ОПТИКА И СПЕКТРОСКОПИЯ, 2010, том 108, № 6, с. 884-889
ТЕРАГЕРЦОВАЯ ОПТИКА И СПЕКТРОСКОПИЯ =
УДК 535.14
ACTIVE TERAHERTZ METAMATERIALS
© 2010 г. H.-T. Chen, J. F. O'Hara, and A. J. Taylor
MPA-CINT, MS K771, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
E-mail: chenht@lanl.gov Rceived December 7, 2009
In this paper we present an overview of research in our group in terahertz (THz) metamaterials and their applications. We have developed a series of planar metamaterials operating at THz frequencies, all of which exhibit a strong resonant response. By incorporating natural materials, e.g., semiconductors, as the substrates or as critical regions of metamaterial elements, we are able to effectively control the metamaterial resonance by the application of external stimuli, e.g., photoexcitation and electrical bias. Such actively controllable metamaterials provide novel functionalities for solid-state device applications with unprecedented performance, such as THz spectroscopy, imaging, and many others.
INTRODUCTION
Metamaterials have been developed during the recent years as artificially structured effective media through a bottom-up approach with specifically designed subwavelength unit cells. They exhibit unusual electromagnetic or optical properties that are very difficult or impossible to realize using naturally existing materials. Particularly, metamaterials can reveal simultaneously negative effective permittivity and permeability, and therefore negative index of refraction [1—5], the properties of "substances" that Veselago envisioned 40 years ago [6]. Exotic properties were predicted and now many of them have been experimentally verified. Thanks to the pioneering works by Pendry and Smith and their colleagues [1, 2, 7—9], worldwide interest continues to grow in metamaterials and has resulted in the emergence of new phenomenology and novel metamaterial structures operating over many decades of the electromagnetic spectrum [3—5]. Phenomena such as negative index of reflection [1, 2], super-resolution in optical imaging [10, 11], electromagnetic invisibility [12], and enhanced terahertz (THz) functionalities [13—17] are of particular interest and importance.
However, metamaterials are still in their infancy, so the ultimate real world applications still require much development and may be hard to predict. In the microwave, infrared, and visible regimes, many technologies have already matured and are routinely used in our daily life. Metamaterials may have particular benefit, however, in the THz frequency range, which is loosely defined between 0.1 and 10 THz and is among the least developed regimes in the electromagnetic spectrum. THz radiation is attractive and promising for numerous applications including molecular identification and sensing, material characterization, nondestructive detection, spectroscopy, biomedical imaging, and secure short-range wireless communication. However, at THz frequencies, both the materials' classical electronic response and the quantum photonic
response die off. The resulting "THz gap" is due to the absence of materials with desirable THz response, and the consequent lack of high power compact THz sources, sensitive THz detectors, and many general THz system components such as filters, lenses, switches/modulators, phase shifters, and beam steering devices. Worldwide efforts continue in this field, but progress has been relatively slow because of the very limited number of useful natural materials.
The emergence of metamaterials with engineered and controllable properties provides unique opportunities that may have the potential to overcome these material issues and bridge the THz gap. In this paper, we present an overview of some of the most important demonstrations in our group during the past few years in the area of actively or dynamically controllable THz planar metamaterials and metamaterial-based THz devices and components.
COMPLEMENTARY PLANAR THZ ELECTRIC METAMATERIALS
Split-ring resonators (SRRs) [9] having an artificially designed magnetic resonant response have played the most important role in the development of metamaterials. Three dimensional bulk metamaterials consisting of periodically arranged subwavelength SRRs and other metamaterial elements are usually required to be considered as effective media. For certain applications, however, two-dimensional planar metamaterials are sufficient and greatly ease fabrication difficulties, particularly at THz and higher frequencies. We have designed, modeled, fabricated and characterized a series of THz planar metamaterial structures [18], termed electric SRRs with the unit cells shown in Fig. 1a. These highly symmetric structures consist of a class of subwavelength elements that exhibit a resonant response to the electric field while minimizing or eliminating any response to the magnetic field [19]. We also fabricated and characterized their inversed metamaterial structures termed complementary metamaterials in contrast to the original
metamaterials, both of which exhibit electric resonances [18]. The important difference between original and complementary metamaterials is the inversed regions of metallized and bare patterning. Both the original structures and their complements were fabricated as square planar arrays on semi-insulating gallium arsenide (SI-GaAs) substrates through conventional photolithographic methods and a lift-off process. The metamaterial samples were then characterized by transmission measurements using THz time-domain spectroscopy (THz-TDS) with a bare GaAs wafer as the reference.
The frequency dependent THz transmission amplitude spectra with normal incidence are shown in Fig. 1b. The low frequency resonant response (<1 THz) originates from the electrically excited circulating currents in the ring (anti-ring) structures and results in a pure electric response. All original metamaterials show a sharp resonant transmission minimum as low as 10% at frequencies between 0.5 and 1.0 THz. The complementary metamaterials, on the other hand, show a resonant enhanced transmission as high as 90% at the same frequencies as the transmission dips in the original metamaterials. Furthermore, the THz transmission amplitude spectra of the original and complementary metamaterials are complementary to each other [18], in accordance with the Babinet's principle. The second transmission minimum (original metamaterials) or maximum (complementary metamaterials) originates from the excitation of collective electric di-poles similar to that in cut wires or their complements.
These metamaterials may be of particular interest in applications such as far-infrared spectroscopy, astronomy, laser cavity output couplers, and Fabry-Per-ot interferometers. We have further designed and characterized additional metamaterial elements including rectangular SRRs controlling the coupling between fundamental and higher-order resonances [20], and elliptical SRRs showing anisotropic properties for polarimetric applications [21]. All of these planar metamaterials have founded the basis in our discovery of more advanced THz metamaterials and devices.
ULTRAFAST DYNAMICAL SWITCHING OF METAMATERIAL RESONANCE
Although the metamaterial resonant response is mainly determined by the geometry and dimensions of the metamaterial elements, incorporation of additional materials, e.g., semiconductors, as the substrate or as an integral part of the metamaterial particles, can enable the dynamical or active control over the metamaterial response and therefore produce novel functionalities. For example, SRR arrays fabricated on high resistivity GaAs semiconductor substrate reveal very strong resonances [18]. However, under photoexcitation using near-infrared laser pulses, Padilla et al. demonstrated that the generated photocarriers on the substrate surface create a conducting channel that short-circuits the split gaps, which dynamically deactivates the metamaterial resonance and thereby tunes
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Fig. 1. (a) Schematic geometry of original (O1-O6) and complementary (C1-C6) planar metamaterial unit cells. (b) THz transmission amplitude spectra of the corresponding metamaterials. The polarization of normally incident THz radiation is configured as shown in O1 and C1 for the original (1) and complementary (2) metamaterials, respectively [18].
the values of the effective permittivity and/or permeability [13]. Consequently, the transmission amplitude of the THz radiation was also switched/modulated over a narrow frequency range centered at the resonance.
The advantage of this optical tuning approach is the possibility of extremely fast switching using femtosecond optical pulses. In this case, although the switching rise time is very fast, the recovery time must also be minimized to achieve fast modulation. Intrinsic GaAs has a recovery time is on the order of nanoseconds due to carrier recombination dynamics. Therefore, reducing the photocarrier lifetime is key to realizing the ul-trafast switching of the metamaterial response. Various
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Fig. 2. THz power transmission spectra through the metamaterial sample at various time delays (numbers near curves, in ps) of THz pulse following the photoexcitation. Inset, metamaterial eSRR unit cell; arrows, polarization of the normally incident THz radiation [23].
methods to shorten the carrier lifetime in semiconductors could be employed including radiation damage and low-temperature growth to introduce defects. We used epitaxial grown GaAs:ErAs nanoisland super-lattices as the metamaterial substrate, where the carrier lifetime is strongly correlated with the superlattice period L and can be engineered from sub-picosecond to tens of picoseconds [2
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