Terahertz time domain spectroscopy
Encyclopedia
In physics
, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic
technique in which the properties of a material are probed with short pulses of terahertz radiation
. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude
and the phase
of the terahertz radiation. In this respect, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude.
d laser and last only a few picosecond
s. A single pulse can contain frequency components covering the whole terahertz range from 0.05 to 4 THz. For detection, the electrical field of the terahertz pulse is sampled
and digitized, conceptually similar to the way an audio card transforms electrical voltage levels in an audio signal into numbers that describe the audio waveform. In THz-TDS, the electrical field of the THz pulse interacts in the detector with a much-shorter laser pulse (e.g. 0.1 picoseconds) in a way that produces an electrical signal that is proportional to the electric field of the THz pulse at the time the laser pulse gates the detector on. By repeating this procedure and varying the timing of the gating laser pulse, it is possible to scan the THz pulse and construct its electric field as a function of time. Subsequently, a Fourier transform
is used to extract the frequency spectrum from the time-domain data.
: many materials are transparent to THz, THz radiation is safe for biological tissue
s because it is non-ionizing
(unlike for example X-rays), and images formed with terahertz radiation can have relatively good resolution (less than 1 mm). Also, many interesting materials have unique spectral fingerprints in the terahertz range, which means that terahertz radiation can be used to identify them. Examples which have been demonstrated include several different types of explosives, polymorphic forms of many compounds used as Active Pharmaceutical Ingredients (API) in commercial medications as well as several illegal narcotic
substances. Since many materials are transparent to THz radiation, these items of interest can be observed through visually opaque intervening layers, such as packaging and clothing.
Though not strictly a spectroscopic technique, the ultrashort width of the THz radiation pulses allows for measurements (e.g., thickness, density, defect location) on difficult to probe materials (e.g., foam). The measurement capability shares many similarities to that observed with pulsed ultrasonic systems. Reflections from buried interfaces and defects can be found and precisely imaged. THz measurements are non-contact however.
.
effect and it is particularly strong in high-mobility semiconductors such as InAs.
material. Effectively, the semiconductor changes abruptly from being an insulator into being a conductor. This conduction leads to a sudden electrical current across a biased antenna patterned on the semiconductor. This changing current emits terahertz radiation, similar to what happens in the antenna
of a radio transmitter.
Typically the two antenna electrode
s are patterned on a low temperature gallium arsenide (LT-GaAs), semi-insulating gallium arsenide (SI-GaAs), or other semiconductor (such as InP) substrate
.
In a commonly used scheme, the electrodes are formed into the shape of a simple dipole antenna
with a gap of a few micrometers and have a bias voltage up to 40 V between them. The ultrafast (100 fs) laser pulse, must have a wavelength
that is short enough to excite
electron
s across the bandgap of the semiconductor substrate. This scheme is suitable for illumination with a Ti:sapphire oscillator laser with pulse energies of about 10 nJ. For use with amplified Ti:sapphire lasers
with pulse energies of about 1 mJ, the electrode gap can be increased to several centimeters with a bias voltage of up to 10 kV.
The short duration of THz pulses generated (typically ~2 ps
) are primarily due to the rapid rise of the photo-induced current in the semiconductor and the short carrier lifetime semiconductor materials (e.g., LT-GaAs). This current may persist for only a few hundred femtoseconds, up to several nanoseconds, depending on the material of which the substrate is composed. This is not the only means of generation, but is currently the most common.
Pulses produced by this method have average power levels on the order of nanowatt
s, although the peak power during the pulses can be many orders of magnitude higher. The bandwidth of the resulting THz pulse is primarily limited by how quickly the charge carrier
s can accelerate in the semiconductor material, rather than the duration of the laser pulse.
, a high-intensity ultrashort laser pulse
passes through a transparent crystal material that emits a terahertz pulse without any applied voltages. It is a nonlinear-optical
process, where an appropriate crystal material is quickly electrically polarized
at high optical intensities. This changing electrical polarization emits terahertz radiation.
Because of the high laser intensities that are necessary, this technique is mostly used with amplified Ti:sapphire lasers
. Typical crystal materials are zinc telluride
, gallium phosphide, and gallium selenide.
The bandwidth of pulses generated by optical rectification is limited by the laser pulse duration, terahertz absorption in the crystal material, the thickness of the crystal, and a mismatch between the propagation speed of the laser pulse and the terahertz pulse inside the crystal. Typically, a thicker crystal will generate higher intensities, but lower THz frequencies. With this technique, it is possible to boost the generated frequencies to 40 THz (7.5 µm) or higher, although 2 THz (150 µm) is more commonly used since it requires less complex optical setups.
s, heat detectors cooled to liquid-helium temperatures. Since bolometers can only measure the total energy of a terahertz pulse, rather than its electrical field over time, it is not suitable for use in THz-TDS.
In both detection methods, a part (called the detection pulse) of the same ultrashort laser pulse that was used to generate the terahertz pulse is fed to the detector, where it arrives simultaneously with the terahertz pulse. The detector will produce a different electrical signal depending on whether the detection pulse arrives when the electric field of the THz pulse is low or high. An optical delay line is used to vary the timing of the detection pulse.
Because the measurement technique is coherent, it naturally rejects incoherent
radiation. Additionally, because the time slice of the measurement is extremely narrow, the noise contribution to the measurement is extremely low.
The Signal-to-Noise (S/N) of the resulting time-domain waveform obviously depends on experimental conditions (e.g., averaging time), however due to the coherent sampling techniques described, high S/N values (>70 dB) are routinely seen with 1 minute averaging times.
, where certain crystalline materials become birefringent in the presence of an electric field. The birefringence caused by the electric field of a terahertz pulse leads to a change in the optical polarization of the detection pulse, proportional to the terahertz electric-field strength. With the help of polarizers and photodiode
s, this polarization change is measured.
As with the generation, the bandwidth of the detection is dependent on the laser pulse duration, material properties, and crystal thickness.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
technique in which the properties of a material are probed with short pulses of terahertz radiation
Terahertz radiation
In physics, terahertz radiation refers to electromagnetic waves propagating at frequencies in the terahertz range. It is synonymously termed submillimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, THz...
. The generation and detection scheme is sensitive to the sample material's effect on both the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
and the phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
of the terahertz radiation. In this respect, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude.
Explanation
Typically, the terahertz pulses are generated by an ultrashort pulseUltrashort 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...
d laser and last only a few picosecond
Picosecond
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
s. A single pulse can contain frequency components covering the whole terahertz range from 0.05 to 4 THz. For detection, the electrical field of the terahertz pulse is sampled
Sampling (signal processing)
In signal processing, sampling is the reduction of a continuous signal to a discrete signal. A common example is the conversion of a sound wave to a sequence of samples ....
and digitized, conceptually similar to the way an audio card transforms electrical voltage levels in an audio signal into numbers that describe the audio waveform. In THz-TDS, the electrical field of the THz pulse interacts in the detector with a much-shorter laser pulse (e.g. 0.1 picoseconds) in a way that produces an electrical signal that is proportional to the electric field of the THz pulse at the time the laser pulse gates the detector on. By repeating this procedure and varying the timing of the gating laser pulse, it is possible to scan the THz pulse and construct its electric field as a function of time. Subsequently, a Fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
is used to extract the frequency spectrum from the time-domain data.
Advantages of THz radiation
THz radiation has several distinct advantages over other forms of spectroscopySpectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
: many materials are transparent to THz, THz radiation is safe for biological tissue
Biological tissue
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
s because it is non-ionizing
Non-ionizing radiation
Non-ionizing radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule...
(unlike for example X-rays), and images formed with terahertz radiation can have relatively good resolution (less than 1 mm). Also, many interesting materials have unique spectral fingerprints in the terahertz range, which means that terahertz radiation can be used to identify them. Examples which have been demonstrated include several different types of explosives, polymorphic forms of many compounds used as Active Pharmaceutical Ingredients (API) in commercial medications as well as several illegal narcotic
Narcotic
The term narcotic originally referred medically to any psychoactive compound with any sleep-inducing properties. In the United States of America it has since become associated with opioids, commonly morphine and heroin and their derivatives, such as hydrocodone. The term is, today, imprecisely...
substances. Since many materials are transparent to THz radiation, these items of interest can be observed through visually opaque intervening layers, such as packaging and clothing.
Though not strictly a spectroscopic technique, the ultrashort width of the THz radiation pulses allows for measurements (e.g., thickness, density, defect location) on difficult to probe materials (e.g., foam). The measurement capability shares many similarities to that observed with pulsed ultrasonic systems. Reflections from buried interfaces and defects can be found and precisely imaged. THz measurements are non-contact however.
Generation
There are three widely used techniques for generating terahertz pulses, all based on ultrashort pulses from titanium-sapphire lasersTi-sapphire laser
Ti:sapphire lasers are tunable lasers which emit red and near-infrared light in the range from 650 to 1100 nanometers. These lasers are mainly used in scientific research because of their tunability and their ability to generate ultrashort pulses...
.
Surface emitters
When an ultra-short (100 femtoseconds or shorter) optical pulse illuminates a semiconductor and its wavelength (energy) is above the energy band-gap of the material, it photogenerates mobile carriers. Given that absorption of the pulse is an exponential process most of the carriers are generated near the surface (typically within 1 micrometre). The presence of the surface has two main effects. Firstly it generates a band bending which has the effect of accelerating carriers of different signs in opposite directions (normal to the surface) creating a dipole, this effect is known as surface field emission. Secondly, the presence of the surface itself creates a break of symmetry which results carriers being able move (in average) only into the bulk of the semiconductor, this phenomenon combined with the difference of mobilities of electrons and holes also produces a dipole, this is known as Photo-demberPhoto-dember
In semiconductor physics, the photo-Dember effect consists in the formation of a charge dipole in the vicinity of a semiconductor surface after ultra-fast photo-generation of charge carriers...
effect and it is particularly strong in high-mobility semiconductors such as InAs.
Photoconductive emitters
In a photoconductive emitter, the optical laser pulse (100 femtoseconds or shorter) creates carriers (electron-hole pairs) in a semiconductorSemiconductor
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...
material. Effectively, the semiconductor changes abruptly from being an insulator into being a conductor. This conduction leads to a sudden electrical current across a biased antenna patterned on the semiconductor. This changing current emits terahertz radiation, similar to what happens in the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
of a radio transmitter.
Typically the two antenna electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s are patterned on a low temperature gallium arsenide (LT-GaAs), semi-insulating gallium arsenide (SI-GaAs), or other semiconductor (such as InP) substrate
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
.
In a commonly used scheme, the electrodes are formed into the shape of a simple dipole antenna
Dipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
with a gap of a few micrometers and have a bias voltage up to 40 V between them. The ultrafast (100 fs) laser pulse, must have a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
that is short enough to excite
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s across the bandgap of the semiconductor substrate. This scheme is suitable for illumination with a Ti:sapphire oscillator laser with pulse energies of about 10 nJ. For use with amplified Ti:sapphire lasers
Chirped pulse amplification
Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification...
with pulse energies of about 1 mJ, the electrode gap can be increased to several centimeters with a bias voltage of up to 10 kV.
The short duration of THz pulses generated (typically ~2 ps
Picosecond
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
) are primarily due to the rapid rise of the photo-induced current in the semiconductor and the short carrier lifetime semiconductor materials (e.g., LT-GaAs). This current may persist for only a few hundred femtoseconds, up to several nanoseconds, depending on the material of which the substrate is composed. This is not the only means of generation, but is currently the most common.
Pulses produced by this method have average power levels on the order of nanowatt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s, although the peak power during the pulses can be many orders of magnitude higher. The bandwidth of the resulting THz pulse is primarily limited by how quickly the charge 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...
s can accelerate in the semiconductor material, rather than the duration of the laser pulse.
Optical rectification
In optical rectificationOptical rectification
Electro-optic rectification is a non-linear optical process which consists in the generation of a quasi-DC polarization in a non-linear medium at the passage of an intense optical beam...
, a high-intensity ultrashort laser pulse
Ultrashort pulse laser
An ultrashort pulse laser is a laser that emits ultrashort pulses of light, generally of the order of femtoseconds to ten picoseconds. Thus they are also known as ultrafast lasers. But "ultrafast laser" is a misnomer, since the speed of light is constant in a given medium.Common current ultrashort...
passes through a transparent crystal material that emits a terahertz pulse without any applied voltages. It is a nonlinear-optical
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
process, where an appropriate crystal material is quickly electrically polarized
Polarization density
In classical electromagnetism, polarization density is the vector field that expresses the density of permanent or induced electric dipole moments in a dielectric material. When a dielectric is placed in an external electric field, its molecules gain electric dipole moment and the dielectric is...
at high optical intensities. This changing electrical polarization emits terahertz radiation.
Because of the high laser intensities that are necessary, this technique is mostly used with amplified Ti:sapphire lasers
Chirped pulse amplification
Chirped pulse amplification is a technique for amplifying an ultrashort laser pulse up to the petawatt level with the laser pulse being stretched out temporally and spectrally prior to amplification...
. Typical crystal materials are zinc telluride
Zinc telluride
Zinc telluride is a binary chemical compound with the formula ZnTe. This solid is a semiconductor material with band gap of 2.23–2.25 eV. It is usually a P-type semiconductor. Its crystal structure is cubic, like that for sphalerite and diamond....
, gallium phosphide, and gallium selenide.
The bandwidth of pulses generated by optical rectification is limited by the laser pulse duration, terahertz absorption in the crystal material, the thickness of the crystal, and a mismatch between the propagation speed of the laser pulse and the terahertz pulse inside the crystal. Typically, a thicker crystal will generate higher intensities, but lower THz frequencies. With this technique, it is possible to boost the generated frequencies to 40 THz (7.5 µm) or higher, although 2 THz (150 µm) is more commonly used since it requires less complex optical setups.
Detection
The electrical field of the terahertz pulses is measured in a detector that is simultaneously illuminated with an ultrashort laser pulse. Two common detection schemes are used in THz-TDS: photoconductive sampling and electro-optical sampling. THz pulses can also be detected by bolometerBolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
s, heat detectors cooled to liquid-helium temperatures. Since bolometers can only measure the total energy of a terahertz pulse, rather than its electrical field over time, it is not suitable for use in THz-TDS.
In both detection methods, a part (called the detection pulse) of the same ultrashort laser pulse that was used to generate the terahertz pulse is fed to the detector, where it arrives simultaneously with the terahertz pulse. The detector will produce a different electrical signal depending on whether the detection pulse arrives when the electric field of the THz pulse is low or high. An optical delay line is used to vary the timing of the detection pulse.
Because the measurement technique is coherent, it naturally rejects incoherent
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....
radiation. Additionally, because the time slice of the measurement is extremely narrow, the noise contribution to the measurement is extremely low.
The Signal-to-Noise (S/N) of the resulting time-domain waveform obviously depends on experimental conditions (e.g., averaging time), however due to the coherent sampling techniques described, high S/N values (>70 dB) are routinely seen with 1 minute averaging times.
Photoconductive Detection
Photoconductive detection is similar to photoconductive generation. Here, the bias electrical field across the antenna leads is generated by the electric field of the THz pulse focused onto the antenna, rather than being applied externally. The presence of the THz electric field generates current across the antenna leads, which is usually amplified using a low-bandwidth amplifier. This amplified current is the measured parameter which corresponds to the THz field strength. Again, the carriers in the semiconductor substrate have an extremely short lifetime. Thus, the THz electric field strength is only sampled for an extremely narrow slice (fs's) of the entire electric field waveform.Electro-optical sampling
The materials used for generation of terahertz radiation by optical rectification can also be used for its detection by using the Pockels effectPockels effect
The Pockels effect , or Pockels electro-optic effect, produces birefringence in an optical medium induced by a constant or varying electric field. It is distinguished from the Kerr effect by the fact that the birefringence is proportional to the electric field, whereas in the Kerr effect it is...
, where certain crystalline materials become birefringent in the presence of an electric field. The birefringence caused by the electric field of a terahertz pulse leads to a change in the optical polarization of the detection pulse, proportional to the terahertz electric-field strength. With the help of polarizers and photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
s, this polarization change is measured.
As with the generation, the bandwidth of the detection is dependent on the laser pulse duration, material properties, and crystal thickness.