Photonic crystals (PCs) have periodically modulated refractive indices, and do not allow the propagation of electromagnetic waves in a certain interval of wavelengths. The periodic structure produces a photonic band gap, which is similar to an electronic bandgap, for example, of a Si semiconductor with periodic electron potentials. In general, PCs are classified mainly into three categories, that is, one-dimensional (1D), two-dimensional (2D), three-dimensional (3D) crystals depending on the periodic dimensionality.

1D/2D Photonic Crystals for THz Devices

In our lab, we have been working on various kind of PCs. For dual-mode semiconductor lasers, we analyzed and fabricated multi-section distributed Bragg reflection (DBR) structures, which can be used to integrated CW THz photomixers that can adjust the wavelengths of two lasing modes such that tune the THz wave generated by the grating-coupled dual-mode laser. We used a home-made holographic nanolithography system where an UV laser beam is split into two beams to make interference patterns with sub-µm periods on photoresist (PR) films.

In a similar way, 2D PCs with a hexagonal array composed of nano size spots can be fabricated by using the holographic nanolithography with three coherent UV laser beams (KR patent 10-2007-0045020). This new technique has several advantages compared with conventional method: 1) single laser beam alignment (less sensitive to external perturbation), 2) period tunability by simply changing the incidence angle without additional beam adjustments, and 3) ideal for large-area (~1 cm2) 2D PCs with a wide range periods of 200 nm - 3 um. The following figures shows the SEM images of 2D PCs. The angle of incidence of three laser beams is 45°, and the period of 2D PCs is about 300 nm. The diameters of PR dots and air holes in hexagonal metallic PC are about 80 nm and 120 nm, respectively.

The recent progress in THz wave generation and detection techniques has generated much interest in low loss THz waveguides which are essential for the construction of compact THz devices and measurement systems. However, most of the present THz systems rely on free space propagation and processing of THz waves due to the virtual absence of low loss waveguides at THz frequencies.

The conventional THz guiding structures such as microstrips, coplanar striplines, and coplanar waveguides fabricated on semiconductor substrates can support only a limited bandwidth due to their excessive dispersion and loss. On the other hand, the photonic crystal fiber (PCF) at infrared wavelengths has engendered growing interest since it offers the opportunity to fabricate optical waveguides with enhanced linear and nonlinear optical properties. A typical PCF consists of a waveguiding core and a spatially periodic cladding region. The core is formed by introducing a defect into the photonic crystal structure to create a localized region with optical properties different from the surrounding cladding region.

For THz applications, low loss materials such as plastics need to be used. The THz applications of these PCFs will depend on the types of the defects. The high-index core PCF can transmit broadband THz signals while the air-core PCF can be used as an ultralowloss, narrow band THz waveguide. In our lab, for the first time, we have experimentally demonstrate the fabrication of a plastic photonic crystal fiber PPCF with a high index core at THz frequencies (US patent 7106933, 8009951, 8009952) Furthermore, we show that the fabricated PPCF exhibits low loss single-mode propagation of subpicosecond THz pulses and reasonably low dispersion.

Low-loss, ultrahigh-reflectance mirrors, known as THz supermirrors, will be required for these high-finesse cavities. Optical supermirrors (R > 0.9997) are available at visible and infrared wavelengths for high-sensitivity measurements, such as cavity ring-down spectroscopy (CRDS) and cavity quantum electrodynamics (QED) experiments. CRDS is a very sensitive spectroscopic technique that uses ultrahigh-reflectance mirrors (typically R > 0.999) to detect extremely small absorptions. QED experiments also require high-finesse cavities, which, in their simplest form, consist of a pair of supermirrors. These cavities can enhance the electromagnetic fields of the cavity modes by orders of magnitude.

However, such supermirrors have not yet been realized at THz frequencies. Metallic mirrors are satisfactory for general purposes, but are not suitable for high-sensitivity spectroscopy because of the finite THz conductivities of common metals such as Cu, Au, and Al. On the other hand, one-dimensional (1D) photonic crystal (PC) mirrors with low-loss dielectric layers are ideal candidates for THz supermirrors, since they can almost completely stop wave propagation at certain frequencies due to photonic band gap (PBG) effects, and their reflectance increases with the number of layers. PC mirrors and filters that function in the THz range have been demonstrated by many groups using various materials, such as polymer films, ceramic films, spin crossover materials, and silicon/air films. However, these materials are too absorptive to achieve the necessary reflectance (R > 0.999). In addition, the reflectances of these mirrors have not been accurately measured.

In our lab, for the first time, we have experimentally demonstrated the ultrahigh reflectance (R > 0.9995 and R ^1/2 > 0.9998) of THz supermirrors, based on 1D PC structures fabricated using high-resistivity float-zone (FZ) silicon wafers. The effects of the material absorption and the layer-thickness error on the reflectance were also analyzed.

Liquid Crystals for THz Device Applications

Despite the wide application of liquid crystals (LCs) in the visible frequency range, their properties in the terahertz range have not yet been extensively investigated. In this paper we have investigated the terahertz properties of LCs E7, BL037, RDP-94990 and RDP-97304 using terahertz time-domain-spectroscopy. We find that RDP-94990 has the largest birefringence and smallest absorption in the terahertz range compared to E7 and BL037. We highlight the importance of investigating all parameters, not just the birefringence, when designing fast, efficient and transmissive terahertz LC devices.