PHYSICAL REVIEW B 86, 235144 (2012) Spectroscopy of terahertz radiation using high- Q photonic crystal microcavities V. M. Muravev, P. A. Gusikhin, G. E. Tsydynzhapov, A.A. Fortunatov, and I.V. Kukushkin Institute of Solid State Physics, RAS, Chernogolovka 142432, Russia and Terasense Development Laboratories, Chernogolovka 142432, Russia (Received 11 September 2012; revised manuscript received 10 November 2012; published 27 December 2012) We report observation of high-Q resonance in the photoresponse of a detector embedded in the 2D photonic crystal slab (PCS) microcavity illuminated by terahertz radiation. The detector and PCS are fabricated from a single GaAs wafer in a unified process. The influence of the period of PCS lattice, microcavity geometry, and detector location on the resonant photoresponse is studied. The resonance is found to originate from coupling of the fundamental PCS microcavity photon mode to the detector. The phenomenon can be exploited to devise a spectrometer-on-a-chip for terahertz range. DOI: 10.1103/PhysRevB.86.235144 PACS number(s): 42.50.−p, 42.70.Qs, 42.79.−e, 73.21.−b Recent years have witnessed a surge of research activity in the field of terahertz radiation (100 GHz to 3 THz). In part, such interest is caused by unique properties of THz radiation, which foretell a variety of potential applications. 1 However, its spectral location between optical and microwave frequencies hinders development of compact THz generation and spectroscopic systems. This leads to an increased need for components that can be used to manipulate THz radiation on-chip. However, in comparison with electrons in media, it is difficult to confine or store light and to control its speed. Photonic crystals (PCs) are expected to solve the problems by letting one manipulate the behavior of light in media beyond the conventional limitations. 2–4 However, at present photonics is not versatile enough, and many standard electronic functions such as memory and logic cannot be achieved by THz photonics alone. For this reason, research on hybrid photonic crystal–electronic structures is of great importance. Early experiments on THz PCs have primarily focused on defect-free structures. 5,6 Their main objective was to investigate properties of periodic media in a regime where fabrication was less challenging than at optical frequencies. However, it is clear that just as at optical frequencies, the utility of THz photonic crystals relies on the incorporation of defects to disturb the periodicity and thereby introduce localized cavity photon modes. 7,8 The first terahertz experiments on PCs with embedded defects have demonstrated the existence of a high-Q photon cavity mode. 9–13 Research on hybrid PC- electronic devices was pioneered in the works of electrically pumped photonic-crystal THz quantum cascade lasers. 14,15 The application of PC technology has enabled simultaneous spectral and spatial laser mode engineering. In the present work we realize a hybrid PC-electronic THz detector system in a unified lithographic process. We study its properties and find a frequency selectivity with a quality factor of up to Q = 210, which opens possibilities for a “spectrometer-on-a-chip” for the sub-THz and THz frequency ranges. A broadband detector and two-dimensional (2D) photonic crystal slab (PCS) consisting of a triangular lattice of air holes were fabricated from a single GaAs wafer with embedded GaAs/AlGaAs heterostructure. The wafer had a 20 nm single quantum well located 200 nm below the top crystal surface. Room temperature electron density was n s = 6 × 10 11 cm −2 with corresponding mobility 6000 cm 2 /V s. The detector was placed inside a microcavity composed of three linearly aligned missing air holes (defined as L3) [Fig. 1(a)]. The detector operation principle relied on coupling of incident terahertz radiation to the metallic detector gates that triggers plasma waves in the two-dimensional electron system (2DES). An ac plasmon potential was then rectified into dc photovoltage on the nonlinear defect introduced in the 2DES. The fabrication process was as follows. First, an ordinary optical lithography procedure was performed to fabricate the detector on the top crystal side. Details of the detector geometry can be found elsewhere. 16,17 Then the wafer was thinned down to a thickness of h = 200 μm. After that, anisotropic deep reactive ion etching in the chlorine atmosphere was used to produce pass-through holes to form the PC. The etching was performed from the bottom side of the wafer to protect the detector. The resulting structure and wafer cross-section are shown in Figs. 1(a) and 1(b). PCSs have been made with triangular lattice constants a = 247, 292, 343, 374, 411, 544, and 816 μm. The diameter of holes was d = 0.6a, total sample size 6 × 6 mm 2 . Also, a reference sample of the same size and with the same detector but no PC (therefore, broadband) have been produced. Samples have been fixed by corners on a chip carrier with a through cut under the region of the photonic crystal [Figs. 1(c) and 1(d)]. To measure frequency response of the detector we used a set of tunable backward-wave oscillators operating in the frequency range 0.1 to 0.4 THz and generating continuous wave radiation with typical output power from 10 to 0.1 mW. The radiation was collimated into the parallel beam of 1 cm diameter and directed onto the sample perpendicularly to the PCS plane. The radiation was chopped at 25 Hz frequency, and photoresponse of the detector was measured synchronously at the same frequency. The measurements were carried out at room temperature. Figure 2 shows frequency dependence of photoresponse of the detector for two different samples. The bottom curve corresponds to the reference sample without photonic crystal. Signal modulation arises from interference of the standing electromagnetic waves inside the GaAs crystal, which acts as a resonator. The upper and middle curves display the photoresponse of the detector embedded in the L3 microcavity in the PCS with a period of a = 411 μm. The sample was irradiated from the back side. One curve was measured under the incident THz radiation with longitudinal polarization (E field along the microcavity axis), while another used transverse 235144-1 1098-0121/2012/86(23)/235144(4) ©2012 American Physical Society