Direct Observation of a Transition of a Surface Plasmon Resonance from a Photonic Crystal Effect Weili Zhang, Abul K. Azad, and Jiaguang Han School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, USA Jingzhou Xu, Jian Chen, and X.-C. Zhang Center for THz Research, Rensselaer Polytechnic Institute, Troy, New York 12180, USA (Received 24 March 2006; published 1 May 2007) Transition of surface-plasmon resonance from out-of-plane photonic crystal effect is observed in a semiconductor array of subwavelength holes by optical pump-terahertz probe measurements. The dielectric properties of the photoexcited array are essentially altered by the intense optical excitation due to photogenerated free carriers. As a result, the array becomes metallic and favors the coupling and propagation of surface plasmons. The photoinduced resonant extremes agree well with the Fano model. DOI: 10.1103/PhysRevLett.98.183901 PACS numbers: 42.70.Qs, 42.65.Re, 73.20.Mf Manipulation of electromagnetic waves below wave- length scale is vital in broad disciplinary applications of photonics and has been a challenging task for decades. Recent demonstration of extraordinary transmission of light through an array of metallic nanostructures has opened up a new avenue to subwavelength photonics [1,2]. This fascinating phenomenon, understood as reso- nant excitation of surface plasmons (SPs) at the metal- dielectric interface, has been extensively explored in a wide spectral range, and is very promising in nanolithog- raphy, near-field microscopy, integrated photonic devices, and biochemical sensing [3 –16]. At terahertz frequencies, in addition to metals, semi- conductors with high density of free carriers show metallic properties by having a negative real part of dielectric func- tion (" r2 < 0), and thus can be used as alternate metallic media to support SPs [14,15]. The advantage of semicon- ductors is that their dielectric function can be modified by varying doping concentration, temperature, or optical ex- citation. This in turn enables tuning and switching of SPs. Recently, SP-enhanced terahertz transmission in highly doped semiconductor gratings was observed [14,15] and was effectively tuned to a reduced transmission level by optical switching [17]. However, it is intriguing how SP resonance can be evolutionally developed when the real part of dielectric function of the constituent medium is altered instantaneously from positive, across zero, to negative. In this Letter, we present observation of a characteristic evolution of SP resonance in a semiconductor subwave- length hole array by use of optical pump-terahertz probe measurements. The array was made from lightly doped silicon (Si), which does not support SPs due to low carrier density, but exhibits out-of-plane two-dimensional (2D) photonic crystal effect [18]. When optical excitation is applied to the array, the photogenerated free carriers alter the dielectric properties of Si; the real part of the dielectric constant changes from positive to negative with increasing excitation intensity. As a result, the signature of photonic crystal effect gradually disappears and SP resonance emerges and is developed into extraordinary terahertz transmission. The resonance profiles of the SP modes are well described by the Fano model. The array sample was fabricated from commercially available 30 m thick n-type Si with 10 cm resistivity and 4  10 14 cm 3 carrier concentration. The periodic through holes were processed by standard microfabrication as described previously [15]. The sample is a 10 mm  10 mm-sized array of 80 m  40 m elliptical holes in a square lattice with a periodicity of 160 m, as shown in the inset of Fig. 1. Conventional optical pump-terahertz probe [19] characterization is carried out by use of an electro-optic terahertz system [20]. The terahertz beam is collimated to a diameter of 1:50 mm on the array, while the beam of optical excitation (100 fs, 1 kHz) has a greater size and overlaps well with the terahertz spot. Terahertz transmission measurements were performed at a normal incidence with p-polarized terahertz field. Figure 1(a) illustrates the transmitted terahertz pulses through air reference, blank Si, and the array of both perpendicular and parallel orientations before and after the optical excitation of a 111 mW average power. The perpendicular (parallel) orientation of the array is defined with the longer axis of the elliptical hole perpendicular (parallel) to the terahertz electric field. The transmitted terahertz pulse through the unexcited blank Si shows 85% field transmission if surface reflections are taken into account; then the Si becomes nearly opaque to tera- hertz waves under intense optical excitation due to strong absorption of photogenerated free carriers. The corre- sponding Fourier-transformed spectra are shown in Fig. 1(b). In the absence of optical excitation the array can be considered as a two-dimensional out-of-plane pho- tonic crystal slab that has shown complicated spectral PRL 98, 183901 (2007) PHYSICAL REVIEW LETTERS week ending 4 MAY 2007 0031-9007= 07=98(18)=183901(4) 183901-1  2007 The American Physical Society