One way to rectify laser pulses is by generating a surge of current. The generated terahertz electrical field is proportional to the time derivative of the current. The most common method for generating terahertz radiation uses a semiconductor antenna: a femtosecond laser excites electrons in the conduction band where they are accelerated by an external DC electric field (~10 kV/cm).
In our lab, we are developing radically different methods of generating ultrafast electrical currents. Metal nanostructures are designed allowing our lasers to excite surface plasmons. These surface plasmons concentrate fields and push electrons out of the metal. The resultant current surge produces terahertz radiation.
Nanotechnology leads to new terahertz-radiation sources
A new process has been discovered producing pulsed terahertz-radiation using a nano-engineered material. Terahertz (or millimeter-wave) radiation is important to homeland security by being able to detect explosives, drugs, and concealed weapons.
A nanostructured surface was made with grooves in glass and a few nanometers of gold deposited on top. This was specifically designed such that ultrashort laser pulses could whip up waves in the sea of electrons present in the metal. The nanostructures have the property that they concentrate the waves in the electron sea to great strengths leading to extreme nonlinear phenomena. In our case, the concentrated waves fling electrons out of the gold layer resulting in ultrashort bursts of electricity rushing out of the structure. These bursts of electricity emit terahertz radiation much like a modern version of Heinrich Hertz’ 1886 experiment scaled down in size by a billion times and in time by a trillion. Our work was a proof of principle experiment but already produced as much terahertz radiation as the best nonlinear optical crystals and could lead to commercial applications.
Click on the thumbnails below for a bigger version and caption:
The Flash animation below explains the process by which terahertz radiation is generated. A 100-femtosecond laser pulse strikes the nanostructure under a specific angle allowing it to couple with surface plasmons. Surface plasmons in the gold layer concentrate the fields causing electrons to be pushed out in ultrafast (femtosecond) bursts. These ultrafast electric currents then emit terahertz radiation.
Terahertz radiation (or millimetre waves, with a frequency of 1012 Hz) consists of light waves with a wavelength in between microwaves and normal infrared radiation. Terahertz radiation can be used for imaging of concealed objects such as weapons, medical imaging, and the detection of certain chemicals such as explosives and drugs. All objects at room temperature emit a continuous stream of terahertz radiation. We are particularly interested in terahertz pulses with a duration around a picosecond (10-12 s). Such ultrashort pulses are important for fundamental studies of novel materials, the study of biological processes, and particle accelerators. The generation and application of terahertz pulses has been studied intensely for the past 10 years or so with now about 500 publications a year. The Strathclyde group led by Klaas Wynne has been working in this area since 1996.
Our new contribution (published in Physical Review Letters) is that we have discovered an entirely new process by which picosecond terahertz pulses are produced using a nano-engineered material. A nanostructured surface – consisting of a shallow glass grating with a 50-nanometre thick layer of gold – was designed to couple visible femtosecond (10-15 s) laser pulses to plasmons on the surface of the gold. (Wiki on surface plasmon resonance) Surface plasmons are nanometre scale waves in the sea of electrons in a metal. The nanostructures have the property that they can concentrate laser and plasmon fields to enormous strengths leading to extreme nonlinear phenomena. In our case, the concentrated fields push electrons out of the metal and out of the entire nanostructure. This gives rise to a femtosecond burst of electricity rushing out of the structure. The burst of electricity emits radiation at terahertz frequencies much like a modern ultrafast version of Heinrich Hertz’ 1886 experiment scaled down in size by a billion times. (Wiki on Hertz)
The experiment was a proof of principle experiment but already produced as much terahertz radiation as one of the best nonlinear optical crystals (ZnTe) and could be commercialised. The research group is now making nanostructured surfaces using self-assembly techniques in a quest to find even better terahertz emitters.
The work was supported by grants from EPSRC and the Leverhulme Trust. The work was performed in the Wolfson Nanometrology Laboratory, which is part of the Biomolecular & Chemical Physics group at Strathclyde (BCP, http://bcp.phys.strath.ac.uk/), and in the Strathclyde Electron and Terahertz to Optical Pulse Source (TOPS, http://tops.phys.strath.ac.uk/).
Small molecules or the side-chains of larger molecules (such as proteins) tend to flop about with a period of about 1 ps corresponding to a frequency of 1 THz (terahertz). This is rather important. Chemical and biological reactions take place on a similar timescale for the very simple reason that they are controlled by terahertz fluctuations in the solvent surrounding them. Unfortunately, terahertz spectra in the condensed phase are rather “blobby” (see below) leading some people to refer to their study as blob-spectroscopy (which is a bit rude, really). We take two approaches to these blobs: very very careful measurements and nonlinear spectroscopy.
The far-infrared spectrum of liquid water up to 20 THz taken in our lab using an FTIR spectrometer (1 THz = 30 cm-1). The motions producing this spectrum are diffusive reorientation (< 30 cm-1), hydrogen bond bending and stretching (60 and 180 cm-1), and single-molecule libration (~500 cm-1).
Terahertz spectra can be measured very accurately using linear spectroscopy. A technique is called “linear” if it measures a two-point correlation function that depends on only one time difference. For example, infrared (terahertz) spectroscopy measures the two-point correlation function of the dipole moment: <µ(0)µ(t)>. Unfortunately, detecting far-infrared radiation is noisy. Therefore, we do our most careful measurements using the optical Kerr effect (OKE), which measures the correlation function of the polarisability tensor: <α(0)α(t)>.
OKE is a pump-probe technique that uses very short (~20 fs) near-infrared (800 nm) laser pulses. A pump pulse makes the sample slightly anisotropic by aligning molecules using their polarisability. The probe pulse measures the decay of this anisotropy as a result of rotations, librations, and diffusion. We can now take signals from a few femtoseconds to a few nanosecond corresponding to frequencies <1 GHz to ~20 THz.
OKE spectra of N-methyl acetamide at various temperatures.
May 2014: Our paper "Stokes-Einstein-Debye Failure in Molecular Orientational Diffusion: Exception or Rule?" finally came out in J .Phys. Chem. B, see http://pubs.acs.org/doi/abs/10.1021/jp5012457. It truely has the loveliest Kerr-effect/Raman data I have ever seen.
21 February 2014: Dr Gopakumar (Gopa) Ramakrishna officially started at Research Assistant in the group. Gopa will concentrate on terahertz spectroscopy.
2 December: Today, Dr Mario González Jiménez officially started as a Research Assistant in the group. He'll be working on femtosecond spectroscopy of biomolecules.
1 October 2013: Today, Judith Reichenbach officially started her PhD studies in the group. She'll be working on nucleation using femtosecond spectroscopy.
April 2013: Another EPSRC grant funded on "Solvation dynamics and structure around proteins and peptides: collective network motions or weak interactions"
October 2012: Dr Christopher Syme has started as a research associate in the group. He will be using confocal fluorescence microscopy and fluorescence lifetime imaging to study phase transitions in liquids.
May 2012: Fully funded PhD studentships in the Wynne group. Applications are invited for a number of PhD studentships in the Wynne group. Some of these studentships are part of the Doctoral Training Centre (DTC) in Continuous Manufacturing and Crystallisation (CMAC).
9 May 2012: Our paper "The dynamic crossover in water does not require bulk water" just came out in PCCP, see doi:10.1039/c2cp40703e. In a nutshell, it shows that you only need one water molecule to have bulk water properties (as long as that water molecule can form a water pentamer).
18/4/12: The latest issue of PCCP (Physical Chemistry and Chemical Physics), the top physical chemistry journal of the Royal Society of Chemistry, is dedicated to such ultrafast chemical dynamics. The special issue was guest edited by Prof Klaas Wynne in the School of Chemistry at Glasgow University and his colleague Dr Neil Hunt at the University of Strathclyde. Special issue PCCP on Ultrafast Chemical Dynamics.
October 2011: The Ultrafast Chemical Physics group has won a £0.7M EPSRC grant to study liquid-liquid phase transitions using microscopy in collaboration with Chemical Engineering at Strathclyde. EPSRC grant for UCP group.
7 July 2011: the EPSRC-funded Coherent regenerative amplifier (producing 23-fs 2.7-mJ 800-nm pulses at a repetition rate of 1 kHz) has been reinstalled in our lab again. This is in addition to a new Coherent Micra-10 (producing 15-fs 800-nm pulses at 80 MHz).
May 2011: A Faraday Discussion on 'Mesostructure and dynamics in liquids and solution' will be held in September 2013 most likely in Bristol.The organising committee consists at the moment of Alan Soper (Rutherford Appleton Laboratory), Austen Angell (Arizona State University), Ken Seddon (Queen's Belfast), Stephen Meech (UEA), an Klaas Wynne (Glasgow University).
October 2010: Next Ultrafast Chemical Physics meeting (UCP 2011) set for 14-16 December 2011 at the University of Strathclyde. Confirmed speakers include Prof David Klug (Imperial College, multidimensional spectroscopy), Prof Andrea Cavalleri (University of Oxford, femtosecond X-ray science) and Prof Klaas Wynne (University of Glasgow, terahertz spectroscopy). In addition we have confirmed attendance of Prof Dwayne Miller (University of Toronto) as plenary speaker for the conference.
August 2010: Our paper in JACS (described in Serving nanoparticle “soup”) has been cited 19 times on Web of Science exactly one year after its publication. It describes how using multiple spectroscopies, we discovered mesoscopic structure in room-temperature ionic liquids.
12 May 2008: Groups wins £0.6M EPSRC grant "Two-dimensional terahertz–IR spectroscopy: a unique probe of ultrafast hydrogen-bond dynamics of liquid water and model systems" by KW, JOK, and DJSB.
2 May 2008: Strathclyde will host the "International Workshop on ultrafast physical-chemistry 2008 (UCP ‘08)" on 30/31 October 2008 to be held in the Senate/Court suite. Plenary speaker is Prof Robin Hochstrasser FRSE (University of Pennsylvania). Confirmed invited speakers are Prof Casey Hynes (CNRS, Paris and University of Colorado, Boulder), Prof Charles Schmuttenmaer (Yale), Prof Majed Chergui (Ecole polytechnique fédérale de Lausanne), Prof Mischa Bonn (AMOLF, Amsterdam), Prof Peter Hamm (University of Zurich), and Prof Thomas Elsaesser (Max Born Institute, Berlin). The workshop is organised by Angus J. Bain (UCL), David Klug (Imperial), Steve Meech (UEA), Neil Hunt (Strathclyde), and Klaas Wynne (Strathclyde).
24 April 2008: Our paper "Glasslike Behavior in Aqueous Electrolyte Solutions" came out in J. Chem. Phys. A summary of the paper in simple terms (best attempt anyway) is on the page The science of syrup and traffic jams.
4 March 2008: Visiting professor Robin Hochstrasser of the University of Pennsylvania has been elected Honorary Fellow of the Royal Society of Edinburgh. This is a prestigious fellowship for scientists of great international renown and we are delighted that Robin has been honoured in this way.
18 March 2007: New paper in JACS on terahertz spectra associated with a helix to coil transition in a peptide. Read more about it in the research highlight Observing ‘The Lubricant of Life’