Historical research guide · 0532 · 2012
Active control of electromagnetically induced transparency analogue in terahertz metamaterials
Jianqiang Gu · Ranjan Singh · Xiaojun Liu · Xueqian Zhang · Yingfang Ma · Shuang Zhang · Stefan A. Maier · Zhen Tian · Abul K. Azad · Hou-Tong Chen · Antoinette J. Taylor · Jiaguang Han · Weili Zhang
In one minute
Jianqiang Gu, Ranjan Singh and their colleagues at Tianjin University and Los Alamos build a chip that can be told, on command, how fast to let light through it. Their metamaterial is a grid of aluminium bars and split rings on sapphire. The bar rings loudly when terahertz light hits it; the split rings do not answer directly, but the bar hands them energy anyway, and the two responses cancel. That cancellation punches a narrow transparent window through what would otherwise be an opaque band — the classical twin of electromagnetically induced transparency, an effect that normally needs gas lasers and cryogenics. The new step is the switch. Islands of silicon sit in the split-ring gaps; a flash from an infrared laser makes them conducting, spoils the silent mode, and closes the window in femtoseconds. Light that had been slowed by a factor of ninety-one is released. The team measures the delay, fits it with a coupled-oscillator model and shows the two agree.
Connection
Chapter 4 rests on a simple idea: change the properties of the medium a wave crosses and you change the path and the speed it experiences, without touching the wave itself. This is that idea working on a laboratory bench at room temperature, with a switching time set by a laser pulse — an engineered effective index, tuned by a factor of nearly nine, in a structure a few micrometres thick. For chapter 2 it is a clean demonstration that interference between a driven mode and a silent one is an engineering variable, not just an atomic curiosity.
From the source
A unit cell made of one aluminium cut wire and a pair of split-ring resonators reproduces electromagnetically induced transparency in a purely classical structure: the cut wire carries a localized surface plasmon resonance at 0.74 terahertz and is the bright mode, the split-ring pair carries an inductive-capacitive resonance at the same frequency but is not excited directly by the incident field and is the dark mode, and destructive interference between the two opens a sharp transparency peak of 85 per cent amplitude transmission inside a broad absorption background.Results, Design and characterization of the EIT metamaterial; Figure 1d
Putting photoconductive silicon islands in the split-ring gaps makes the transparency window switchable by light. Under an 800 nanometre, 50 femtosecond pump the silicon conductivity rises from 160 to 4,050 siemens per metre as the average pump power goes from zero to 1,350 milliwatts, and the transparency window falls from 85 per cent to 50 per cent and then vanishes entirely, leaving a single broad plasmon dip as low as 43 per cent.Results, Figure 2a and 2b
Fitting the measured spectra to a coupled Lorentz oscillator model isolates which parameter the light is actually moving: the detuning, the bright-mode damping rate and the coupling strength barely change with pump power, while the dark-mode damping rate climbs from 0.025 terahertz without pump — half the bright-mode value — to 0.271 terahertz at full pump, four times the bright-mode value. The switch is loss injected into the dark mode, which spoils the destructive interference the transparency depends on.Results, Theoretical calculations; Figure 3
The structure is a slow-light element and the slowing is measured, not inferred: the terahertz wave packet centred at 0.74 terahertz emerges 5.74 picoseconds late including its passage through the substrate, equivalent to 1.72 millimetres of free-space propagation, and group delays retrieved from the measurements and from the analytic model agree closely across the whole tuning range.Discussion; Figure 5a and 5b
Expressed as a group refractive index and assuming a 6 micrometre effective film thickness, the device tunes from 91 with no optical pump to 10.5 at 500 milliwatts of photoexcitation at 0.74 terahertz — the highest of those values comparable to the best group indices previously reported for transparency metamaterials, but here reached in a structure whose value can be changed while it operates.Discussion, paragraph on tunable group index
The authors name the direction they expect the work to open: chip-scale ultrafast components for optical buffering and terahertz active filtering, ultrasensitive sensors and nonlinear devices, and electrically and mechanically tunable versions of the same transparency metamaterial that would not need an optical pump at all.Introduction, final paragraph; Discussion, summary paragraph
The way in
https://doi.org/10.1038/ncomms2153How to cite it
Jianqiang Gu, Ranjan Singh, Xiaojun Liu, Xueqian Zhang, Yingfang Ma, Shuang Zhang, Stefan A. Maier, Zhen Tian, Abul K. Azad, Hou-Tong Chen, Antoinette J. Taylor, Jiaguang Han, Weili Zhang (2012) Active control of electromagnetically induced transparency analogue in terahertz metamaterials doi:10.1038/ncomms2153