STM-D-0532Paper2012Published and peer-reviewed

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

Abstract and summary · read the original at the source

In one page

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.

Why it matters hereChapter 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.

What it claims

01A 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
Settled physics
02Putting 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
Published and peer-reviewed
03Fitting 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
Published and peer-reviewed
04The 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
Published and peer-reviewed
05Expressed 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
Published and peer-reviewed
06The 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
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Read it · abstract

Abstract

Recently reported metamaterial analogues of electromagnetically induced transparency enable a unique route to endow classical optical structures with aspects of quantum optical systems. This method opens up many fascinating prospects on novel optical components, such as slow light units, highly sensitive sensors and nonlinear devices. In particular, optical control of electromagnetically induced transparency in metamaterials promises essential application opportunities in optical networks and terahertz communications. Here we present active optical control of metamaterial-induced transparency through active tuning of the dark mode. By integrating photoconductive silicon into the metamaterial unit cell, a giant switching of the transparency window occurs under excitation of ultrafast optical pulses, allowing for an optically tunable group delay of the terahertz light. This work opens up the possibility for designing novel chip-scale ultrafast devices that would find utility in optical buffering and terahertz active filtering.

The way in

https://doi.org/10.1038/ncomms2153Published as Nature Communications 3, 1151 (2012), received 19 June 2012, accepted 20 September 2012, published 23 October 2012. Every page of the article carries the line ’© 2012 Macmillan Publishers Limited. All rights reserved.’ and the paper points readers to the Nature Publishing Group reprints and permissions page; no Creative Commons statement appears in the text or on the publisher record, so this sheet carries the summary, the claims and the authors’ own abstract and sends the reader to the source. Work done at Tianjin University, the Center for Integrated Nanotechnologies at Los Alamos National Laboratory, the University of Birmingham, Imperial College London and Oklahoma State University; Gu and Singh contributed equally. Registry note: the fetched metadata record for this DOI carried an empty abstract field and only the first eight of the thirteen authors; the abstract below and the full author list are taken from the paper itself.

How 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

Where it sits in the curriculum

What the vacuum is The metric, warp drives and wormholes