to interrogate many THz-frequency phenomena including molecular rotations in a gas, vibrations in a molecular crystal (like sugar), and electronic transitions in nanostructures such as quantum wells or quantum dots. It can also be used to probe a variety of more exotic condensed phase phenomena including Cooper pairs, polarons, and magnons. In addition, THz is has practical applications as it is the frontier in high-speed electronics, and may prove useful as a replacement for x-ray scanners in airports.

THz frequencies lie above what is easily accessible with fast electronics and below what is easily accessible with tunable lasers, so THz technology is less advanced than in other regions of the electromagnetic spectrum. There are many challenges associated with generating and detecting THz radiation, and typically expensive ultrafast lasers are required. Guiding the THz is also difficult: wires cannot carry high enough frequencies and the beam diverges so quickly it is difficult to route the THz with optics.

The Nelson group has developed the THz polaritonics platform to address these issues, where generation, amplification, control, and detection occur in a single compact chip. Here, the chip is a 30 - 50 um thick slab of lithium niobate (LiNbO3) crystal. An intense optical pump pulse passes through the slab where it generates THz-frequency phonon-polariton wave via a nonlinear optical response in the crystal. The generated THz wave is guided down the slab, where it will interact with a machined air-gap, metallic microstructure, or sample deposited on the surface.

THz Imaging -- A key capability of the polaritonics chip is the ability to measure the full spatial profile of the electric field of the THz wave at each point in time as the wave propagates at the speed of light [2]. This information can be played back as a video showing interactions of the wave with structures in or on the chip, providing exceptional insight into the behavior of photonic components. This is possible because LiNbO3 is an electro-optic crystal, so the THz field induces a change in the index of refraction, which shifts the phase of the expanded optical probe beam used to detect the THz wave: E(x,y) ∝ Δn(x,y) ∝ Δφ(x,y)

We then use a phase sensitive imaging technique to record the induced shift, and step the time delay between pump and probe pulses to build up the full evolution of the wave. See the movie theater for examples of movies collected using this technique. (a) in the Figure 2 shows the optical setup for phase contrast imaging. A phase mask in the Fourier plane of the lens introduces a 90 degree phase shift between the main beam and the diffracted light, which leads to interference and thus phase-to-amplitude conversion in the image plane. (b) shows one frame from such a movie. The light-gray rectangles are air gaps cut into the LiNbO3 slab. On the left, part of the THz wave has reflected off the air gaps and on the right some has transmitted through the thin bridges between gaps. Both reflected and transmitted waves are undergoing diffraction and interference.

THz Antennas -- We recently used the polaritonics platform to study metal antennas deposited on the surface of the LiNbO3 slab [3]. Pairs of half-wave antennas aligned end-to-end and separated by a small gap, like the one shown in (a) in the figure below, provide very large field enhancements in regions much smaller than the diffraction limit. We studied these antennas for three purposes: 1. to develop a component for future high-speed devices that can interconvert propagating electromagnetic waves and subwavelength electronics, 2. to harness the antenna's field enhancement to generate very high amplitude electric fields for future nonlinear THz experiments, and 3. to improve fundamental understanding of antenna behavior for experiments at all frequency ranges.

Photonic Crystals -- A photonic crystal is made of two or more materials that have different refractive indices arranged in a periodic fashion. Most commonly they are made by either cutting holes in a dielectric material or by placing rods of a dielectric in air. A cartoon is shown in Figure 7. Because of the periodicity of the material, photonic crystals exhibit optical properties not found in normal dielectrics. The most notable new characteristic is the presence of a photonic bandgap, i.e. a range of frequencies for which light cannot propagate inside of the crystal. If light incident on a crystal has a frequency that is in the bandgap it will be rejected with very high efficiency.

hotonic crystals can be used to build bends for redirecting light, splitters to divide light into different channels, and various kinds of filters. Previous work has focused on studying photonic crystals in the microwave regime for use in telecommunications. However, almost all this has focused on studying the light after it has propagated through the crystals, and it has been difficult to measure the fields directly within crystals themselves. Our work takes advantage of the imaging techniques that we have developed to study the fields within photonic crystals. These measurements can be compared with simulation and for time resolved studies by watching the fields as they propagate in the crystal. Using laser machining [4], we are able to fabricate THz-frequency photonic crystals by cutting air holes into thin lithium niobate slabs. We also work on developing new materials which can use electrical voltages and optical pulses to change the performance of the photonic crystals.