Terahertz Radiation - Revision history

Cmditradmin at 22:49, 3 November 2011

2011-11-03T22:49:02Z

← Older revision		Revision as of 15:49, 3 November 2011
Line 18:		Line 18:
	==History of THz Science==		==History of THz Science==
	[[Image:Thz_setup.jpg\|thumb\|300px\|A THz-TDS/OPTP experimental setup]]		[[Image:Thz_setup.jpg\|thumb\|300px\|A THz-TDS/OPTP experimental setup]]
	Far-infrared studies have occurred since the invention of the bolometer in 1881 by astronomer Samuel Langley. Much of the early far-infrared work was done by Rubens, who discovered reststrahlen sources enabling measurements crucial to Planck's blackbody radiation theory. Far-infrared transmission studies of superconductors confirmed the energy gap predicted by Bardeen,Cooper, and Schrieffer's superconductivity theory. Since the invention of frequency-division multiplexing in 1949, Fourier-transform infrared spectroscopy became a standard tool in physical chemistry. Over the years, numerous sources and detectors have been discovered e.g. Golay cell, water-vapor laser, pyroelectric detector, synchrotrons, free-electron lasers, and quantum cascade lasers.		Far-infrared studies have occurred since the invention of the bolometer in 1881 by astronomer Samuel Langley. Much of the early far-infrared work was done by Rubens, who discovered reststrahlen sources enabling measurements crucial to Planck's blackbody radiation theory. Far-infrared transmission studies of superconductors confirmed the energy gap predicted by Bardeen, Cooper, and Schrieffer's superconductivity theory. Since the invention of frequency-division multiplexing in 1949, Fourier-transform infrared spectroscopy became a standard tool in physical chemistry. Over the years, numerous sources and detectors have been discovered e.g. Golay cell, water-vapor laser, pyroelectric detector, synchrotrons, free-electron lasers, and quantum cascade lasers.


Line 35:		Line 35:
	<math>E_{THz}\propto\partial J/\partial t</math>		<math>E_{THz}\propto\partial J/\partial t</math>

	Though PDAs are bright THz sources, finite carrier lifetimes limit the bandwidth available from photoconductive materials to less than that ~~acheivable~~ via OR in NLO materials. Detection of the THz pulse occurs, essentially, through the opposite effect. An ultra-short pulse generates short-lived charge carriers, which are subsequently accelerated by the THz electric field. This generates a transient current between two electrodes, which is proportional to the amplitude and direction of the THz electric field making this a coherent detection technique. A delay stage allows for the one pulse to be moved in time with respect to the other, similar to a correlation, such that the THz waveform is sampled point-by-point by the "probe" pulse		Though PDAs are bright THz sources, finite carrier lifetimes limit the bandwidth available from photoconductive materials to less than that achievable via OR in NLO materials. Detection of the THz pulse occurs, essentially, through the opposite effect. An ultra-short pulse generates short-lived charge carriers, which are subsequently accelerated by the THz electric field. This generates a transient current between two electrodes, which is proportional to the amplitude and direction of the THz electric field making this a coherent detection technique. A delay stage allows for the one pulse to be moved in time with respect to the other, similar to a correlation, such that the THz waveform is sampled point-by-point by the "probe" pulse
	<br clear='all'>		<br clear='all'>

	===Optical Rectification in a Nonlinear Optical Medium===		===Optical Rectification in a Nonlinear Optical Medium===
	[[Image:THz_OR.png\|thumb\|300px\|THz "Optical-rectification" generates broadband THz pulses via difference frequency mixing of the frequency components of an ultra-short pulse]]		[[Image:THz_OR.png\|thumb\|300px\|THz "Optical-rectification" generates broadband THz pulses via difference frequency mixing of the frequency components of an ultra-short pulse]]
	OR is more properly though of as difference frequency mixing. In a NLO material, the polarization induced by an AC electric field will not reverse its sign at the same time as the driving field, leading to a nonlinear polarization and thus frequency mixing. If the field is applied with a ~~femptosecond~~ laser pulse, with a large spectral bandwidth, difference frequency mixing of different frequency components results in electromagnetic waves in the terahertz region. This is the optical analog of the audible beat frequency generated by two struck tuning forks. The emitted field is related to the polarization, <em>P</em>, by		OR is more properly though of as difference frequency mixing. In a NLO material, the polarization induced by an AC electric field will not reverse its sign at the same time as the driving field, leading to a nonlinear polarization and thus frequency mixing. If the field is applied with a femtosecond laser pulse, with a large spectral bandwidth, difference frequency mixing of different frequency components results in electromagnetic waves in the terahertz region. This is the optical analog of the audible beat frequency generated by two struck tuning forks. The emitted field is related to the polarization, <em>P</em>, by
	<center><math>E_{THz}\propto\partial^{2} P/\partial t^{2}</math></center>		<center><math>E_{THz}\propto\partial^{2} P/\partial t^{2}</math></center>
	The efficiency of THz generation depends on the second order nonlinear susceptibility of the medium. Popular materials include: inorganic NLO crystals (e.g. ZnTe, GaP), organic NLO crystals (e.g. DAST), NLO guest-host electro-optic (EO) polymers.		The efficiency of THz generation depends on the second order nonlinear susceptibility of the medium. Popular materials include: inorganic NLO crystals (e.g. ZnTe, GaP), organic NLO crystals (e.g. DAST), NLO guest-host electro-optic (EO) polymers.
Line 56:		Line 56:
	== EO Polymers for THz==		== EO Polymers for THz==
	[[Image:THz_Polymer_vs_ZnTe.png‎ \|thumb\|250px\| Poor phase matching due to a 5.3 THz phonon limits the bandwidth available using the inorganic crystal ZnTe as a THz sensor. Improved phase matching and a coherence length of 50 <math>\mu</math>m at 10 THz allows for broadband THz detection in this EO polymer ]]		[[Image:THz_Polymer_vs_ZnTe.png‎ \|thumb\|250px\| Poor phase matching due to a 5.3 THz phonon limits the bandwidth available using the inorganic crystal ZnTe as a THz sensor. Improved phase matching and a coherence length of 50 <math>\mu</math>m at 10 THz allows for broadband THz detection in this EO polymer ]]
	EO poled guest-host polymers have the potential to ~~acheive~~ orders of magnitude higher optical nonlinearities than crystalline materials. Poled polymers have ~~acheived~~ electro-optic coefficients >400 pm/V, which is two orders of magnitude larger than the inorganic crystal ZnTe, a standard material for THz generation and detection. The tunable optical and NLO properties of poled polymers, along with good phase matching, makes them valuable for THz applications.<ref>A. M. Sinyukov, L. M. Hayden, J. Phys. Chem. B 108, 8515-8522 (2004)</ref> For example, poled polymers have been used to achieve a broad bandwidth THz system at telecommunications wavelengths,<ref>C. V. McLaughlin, et al., Appl. Phys. Lett. 92, 151107 (2008)</ref> where compact laser systems are available. Due to their relatively flat THz index of refraction, proper choice of the "pump" wavelength would allow for phase matching across the THz band even for very thick polymers.		EO poled guest-host polymers have the potential to achieve orders of magnitude higher optical nonlinearities than crystalline materials. Poled polymers have achieved electro-optic coefficients >400 pm/V, which is two orders of magnitude larger than the inorganic crystal ZnTe, a standard material for THz generation and detection. The tunable optical and NLO properties of poled polymers, along with good phase matching, makes them valuable for THz applications.<ref>A. M. Sinyukov, L. M. Hayden, J. Phys. Chem. B 108, 8515-8522 (2004)</ref> For example, poled polymers have been used to achieve a broad bandwidth THz system at telecommunications wavelengths,<ref>C. V. McLaughlin, et al., Appl. Phys. Lett. 92, 151107 (2008)</ref> where compact laser systems are available. Due to their relatively flat THz index of refraction, proper choice of the "pump" wavelength would allow for phase matching across the THz band even for very thick polymers.

	See [[Second-order_NLO_Materials#Poling_of_Chromophores_in_Polymers\|Poling Polymers Basics]]		See [[Second-order_NLO_Materials#Poling_of_Chromophores_in_Polymers\|Poling Polymers Basics]]
Line 135:		Line 135:

	[[Image:Polymer_dynamics.png\|thumb\|250px\|The conductivity observed in an amorphous polymer at various delays after photo-excitation shows an evolution from dispersive transport to a bound excitonic response..]]		[[Image:Polymer_dynamics.png\|thumb\|250px\|The conductivity observed in an amorphous polymer at various delays after photo-excitation shows an evolution from dispersive transport to a bound excitonic response..]]
	Here, OPTP spectroscopy has been used to examine carrier dynamics in an amorphous semiconducting polymer film. The complex conductivity		Here, OPTP spectroscopy has been used to examine carrier dynamics in an amorphous semiconducting polymer film. The complex conductivity evolves over time until the real conductivity is nearly zero and the imaginary conductivity is negative and linear with frequency. This signature has come to be associated with bound excitons. At early delays, a variety of conductivity models could describe the dispersive transport seen, including the Drude-Smith model, Debye relaxation, and the Lorentz oscillator model.
	evolves over time until the real conductivity is nearly zero and the imaginary conductivity is negative and linear with frequency. This signature has come to be associated with bound excitons. At early delays, a variety of conductivity models could describe the dispersive transport seen, including the Drude-Smith model, Debye relaxation, and the Lorentz oscillator model.
	<br clear='all'>		<br clear='all'>

Cmditradmin: /* Modes of Operation */

2010-07-20T18:22:34Z

Modes of Operation

← Older revision		Revision as of 11:22, 20 July 2010
Line 123:		Line 123:


	see e.g. F. A. Hegmann, et. al., "Probing Organic Semiconductors with Terahertz Pulses." In Photophysics of Molecular Materials; Lanzani, G., Ed.; WILEY-VCH: Weinheim, 2006.		see F. A. Hegmanne.<ref>F. A. Hegmann, et. al., "Probing Organic Semiconductors with Terahertz Pulses." In Photophysics of Molecular Materials; Lanzani, G., Ed.; WILEY-VCH: Weinheim, 2006.</ref>

	=== Examples in Materials Science ===		=== Examples in Materials Science ===
	THz-TDS and OPTP spectroscopy have been used for a number of materials science studies, from the properties of dielectrics, solvation dynamics, metamaterials, insulator-metal transitions, high-temperature superconductivity, colossal magnetoresistance, and carrier dynamics in semiconductors, nanostructures, and organics. Below, a cross-section of materials studies examples are listed to illustrate the potential of THz spectroscopy.		THz-TDS and OPTP spectroscopy have been used for a number of materials science studies, from the properties of dielectrics, solvation dynamics, metamaterials, insulator-metal transitions, high-temperature superconductivity, colossal magnetoresistance, and carrier dynamics in semiconductors, nanostructures, and organics. Below, a cross-section of materials studies examples are listed to illustrate the potential of THz spectroscopy.

Cmditradmin: /* THz Time Domain Spectroscopy */

2010-07-20T16:18:08Z

THz Time Domain Spectroscopy

← Older revision		Revision as of 09:18, 20 July 2010
Line 85:		Line 85:
	=== THz Time Domain Spectroscopy ===		=== THz Time Domain Spectroscopy ===
	[[Image:TDS_cartoon.png\|thumb\|300px\|In THz Time Domain Spectroscopy, a ~1ps single-cycle THz pulse is propagated through a sample.]]		[[Image:TDS_cartoon.png\|thumb\|300px\|In THz Time Domain Spectroscopy, a ~1ps single-cycle THz pulse is propagated through a sample.]]

			[[Image:OPTP.png\|thumb\|400px\|Optical-Pump THz Probe Spectroscopy ]]

	In THz-TDS, the transmission of the THz waveform (electric field) through a sample is measured and compared to a reference, typically the transmission through the experiment without a sample present. Modeling normal incidence transmission through the sample with Fresnel coefficients,<ref>L. Duvillaret, et al., IEEE J. Quant. Elect. 2, 739 (1996)</ref> and assuming <math>\kappa << n</math>, the index of refraction, <em>n</em>, and the power absorption coefficient <math>\alpha</math> are		In THz-TDS, the transmission of the THz waveform (electric field) through a sample is measured and compared to a reference, typically the transmission through the experiment without a sample present. Modeling normal incidence transmission through the sample with Fresnel coefficients,<ref>L. Duvillaret, et al., IEEE J. Quant. Elect. 2, 739 (1996)</ref> and assuming <math>\kappa << n</math>, the index of refraction, <em>n</em>, and the power absorption coefficient <math>\alpha</math> are
	<center><math>n(\nu)\approx1+\frac{c}{d\omega}\left\|\Delta\Phi \right\|</math></center>		<center><math>n(\nu)\approx1+\frac{c}{d\omega}\left\|\Delta\Phi \right\|</math></center>

Cmditradmin: /* EO Polymers for THz */

2010-06-24T17:05:51Z

EO Polymers for THz

← Older revision		Revision as of 10:05, 24 June 2010
Line 58:		Line 58:
	EO poled guest-host polymers have the potential to acheive orders of magnitude higher optical nonlinearities than crystalline materials. Poled polymers have acheived electro-optic coefficients >400 pm/V, which is two orders of magnitude larger than the inorganic crystal ZnTe, a standard material for THz generation and detection. The tunable optical and NLO properties of poled polymers, along with good phase matching, makes them valuable for THz applications.<ref>A. M. Sinyukov, L. M. Hayden, J. Phys. Chem. B 108, 8515-8522 (2004)</ref> For example, poled polymers have been used to achieve a broad bandwidth THz system at telecommunications wavelengths,<ref>C. V. McLaughlin, et al., Appl. Phys. Lett. 92, 151107 (2008)</ref> where compact laser systems are available. Due to their relatively flat THz index of refraction, proper choice of the "pump" wavelength would allow for phase matching across the THz band even for very thick polymers.		EO poled guest-host polymers have the potential to acheive orders of magnitude higher optical nonlinearities than crystalline materials. Poled polymers have acheived electro-optic coefficients >400 pm/V, which is two orders of magnitude larger than the inorganic crystal ZnTe, a standard material for THz generation and detection. The tunable optical and NLO properties of poled polymers, along with good phase matching, makes them valuable for THz applications.<ref>A. M. Sinyukov, L. M. Hayden, J. Phys. Chem. B 108, 8515-8522 (2004)</ref> For example, poled polymers have been used to achieve a broad bandwidth THz system at telecommunications wavelengths,<ref>C. V. McLaughlin, et al., Appl. Phys. Lett. 92, 151107 (2008)</ref> where compact laser systems are available. Due to their relatively flat THz index of refraction, proper choice of the "pump" wavelength would allow for phase matching across the THz band even for very thick polymers.

	See [[~~Organic_Photonics_Applications~~#~~Poling_Polymers_Basics~~\|Poling Polymers Basics]]		See [[Second-order_NLO_Materials#Poling_of_Chromophores_in_Polymers\|Poling Polymers Basics]]

	<br clear='all'>		<br clear='all'>

Cmditradmin: /* Photoconductive Dipole Antennae */

2010-06-24T17:02:01Z

Photoconductive Dipole Antennae

← Older revision		Revision as of 10:02, 24 June 2010
Line 34:		Line 34:

	<math>E_{THz}\propto\partial J/\partial t</math>		<math>E_{THz}\propto\partial J/\partial t</math>

	Though PDAs are bright THz sources, finite carrier lifetimes limit the bandwidth available from photoconductive materials to less than that acheivable via OR in NLO materials. Detection of the THz pulse occurs, essentially, through the opposite effect. An ultra-short pulse generates short-lived charge carriers, which are subsequently accelerated by the THz electric field. This generates a transient current between two electrodes, which is proportional to the amplitude and direction of the THz electric field making this a coherent detection technique. A delay stage allows for the one pulse to be moved in time with respect to the other, similar to a correlation, such that the THz waveform is sampled point-by-point by the "probe" pulse		Though PDAs are bright THz sources, finite carrier lifetimes limit the bandwidth available from photoconductive materials to less than that acheivable via OR in NLO materials. Detection of the THz pulse occurs, essentially, through the opposite effect. An ultra-short pulse generates short-lived charge carriers, which are subsequently accelerated by the THz electric field. This generates a transient current between two electrodes, which is proportional to the amplitude and direction of the THz electric field making this a coherent detection technique. A delay stage allows for the one pulse to be moved in time with respect to the other, similar to a correlation, such that the THz waveform is sampled point-by-point by the "probe" pulse
	<br clear='all'>		<br clear='all'>

Cmditradmin at 20:22, 29 December 2009

2009-12-29T20:22:11Z

← Older revision		Revision as of 13:22, 29 December 2009
Line 161:		Line 161:
	== References ==		== References ==
	<references/>		<references/>
			[[category:second order NLO]]
			[[category:photonics applications]]
	<table id="toc" style="width: 100%">		<table id="toc" style="width: 100%">
	<tr>		<tr>

Cmditradmin: /* Photoconductive Dipole Antennae */

2009-11-30T17:43:58Z

Photoconductive Dipole Antennae

← Older revision		Revision as of 10:43, 30 November 2009
Line 30:		Line 30:
	===Photoconductive Dipole Antennae===		===Photoconductive Dipole Antennae===
	[[Image:THz_PDA.png‎\|thumb\|400px\|Schematic of photoconductive dipole antenna emitter and sensor]]		[[Image:THz_PDA.png‎\|thumb\|400px\|Schematic of photoconductive dipole antenna emitter and sensor]]
	In a PDA, THz radiation is emitted by an ultra-fast transient photo-current created in a biased semiconductor, typically GaAs. The semiconductor is photo-excited with an ultra-short pulse to generate short-lived charges, which are accelerated with an applied electric field. The emitted field, <math>E_{THz}</math> is related to the current, <em>J</em>, by		In a PDA, THz radiation is emitted by an ultra-fast transient photo-current created in a biased semiconductor, typically GaAs. The semiconductor is photo-excited with an ultra-short pulse to generate short-lived charges, which are accelerated with an applied electric field. The emitted field,
	~~<center>~~<math>E_{THz}\propto\partial J/\partial t</math~~></center~~>		<math>E_{THz}</math> is related to the current, <em>J</em>, by

			<math>E_{THz}\propto\partial J/\partial t</math>
	Though PDAs are bright THz sources, finite carrier lifetimes limit the bandwidth available from photoconductive materials to less than that acheivable via OR in NLO materials. Detection of the THz pulse occurs, essentially, through the opposite effect. An ultra-short pulse generates short-lived charge carriers, which are subsequently accelerated by the THz electric field. This generates a transient current between two electrodes, which is proportional to the amplitude and direction of the THz electric field making this a coherent detection technique. A delay stage allows for the one pulse to be moved in time with respect to the other, similar to a correlation, such that the THz waveform is sampled point-by-point by the "probe" pulse		Though PDAs are bright THz sources, finite carrier lifetimes limit the bandwidth available from photoconductive materials to less than that acheivable via OR in NLO materials. Detection of the THz pulse occurs, essentially, through the opposite effect. An ultra-short pulse generates short-lived charge carriers, which are subsequently accelerated by the THz electric field. This generates a transient current between two electrodes, which is proportional to the amplitude and direction of the THz electric field making this a coherent detection technique. A delay stage allows for the one pulse to be moved in time with respect to the other, similar to a correlation, such that the THz waveform is sampled point-by-point by the "probe" pulse
	<br clear='all'>		<br clear='all'>

	===Optical Rectification in a Nonlinear Optical Medium===		===Optical Rectification in a Nonlinear Optical Medium===
	[[Image:THz_OR.png\|thumb\|300px\|THz "Optical-rectification" generates broadband THz pulses via difference frequency mixing of the frequency components of an ultra-short pulse]]		[[Image:THz_OR.png\|thumb\|300px\|THz "Optical-rectification" generates broadband THz pulses via difference frequency mixing of the frequency components of an ultra-short pulse]]

Pcunni1 at 21:27, 6 November 2009

2009-11-06T21:27:34Z

← Older revision		Revision as of 14:27, 6 November 2009
Line 141:		Line 141:
	<br clear='all'>		<br clear='all'>

			====Temperature Dependent Conduction====
			[[Image:HBCCO_Tdep.png‎\|thumb\|300px\|The imaginary conductivity of the high-temperature superconductor HBCCO shows a sharp increase below the critical temperature due to Cooper pairing]]
			Here, THz TDS was used to study the temperature dependent dielectric properties, i.e. conductivity, of the high-temperature superconductor HBCCO. A marked increase in the imaginary component occurs below the critical temperature, where Cooper pairing becomes efficient. At low temperatures, the conductivity is described by the properties of both superconducting Cooper-pairs and normal state electrons, while at high temperature the conductivity is Drude-like. OPTP can be used to study Cooper-pairing dynamics, as optical excitation breaks apart Cooper-pairs into metastable quasiparticles.
			<br clear='all'>
	== External Links ==		== External Links ==

Pcunni1 at 19:54, 8 October 2009

2009-10-08T19:54:19Z

← Older revision		Revision as of 12:54, 8 October 2009
Line 84:		Line 84:
	<center><math>n(\nu)\approx1+\frac{c}{d\omega}\left\|\Delta\Phi \right\|</math></center>		<center><math>n(\nu)\approx1+\frac{c}{d\omega}\left\|\Delta\Phi \right\|</math></center>

	<center><math>\alpha(\nu)\approx-\frac{1}{2 d}ln\left(\frac{1}{T}\frac{Mag_{sample}}{Mag_{ref}} \right)</math></center>		<center><math>\alpha(\nu)\approx-\frac{2}{d}ln\left(\frac{1}{T}\frac{Mag_{sample}}{Mag_{ref}} \right)</math></center>

	where <math>\omega = 2\pi\nu</math>, <math>\alpha =\omega \kappa / 2 c</math>, <em>d</em> is the sample thickness, <em>c</em> is the speed of light, <em>T</em> are the Fresnel transmission coefficients to correct for reflective losses, <math>\Delta\Phi</math> is the measured change in THz phase, and <math>Mag_{sample}</math> is the magnitude of the THz transmission through the sample.		where <math>\omega = 2\pi\nu</math>, <math>\alpha =\omega \kappa / 2 c</math>, <em>d</em> is the sample thickness, <em>c</em> is the speed of light, <em>T</em> are the Fresnel transmission coefficients to correct for reflective losses, <math>\Delta\Phi</math> is the measured change in THz phase, and <math>Mag_{sample}</math> is the magnitude of the THz transmission through the sample.
Line 115:		Line 115:
	<br clear='all'>		<br clear='all'>


			see e.g. F. A. Hegmann, et. al., "Probing Organic Semiconductors with Terahertz Pulses." In Photophysics of Molecular Materials; Lanzani, G., Ed.; WILEY-VCH: Weinheim, 2006.
	=== Examples in Materials Science ===		=== Examples in Materials Science ===
	THz-TDS and OPTP spectroscopy have been used for a number of materials science studies, from the properties of dielectrics, solvation dynamics, metamaterials, insulator-metal transitions, high-temperature superconductivity, colossal magnetoresistance, and carrier dynamics in semiconductors, nanostructures, and organics. Below, a cross-section of materials studies examples are listed to illustrate the potential of THz spectroscopy.		THz-TDS and OPTP spectroscopy have been used for a number of materials science studies, from the properties of dielectrics, solvation dynamics, metamaterials, insulator-metal transitions, high-temperature superconductivity, colossal magnetoresistance, and carrier dynamics in semiconductors, nanostructures, and organics. Below, a cross-section of materials studies examples are listed to illustrate the potential of THz spectroscopy.

Pcunni1 at 21:18, 4 October 2009

2009-10-04T21:18:14Z

Show changes