FAST TRACK COMMUNICATION Terahertz metamaterials on free-standing highly-flexible polyimide substrates

We have fabricated resonant terahertz metamaterials on free-standing polyimide substrates. The low-loss polyimide substrates can be as thin as 5.5μm yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimeters. Our results provide a path forward for creating multi-layer non-planar metamaterials at terahertz frequencies. (Some figures in this article are in colour only in the electronic version) The advent of metamaterial composites has given rise to numerous electromagnetic functionalities previously unimagined. This includes negative refractive index, superlensing, cloaking and quite generally, the fabrication of metamaterials which have been designed using coordinate transformation approaches [1–5]. Many of these ideas were initially implemented at microwave frequencies where fabrication of multilayer composites has become increasingly sophisticated during the past several years. This has resulted in dramatically reduced times from conceptualization and electromagnetic simulation to, ultimately, fabrication and characterization. However, the fabrication of subwavelength unit cells becomes increasingly challenging in moving from the microwave to visible region of the electromagnetic spectrum though important progress has been made [6–9]. To date, the majority of this work has been on planar composites. At terahertz (THz) frequencies and above, creating multiple unit cell structures in the direction of propagation and taking full advantage of coordinate transformation MM design to realize non-planar MM composites requires the development of new fabrication strategies. The far-infrared, or terahertz, is a promising region to investigate novel approaches to metamaterials fabrication. First, the unit cells are on the order of tens of micrometres which is amenable to novel microfabrication approaches. Second, and perhaps of greater importance, there is a strong technological impetus to create sources, detectors and components at terahertz frequencies to realize the unique potential of THz radiation [10–13]. Metamaterials are expected to play an important role in this regard as evidenced by recent demonstrations of MM-based modulators and frequency tunable filters [14–16]. An important step in the progression of functional THz MM composites is the fabrication of multilayer structures. For example, a strongly resonant THz MM absorber consisting of two layers spaced by approximately 6μm [24] was recently demonstrated (see also [17] for the microwave perfect absorber). The entire structure, however, was on top of a thick GaAs supporting substrate. There has also been other work at THz frequencies using polymer spin-coating based techniques to fabricate metamaterials, but the focus of this work was not on ultrathin flexible substrates [18, 19]. Closely related to creating THz metamaterial composites is the seminal work on frequency selective surfaces [20–22]. More recently, there has been a report on creating polyimidebased multilayer metallic photonic band-gap (MPBG) structures at terahertz frequencies [23]. The MPBG structure 0022-3727/08/232004+05$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 41 (2008) 232004 Fast Track Communication Figure 1. Photographs of the free-standing electric metamaterials fabricated on polyimide substrate. The thin samples naturally roll up into a cylinder unless supported on a frame. (Inset) A flexible ‘skin’ applied to a finger. (Colour online.) served as a −35 dB notch filter at ∼4.5 THz. The primary difference between [23] and the present publication is that (a) we are focusing on split ring resonators and (b) our structures are easily peeled from the substrate while in [23], a citric acid/hydrogen peroxide etch was used to remove the substrate. It is important to extend such work and develop MM on thin flexible substrates that are considerably thinner than the lateral unit cell dimensions in moving towards creating multilayer non-planar metamaterials such as cloaks, concentrators or resonant absorbers at terahertz frequencies. In this letter, we demonstrate resonant terahertz metamaterials on free-standing low-loss polyimide substrates as thin as 5.5μm yielding robust large-area metamaterials which are easily wrapped into cylinders with a radius of a few millimetres. The MM structures were fabricated by depositing a 200nm-thick gold with a 10 nm thick adhesion layer of titanium on a polyimide substrate. The liquid polyimide of PI-5878G (HD MicroSystemsTM) was spin-coated on a 2 inch silicon wafer to form the substrate. In this work, we fabricated our samples with two different thicknesses, namely 5.5 and 11μm. The thickness of the polyimide substrate can be precisely controlled by adjusting the spin rate and curing temperature. AZ5214e image reversal photoresist was patterned using direct laser writing with a Heidelberg DWL 66 laser writer. 200 nm thick Au/Ti was E-beam evaporated followed by rinsing in acetone for several minutes. As a final step, the MM structures patterned on the polyimide substrate were peeled off of the silicon substrate. The as-fabricated 2 inch diameter samples show extreme mechanical flexibility, as shown in figure 1. Terahertz time-domain spectroscopy (THz-TDS) was used to characterize the metamaterial response. The transmission of the THz electric field, at normal incidence, was measured for the sample and a reference, which in the present case is simply air. The electric field spectral amplitude and phase are calculated through Fourier transformation of the time-domain pulses. Dividing the sample by the reference yields the complex spectral transmission [13]. Prior to measurement, the free-standing MM samples were diced into 1 cm × 1 cm squares and mounted at normal incidence with H E 1