Prasahnt Sivarajah

Direct experimental visualization of waves and band structure in 2D photonic crystal slabs

We demonstrate for the first time the ability to perform time resolved imaging of terahertz (THz) waves propagating within a photonic crystal (PhC) slab. For photonic lattices with different orientations and symmetries, we used the electrooptic effect to record the full spatiotemporal evolution of THz fields across a broad spectral range spanning the photonic band gap. In addition to revealing real-space behavior, the data let us directly map the band diagrams of the PhCs. The data, which are in good agreement with theoretical calculations, display a rich set of effects including photonic band gaps, eigenmodes and leaky modes. S Online supplementary data available from stacks.iop.org/NJP/16/053003/ mmedia

High-Resolution, Low-Noise Imaging in THz Polaritonics

Time-resolved imaging of propagating electromagnetic waves at terahertz (THz) frequencies provides deep insights into waves and their interaction with a variety of photonic elements. As new components for THz control are developed, such as metamaterial microstructures that display deep sub-wavelength E-field localization, finer spatial resolution and more sensitive imaging techniques are required to study them. Here we introduce key advances in the optical design and lock-in image acquisition at 500 Hz for the complementary imaging techniques of phase contrast and polarization gating. Compared to other methods, this leads to a 4-fold improvement in resolution and up to 5-fold reduction in noise through suppression of low frequency laser fluctuations. With a resolution better than 1.5 μm (λ/100 at 0.5 THz) and a noise floor of 0.2%, phase contrast imaging presents new opportunities for studying very fine structures and near-fields in the THz regime. For most other experiments, polarization gating imaging is preferred because its noise floor is lower at 0.12% and its <; 5 μm resolution is typically more than sufficient.

THz generation using a reflective stair-step echelon

We present a novel method for THz generation in lithium niobate using a reflective stair-step echelon structure. The echelon produces a discretely tilted pulse front with less angular dispersion compared to a high groove-density grating. The THz output was characterized using both a 1-lens and 3-lens imaging system to set the tilt angle at room and cryogenic temperatures. Using broadband 800 nm pulses with a pulse energy of 0.95 mJ and a pulse duration of 70 fs (24 nm FWHM bandwidth, 39 fs transform limited width), we produced THz pulses with field strengths as high as 500 kV/cm and pulse energies as high as 3.1 μJ. The highest conversion efficiency we obtained was 0.33%. In addition, we find that the echelon is easily implemented into an experimental setup for quick alignment and optimization.

What is the Brillouin zone of an anisotropic photonic crystal

The concept of the Brillouin zone (BZ) in relation to a photonic crystal fabricated in an optically anisotropic material is explored both experimentally and theoretically. In experiment, we used femtosecond laser pulses to excite THz polaritons and image their propagation in lithium niobate and lithium tantalate photonic crystal (PhC) slabs. We directly measured the dispersion relation inside PhCs and observed that the lowest bandgap expected to form at the BZ boundary forms inside the BZ in the anisotropic lithium niobate PhC. Our analysis shows that in an anisotropic material the BZ - defined as the Wigner-Seitz cell in the reciprocal lattice - is no longer bounded by Bragg planes and thus does not conform to the original definition of the BZ by Brillouin. We construct an alternative Brillouin zone defined by Bragg planes and show its utility in identifying features of the dispersion bands. We show that for an anisotropic 2D PhC without dispersion, the Bragg plane BZ can be constructed by applying the Wigner-Seitz method to a stretched or compressed reciprocal lattice. We also show that in the presence of the dispersion in the underlying material or in a slab waveguide, the Bragg planes are generally represented by curved surfaces rather than planes. The concept of constructing a BZ with Bragg planes should prove useful in understanding the formation of dispersion bands in anisotropic PhCs and in selectively tailoring their optical properties.

Circumventing limitations of tilted-pulse-front terahertz generation using a stair-step echelon

Ultrafast terahertz (THz) transients are of great interest for linear and nonlinear THz spectroscopy and compact particle acceleration. Tilted-pulse-front (TPF)THz generation in lithium niobate (LN) has become ubiquitous due to its compatibility with easily accessible high-power 800 nm/1 μm laser technology. However, the use of diffraction grating (DG) based TPFs (DG-TPFs) present formidable imaging challenges, particularly for large pump bandwidths and beam sizes. Furthermore, as the optical pump spectrum red-shifts and broadens due to repeated down-conversion to THz frequencies (referred to as cascading), a spatio-temporal break-up of the pump pulse results. This limits the energy conversion efficiency (η) and produces THz pulses with spatio-temporal distortions.