Picosecond ultrasonics - AbsoluteAstronomy.com

Picosecond ultrasonics is a type of ultrasonics

Ultrasonics

Ultrasonics is a term meaning the application of ultrasound. It is often used in industry as a shorthand term for any equipment employing ultrasonic principles....

that uses ultra-high frequency ultrasound generated by ultrashort

Ultrashort pulse

In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a femtosecond . Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators...

light pulses. It is a non-destructive

Nondestructive testing

Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....

technique in which picosecond acoustic

Acoustics

Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...

pulses penetrate into thin films or nanostructures to reveal internal features such as film thickness as well as cracks

Fracture

A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

, delaminations and voids. It can also be used to probe liquids. The technique is also referred to as picosecond laser ultrasonics or laser picosecond acoustics.

Introduction

When an ultrashort light pulse, known as the pump

Time-resolved spectroscopy

In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied that occur after illumination of a material, but in principle, the technique can be applied to...

pulse, is focused onto a thin opaque film on a substrate, the optical absorption results in a thermal expansion

Thermal expansion

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

that launches an elastic

Elasticity (physics)

In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....

strain pulse. This strain

Strain

Strain can refer to:* Strain , variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes* Strain , a chemical stress of a molecule...

pulse mainly consists of longitudinal

Longitudinal wave

Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...

acoustic phonon

Phonon

In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids...

s that propagate directly into the film as a coherent

Coherence (physics)

In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....

pulse. For example, in the case of an aluminium film this pulse will have a typical frequency and bandwidth both ~ 100 GHz, a duration of ~ 10 ps, a wavelength of ~100 nm, and a strain amplitude of ~ 10^-4 when using optical pulses of duration ~ 100 fs and energy ~ 1 nJ focused to a ~ 50 μm spot.

After acoustic reflection from the film-substrate interface, the strain pulse returns to the film surface, where it can be detected by a delayed optical probe

Time-resolved spectroscopy

pulse through optical reflectance or (for films that are thin enough) transmittance changes. This time-resolved

Time-resolved spectroscopy

method for generation and photoelastic

Photoelasticity

Photoelasticity is an experimental method to determine the stress distribution in a material. The method is mostly used in cases where mathematical methods become quite cumbersome. Unlike the analytical methods of stress determination, photoelasticity gives a fairly accurate picture of stress...

detection of coherent picosecond acoustic phonon pulses was proposed by Christian Thomsen and coworkers in a collaboration between Brown University

Brown University

Brown University is a private, Ivy League university located in Providence, Rhode Island, United States. Founded in 1764 prior to American independence from the British Empire as the College in the English Colony of Rhode Island and Providence Plantations early in the reign of King George III ,...

and Bell Laboratories in 1984.

Initial development took place in Humphrey Maris

Humphrey Maris

Humphrey Maris is a physicist and a professor at Brown University. He studies cryogenics. In 1991 he was made the George Chase Professor of Natural Science....

’s group at Brown University and elsewhere in the late 1980’s.
In the early 1990’s the method was extended in scope at Nippon Steel Corp.

Nippon Steel

, also referred to as , was formed in 1970. Nippon Steel Corporation is the world's 4th largest steel producer by volume.-Early years:Nippon Steel was created by the merger of two giants, Yawata Iron & Steel and Fuji Iron & Steel...

by direct sensing of the picosecond surface vibrations of the film caused by the returning strain pulses, resulting in improved detection sensitivity in many cases. Advances after the year 2000 include the generation of picosecond acoustic solitons by the use of millimeter propagation distances and the generation of picosecond shear waves by the use of anisotropic

Anisotropy

Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...

materials or small (~1 μm) optical spot sizes. Acoustic frequencies up to the terahertz range in solids and up to ~ 10 GHz in liquids have been reported.

Apart from thermal expansion, generation through the deformation potential or through piezoelectricity

Piezoelectricity

Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

is possible. Picosecond ultrasonics is currently used as a thin film metrology technique for probing films of sub-micrometer thicknesses with nanometer resolution in depth, that sees widespread use in the semiconductor

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

processing industry.

Generation

The absorption of an incident optical pump pulse sets up a local thermal stress near the surface of the sample. This stress launches an elastic strain pulse that propagates into the sample. The exact depth for the stress generation depends, in particular, on the material involved and the optical pump wavelength. In metals and semiconductors, for example, ultrashort-timescale thermal

Thermal diffusion

Thermal diffusion may refer to:* Thermal diffusion, an obsolete method of uranium enrichment* Brownian motion .* Diffusion in a temperature gradient ....

and carrier

Charge carrier

In physics, a charge carrier is a free particle carrying an electric charge, especially the particles that carry electric currents in electrical conductors. Examples are electrons and ions...

diffusion tends to increase the depth that is initially heated within the first ~1 ps.

Acoustic pulses are generated with a temporal duration approximately equal to the acoustic transit time across this initially heated depth, in general greater than the optical absorption depth

Absorption (electromagnetic radiation)

In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

. For example, the optical absorption depths in Al and GaAs are ~10 nm for blue light, but the electron diffusion depths are ~50 and 100 nm, respectively. The diffusion depth determines the spatial extent of the strain pulse in the through-thickness direction.

The main generation mechanism for metals is thermal expansion, whereas for semiconductors it is often the deformation potential mechanism. In piezoelectric materials the inverse piezoelectric effect, arising from the production of internal electric fields induced by charge

Electric charge

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

separation, may dominate.

When the optical spot diameter D, for example D~10 µm, at the surface of an elastically isotropic and flat sample is much greater than the initially heated depth, one can approximate the acoustic field propagating into the solid by a one-dimensional problem, provided that one does not work with strain propagation depths that are too large (~D²/Λ=Rayleigh length

Rayleigh length

In optics and especially laser science, the Rayleigh length or Rayleigh range is the distance along the propagation direction of a beam from the waist to the place where the area of the cross section is doubled. A related parameter is the confocal parameter, b, which is twice the Rayleigh length...

, where Λ is the acoustic wavelength). In this configuration—the one originally proposed for picosecond ultrasonics—only longitudinal acoustic strain pulses need to be considered. The strain pulse forms a pancake-like region of longitudinal strain that propagates directly into the solid away from the surface.

For small spot sizes approaching the optical diffraction

Diffraction

Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

limit, for example D~1 µm, it may be necessary to consider the three-dimensional nature of the problem. In this case acoustic mode-conversion at surfaces and interfaces and acoustic diffraction play an important role, resulting in the involvement of both shear and longitudinal polarizations. The strain pulse separates into different polarization components and spreads out laterally (for distances >D²/Λ) as it propagates down into the sample, resulting in a more complicated, three-dimensional strain distribution.

The use of both shear and longitudinal pulses is advantageous for elastic constant or sound velocity determination. Shear waves may also be generated by the use of elastically anisotropic solids cut at oblique angles to the crystal

Crystal

A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...

axes. This allows shear or quasi-shear waves to be generated with a large amplitude in the through-thickness direction.

It is also possible to generate strain pulses whose shape does not vary on propagation. These so-called acoustic solitons have been demonstrated at low temperatures over propagation distances of a few millimeters. They result from a delicate balance between acoustic dispersion

Dispersion relation

In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....

and nonlinear

Nonlinear acoustics

Non-linear acoustics is a branch of physics dealing with sound waves being distorted as they travel.-Introduction:A sound wave propagates through a material as a localized pressure change...

effects.

Detection

Strain pulses returning to the surface from buried interfaces or other sub-surface acoustically inhomogeneous regions are detected as a series of echoes. For example, strain pulses propagating back and forth through a thin film produce a decaying series of echoes, from which one may derive, in particular, the film thickness, the ultrasonic attenuation

Attenuation

In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

or the ultrasonic dispersion.

The original detection mechanism used in picosecond ultrasonics is based on the photoelastic effect. The refractive index

Refractive index

In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

and extinction coefficient near the surface of the solid are perturbed by the returning strain pulses (within the optical absorption depth of the probe light), resulting in changes in the optical reflectance or transmission. The measured temporal echo shape results from a spatial integral involving both the probe light optical absorption profile and the strain pulse spatial profile (see below).

Detection involving the surface displacement is also possible if the optical phase is variation is recorded. In this case the echo shape when measured through the optical phase variation is proportional to a spatial integral of the strain distribution (see below). Surface displacement detection has been demonstrated with ultrafast optical beam deflection and with interferometry

Interferometry

Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

.

For a homogeneous isotropic sample in vacuum with normal optical incidence, the optical amplitude reflectance (r) modulation can be expressed as

where

(n the refractive index and κ the extinction coefficient) is the complex refractive index for the probe light in the sample, k is the wave number of the probe light in vacuum, η(z, t) is the spatiotemporal longitudinal strain variation,

is the photoelastic constant, z is the depth in the sample, t is the time and u is the surface displacement of the sample (in the +z direction):

To obtain the variation in optical reflectivity for intensity R one uses

, whereas to obtain the variation in optical phase one uses

.

The theory of optical detection in multilayer samples, including both interface motion and the photoelastic effect, is now well-developed. The control of the polarization state and angle of incidence of the probe light has been shown to be useful for detecting shear acoustic waves.

Applications and future challenges

Picosecond ultrasonics has been applied successfully to analyze a variety of materials, both solid and liquid. It is increasingly being applied to nanostructures, including sub-micrometre films, multilayers, quantum well

Quantum well

A quantum well is a potential well with only discrete energy values.One technology to create quantization is to confine particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region...

s, semiconductor heterostructures

Heterojunction

A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction...

and nano-cavities.^,

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Introduction

Generation

Detection

Applications and future challenges

See also

External links