Showing papers by "John F. O'Hara published in 2005"

Prism coupling to terahertz surface plasmon polaritons

Abstract: Silicon prisms are used to couple free-space broadband terahertz into surface plasmon polariton (SPP) modes, otherwise known as surface electromagnetic waves (SEW) or Zenneck waves, on to polished metal surfaces. We show that coupling to surface waves occurs via edge-diffraction and hump-coupling and that attenuated total reflection is not an important coupling mechanism. Coupling and decoupling to a broadband, single-cycle SPP pulse is demonstrated with an energy efficiency of approximately 3.5%. Measurements of SPPs through thin plastic films reveal a strong surface sensitivity and suggest new configurations for maximizing THz SPP utility.

Terahertz surface plasmon polariton coupling on metallic grating structures

Abstract: We report efficient coupling and decoupling of free-space terahertz radiation to surface plasmon polaritons on various metallic gratings

Terahertz surface plasmon polariton generation with metallic gratings and silicon prisms

Abstract: Terahertz time-domain spectroscopy is used to generate surface plasmon polaritons via metallic gratings and silicon prisms. Grating measurements indicate efficient, narrowband coupling. Prism measurements show broadband coupling with substantial promise for guided-wave spectroscopy applications.

Terahertz Surface Plasmon Polariton Coupling via Gratings and Prisms

Abstract: Terahertz time-domain spectroscopy is used to study coupling to surface plasmon polaritons via silicon prisms and metallic gratings. Grating measurements indicate efficient, narrowband coupling, while prism measurements show broadband coupling and propagation over ~6.3cm.

Enhanced THz detection using self-assembled ErAs nanoislands

Abstract: We assess the potential of self-assembled ErAs:GaAs nanoislands for THz detection using optical-pump THz-probe spectroscopy and test THz detectors based on these devices. Enhanced performance is demonstrated for ErAs:GaAs over radiation-damaged silicon-on-sapphire based photoconductive detectors. A major focus of current research in ultrafast optoelectronics is the development of new sources and detectors at terahertz (THz) frequencies. Traditionally, low temperature grown GaAs (LT-GaAs) [1,2] and radiationdamaged silicon-on-sapphire (RD-SOS) [3] have been used in photoconductive (PC) antennas to detect THz radiation due to their fast carrier trapping times. However, the development of self-assembled ErAs nanoislands embedded in a GaAs matrix [4] offers an alternative for THz applications with several advantages. These devices are grown by molecular beam epitaxy (MBE) in a superlattice structure on a semiconducting GaAs (100) substrate. The lifetime of photo-excited carriers can be controlled by varying the spacing between ErAs nanoisland layers (the superlattice period L) [4]. By adjusting the growth temperature and ErAs content, the trap density and dark resistance can be independently tuned. Recently, ErAs:InGaAs superlattices have also been demonstrated as photomixers at 1.55 μm [5], enabling THz generation with widely available 1.55 μm semiconductor and fiber lasers. The ability to independently tune material properties makes ErAs nanoisland-based devices useful as PC detectors in THz time-domain spectroscopy (THz-TDS). In this work, we use optical-pump THz-probe spectroscopy to estimate the potential of MBE grown ErAs:GaAs superlattices for THz detection by obtaining the optically induced time-dependent THz conductivity upon 800 nm excitation. We verify this by fabricating and comparing two nearly identical THz detectors, one made from ErAs:GaAs and one from RD-SOS. Our results show that short, high bandwidth THz pulses can be detected with significantly enhanced efficiency using ErAs:GaAs superlattices in a PC antenna structure. The sample used in this study was grown by MBE on a semi-insulating (100) GaAs substrate [4]. The sample has X=80 superlattice periods, each consisting of 1.2 monolayers of ErAs and L=25 nm of GaAs (Figure 1). Further growth details are given in ref. [4]. Measurements on a reference GaAs substrate were also performed for comparison. Fig. 1. ErAs:GaAs superlattice sample structure. Optical-pump THz-probe experiments were performed using a 1 kHz regeneratively amplified Ti:sapphire laser system producing 2 mJ, 50 fs pulses at 800 nm. An incident fluence of 19 μJ/cm was used to excite the sample. The sample was placed under vacuum and the system was purged with nitrogen at 295 K. The measurement of the optically induced change in the electric field, ∆E(t)/E0, for the ErAs:GaAs superlattice sample is shown in Figure 2(a). The ∆E(t)/E0 signal can be fit at early times by the convolution of a Gaussian function (corresponding to the THz pulse) and a fast single exponential decay (corresponding to carrier trapping in the ErAs nanoislands). Physically, this corresponds to the excitation of carriers in the GaAs layers and their rapid trapping in the ErAs nanoislands on a sub-ps time scale while the THz pulse traverses the sample. Fitting the ∆E(t)/E0 data measured on the ErAs:GaAs superlattices is done by varying only one parameter, the carrier capture lifetime τ and by using known values for the photo-excited carrier density N0 and the THz pulse width (obtained from bulk GaAs measurements). The resulting carrier capture time was 839 fs. Using the known optical fluence, the bulk GaAs mobility and the time-dependent carrier density N(t)=N0e(-t/τ) we can also obtain the JFH1-3