Ultrasonics 56 (2015) 1–2
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Preface
Ultrafast acoustics started thirty years ago when a pump–probe experiment led on semiconductor thin films showed that absorption of very short laser pulses could create ultrafast acoustic transients within a material [1]; soon after it was shown that picosecond acoustic pulses could be generated and detected using the same way. Surprisingly, this breakthrough occurred outside the ‘‘laser ultrasonics’’ community which at about the same time was obtaining the first nanosecond acoustic pulses [2,3]. This newly opened field, so called ‘‘picosecond acoustics’’ grew rapidly since measurements in very thin films (a few tens of nanometers) or at very high frequencies (a few hundreds Gigahertz) became suddenly available. Industrial applications occurred quite early, at the beginning of the nineties, with the opaque film metrology for semiconductors manufacturers. This technique also spread out in fundamental research laboratories when cheaper and reliable femtosecond optical sources became available. It was the time for detailed investigations of short acoustic pulses generation processes in different media: metals, semiconductors, piezoelectric materials, multilayers, etc. At the same time, new detection techniques appeared beyond the original time-resolved reflectivity
interferometry allowed the detection of surface waves propagating far away from the laser pump spot. The possibility for acoustic field mapping allowed the observation of phonon focusing in anisotropic crystals or the study of acoustic propagation within 2D phononic crystals. It has also been shown that ultrafast acoustics is not limited to very thin films but can also brought to light very interesting phenomena in bulk systems like solitons generation due to dispersion and nonlinear effects. Regarding this last effect, it can be considered that propagation of very short acoustic pulses in pure crystals at low temperature offers one of the best model systems for the KdV equation. Both non linearity and dispersion come from the intrinsic properties of crystalline lattices and the large ratio between acoustic beam diameter and the typical acoustic wavelength allows insuring the 1D behavior of the propagation. Strain as large as 10 3 can be easily reached and subpicosecond acoustic solitons (with a spatial extension of a few nanometers) can be obtained (also they have no yet been directly observed). These studies gave rise to a number of works where short and intense acoustic pulses were used to rapidly modulate optical, electronics or magnetics properties of nano-devices.
measurements: laser deflection, interferometry, ultrafast ellipsometry, external or built-in optical cavities, etc. The study of detection processes also demonstrated the interest to vary the probe wavelength in order to be close to an optical cavity mode, a plasmon resonance in metallic nanoparticles or electronic transitions in semiconducting nanostructures. More recently new very promising techniques appeared in this field with the asynchronous detection scheme or time-resolved X-ray scattering. While the very first experiments in picosecond acoustics were restricted to longitudinal waves, the advent of new detection schemes like
One reason of the success of ultrafast acoustics lies in the fruitful interplay between this field and nanotechnologies and nanosciences. Indeed ultrafast acoustic bridges the gap between piezo-transducer and inelastic neutron or X-ray scattering in frequency domain and matches adequately the scale of nanotechnologies. Elastic properties of many nano-objects or nanostructures and their connection with surroundings can now be studied while the tremendous progress in nano-technologies and molecular beam epitaxy gives rise to ultra-high frequency acoustic emitters and detectors either for bulk or surface waves or to acoustic
http://dx.doi.org/10.1016/j.ultras.2014.10.001 0041-624X/Ó 2014 Published by Elsevier B.V.
2
Preface / Ultrasonics 56 (2015) 1–2
nano-sources. Metallic nanoparticles or cylinders and quantum dots could be used as point sources for subterahertz acoustic waves. In the same way semiconductor superlattices allowed coherent acoustic experiments in the terahertz range on macroscopic distances. Although many things have been done since the very beginning of ultrafast acoustics, new routes appeared recently or have to be opened. For instance, thin film metrology under very high pressure in diamond anvils or ultrafast acoustics in soft matter and biology look very promising. New ways for generation of high frequency surface waves could also be very interesting for surface metrology. The nanometric acoustic wavelengths which can be used in ultrafast acoustics have not yet been used for the imaging of embedded nanostructures but acoustics microscopy with submicrometric resolution is still under progress. One of the limitation of ultrafast acoustics which come from the use of femtosecond oscillators could be removed very soon with the advent of all-electrical acoustic waves generators and detectors of hundreds Gigahertz: SASER operating at 325 GHz with an electrical pumping has already been demonstrated [4]. This special issue aims to establish the state-of-the-art of ultrafast acoustics. It starts with extensive reviews of the basics
principles of ultrafast acoustics: short acoustic pulse generation, propagation and detection. Then it goes on with examples of the most successful applications of ultrafast acoustics in nanosciences. It ends with examples of the latest developments, illustrating the ultrafast acoustics outreach. References [1] C. Thomsen, J. Strait, Z. Vardeny, H.J. Maris, J. Tauc, Coherent phonon generation and detection by picosecond light pulses, Phys. Rev. Lett. 53 (1984) 989–992. [2] A.C. Tam, Pulsed-laser generation of ultrashort acoustic pulses: application for thin-film ultrasonic measurements, Appl. Phys. Lett. 45 (1984) 510–512. [3] G.M. Sessler, R. Gerhard-Multhaupt, Optoacoustic generation and electrical detection of subnanosecond acoustic pulses, J. Appl. Phys. 58 (1985) 119–121. [4] W. Maryam, A.V. Akimov, R.P. Campion, A.J. Kent, Dynamics of a vertical cavity quantum cascade phonon laser structure, Nat. Commun. 4 (2013) 2184.
Emmanuel Péronne Bernard Perrin Institut des Nanosciences de Paris, Pierre & Marie Curie University and CNRS, Paris, France Available online 5 October 2014
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Preface
Ultrafast acoustics started thirty years ago when a pump–probe experiment led on semiconductor thin films showed that absorption of very short laser pulses could create ultrafast acoustic transients within a material [1]; soon after it was shown that picosecond acoustic pulses could be generated and detected using the same way. Surprisingly, this breakthrough occurred outside the ‘‘laser ultrasonics’’ community which at about the same time was obtaining the first nanosecond acoustic pulses [2,3]. This newly opened field, so called ‘‘picosecond acoustics’’ grew rapidly since measurements in very thin films (a few tens of nanometers) or at very high frequencies (a few hundreds Gigahertz) became suddenly available. Industrial applications occurred quite early, at the beginning of the nineties, with the opaque film metrology for semiconductors manufacturers. This technique also spread out in fundamental research laboratories when cheaper and reliable femtosecond optical sources became available. It was the time for detailed investigations of short acoustic pulses generation processes in different media: metals, semiconductors, piezoelectric materials, multilayers, etc. At the same time, new detection techniques appeared beyond the original time-resolved reflectivity
interferometry allowed the detection of surface waves propagating far away from the laser pump spot. The possibility for acoustic field mapping allowed the observation of phonon focusing in anisotropic crystals or the study of acoustic propagation within 2D phononic crystals. It has also been shown that ultrafast acoustics is not limited to very thin films but can also brought to light very interesting phenomena in bulk systems like solitons generation due to dispersion and nonlinear effects. Regarding this last effect, it can be considered that propagation of very short acoustic pulses in pure crystals at low temperature offers one of the best model systems for the KdV equation. Both non linearity and dispersion come from the intrinsic properties of crystalline lattices and the large ratio between acoustic beam diameter and the typical acoustic wavelength allows insuring the 1D behavior of the propagation. Strain as large as 10 3 can be easily reached and subpicosecond acoustic solitons (with a spatial extension of a few nanometers) can be obtained (also they have no yet been directly observed). These studies gave rise to a number of works where short and intense acoustic pulses were used to rapidly modulate optical, electronics or magnetics properties of nano-devices.
measurements: laser deflection, interferometry, ultrafast ellipsometry, external or built-in optical cavities, etc. The study of detection processes also demonstrated the interest to vary the probe wavelength in order to be close to an optical cavity mode, a plasmon resonance in metallic nanoparticles or electronic transitions in semiconducting nanostructures. More recently new very promising techniques appeared in this field with the asynchronous detection scheme or time-resolved X-ray scattering. While the very first experiments in picosecond acoustics were restricted to longitudinal waves, the advent of new detection schemes like
One reason of the success of ultrafast acoustics lies in the fruitful interplay between this field and nanotechnologies and nanosciences. Indeed ultrafast acoustic bridges the gap between piezo-transducer and inelastic neutron or X-ray scattering in frequency domain and matches adequately the scale of nanotechnologies. Elastic properties of many nano-objects or nanostructures and their connection with surroundings can now be studied while the tremendous progress in nano-technologies and molecular beam epitaxy gives rise to ultra-high frequency acoustic emitters and detectors either for bulk or surface waves or to acoustic
http://dx.doi.org/10.1016/j.ultras.2014.10.001 0041-624X/Ó 2014 Published by Elsevier B.V.
2
Preface / Ultrasonics 56 (2015) 1–2
nano-sources. Metallic nanoparticles or cylinders and quantum dots could be used as point sources for subterahertz acoustic waves. In the same way semiconductor superlattices allowed coherent acoustic experiments in the terahertz range on macroscopic distances. Although many things have been done since the very beginning of ultrafast acoustics, new routes appeared recently or have to be opened. For instance, thin film metrology under very high pressure in diamond anvils or ultrafast acoustics in soft matter and biology look very promising. New ways for generation of high frequency surface waves could also be very interesting for surface metrology. The nanometric acoustic wavelengths which can be used in ultrafast acoustics have not yet been used for the imaging of embedded nanostructures but acoustics microscopy with submicrometric resolution is still under progress. One of the limitation of ultrafast acoustics which come from the use of femtosecond oscillators could be removed very soon with the advent of all-electrical acoustic waves generators and detectors of hundreds Gigahertz: SASER operating at 325 GHz with an electrical pumping has already been demonstrated [4]. This special issue aims to establish the state-of-the-art of ultrafast acoustics. It starts with extensive reviews of the basics
principles of ultrafast acoustics: short acoustic pulse generation, propagation and detection. Then it goes on with examples of the most successful applications of ultrafast acoustics in nanosciences. It ends with examples of the latest developments, illustrating the ultrafast acoustics outreach. References [1] C. Thomsen, J. Strait, Z. Vardeny, H.J. Maris, J. Tauc, Coherent phonon generation and detection by picosecond light pulses, Phys. Rev. Lett. 53 (1984) 989–992. [2] A.C. Tam, Pulsed-laser generation of ultrashort acoustic pulses: application for thin-film ultrasonic measurements, Appl. Phys. Lett. 45 (1984) 510–512. [3] G.M. Sessler, R. Gerhard-Multhaupt, Optoacoustic generation and electrical detection of subnanosecond acoustic pulses, J. Appl. Phys. 58 (1985) 119–121. [4] W. Maryam, A.V. Akimov, R.P. Campion, A.J. Kent, Dynamics of a vertical cavity quantum cascade phonon laser structure, Nat. Commun. 4 (2013) 2184.
Emmanuel Péronne Bernard Perrin Institut des Nanosciences de Paris, Pierre & Marie Curie University and CNRS, Paris, France Available online 5 October 2014
