THz Generation, Control and Detection on a Chip
Benjamin Ofori-Okai, Prasahnt Sivarajah, and Stephanie Teo
The Polaritonics platform
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 (see Figure 1 below).
Figure 1 Polaritonics geometry. The THz is generated by an ultrafast optical pump pulse and then guided down the crystal slab, where it can interact with a 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 [Werley 2010]. 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: 
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.
Figure 2. THz imaging. (a) Phase contrast imaging. (b) An image of a THz wave reflecting off and transmitting through 5 slits, clearly demonstrating diffraction and interference.
THz Antennas
We recently used the polaritonics platform to study metal antennas deposited on the surface of the LiNbO3 slab [Werley 2012]. 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.
Figure 3. A THz antenna. (a) Diagram of a pair of half-wave antennas, showing field localization and enhancement at the antenna ends and in the small gap between them. (b) An image of a resonant THz wave interacting with such an antenna deposited on the surface of the lithium niobate slab. (c) A magnified view of (b) showing field localization and enhancement at the antenna ends and gap.
We used polaritonics imaging to map the THz field with λ/100 spatial resolution and fully understand the near-field profiled of the antenna [see (b) and (c) in Figure 3]. We directly measured E-field enhancements up to 40-fold and developed some simple models to predict field enhancement as a function of antenna length and gap size. These insights are applicable at all frequency ranges and will aid the design of antennas for various applications including single-molecule fluorescence, surface enhanced Raman spectroscopy, near-field scanning optical microscopy. In particular, our group want to use the extremely intense fields in the antenna gap to perform nonlinear THz spectroscopy.
THz Metamaterials
In 1967, Veselago postulated the existence of unnatural materials that have simultaneous negative permittivity, ε, and permeability, µ. These materials would have a negative refractive index (see Fig. 4). Only many decades later were these materials proven to exist by Smith and Pendry in a composite medium now termed metamaterials. In the same way that a material is made up of atoms that give rise to characteristic macroscopic properties, µ and ε, a metamaterial is made up of periodic microstructures or artificial atoms that give an effective response, µeff and εeff. Some interesting phenomena that result from such materials include negative refraction, superlensing, and cloaking.
Figure 4. Refraction with a negative index. (a) Refraction in a conventional positive index material with rays on opposite sides of the normal and the same sign of phase velocity. (b) Refraction in a negative index material with rays on the same side of the normal and negative phase velocity.
Electric and magnetic microstructures that are resonant at THz frequencies can be fabricated using lithography, where the dimensions and periodic spacings are smaller than the wavelength of light. Examples of these microstructures are shown in Fig. 5. Microstructures generally behave as damped harmonic oscillators, which leads to a negative response at frequencies near the resonant frequency w0.
Figure 5. Examples of electric and magnetic microstructures and the real part of the frequency-dependent permittivity and permeability function.
We are interested in learning more about the properties of negative index materials and how we can apply them to new systems and devices. With our capabilities in imaging fields, it may be possible to visualize negative refraction in a slab of LiNbO3, where a 2D array of metamaterials is lithographically deposited on the surface (see Fig. 6 for experimental geometry). Much like in the antenna study, using phase-contrast imaging [Werley 2010] we can temporally and spatially resolve the near- and far-field behavior of such materials.
Figure 6. Experimental geometry of a negative index slab waveguide, where an evanescent rightward propagating THz wave interacts with a 2D array of metamaterials.
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.
Figure 7. (a) A cartoon of a photonic crystal. The white circles are air holes which are cut into a piece of dielectric material. (b) Illustration of the photonic bandgap. In this case, the frequency of the red light is in the bandgap of the material and hence it is rejected while the green is transmitted through.
Photonic 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, 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.
References
[Werley 2010] C. A. Werley, Q. Wu, K-H Lin, C. R. Tait, A. Dorn, and K. A. Nelson, Comparison of phase-sensitive imaging techniques for studying terahertz waves in structured LN, J. Opt. Soc. Am. B 27, 2350-2359 (2010).
[Werley 2012] C. A. Werley, K. Fan, A. C. Strikwerda, S. M. Teo, X. Zhang, R. D. Averitt, and K. A. Nelson, Time-resolved imaging of near-fields in THz antennas and direct quantitative measurement of field enhancement, Opt. Express 20, 8551-8567 (2012).
[Ward 2006] D. W. Ward, E. R. Statz, and K. A. Nelson, Fabrication of polaritonic structures in LiNbO3 and LiTaO3 using femtosecond laser machining, Appl. Phys. A 86, 49-54 (2006).
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