Thin flexible multi-octave metamaterial absorber for millimeter wavelengths

The development of radiation-absorbent materials and devices for millimeter and submillimeter astronomy instruments is a research area of significant interest that has substantial engineering challenges. Alongside a low-profile structure and ultra-wideband performance in a wide range of angles of incidence, advanced absorbers in cosmic microwave background (CMB) instruments are aimed at reducing optical systematics, notably instrument polarization, far beyond previously achievable specifications. This paper presents a metamaterial-inspired flat conformable absorber design operating in a wide frequency range of 80–400 GHz. The structure comprises a combination of subwavelength metal-mesh capacitive and inductive grids and dielectric layers, using the magnetic mirror concept for a large bandwidth. The overall stack thickness is a quarter of the longest operating wavelength and is close to the theoretical limit stipulated by Rozanov’s criterion. The test device is designed to operate at a 22.5° incidence. The iterative numerical-experimental design procedure of the new metamaterial absorber is discussed in detail, as well as the practical challenges of its manufacture. A well-established mesh-filter fabrication process has been successfully employed for prototype fabrication, which ensures cryogenic operation of the hot-pressed quasi-optical devices. The final prototype, extensively tested in quasi-optical testbeds using a Fourier transform spectrometer and a vector network analyzer, demonstrated performance closely matching the finite-element analysis simulations; that is, greater than 99% absorbance for both polarizations, with only a 0.2% difference, across the frequency band of 80-400 GHz. The angular stability for up to ±10∘ has been confirmed by simulations. To the best of our knowledge, this is the first successful implementation of a low-profile, ultra-wideband metamaterial absorber for this frequency range and operating conditions

Calculation of the characteristics of coplanar resonators for kinetic inductance detectors

Photon detectors based on the change of kinetic inductance of a thin superconducting film have a number of applications, particularly in astronomy, owing to their high sensitivity and ease of integration into large arrays. Here we discuss in detail the analysis of kinetic inductance detectors that use thin film microwave coplanar resonators. Photon absorption decreases the electron pair density, increasing the magnetic penetration depth ?, which causes a decrease in the resonant frequency (or phase) of an irradiated resonator. To quantify this effect, we first compute the resonator current distribution, from which the ?-dependent parameters (such as kinetic inductance) are calculated. Optimum responsivity for phase measurement is achieved by using the thinnest film with the narrowest center conductor width at the lowest possible temperature. However, the responsivity is compromised by extrinsic microwave losses, in particular due to residual surface resistance, which is likely to be significant in the thinnest films

Antenna-coupled direct detector for millimetre and submillimetre astronomy based on 2D electron gas in semiconducting heterostructure

The energy resolution of a detector is related to the figure of merit NEP×√τ which is proportional to the heatcapacity of the detector. Hot electron (cold electron) devices have much lower heat capacity than bolometers withsilicon nitride based thermal isolation. Traditional hot electron bolometers (HEB) require sub-micron fabricationfor use at submm wavelengths and it is difficult to simultaneously couple radiation and read out these devices.The 2D electron gas (2DEG) in a semiconductor heterojunction effectively acts as a metal film with a thicknessof a few angstroms and a tunable density and electron mobility. We describe a HEB that uses a 2DEG as anabsorber and present simulations of optical coupling schemes for this type of detector including an antennacoupled to a coplanar waveguide with distributed 2DEG absorbers

A cold electron bolometer using a two-dimensional electron gas absorber

We describe a new type of Terahertz (THz) detector using a two-dimensional electron gas (2DEG) hot electron bolometer where the temperature of the electrons read out using superconducting tunnel junctions connected to the 2DEG (similar to a SINIS detector). We present measurements of the electron-phonon thermal conductivity in a high-mobility GaAs/AlGaAs 2DEG sample at 4.2 K as a function of electron temperature and magnetic field. From these measurements we estimate the sensitivity of an element in a filled array of S-2DEG-S detectors at 4.2 K to be approximately 10-14 W/[square root]Hz with a response time of 1 ns. Using measured parameters for the normal resistance of the S-2DEG-S contacts, we calculate the effect of using a voltage bias to cool the electrons in the absorber significantly from a 300 mK base temperature. In this configuration, these detectors can achieve sufficient sensitivity to detect individual THz photons

Lumped element kinetic inductance detectors

Kinetic Inductance Detectors (KIDs) provide a promising solution to the problem of producing large format arrays of ultra sensitive detectors for astronomy. Traditionally KIDs have been constructed from superconducting quarter-wave resonant elements capacitively coupled to a co-planar feed line [1]. Photon detection is achieved by measuring the change in quasi-particle density caused by the splitting of Cooper pairs in the superconducting resonant element. This change in quasi-particle density alters the kinetic inductance, and hence the resonant frequency of the resonant element. This arrangement requires the quasi-particles generated by photon absorption to be concentrated at positions of high current density in the resonator. This is usually achieved through antenna coupling or quasi-particle trapping. For these detectors to work at wavelengths shorter than around 500 μm where antenna coupling can introduce a significant loss of efficiency, then a direct absorption method needs to be considered. One solution to this problem is the Lumped Element KID (LEKID), which shows no current variation along its length and can be arranged into a photon absorbing area coupled to free space and therefore requiring no antennas or quasi-particle trapping. This paper outlines the relevant microwave theory of a LEKID, along with theoretical and measured performance for these devices

THz direct detector with 2D electron gas periodic structure absorber

We describe the performance of a direct detector that uses the high mobility 2D electron gas (2DEG) formed at the AlGaAs/GaAs interface as a frequency selective absorber. The 2DEG mesa-structure is etched to form a planar periodic structure with resonant absorption properties in the submm - THz region. Electrons in the 2DEG are heated by incoming radiation above the lattice temperature and the temperature of the hot electrons is measured by Superconducting - 2DEG - Superconducting (S-2DEG-S) tunnel junctions. The estimated noise equivalent power for such a detector at 100 mK is in order of 10-18 W/Hz1/2. In this paper we present the spectral measurements and simulated results of absorption properties at 4.2 K for a resonant mesa geometry. The thermal conductance and time constant of 2D electrons are studied at 450 mK-4.2 K. We measure an electron-phonon conductance on the order of 10-17 W/K per electron at 450 mK which gives a low value of heat conductance 2DEG relative to normal metal absorbers due to the low 2DEG electron density. These devices have a combination of sensitivity and speed which makes them possible candidates for the components in future astrophysical THz instruments

Photoemission studies of the surfactant-aided growth of Ge on Te-terminated Si(100)

The interactions of Ge adatoms with a Si(100) surface terminated by an ordered layer of Te have been studied in detail using XPS, SXPS, STM and LEED. It has been demonstrated that the Te layer has a surfactant action on the growth mode of the Ge in that the two dimensional growth regime is extended to at least 200 Å and the Te is seen to segregate to the growing Ge surface. The surface reconstruction of the Ge layer changes from (1 × 1) in the initial stages to (2 × 2) as growth proceeds and the surface population of Te is reduced. SXPS line shape analysis has indicated that the initial stages of Ge incorporation are characterised by the formation of small islands above those surface Si sites not fully coordinated with Te. Continued growth of such islands is, however, restricted due to their high surface free energy with respect to the surrounding Te-terminated areas. Ge atoms therefore site-exchange with Te atoms in bridge sites, thus becoming incorporated onto the Si lattice and displacing the Te to bridge sites on the growing surface. In this manner islanding is prevented and two-dimensional growth continues beyond the critical thickness. No evidence is seen for any significant incorporation of the Te within the growing Ge layer

Pixel design for FIR/mm/submm astronomy constituted of a set of antenna-coupled superconducting detectors feeding a metamaterial-based lenslet

Astronomy in the millimetre and sub-millimetre spectrum aims at unveiling the processes behind the origin and evolution of our Universe at various scales, from protostars to the Cosmic Microwave Background. To carry out such observations, novel imaging and spectroscopy instruments covering the Tera-Hertz range are needed. At present, high-resolution spectroscopy is carried out with heterodyne detectors, using either Superconductor-Insulator-Superconductor-mixers or Hot-Electron Bolometer mixers, inherently limited in bandwidth and difficult to multiplex. Imaging is accomplished with detectors working near photo-noise level, primarily Transition Edge Sensors or Microwave Kinetic Inductance Detectors arrays, for which the focal optics needs scaling to reach the sensitivity needed for future cosmology experiments. In this work, the simulated design of a single pixel constituted of a set of antenna-coupled detectors receiving the light focused by a metamaterial-based phase-engineered lenslet is presented. This technology relies on standard lithography fabrication techniques and enables compact and broadband imaging and spectroscopy on-chip

Water-gated organic nanowire transistors

We gated both p-type, and n-type, organic nanowire (NW) films with an aqueous electric double layer (EDL) in thin-film transistor (TFT) architectures. For p-type NWs, we used poly(3-hexylthiophene) (P3HT) NWs grown via two different routes. Both can be gated with water, resulting in TFTs with threshold lower than for conventionally cast P3HT films under the same gating conditions. However, TFT drain currents are lower for NWs than for conventional P3HT films, which agrees with similar observations for ‘dry’ gated TFTs. For n-type NWs, we have grown ‘nanobelts’ of poly(benzimidazobenzophenanthroline) (BBL) by a solvent/non-solvent mixing route with later displacement of the solvent, and dispersion in a non-solvent. Water-gating such films initially failed to give an observable drain current. However, BBL nanobelts can be gated with the aprotic solvent acetonitrile, giving high n-type drain currents, which are further increased by adding salt. Remarkably, after first gating BBL NW films with acetonitrile, they can then be gated by water, giving very high drain currents. This behaviour is transient on a timescale of minutes. We believe this observation is caused by a thin protective acetonitrile film remaining on the nanobelt surface