Laser ablation boosts terahertz emission

Laser ablation boosts terahertz emission

From almost instantaneous wireless transfer of huge amounts of data and easy detection of explosives, weapons, or harmful gases, to safe 3-D medical imaging and new advances in spectroscopy --technologies based on terahertz (THz) radiation, the electro-magnetic band with wavelengths from 0.1 to 1 mm, can transform science fiction into reality. However, scientists and engineers still do not have cheap and efficient solutions for mass production of THz-based devices.

For years, the THz portion of the spectrum remained unused, giving rise to the term "terahertz gap". Research, recently published in Optics Letters by the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), suggests one possible solution for this problem: a method to increase efficiency of THz emission gallium arsenide (GaAs)-based devices.

THz radiation lies between infrared and microwave radiation in the electro-magnetic spectrum. It is absorbed by water --which limits the use of THz devices in the Earth's atmosphere, laden with water vapour, to short distances--but it can penetrate fabrics, paper, cardboard, plastics, wood, and ceramics.

Many materials have a unique "fingerprint"in the THz band allowing their easy identification with THz scanners. Moreover, unlike X-rays and ultraviolet light, THz radiation is safe for live tissues and DNA, due to its non-ionising properties. THz technology could be a next important breakthrough in medicine, security, chemistry, and information technology.

Generation of THz waves is difficult since the frequency is too high for conventional radio transmitters, but too low for optical transmitters, like the majority of lasers. Therefore, researchers have to come up with new innovative devices.

One of the most frequently used THz emitters is a photoconductive antenna, comprising two electric contacts and a thin film of semiconductor, often GaAs, between them. When the antenna is exposed to a short pulse from a laser, the photons excite electrons in the semiconductor, and a short burst of THz radiation is produced. Thus the energy of the laser beam is transformed into a THz electro-magnetic wave.

OIST researchers showed that micro-structure of the semiconductor surface plays an important role in this process. Femtosecond-laser-ablation, in which the material is exposed to ultrashort bursts of high energy, creates Micrometer-scale grooves and ripples on the surface of GaAs. "The light gets trapped in these ripples", says Athanasios Margiolakis, a Special Research Student at OIST. Since more light is absorbed by the ablated material, the efficiency of THz emission, given a sufficiently powerful laser, increases by 65%.
Other properties of the material change as well.

For example, ablated GaAs shows only a third of the electrical current of non-ablated GaAs. "We observe counter-intuitive phenomena,"the researchers write, "One generally expects that the material showing the higher photocurrent would give the best THz emitter." They explain this phenomenon by shorter carrier lifetimes. That is, electrons in ablated samples return to non-agitated states much faster than in control samples.

Dr Julien Madéo, one of the OIST team members, says that "femtosecond-laser ablation allows us to engineer the properties of materials and to overcome their intrinsic limitations, leading, for example, to near 100% photon absorption as well as broader absorption bandwidth, control of the electron concentration and lifetime". This technique is a fast, lower-cost alternative to existing methods of manufacturing materials for THz applications.

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Gold Antennas : molecular bonds

 

Gold Antennas : molecular bonds

Micrometer-long gold antennas (lower left) absorb and re-radiate infrared radiation (top center). A layer of plastic immediately above the antennas (bottom center) experiences near-field electromagnetic radiation that excites molecular bonds (top right)

The antenna inside your smart phone efficiently picks up the electromagnetic transmissions used by the device. Borrowing this trick, researchers have used tiny gold antennas to boost the power of infrared techniques that record the twists and turns of chemical bonds. The method could track the workings of cell membranes and help chemists develop responsive surface coatings that react to changing conditions.

Chemical bonds typically vibrate, stretch, or wiggle when they absorb specific wavelengths of infrared (IR) light, making IR spectroscopy a versatile tool for molecular studies. In a recently developed technique known as nonlinear vibrational spectroscopy, an ultrashort pulse of IR light, lasting tens of femtoseconds or less, sets a bond in motion. Then a second pulse hits the bond a few picoseconds later.

Because the bond is now moving in some way, it absorbs the light differently than it did the first time. By hitting a bond with a series of ultrashort pulses and detecting the transmitted light, researchers can create snapshots of the bond’s ultrafast motion. Such methods have been used to observe the formation and breaking of hydrogen bonds, the unfolding of proteins when exposed to heat, and the disassociation of ion pairs in water.

In theory, this method could also be useful in studying thin molecular surfaces, such as the lipid bilayer of a cell membrane. But there’s a problem: most molecular bonds are inefficient at capturing IR radiation. Although a bond’s vibration frequencies correspond to those of IR light, its size—less than a nanometer—is much smaller than micrometer-scale IR wavelengths.

This mismatch makes chemical bonds poor antennas for IR radiation, and small biological samples do not contain enough molecules to generate a detectable response for nonlinear vibrational spectroscopy to work. To get around this difficulty, Yves Rezus and his colleagues at the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam decided to provide some help in the form of tiny gold antennas.

Rezus and his colleagues fabricated gold strips, 200 nanometers (nm) wide, 100 nm thick, and 2,000 nm long, on a transparent base. Preliminary experiments showed that strips with these dimensions are good antennas for 5800-nm IR light, a wavelength the researchers selected because it excites carbon-oxygen (C-O) bonds. They then deposited on these antennas a 5-nm-thick layer of polymethylmethacrylate (PMMA), a transparent plastic also known as Lucite or Plexiglas that contains a high density of C-O bonds.

Using a pump-probe technique in which one laser pulse excites the sample and a second tests how the sample changes in response, the researchers found that the PMMA plus gold antenna arrangement yielded a signal that was ten thousand times greater than the PMMA alone. “We went from something that was barely detectable to—Bam!—this huge signal,” says Rezus.

This enhancement occurs because of “near-field” electromagnetic effects close to the antennas. The gold strips absorb and re-radiate IR radiation, but at distances of more than a few wavelengths, the reradiated waves are similar to the incoming radiation and no better at exciting C-O bonds. However, each PMMA film is within a few nanometers of an antenna, where the electric and magnetic fields take on a complex form that more efficiently excites C-O bonds in the PMMA.

Although other researchers have explored near-field effects of small antennas, this is the first application to nonlinear vibrational spectroscopy. The team believes that by fine-tuning the shape of the antennas they could boost the sensitivity of the technique by another factor of 10, making it useful for investigating biological materials that have fewer C-O bonds than PMMA. Rezus says the technique could probe how drugs permeate through cell membranes or capture the inner workings of so-called responsive surfaces—materials that alter their color in the presence of a chemical or repel dirt or odors.

Javier Aizpurua of the Materials Physics Center in San Sebastián, Spain, who studies metallic nanoantennas and ultrafast phenomena, says the method is “a very nice new thing that opens the door to addressing the dynamics of very low amounts of molecules.”

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