Dual Vivaldi UWB nanoantenna for optical applications

Transcription

1 Dual Vivaldi UWB nanoantenna for optical applications Zeev Iluz, Yuval Yifat, Doron Bar-Lev, Michal Eitan, Yoni Kantarovsky, Yuav Blue, Yael Hanein, Koby Scheuer, and Amir Boag School of Electrical Engineering Tel Aviv University, Tel Aviv 69978, Israel 1

2 Outline The motivation: solar energy harvesting using optical nanoantennas The Dual Vivaldi antenna geometry simulation results The fabrication process The measurements setup using far field instead of near field Design verification Future research 2

3 The motivation The energy from 1hr of sunlight striking the earth ( J ) ~ 1 year of consumed energy worldwide ( J in 2001*) Two main commercial technologies: Concentrating solar power (CSP) systems Photovoltaics (PV) World insolation map A CSP System Typical Solar Cell Both technologies at present have low efficiency! *The UN Development Program (2003) World Energy Assessment Report 3 3 Wednesday,

4 Alternative approach: optical rectenna system Any optical rectenna system will include: 1. Receiving antenna 2. Non linear load that rectifies the AC field induced at antenna terminals 3. In 1964, Raytheon demonstrated a helicopter powered by 2.45 GHz rectenna system. The helicopter flew for over 10 hours 4

5 General Concept NanoAntenna + high-frequency diode EM radiation excites AC in nano-antenna. The high-frequency diode rectifies the AC current. The outcome IR detection + Second Harmonic Generation. 5

6 Guidelines for efficient IR rectenna 1. Wideband (both impedance matching & radiation efficiency) 2. Integrated antenna-to-waveguide device (matching manipulations) 3. DC power lines that do not interact with antenna operation (array configuration) 6

7 The Dual Vivaldi antenna Classical Vivaldi - slot antenna with exponential taper UWB impedance matching End-fire radiation Our approach: two end-fire Vivaldi antennas, placed opposite to one another Peak gain at the antenna broadside direction. 7

8 The simulation setup: CST MWS - Finite Elements Frequency Domain solver. Unit cell boundary condition: dx=1790 nm, dy=470 nm higher modes (grating lobes) at the azimuth plane. Materials: the complex indices of refraction for all metals was imported. Two phase simulations: 1. Place ports at the slot line edge S parameters. 2. Normal incident scattering EM field. 8

9 Phase 1: both parallel plate waveguide gaps were excited coherently and in phase, using ports across the gaps: Port 2 Port 1 9

10 Dual Vivaldi Antenna: S parameter simulation results The return loss > 9.5 db between (129% impedance bandwidth) μm 10

11 The fabrication process The antennas structure, composed of a 7 nm adhesion promotion layer of Cr followed by 33 nm of Au, was patterned using E-beam lithography. Both Open and short circuits were fabricated. 11

12 Initial fabrication results c H g W W[nm] H[nm] g [nm] c[nm] dx[um] dy[um] SINGLE ANTENNA SPECIFICATION Ant Design Ant Measured ARRAY SPECIFICATION

13 Array Fabrication Open Circuit Short Circuit The antenna arrays size is 150 µm X 150 µm, with approximately 26,000 elements. 13

14 Measurements setup To equally excite all nano-antennas in the array (in amplitude and in phase), the array was positioned at the waist of the incident Gaussian beam. The lateral pitch of the arrays the 1 st order Bragg diffraction lobe at λ= nm for a normally incident excitation beam. The spectral response of the antennas was obtained by sweeping the excitation wavelength through the measurement range for different rotation positions of the array. By rotating the stage, the entire optical spectrum of the laser was covered. 14

15 Measurements at 780 nm were performed using a similar optical setup with a few differences: (a) a Si detector was placed at an angle of 26º relative to the incident beam in accordance with the Bragg condition. (b) the laser source was a non-tunable, wideband ultra-fast laser. 15

16 Design verification 16

17 Efficiency: Only the ±1 lobe can be measured. Excellent agreement between simulation and measurements. Summing all lobes results in radiation efficiency exceeding 90%. 17

18 Future research CNT diodes A single CNT connecting Ti electrode (Schottky) with Pt electrode (Ohmic) on a Quartz substrate. The minimal distance between the electrodes is less than 1mm. 18

19 TAU Nanorectenna Dual Vivaldi broken Antenna (model + experiment) Dual Vivaldi broken Antenna (2 Metals + Carbon nanotubes) NiAu TiAu Made at TAU 19

20 Short Vivaldi + MIM Au Sub nm isolation layer Sub nm isolation layer Al Au Al 20

21 Thank you 21