Interview conducted by Gary Thomas
David Daughton, Applications Scientist at Lake Shore Cryotronics, reveals some of the benefits of an often overlooked resource for the materials science community: terahertz (THz) energy.
GT: Could you briefly introduce terahertz (THz) energy and its key properties?
DD: Terahertz refers to the portion of the electromagnetic spectrum which lies between the microwave and infrared frequency light.
The definitions are going to vary depending on who in the terahertz community that you talk to, but I generally like to think that terahertz frequency light is anything that lies between about 0.1 terahertz and 10 terahertz. This corresponds to a wavelength between 3 millimeters at the low end of that frequency domain and about 30 microns at the high end.
DD: Unlike the infrared and microwave frequencies, there has been a historical lack of reliable terahertz sources and detectors, and that lack of reliable sources has limited a widespread adoption of Terahertz spectroscopies for materials characterization, compared with IR or microwave characterization techniques.
In the past 20 or 25 years, there’s been a number of terahertz sourcing technologies that have emerged and those technologies have matured to a point where terahertz is becoming a viable characterization tool.
There are tradeoffs between the various sources, but each particular application that you’re targeting with terahertz spectroscopy will really determine the kind of sourcing and detection that users are going to adopt.
GT: What are some of the unique measurement capabilities of THz spectroscopy compared with other optical characterisation technologies?
DD: Because terahertz is an electromagnetic wave, a lot of the conventional optical techniques that are used -- such as absorbance, reflectance, polarimetry, wave guiding, even near field microscopy -- can be extended into the terahertz regime.
I think what is unique is that terahertz interacts with materials in ways that are different than other electromagnetic radiation. For example, a lot of plastics, ceramics and even papers are transparent to terahertz, while, for ordinary light, those same materials are opaque.
Terahertz is also extraordinarily sensitive to water content, and so if the material has even a small amount of water content, it’s fairly absorptive to terahertz light, meaning there are ways of characterizing water content in materials using THz spectroscopy.
For Lake Shore, we’re interested in materials like semiconductors, and with terahertz light, the energy is sufficiently low so it can couple to the free carriers in many of the common semiconductors and characterize the mobility and carrier concentration..
Unlike visible light characterization, the THz energy is much smaller than the band gaps in most materials, and the analysis of THz absorption spectra need not include terms to describe photogenerated carriers.
GT: THz-TDS systems are commonly used in materials characterisation studies – could you explain how these systems work?
DD: Terahertz time-domain sources (THz-TDS) use optical pulses from mode-locked femtosecond lasers. The idea is that you convert that monochromatic, femtosecond pulse into a broadband, picosecond terahertz pulse which contains all frequencies, or a large number of frequencies, in the terahertz regime.
And so, for a transmission measurement, what you’ll do is generate that terahertz pulse and then propagate it through the sample before sending it into some sort of terahertz detector.
That detection can be done either with what’s called a photoconductive switch, a non-linear device or an electro-optic crystal. The idea is that you use a portion of the femtosecond energy to generate the THz and another portion to time gate the detector in order to measure both the amplitude as well as the time-delay of the terahertz pulse which passes through your sample.
From that, you’re able to derive the frequency dependent absorption and the phase delay from the Fourier Transform of your terahertz pulse. From those two bits of information, you can apply various models to understand how that terahertz pulse has interacted with the material.
I think the key thing with terahertz time domain spectroscopy is that, typically, you get somewhere between 3 and 10 gigahertz of spectral resolution and that actually comes out of the properties of the Fourier transform.
GT: How can “photomixing” aid these systems?
DD: Photomixing is related to some of the terahertz time domain sources in that we use the same semiconductor material, which has a fast carrier relaxation rate. With photo-conductive mixing, rather than using one of these costly femtosecond lasers, you use two low-cost continuous wave lasers and you mix those two together onto an antenna using these fast carrier recombination rate materials.
By using similar terahertz sourcing technology used in some of these terahertz time domain sources but with lower cost laser systems, you’re able to develop a broad bandwidth, a continuously tunable terahertz source that offers better spectral resolution compared to those terahertz time domain systems -- often as low as about 100 megahertz.
One of the advantages is that because the photomixing generates a single frequency at a time, rather than all frequencies at the same time like a terahertz time domain source, you can focus your efforts on looking at a narrow spectral region. For example, if there’s a narrow absorption in a particular part of the spectral region, you can focus your acquisition time on that narrow spectral region instead of the entire spectral region.
GT: How can THz spectroscopy assist with the characterisation of dielectric materials?
DD: Most non-polar dielectrics are transparent to terahertz light, so from a spectroscopic standpoint, you can derive the absorption and the index at terahertz frequencies which can drive other terahertz applications such as making lenses or windows.
However, I think one of the key strengths of terahertz spectroscopy is using it as a non-destructive evaluation tool. For dielectric materials, for example, you could think about having a multilayer that’s glued together; because the terahertz light can pass through the multiple layers and picks up information from all of the layers, you could use terahertz spectroscopy to characterize the chemical composition of the adhesion layer over time in this composite structure.
Another possibility is polarization sensitive terahertz spectroscopy being used during manufacturing to look at the alignment of fibers in composite dielectrics. So there are lots of opportunities to look at dielectric materials, especially ones that have a little bit more structure to them, with terahertz spectroscopy and taking advantage of the transparency.
GT: How can THz spectroscopy aid the semiconductor industry?
On the semiconductor front, terahertz imaging has been a very hot topic of late and a number of manufacturers have been looking at terahertz imaging as a way of conducting fault detection in circuits. It’s a little bit away from the terahertz spectroscopy side of things, but it’s really an important application of terahertz energy.
I believe that the strength of terahertz spectroscopy for the semiconductor industry is really in developing new materials and new processes. On the roadmap of semiconductor development are things like high k dielectrics and new thin film deposition techniques, as well as a move away from silicon, so a new characterization tool that can be sensitive to some of the things that go on in those materials could be useful.
GT: How is Lake Shore Cryotronics utilising the unique capabilities of THz spectroscopy?
DD: Lake Shore is in the business of working with materials development scientists, both in academia and industry, to develop systems that they can use to characterize materials. We’ve been building and developing vibrating simple magnetometers and Hall systems for years now, and when terahertz started showing up on our radar, we immediately said “well, how can this help our core customer?”
Over the last few years, we’ve been developing a terahertz frequency characterization system product which bundles continuous-wave terahertz spectroscopy using photomixers with variable cryogenic temperatures and high magnetic field.
The design of the platform and the software is targeting users that are in the materials development community, not necessarily THz experts, who are looking for a platform that they can use to characterize the terahertz frequency properties of their pre-device stage materials.
DD: I think the major change has been accessibility to terahertz technologies as a whole. 20 years ago you had to be a laser expert or have several degrees in microwave engineering in order to really access the technologies. These systems were built in labs for labs and they were large and not very cost effective.
Over the past 20 years, the number and variety of sources has grown rapidly. This has resulted in increased accessibility for what I’d call non-terahertz experts --- people who are looking at biological materials, semiconductors, or new composite materials who aren’t necessarily in the business of building a terahertz spectrometer but are interested in using its unique capabilities to look at their materials.
GT: What are some of the current limitations to THz spectroscopy and how can these be overcome?
DD: I think the biggest limitation to terahertz spectroscopy is in sourcing. As I previously mentioned, with the varieties of terahertz sources that are out there, there’s limitations to each of them, be it power, spectral bandwidth, or spectral resolution, and so what often happens is an application will pick the terahertz source.
I think over time and with the efforts of researchers in this field, what’s going to happen is an evolution towards a cost-effective, high-power, continuously tunable, high spectral resolution solution for terahertz spectroscopy. I think that the biggest limitation for users outside of the terahertz community that want to bring terahertz into the labs is that the systems are either severely limited or they’re really expensive. So I think, as technology evolves, the sources will become cheaper and there will be bundling of these sources into platforms that are similar to UV-VIS, Raman, or other light-based characterization systems, which are commonplace in biology, chemistry, and materials research settings.
This is why started where we have with our terahertz development system; it is really about coming up with the best sourcing technology we can that’s both cost effective and build it into a system that can be easily used by a technical customer-base.
David Daughton is an Applications Scientist at Lake Shore Cryotronics in Westerville, OH. David joined Lake Shore in 2011 to develop new measurement platforms for terahertz frequency characterization of electronic and magnetic materials and support customer applications in Lake Shore’s cryogenic probe station products.
He received B.S. and M.S. degrees from the University of Delaware, where he explored the effects of disorder on the superfluid and solid phases of helium, and a Ph.D. from the Ohio State University for scanning tunneling microscopy and optical studies of carbon-based electronic materials.
David Daughton, Applications Scientist at Lake Shore Cryotronics, reveals some of the benefits of an often overlooked resource for the materials science community: terahertz (THz) energy.
GT: Could you briefly introduce terahertz (THz) energy and its key properties?
DD: Terahertz refers to the portion of the electromagnetic spectrum which lies between the microwave and infrared frequency light.
The definitions are going to vary depending on who in the terahertz community that you talk to, but I generally like to think that terahertz frequency light is anything that lies between about 0.1 terahertz and 10 terahertz. This corresponds to a wavelength between 3 millimeters at the low end of that frequency domain and about 30 microns at the high end.
Diagram showing the position of terahertz (THz) energy in the electromagnetic spectrum. Image credit: Lake Shore Cryotronics.
GT: How much work has been previously undertaken using terahertz energy relative to the other regions of the electromagnetic spectrum?DD: Unlike the infrared and microwave frequencies, there has been a historical lack of reliable terahertz sources and detectors, and that lack of reliable sources has limited a widespread adoption of Terahertz spectroscopies for materials characterization, compared with IR or microwave characterization techniques.
In the past 20 or 25 years, there’s been a number of terahertz sourcing technologies that have emerged and those technologies have matured to a point where terahertz is becoming a viable characterization tool.
There are tradeoffs between the various sources, but each particular application that you’re targeting with terahertz spectroscopy will really determine the kind of sourcing and detection that users are going to adopt.
GT: What are some of the unique measurement capabilities of THz spectroscopy compared with other optical characterisation technologies?
DD: Because terahertz is an electromagnetic wave, a lot of the conventional optical techniques that are used -- such as absorbance, reflectance, polarimetry, wave guiding, even near field microscopy -- can be extended into the terahertz regime.
I think what is unique is that terahertz interacts with materials in ways that are different than other electromagnetic radiation. For example, a lot of plastics, ceramics and even papers are transparent to terahertz, while, for ordinary light, those same materials are opaque.
Terahertz is also extraordinarily sensitive to water content, and so if the material has even a small amount of water content, it’s fairly absorptive to terahertz light, meaning there are ways of characterizing water content in materials using THz spectroscopy.
For Lake Shore, we’re interested in materials like semiconductors, and with terahertz light, the energy is sufficiently low so it can couple to the free carriers in many of the common semiconductors and characterize the mobility and carrier concentration..
Unlike visible light characterization, the THz energy is much smaller than the band gaps in most materials, and the analysis of THz absorption spectra need not include terms to describe photogenerated carriers.
GT: THz-TDS systems are commonly used in materials characterisation studies – could you explain how these systems work?
DD: Terahertz time-domain sources (THz-TDS) use optical pulses from mode-locked femtosecond lasers. The idea is that you convert that monochromatic, femtosecond pulse into a broadband, picosecond terahertz pulse which contains all frequencies, or a large number of frequencies, in the terahertz regime.
And so, for a transmission measurement, what you’ll do is generate that terahertz pulse and then propagate it through the sample before sending it into some sort of terahertz detector.
That detection can be done either with what’s called a photoconductive switch, a non-linear device or an electro-optic crystal. The idea is that you use a portion of the femtosecond energy to generate the THz and another portion to time gate the detector in order to measure both the amplitude as well as the time-delay of the terahertz pulse which passes through your sample.
From that, you’re able to derive the frequency dependent absorption and the phase delay from the Fourier Transform of your terahertz pulse. From those two bits of information, you can apply various models to understand how that terahertz pulse has interacted with the material.
I think the key thing with terahertz time domain spectroscopy is that, typically, you get somewhere between 3 and 10 gigahertz of spectral resolution and that actually comes out of the properties of the Fourier transform.
GT: How can “photomixing” aid these systems?
DD: Photomixing is related to some of the terahertz time domain sources in that we use the same semiconductor material, which has a fast carrier relaxation rate. With photo-conductive mixing, rather than using one of these costly femtosecond lasers, you use two low-cost continuous wave lasers and you mix those two together onto an antenna using these fast carrier recombination rate materials.
By using similar terahertz sourcing technology used in some of these terahertz time domain sources but with lower cost laser systems, you’re able to develop a broad bandwidth, a continuously tunable terahertz source that offers better spectral resolution compared to those terahertz time domain systems -- often as low as about 100 megahertz.
One of the advantages is that because the photomixing generates a single frequency at a time, rather than all frequencies at the same time like a terahertz time domain source, you can focus your efforts on looking at a narrow spectral region. For example, if there’s a narrow absorption in a particular part of the spectral region, you can focus your acquisition time on that narrow spectral region instead of the entire spectral region.
GT: How can THz spectroscopy assist with the characterisation of dielectric materials?
DD: Most non-polar dielectrics are transparent to terahertz light, so from a spectroscopic standpoint, you can derive the absorption and the index at terahertz frequencies which can drive other terahertz applications such as making lenses or windows.
However, I think one of the key strengths of terahertz spectroscopy is using it as a non-destructive evaluation tool. For dielectric materials, for example, you could think about having a multilayer that’s glued together; because the terahertz light can pass through the multiple layers and picks up information from all of the layers, you could use terahertz spectroscopy to characterize the chemical composition of the adhesion layer over time in this composite structure.
Another possibility is polarization sensitive terahertz spectroscopy being used during manufacturing to look at the alignment of fibers in composite dielectrics. So there are lots of opportunities to look at dielectric materials, especially ones that have a little bit more structure to them, with terahertz spectroscopy and taking advantage of the transparency.
GT: How can THz spectroscopy aid the semiconductor industry?
On the semiconductor front, terahertz imaging has been a very hot topic of late and a number of manufacturers have been looking at terahertz imaging as a way of conducting fault detection in circuits. It’s a little bit away from the terahertz spectroscopy side of things, but it’s really an important application of terahertz energy.
I believe that the strength of terahertz spectroscopy for the semiconductor industry is really in developing new materials and new processes. On the roadmap of semiconductor development are things like high k dielectrics and new thin film deposition techniques, as well as a move away from silicon, so a new characterization tool that can be sensitive to some of the things that go on in those materials could be useful.
GT: How is Lake Shore Cryotronics utilising the unique capabilities of THz spectroscopy?
DD: Lake Shore is in the business of working with materials development scientists, both in academia and industry, to develop systems that they can use to characterize materials. We’ve been building and developing vibrating simple magnetometers and Hall systems for years now, and when terahertz started showing up on our radar, we immediately said “well, how can this help our core customer?”
Over the last few years, we’ve been developing a terahertz frequency characterization system product which bundles continuous-wave terahertz spectroscopy using photomixers with variable cryogenic temperatures and high magnetic field.
The design of the platform and the software is targeting users that are in the materials development community, not necessarily THz experts, who are looking for a platform that they can use to characterize the terahertz frequency properties of their pre-device stage materials.
Terahertz spectroscopy system from Lake Shore Cryotronics. Image credit Lake Shore Cryotronics.
GT: How has THz technology evolved over the last 20 years?DD: I think the major change has been accessibility to terahertz technologies as a whole. 20 years ago you had to be a laser expert or have several degrees in microwave engineering in order to really access the technologies. These systems were built in labs for labs and they were large and not very cost effective.
Over the past 20 years, the number and variety of sources has grown rapidly. This has resulted in increased accessibility for what I’d call non-terahertz experts --- people who are looking at biological materials, semiconductors, or new composite materials who aren’t necessarily in the business of building a terahertz spectrometer but are interested in using its unique capabilities to look at their materials.
GT: What are some of the current limitations to THz spectroscopy and how can these be overcome?
DD: I think the biggest limitation to terahertz spectroscopy is in sourcing. As I previously mentioned, with the varieties of terahertz sources that are out there, there’s limitations to each of them, be it power, spectral bandwidth, or spectral resolution, and so what often happens is an application will pick the terahertz source.
I think over time and with the efforts of researchers in this field, what’s going to happen is an evolution towards a cost-effective, high-power, continuously tunable, high spectral resolution solution for terahertz spectroscopy. I think that the biggest limitation for users outside of the terahertz community that want to bring terahertz into the labs is that the systems are either severely limited or they’re really expensive. So I think, as technology evolves, the sources will become cheaper and there will be bundling of these sources into platforms that are similar to UV-VIS, Raman, or other light-based characterization systems, which are commonplace in biology, chemistry, and materials research settings.
This is why started where we have with our terahertz development system; it is really about coming up with the best sourcing technology we can that’s both cost effective and build it into a system that can be easily used by a technical customer-base.
About David Daughton
David Daughton is an Applications Scientist at Lake Shore Cryotronics in Westerville, OH. David joined Lake Shore in 2011 to develop new measurement platforms for terahertz frequency characterization of electronic and magnetic materials and support customer applications in Lake Shore’s cryogenic probe station products.
He received B.S. and M.S. degrees from the University of Delaware, where he explored the effects of disorder on the superfluid and solid phases of helium, and a Ph.D. from the Ohio State University for scanning tunneling microscopy and optical studies of carbon-based electronic materials.
1 comment:
This article is Great! Thank you Dr. Daughton for taking time to help many of us, better understand THz.
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