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THz Air-Breakdown-Coherent-Detection (THz ABCD)


The most significant contribution of Prof. Zhang’s THz air photonic research has been the demonstration of a high field, broadband THz spectrometer which uses air as the THz wave emitter and sensor. A focused optical pulse creates an atmospheric plasma, which produces very strong (105 V/cm) and ultra-broadband THz waves (0.1 to 10 THz) in the far field. Using the air-biased-coherent-detection (ABCD) method, the air can also detect pulsed THz waves through the reciprocal process of THz wave generation. This is a significant development, since it was the first time a coherent electro-optic detector covered the entire THz gap. Prof. Zhang’s group systematically studied the use of air and selected gases for THz wave generation and detection, and a full description using a quantum model was developed.

In recent decades, the field of THz photonics (the generation and detection of terahertz waves using lasers) has relied on solid-state materials, since they provide the requisite, large optical nonlinearities or appropriate band gaps. However, these materials tend to have strong absorption modes and dispersion in the THz range, limiting the reachable wavelengths of radiation. In addition, as the intensities achieved by the lasers used in the experiments increase, solid-state emitters are limited by their damage thresholds, effectively limiting the efficiency of the nonlinear THz generation effects. Gases, however, offer a way around these problems, since they have much lower dispersion and absorption and are easily replaceable after they are “damaged” (i.e. ionized) by the laser.

NSF funded IGERT trainee Nick Karpowicz and researchers including IGERT faculty Xi-Cheng Zhang at Rensselaer Polytechnic Institute have been working on the understanding, design and construction of a THz air-biased-coherent-detection (THz ABCD). THz waves can be detected coherently in gases using a novel heterodyne technique, providing the full spectrum and phase of the THz radiation. This is already being put to use in our lab in numerous investigations by multiple researchers. Nick also described the method of generating terahertz pulses in gases, and was the first to use a fully quantum-mechanical model to solve the laser-atom interaction and determine how the excitation of an atom in a gas can result in an asymmetric ionization process that yields an intense THz transient and a set of free-electron wave packets.

In the June 13, 2008 issue of Science an article entitled, “New Efforts to Detect Explosives Require Advances on Many Fronts”, featured Prof. Zhang’s work using THz-ABCD spectroscopy to exploit THz properties and detect explosives from distances of 100 meters or more.

Figure 1 plots THz temporal waveform generated and detected by using air with THz ABCD method. The spectrum covers the entire THz gap (0.3 THz to 10 THz).

Address Goals

The rapid advancement in the generation and detection of terahertz (THz) radiation in recent years has opened new doorways to both scientific discovery and new technologies to fulfill some of society’s most urgent needs. Nick Karpowicz’s research on “Physics and Utilization of Terahertz Gas Photonics” represents a great step forward in both the capabilities of terahertz measurements and the fundamental understanding of the physical processes involved. It presents THz systems with unprecedented continuous bandwidths, finally bridging the entire “THz gap” with a single measurement. Simultaneously, a rigorous physical model is constructed that describes how the THz waves are generated and detected.

In performing the research behind his thesis, Nick demonstrated a strong commitment to thoroughness and the pursuit of knowledge, in keeping with the best of Rensselaer’s traditions. In order to better understand the THz generation process, he independently designed and built a gas jet system, which allowed for a much cleaner measurement. In these results, a curious “wiggle” was visible after the terahertz pulse, which could not be explained by any of the existing theories. While it would have been easy to ignore this or hand-wave it away, he spent months writing and implementing a first-principles solution of the time-dependent Schrödinger equation to accurately describe the interaction. These results became a central part of his thesis and that “wiggle” turned out to be the echo described in his recent publication in the prestigious journal Physical Review Letters, “Coherent Terahertz Echo of Tunnel Ionization in Gases,” which provides new insight not just into terahertz technology, but the formation of a laser-induced plasma in a gas, as well. Recently, editors from three international journals have invited Nick to submit invited papers. Led by Dr. Jianming Dai, Nick and Xi-Cheng Zhang’ recently submitted a paper to Physics Review Letters on “Coherent Polarization Control of Terahertz Waves Generated from Two-Color Laser-Induced Gas Plasma”.

The fundamental research on THz ABCD has led a technology innovation. Dr. Zhang’s team developed this method, and transferred the design to a small company, Zomega THz Corp. Recently, NSF awarded SBIR Phase I (2008) and Phase II (2009) to Zomega to build a compact THz ABCD spectrometer.