Terahertz detection and carbon nanotubes

Thursday, October 2, 2014

A novel imaging and tissue analysis scheme in the terahertz frequency band has the benefits of simplicity, coherence, and high sensitivity.

2 October 2014, SPIE Newsroom. DOI: 10.1117/2.1201409.005613
Terahertz radiation has an inherently low penetration depth in hydrated biological tissue, so skin is an ideal target for imaging at terahertz frequencies. Examining structures within the surface layers of skin is a mainstay of diagnosis in pathology, and using several different frequency regions to image these can provide complementary information about skin structure and function. The research community has successfully developed diagnostic methods based on the response of tissue in the terahertz spectral region.1However, to date, the absence of a compact, robust, inexpensive sensing solution has impeded general adoption of terahertz radiation for biological applications.
The quantum cascade laser (QCL) is one of the most promising radiation sources for imaging at terahertz frequencies. As with other lasers, QCLs exhibit self-mixing (SM), whereby re-injection of emitted radiation into the laser cavity affects the laser operating parameters. We can exploit this phenomenon for sensing purposes, using it to measure displacement, velocity, and fluid flow, as well as for coherent and incoherent imaging.
Here we present an imaging and tissue analysis scheme in the terahertz band that exploits the interferometric nature of coherent optical feedback in a terahertz QCL. SM interferometry is essentially a homodyne detection scheme, where the stable local oscillator signal (the QCL emission) is combined with the time-delayed version of itself. Figure 1 shows the basic structure and operating principles of our SM interferometer. The re-injected light interferes (‘ mixes’) with the intra-cavity electric field, causing small variations in the fundamental laser parameters, including the threshold gain, emitted power, lasing spectrum, and laser terminal voltage.2While optical feedback affects almost all laser parameters, the two we can most conveniently monitor are the emitted optical power and the voltage across the laser terminals. Of these, monitoring the laser terminal voltage is preferable as it obviates the need for an external terahertz detector.3The small voltage variation (referred to as the SM signal) depends on both the amplitude and phase of the electric field of the reflected laser beam. This results in a highly sensitive and compact sensing technique that can probe information about the complex reflectivity or refractive index of the external target.
Figure 1. Schematic diagram of the setup for tissue imaging experiments. PC DAQ: Data acquisition. Mod i/p: Modulation input. QCL: Quantum cascade laser.
We can create SM signals by exploiting temporal variations in optical length of the external cavity (through variation of its refractive index or the physical length), complex reflectivity of the target, or the laser frequency. The imaging process involves illuminating static objects with terahertz radiation. In other words, the objects being imaged are not changing during the signal acquisition. Therefore, we require some type of modulation to generate an SM signal. Here we opt for slow frequency modulation,4which results in an SM waveform imprinted with the complex refractive index of the target. The sensitivity of this scheme is discussed in detail elsewhere.5
We performed tissue imaging experiments using porcine tissue and murine skin. We first acquired a 2D array of time-domain waveforms and then reduced each waveform to a corresponding array of numbers, forming an image. Figure 2 shows results of the reflection-mode imaging of a 3mm-diameter skin biopsy from the transgenic laboratory mouse strain Cdk4::Tyr-NRAS.6, 7 The sample in Figure 2(a) shows a barely visible stage 1 melanoma lesion on the left hand side of the sample. Figure 2(b) shows a signal strength reduction of the terahertz waveforms, while Figure 2(c) shows a corresponding phase reduction from the array of SM waveforms. We can relate the tumor plaque in this sample to features observable in the pair of terahertz image reductions.
Figure 2. (a) Photograph of the 3mm diameter skin biopsy from the Cdk4::Tyr-NRAS sample. (b) Trimmed total variation, showing a signal strength reduction of the terahertz self-mixing waveforms. (c) Trimmed last peak position, showing a phase-like reduction of the terahertz self-mixing waveforms.
In summary, we developed a highly sensitive and compact scheme that shows promise for biological imaging in the terahertz frequency range. Our approach offers the possibility of early detection of skin changes before they become apparent in visible images. Our results also point toward techniques for the characterization of healthy tissue types for the study of normal physiology and possible therapeutic approaches. Additionally, imaging of skin malignancy in animal models at terahertz wavelengths shows earlier and potentially more powerful discrimination than is currently possible in the visible and IR regions of the spectrum.
Continuous improvements in terahertz QCL technology will lead to devices operating at higher temperatures and consequently more robust and compact self-mixing instruments for tissue characterization. In future work, we will investigate the origin of contrast in biological tissues based on water content and distribution, and we will assess the innate molecular response at terahertz frequencies. A clearer understanding of these factors will guide us towards discriminating a variety of skin pathologies.
This research was supported under the Australian Research Council's Discovery Projects funding scheme (DP 120 103703) and a Cancer Research UK Leeds Centre Development Fund Equipment award (Grant number C37059/A16369). We also acknowledge support of the ERC ‘NOTES’ and ‘TOSCA’ programs, the Royal Society, the Wolfson Foundation, and the European Cooperation in Science and Technology (COST) Action BM1205. Author Yah Leng Lim acknowledges support under the Queensland Government's Smart Futures Fellowships program.

Aleksandar D. Rakic, Karl Bertling, Yah Leng Lim, Stephen J. Wilson, Milan Nikolic, Thomas Taimre
School of Information Technology and Electrical Engineering, The University of Queensland
Brisbane, Australia
Dragan Indjin, Alexander Valavanis, Edmund H. Linfield, A. Giles Davies
University of Leeds
Leeds, UK
Graeme Walker, Blake Ferguson
Queensland Institute of Medical Research
Herston, Australia
Tarl W. Prow, Helmut Schaider, H. Peter Soyer
Translational Research Institute
Brisbane, Australia

1. C. Yu, S. Fan, Y. Sun, E. Pickwell-MacPherson, The potential of terahertz imaging for cancer diagnosis: A review of investigations to date, Quant. Imag. Med. Surg. 2(1), p. 33-45, 2012.
2. G. Giuliani, M. Norgia, S. Donati, T. Bosch, Laser diode self-mixing technique for sensing applications, J. Opt. A, Pure Appl. Opt. 4(6), p. S283-S294, 2002.
3. P. Dean, Y. L. Lim, A. Valavanis, R. Kliese, M. Nikolić, P. Suraj Khanna, M. Lachab, Terahertz imaging through self-mixing in a quantum cascade laser, Opt. Lett. 36(13), p. 2587-2589, 2011.
4. A. D. Rakić, D. Aleksandar, T. Taimre, K. Bertling, Y. L. Lim, P. Dean, D. Indjin, Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis, Opt. Express 21(19), p. 22194-22205, 2013.
5. T. Taimre, K. Bertling, Y. L. Lim, P. Dean, D. Indjin, A. D. Rakić, Methodology for materials analysis using swept-frequency feedback interferometry with terahertz frequency quantum cascade lasers, Opt. Express 22(15), p. 18633-18647, 2014.
6. E. M. Wurm, L.L. Lin, B. Ferguson, D. Lambie, T. W. Prow, G. J. Walker, H. P. Soyer, A blueprint for staging of murine melanocytic lesions based on the Cdk4(R24C/R24C)::Tyr-NRAS(Q)(61K) model, Exp. Dermatol. 21(9), p. 676-681, 2012.
7. E. Chai, B. Ferguson, T. Prow, P. Soyer, G. Walker, Three-dimensional modelling for estimation of nevus count and probability of nevus-melanoma progression in a murine model, Pigm. Cell Mel. Res. 27(2), p. 317-319, 2014.

Abstract-Terahertz electromagnetic wave generation and amplification by an electron beam in the elliptical plasma waveguides with dielectric rod

The propagation of electromagnetic waves in an elliptical plasma waveguide including stronglymagnetized plasma column and a dielectric rod is investigated. The dispersion relation of guided hybrid electromagnetic waves is obtained. Excitation of the waves by a thin annular relativistic elliptical electron beam will be studied. The time growth rate of electromagnetic waves is obtained. The effects of relative permittivity constant of dielectric rod, radius ofdielectric rod, accelerating voltage, and current density of the annular elliptical beam on the growth rate and the frequency spectra are numerically presented.

Wednesday, October 1, 2014

Abstract-Coulomb-driven terahertz-frequency intrinsic current oscillations in a double-barrier tunneling structure

O. Jonasson and I. Knezevic


We investigate time-dependent, room-temperature quantum electronic transport in GaAs/AlGaAs double-barrier tunneling structures (DBTSs). The open-boundary Wigner-Boltzmann transport equation is solved by the stochastic ensemble Monte Carlo technique, coupled with Poisson's equation and including electron scattering with phonons and ionized dopants. We observe well-resolved and persistent THz-frequency current density oscillations in uniformly-doped, dc-biased DBTSs at room temperature. We show that the origin of these intrinsic current oscillations is not consistent with previously proposed models, which predicted an oscillation frequency given by the average energy difference between the quasi-bound states localized in the emitter and main quantum wells. Instead, the current oscillations are driven by the long-range Coulomb interactions, with the oscillation frequency determined by the ratio of the charges stored in the emitter and main quantum wells. We discuss the tunability of the frequency by varying the doping density and profile.

Graduate Student Shang Hua Yang Received a IEEE Antennas and Propagation Society Doctoral Research Award


Shang Hua Yang has been selected to receive a Doctoral Research Award from the IEEE Antennas and Propagation Society for his research project, Three-Dimensional Plasmonic Photoconductive Antennas for High-Power Terahertz Generation.

Shang Hua is an electrical engineering Ph.D. student working with Prof. Mona Jarrahi at Terahertz Electronics Laboratory. His research is focused on designing plasmonic nanostructures to enhance efficiency of conventional photoconductive terahertz emitters. For his doctoral research, he has demonstrated the most efficient laser-driven terahertz radiation source.

About the Award: The IEEE Antennas and Propagation Society grants up to ten Ph.D Research Awards each year. The selection committee evaluates each applicant based on his or her research project, academic record, and potential to contribute to the electromagnetics profession in the future. The award consists of a $2500 fellowship.

Compact and broadly tunable semiconductor terahertz radiation sources

Difference-frequency generation in quantum cascade lasers enables room-temperature emission from 1 to THz.6
1 October 2014, SPIE Newsroom. DOI: 10.1117/2.1201409.005632
Terahertz radiation, with frequencies of 0.3–10THz, displays several unique characteristics. For instance, this radiation (also known as T-rays) can be transmitted through materials that block IR and visible light (e.g., plastics, ceramics, and paper), and can be used to identify chemical or biological substances that have strong terahertz absorption signatures. T-ray sensors have a number of potential applications in a variety of industries. They would be useful in the manufacturing industry for improved quality control and for the protection of soldiers and civilians from chemical/biological attacks in the defense industry. In addition, T-ray imagers could be used in the medical industry for dental cavity screening, or for the identification of cancerous skin and tissue. A major technological breakthrough, however, is still required to make these applications a reality. Terahertz radiation sources are bulky, complex in design and operation, and expensive to manufacture, but real-world spectroscopy and imaging applications require sources that are compact, cost-effective, that emit radiation with high spectral purity and high power, and that can be tuned over a wide wavelength range.
Purchase SPIE Field Guide to Optical Fiber TechnologySemiconductor terahertz sources are an appealing solution to this problem, given their compact size and ruggedness, simple electrical-powered operation, and propensity for mass production. Many commercial sources (e.g., multiplier chains, photomixers, and photoconductive switches), however, experience a rapid drop in terahertz power at frequencies greater than 2THz because of conversion efficiency and carrier velocity constraints. Quantum cascade lasers (QCLs) are a class of compact, semiconductor lasers that are made up of hundreds of repetitions of quantum wells and barriers. The processing steps and materials that are used in their construction are almost identical to telecommunication diode lasers. QCLs can therefore be mass-produced in existing telecom diode foundries.1 Traditional terahertz QCLs, however, can only operate at cryogenic temperatures, and they have inherently small tuning bandwidths.
We have developed room-temperature electrically pumped sources that operate in the 1–6THz range using QCLs.1–7 In our approach, we leverage the performance features (i.e., the ability to operate with high power at room temperature) of mid-IR QCLs (with wavelengths of 5–12μm). We use difference-frequency generation (DFG) in mid-IR QCLs on indium phosphide (InP) substrates to create the room-temperature terahertz sources.1–7Terahertz DFG is a nonlinear optical process, in which two light beams (at two different frequencies, ω1 and ω2) interact with a nonlinear optical medium to generate radiation at a frequency equal to the difference between ω1 and ω2 (see Figure 1).2,3
Figure 1. A Cherenkov emission waveguide scheme for a difference-frequency-generated (DFG) quantum cascade laser (QCL) device. The active region is blue, the mid-IR pumps are purple, the indium phosphide substrate is light gray, and the terahertz waves are shown in red. Two mid-IR beams with frequencies ω1and ω2 interact in the nonlinear optical medium to generate radiation at terahertz frequencies (ωTHz). θC: Angle of Cherenkov emission, with respect to the propagation direction of the mid-IR pumps.
We have designed the active regions of our terahertz DFG-QCL sources to support lasing at two mid-IR frequencies separated by as much as 6THz, and to lase with a wavelength of about 9μm. We have also quantum-engineered the laser active region to have giant optical nonlinearity that is two to four orders of magnitude larger than more commonly used nonlinear crystals. We achieved this by tailoring the energy separation of the confined quantum well states, which are involved in the nonlinear interaction, to coincide with the energy of the mid-IR pumps.2, 5 In our sources we use a Cherenkov emission waveguide configuration that we have adapted for terahertz DFG-QCL technology.4 The Cherenkov emission is characterized by nonlinear material radiating terahertz waves into the device substrate at an angle to the propagation direction of the mid-IR pumps (see Figure 1). This scheme is well suited for broadband terahertz output, given that the phase-matching conditions can be satisfied from 1 to 6THz. We grow our devices on a semi-insulating InP substrate that satisfies the Cherenkov phase matching requirements and has a low terahertz absorption loss.4 We are able to couple the T-rays into free-space via a mechanically polished substrate output facet (see Figure 1).
Using our novel methodology, we fabricated sources into 22μm-wide ridge waveguides and cleaved the wafer into 2–3mm-long lasers. We realized single-frequency terahertz tuning by defining a monolithic diffraction grating in the device's upper waveguide layer that fixes one mid-IR pump at a particular frequency. We subsequently mounted a laser in an external cavity system—as shown in Figure 2(a)—with an external diffraction grating, which provided feedback for the second mid-IR pump. We can rotate an external diffraction grating to tune the second mid-IR pump frequency. The terahertz output from the device follows the difference between the two mid-IR pump frequencies. Our source is biased with 50ns-wide pulse widths and a 50kHz repetition rate. We have demonstrated that this room-temperature external-cavity tuned system5, 7 can provide continuously-tunable terahertz output in the 1.4–5.9THz range, with a 90μW peak power output. Terahertz emission spectra for different grating positions are shown in Figure 2(b). A record 4.5THz tuning bandwidth has been achieved with these devices.7
Figure 2. Illustrating the external-cavity terahertz tuning system. (a) A schematic of the system. (b) A typical terahertz tuning spectrum from the system. (c) Photograph of the prototype system. DFB: Distributed feedback.
More recently, we demonstrated monolithic electrically tunable terahertz emission6 by writing two separate distributed gratings in the device waveguide. The front grating operates as a distributed feedback (DFB) grating, while the back grating operates as a distributed Bragg reflector (DBR) grating. These gratings enable us to fix mid-IR pumps at two different frequencies, as shown in Figure 3(a). We etched a trench between each grating section to electrically isolate them for independent mid-IR wavelength tuning. The QCL is pulsed-biased through the front DFB section, and an additional direct current bias can be applied to each grating section, to thermally tune their respective mid-IR pumps. Emission spectra from a device that tunes from 3.44–4.02THz is shown in Figure 3(b). The tuning range of 580GHz (3.44–4.02THz) is the largest single-mode monolithic tuning that has yet been demonstrated using a semiconductor source in this frequency range.
Figure 3. Illustrating the monolithic tunable device. (a) A schematic diagram of the device. (b) A typical terahertz tuning spectrum from the device. (c) Photograph of the terahertz DFG-QCL chip that contains several tunable laser sources. DC: Direct current.
We have developed a new approach to the design of terahertz radiation sources. Our devices provide, by far, the largest tuning bandwidth for terahertz semiconductor sources, and they can be produced in mass quantities at existing diode laser foundries. By combining our devices with pyroelectric or heterodyne-based terahertz detectors, we envision the development of highly compact semiconductor-based terahertz systems for chemical and biological sensing, spectroscopy, and materials inspection applications. In our ongoing research we are investigating ways to further increase the power output of our devices. These include improving the terahertz outcoupling (we estimate that nearly 90% of the generated terahertz radiation currently gets trapped inside the QCLs), and implementing active regions with stronger optical nonlinearity.

Karun Vijayraghavan, Yifan Jiang, Seungyong Jung, Mikhail Belkin
The University of Texas at Austin
Department of Electrical and Computer Engineering, TX
Karun Vijayraghavan is completing his PhD in electrical engineering in Mikhail Belkin's group, and is looking to work on the commercialization of terahertz DFG-QCL technology.
Yifan Jiang is a PhD student in Mikhail Belkin's group, where she works on the development of high-performance broadly tunable terahertz DFG-QCL sources.
Seungyong Jung has been a postdoctoral fellow in Mikhail Belkin's group since 2013. His research interests include mid-IR and terahertz optoelectronic devices and their applications.
Mikhail Belkin is an associate professor whose research interests are focused on mid-IR and terahertz photonics, nanospectroscopy, and nonlinear optics.

1. K. Vijayraghavan, M. Jang, A. Jiang, X. Wang, M. Troccoli, M. A. Belkin, THz difference-frequency generation in MOVPE-grown quantum cascade lasers, Photon. Technol. Lett.26, p. 391-394, 2014.
2. M. A. Belkin, F. Capasso, A. Belyanin, D. L. Sivco, A. Y. Cho, D. C. Oakley, C. J. Vineis, G. W. Turner, Terahertz quantum-cascade-laser source based on intracavity difference-frequency generation, Nat. Photon. 1, p. 288-292, 2007.
3. M. A. Belkin, F. Capasso, F. Xie, A. Belyanin, M. Fischer, A. Wittmann, J. Faist, Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation, Appl. Phys. Lett. 92, p. 201101, 2008. doi:10.1063/1.2919051
4. K. Vijayraghavan, R. W. Adams, A. Vizbaras, M. Jang, C. Grasse, G. Boehm, M. C. Amann, M. A. Belkin, Terahertz sources based on Čerenkov difference-frequency generation in quantum cascade lasers, Appl. Phys. Lett. 100, p. 251104, 2012.doi:10.1063/1.4729042
5. K. Vijayraghavan, Y. Jiang, M. Jang, A. Jiang, K. Choutagunta, A. Vizbaras, F. Demmerle, G. Boehm, M. C. Amann, M. A. Belkin, Broadly tunable terahertz generation in mid-infrared quantum cascade lasers, Nat. Commun. 4, p. 2021, 2013.doi:10.1038/ncomms3021
6. S. Jung, A. Jiang, Y. Jiang, K. Vijayraghavan, X. Wang, M. Troccoli, M. Belkin, Broadly tunable monolithic room temperature terahertz quantum cascade laser sources, Nat. Commun. 5, p. 4267, 2013. doi:10.1038/ncomms5267
7. Y. Jiang, K. Vijayraghavan, S. Jung, F. Demmerle, G. Boehm, M. C. Amann, M. A. Belkin, External cavity terahertz quantum cascade laser sources based on intra-cavity frequency mixing with 1.2-5.9THz tuning range, J. Opt. 16, p. 094002, 2014. doi:10.1088/2040-8978/16/9/094002