Introduction
Developments of anti-reflection implementation in the terahertz frequency range
Quarter-wave coating
Fig.2 Simulated transmissions of model plane-parallel anti-reflection coated silicon window and anti-reflection coated wedge silicon window are plotted in short dash and long dash, respectively. FTS-measurement of small silicon window sample is represented by solid line and THOMAS-measurement of aircraft-window is represented by diamond symbol, edited for clarity [1] |
Absorptive metallic coating
Multi-layer coating
Fig.7 Transmittance of multi-layer silicon oxide coating on germanium substrate: solid circle is designed value, triangle is calculated value from SEM measurement, solid square is calculated value base on SEM measurement and EDS analysis, and solid curve is measured value under FTIR, edited for clarity [7] |
Metamaterial device
Sub-wavelength surface relief structures
Gradient index photonic structures
Design inception
Tab.1 Design parameters of three-layer structure from gradient index anti-reflection theory |
layer | 1 (air) | 2 | 3 | 4 | 5 (silicon) |
---|---|---|---|---|---|
effective index of refraction | 1.0 | 1.55 | 2.25 | 2.95 | 3.42 |
thickness (per unit wavelength) | - | 0.3156 | 0.225 | 0.172 | - |
Structure simulation
Tab.2 Complete fabrication parameters of the three-layer structures |
layer | 1 (air) | 2 | 3 | 4 | 5 (silicon) |
---|---|---|---|---|---|
refractive index | 1.0 | 1.55 | 2.25 | 2.95 | 3.42 |
layer height | - | 8.9 µm | 6.3 µm | 4.8 µm | - |
air ratio/% | 100 | 77.44 | 42.25 | 10.89 | 0 |
hole dimension (Λ = 20 µm) | - | 17.6 µm × 17.6 µm | 13.0 µm × 13.0 µm | 6.6 µm × 6.6 µm | - |
hole dimension (Λ = 15 µm) | - | 13.2 µm × 13.2 µm | 9.7 µm × 9.7 µm | 4.9 µm × 4.9 µm | - |
Structure fabrication
Performance evaluation
Terahertz time-domain spectroscopy (THz-TDS)
Tab.3 THz-ABCD system specification |
laser | SP Hurricane amplifier: 1 kHz repetition rate 750 mW power 100 fs pulse duration |
---|---|
terahertz emitter | nitrogen purged |
biased electric field | ~20 kV/cm |
terahertz receiver | nitrogen purged; fused silica lens |
modulation frequency | 500 Hz |
lock-in time constant | 100 ms |
measurement type | normal reflection (incident angle: 0°) |
Reflection and transmission evaluation
Polarization and incident angle measurement
Design sensitivity
Tab.4 Summary of parameters with their corresponding relative refractive indices |
original design | -5% discrepancy | + 5% discrepancy | |
---|---|---|---|
layer 2 | a2 = 13.2 µm, n2 = 1.55 | a2,0 = 12.54 µm, n2,0 = 1.70 | a2,1 = 13.86 µm, n2,1 = 1.37 |
layer 3 | a3 = 9.7 µm, n3 = 2.25 | a3,0 = 9.22 µm, n3,0 = 2.33 | a3,1 = 10.19 µm, n3,1 = 2.17 |
layer 4 | a4 = 4.9 µm, n4 = 2.95 | a4,0 = 4.66 µm, n4,0 = 2.98 | a4,1 = 5.15 µm, n4,1 = 2.92 |