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Tunable Ultrafast Lasers Probe Nanostructures

High frequency photoacoustic waves study nano-scales objects and their lattices

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Introduction

Nanoscale objects and structures are currently attracting much interest. They possess unique properties that can be applied to fields as diverse as microbiology and quantum electronics. But their size scale also means that their characteristic properties such as vibrational resonant modes cannot be measured by traditional techniques. One tool that is proving useful is ultrafast photoacoustics. This article examines how this technique is being applied to the study of nanostructures and how it is enabled by tunable ultrafast laser sources.

Ultrafast Photoacoustics

When an ultrafast laser pulse hits a highly absorbing target, the entire pulse energy can be absorbed in a thin surface layer of just a few nanometers, causing intense local heating. If this material is a metal such as aluminum, then the layer undergoes rapid expansion, creating an acoustic shock wave with a very high frequency. For example, if the laser pulse is in the femtosecond domain and the expansion is limited to a layer thickness of say 10 nm, then frequencies can exceed 200 GHz. Researchers realized that these waves could be used to perform sonar with a completely new spatial/temporal resolution. For example, the thickness of films in integrated circuits can be sensed by the time delay on the returning acoustic echo from the interfaces between the different materials layers.

There are two ways to detect the echo pulse. The input laser pulse is split into a pump and probe pulse, the latter passing through a variable delay line. When the acoustic echo passes through the outer metal layer, it strains the electronic structure of the lattice, thereby perturbing the refractive index of the metal, and hence changing its reflectivity. This is detected as a function of pump-probe delay. Alternatively, if the material is coated with a transparent outer layer, the probe pulse interacts with this layer like a very thin Fabry-Perot etalon, where the reflected intensity is a result of interference between the front and rear surface reflections. Again the reflected acoustic pulse changes the measured optical signal by slightly changing the layer index and hence affecting the phase difference.

The Need for Laser Tunability

Beginning in 1999, a group led by Professor Arnaud Devos at the Institut d'Electronique de Microélectronique et de Nanotechnologie (Lille, France) began using this technique to look at nanostructures. They soon made a key discovery that both detection techniques were sensitive to the wavelength of the ultrafast laser [refs 1,2]. Devos explains, "In the case of metals, the signal sensitivity is very dependent on its proximity to electronic transitions." He cites the example of aluminum which has a low reflectivity at 850 nm because of an electronic resonance. "At 800 nm we get a nice positive signal, and at 900 nm we see a strong negative signal, but at 850 nm we cannot get a photoacoustic signal." In the case of a transparent detection layer, the signal is highly dependent on wavelength because the layer acts as a Fabry-Perot etalon with transmission/reflection modes.

Based on this breakthrough, Devos' group began studying photoacoustics as a function of wavelength, initially using a Coherent Mira laser. According to Devos, "By measuring the photacoustic signal at different wavelengths we get a significant improvement in measurement precision. Moreover, this wavelength dependence gives us another set of data about the sample under study."

Recently, Devos group switched to using one of the new generation of tunable one-box ultrafast lasers (Coherent Chameleon Ultra). "We chose this laser for several reasons. First it offers very fast and fully automated tuning over a very wide wavelength range: 680-1090 nm. This is important as we typically take data at up to 10 different wavelengths in each experiment. Plus we get femtosecond pulses from a closed, one-box laser without the need for frequent tweaking and adjustment. So my students can concentrate on taking and analyzing data rather than becoming laser experts."

Studying Nanostructures

One area of study for the Devos group is 10 nm diameter gold nano-spheres. Unlike a conventional molecule, this can have vibrational modes in the 100-200 GHz window. There is no other simple way to study the vibrational spectrum. Devos is particularly interested in synthetic crystals and lattices created by repetitive patterns of nanostructures. With lattice dimensions on the order of a few 100 nm, these phononic crystals have high frequency lattice modes that cannot be probed by traditional spectroscopic methods.

To study individual structures, the team deposits a layer of these on to a thin metal film or glass/metal film. In the case of phononic crystals, these are created using electron beam lithography, again on top of thin metal or glass/metal substrates.

Clearly, this is important fundamental research, but it could also have some long-term potential commercial applications. One example might be to use the phononic crystals as RF filters for GHz mobile phones.

To summarize, as with several other ultrafast applications like multiphoton microscopy and THz imaging, the evolution of femtosecond lasers has enabled scientists to probe samples in a unique way. And the development of simplified, turnkey laser tools allows these researchers to focus on science rather than on laser technology.

Figure

Figure 1. Schematic diagram of the experimental setup used for performing ultrafast photoacoustics using a Chameleon. The laser beam is split in two parts, the pump and the probe beam. The pump launchs a short acoustic pulse which propagates in the structure. The probe detects the echo coming back at the free surface. In this particular case note that the probe beam is frequency doubled in order to detect acoustic wave in the blue range.

Figure 2. Illustration of the high sensitivity of the acoustic signal to a change in the laser-wavelength. As shown here, the signal can experience a sign change when the laser is tuned around particular wavelength. An excellent agreement is found between experiment (dots) and theory (red line). With this type of data, ultrafast photoacoustics delivers a higher precision in thickness measurement which would be needed for the development of new RF filter technology.

References

1. "Evidence of Laser-Wavelength Effect in Picosecond Ultrasonics: Possible Connection With Interband Transitions,"A. Devos and C. Lerouge, Phys. Rev. Lett., vol 86, p2669 (2001).
2. "High-laser-wavelength sensitivity of the picosecond ultrasonic response in transparent thin films," A. Devos,* J.-F. Robillard, R. Côte, and P. Emery,. B vol74, p 064114 (2006).

Authors: David Armstrong and Jean-Luc Tapie, Coherent Inc., Santa Clara CA. Email david.armstrong@coherent.com Jean-luc.tapie@coherent.com www.coherent.com