|Acquisition Program: || Objective: ||To develop and demonstrate a compact, portable, widely-tunable, and narrowband terahertz (THz) emitter, based on frequency mixing of two lasing beams in a nonlinear crystal being placed inside a dual-frequency solid-state laser cavity.
|| Description: ||THz spectroscopy holds promise in the identifications and detections of biological and chemical species . However, such applications have been impeded primarily by the lack of a miniature, portable, coherent, high-power, and widely-tunable THz source [2-5]. It was demonstrated in the past that among all the schemes for THz generation, frequency mixing of two laser beams in a nonlinear crystal can be quite efficient . However, since the lasers emitting the two frequencies are bulky, the corresponding THz source is not portable. Recently, a single-mode ultra-compact Nd:YAG microchip laser has produced ultra-high peak powers . However, since a Nd:YAG laser has a rather narrow lasing bandwidth, it cannot be used to produce two different frequencies necessary for the generation of THz waves in a nonlinear crystal. Other solid-state lasers such as those based on a Nd:LSB laser crystal can generate lasing frequencies tunable within a broad frequency range [8-9]. Recently, a CW dual-frequency Nd:LSB microchip laser was implemented and used to generate THz waves . In order to significantly scale up THz output powers, nanosecond laser pulses should be used. In addition, frequency mixing inside a dual-frequency solid-state laser cavity (i.e. intracavity frequency mixing) can dramatically improve the THz output powers. Since the THz output peak power based on frequency mixing is proportional to the product of the input peak powers for the two mixing beams, nanosecond laser pulses and intracavity frequency mixing can be employed to increase the THz output peak powers by four orders of magnitude. As a result, the average THz output powers are expected to approach the mW level. Moreover, novel structures such as plasmonic metallic gratings  and stacks  can be incorporated to modify dispersion of a nonlinear crystal, spatially confine THz waves, and utilize quasi-phase-matching, and therefore, to further improve the conversion efficiency for the THz generation. Therefore, a research and development effort is expected that will utilize nonlinear mixing and a dual-frequency solid-state cavity to demonstrate significantly enhanced THz emission performance and to demonstrate the effectiveness of the source technology within a military-relevant sensing and/or monitoring application.
|| ||PHASE I: In Phase I, laser devices should designed, fabricated, and tested that are capable of CW dual-frequency and transform-limited Q-switched dual-frequency operation. Here, laser technologies should be investigated that are amenable to significantly up scaling the output power at terahertz frequencies. Note that lasers based upon Nd:LSB crystals are one potential candidate but other alternatives that offer the same or superior performance are acceptable. Detailed feasibility studies on intracavity THz generation based on the performances of the CW and Q-switched laser technologies investigated above should be executed and the results should be used to construct a development plan for a military-relevant sensors demonstration.
|| ||PHASE II: In Phase II, the methodology of THz generation inside the cavities of both CW and Q-switched dual-frequency lasers should be achieved, followed by the characterization and optimization of the THz emitter. Novel configurations and structures for achieving THz generation and enhancement such as total internal reflection, non-collinear propagation, backward propagation, plasmonic metallic gratings, and cavity should be investigated. By the end of Phase II, the first-generation electrically-driven miniature and monolithic THz emitter operating at room temperature should be demonstrated that achieve (or closely approach) the following operational capabilities: an average output power of 1 mW, a peak output power of 20 W; a continuously frequency-tuning range of 150 GHz – 1 THz (5-33.3 cm-1); a linewidth of 100 MHz; a repetition rate of 10 kHz; and, a system dimensional footprint of 10"×6"×4" or smaller. The Phase II effort should also include a sensing/monitoring demonstration that illustrates the advantages of the technology for a military-relevant application, and the preference is for this activity to be connected to a U.S. Army research laboratory or center.
|| ||PHASE III: The resulting THz source technology for be useful in the development of compact THz sensor systems for such military and private sector applications as sensing and monitoring of biological species, explosives, and hazardous chemicals as well as for nondestructive evaluation and medical diagnostics. The proposed technology development would also find dual-use applications in other highly specialized areas such as providing for ultra wideband communication capabilities for short-range, covert and/or space-based communications, and providing components for ultra fast/high-frequency data processing and computation.
|| References: ||
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8. W. K. Jang, T. Taira, Y. Sato, and Y. M. Yu, “Laser emission under 4F5/2 and 4F3/2 pumping in Nd:LSB micro-laser,” Jap. J. Appl. Phys. 43, L70-L72 (2004).
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11. Q. Gan, Z. Fu, Y. J. Ding, and F. J. Bartoli, “Ultra-wideband slow light system based on THz plasmonic graded metallic grating structures,” Phys. Rev. Lett. 100, 256803/1-4 (2008).
12. Y. Jiang, Y. J. Ding, and I. B. Zotova, “Power scaling of coherent terahertz pulses by stacking GaAs wafers,” Appl. Phys. Lett. 93, 241102/1-3 (2008).
|Keywords: ||Terahertz, Nd:LSB laser, dual-frequency laser, intracavity, frequency mixing, spectroscopy, biological, chemical, explosive, nondestructive evaluation, medical diagnostics, manufacturing materials|