Using Three Incommensurate Wave Components to Generate Terahertz Radiation
Faculty Mentor: Jeremy Johnson, Department of Chemistry and Biochemistry
Electromagnetic radiation is a wave composed of an electric field and a magnetic field.
Examples of electromagnetic radiation include visible light, ultraviolet light, Xrays,
infrared light, microwaves, and more. Electromagnetic radiation is an indispensable tool for both
controlling and studying matter and is applied to countless fields such as electronics
manufacturing and medical imaging. Over the past two decades terahertz (THz) radiation, a form
of electromagnetic radiation, has become an intense area of research. This surge in activity in
due in part to technological advances in generating THz radiation, but is also due to the myriad
of potential applications of THz radiation. For example, THz radiation has current and proposed
applications in security imaging, 1 biomolecule identification, 2 cancer and tumor imaging, 3,4 and
semiconductor and material characterization. 5,6 Furthermore, THz radiation is completely
harmless to biological tissue. However, THz technology and application is limited by the ability
to produce, control, and detect THz radiation. 7,8 Our experiment involves producing THz
radiation and may provide interesting insights into the mechanism of THz generation.
One method of creating THz radiation requires three conditions: i) A laser must be focused to
create a plasma filament, a gas of free electrons which have been separated from the nuclei of
atoms in the air, ii) the laser must consist of two short pulses that simultaneously create the
plasma, and iii) the two pulses must each have commensurate wavelengths, e.g., the wavelength
of one pulse is half the wavelength of the other pulse. This method of generating THz is well
established, and it has been shown that deviating from any of these conditions entirely quenches
the output of THz radiation.
However, we have shown in our experiments that we can deviate from the third condition
(commensurate wavelengths) if we add a third pulse. In this experiment two of the pulses had
wavelengths of 800 nm and 1300 nm, and the third pulse had a range of wavelengths. Figure 1
displays the results of this experiment compared with only using two pulses (data from
Vvendenski et al. 9 ). The red markers and the corresponding black Gaussian curve are
Vvendenski et al.’s results. Vvendenski et al. used an 800 nm pulse and varied the wavelength of
the second pulse. As they deviated from 1600 nm (the commensurate wavelength) in the second
pulse, the output of THz radiation is quickly quenched. Our results (blue markers) are roughly
constant from 1300 nm to 2000 nm, the entire range that we tested. (note: Our laser is less
powerful above 1800 nm which may explain the slight decrease in output.) This demonstrates
that generating THz radiation does not require commensurate wavelengths when three input
pulses are used.
To further demonstrate that three incommensurate pulses can create THz radiation we conducted
experiments using three pulses of incommensurate wavelengths (e.g. 800 nm, 1300 nm, and
1800 nm), but controlled how strong each individual pulse was. When all three pulses were
present we observed strong THz radiation output, and if any of the three pulses were absent THz
radiation was not produced. However, we further observed that if two of the pulses were strong
and one of the pulses was weak then THz radiation was weakly produced. This further suggests
that three pulses of incommensurate wavelengths produce THz radiation.
In summary, we have demonstrated a new condition that produces THz radiation: three pulses
(regardless of wavelength) focused simultaneously to create a plasma filament. This contrasts
with the well established method of creating THz radiation, which requires commensurate
wavelengths. These findings have a twofold contribution to science and technology: i) in some
settings generating THz radiation with this method may be more convenient, allowing
industrialization of THz technology, and ii) these findings may further inform theories of the
mechanism of generating THz radiation. This progress contributes to society by furthering the
potential applications of THz technology in fields such as security, medicine, and electronics.
1. J. F. Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, and D. Zimdars, “THz imaging and sensing for security
applications–explosives, weapons and drugs,” Semicond. Sci. Technol. 20(7), S266 (2005).
2. M. Brucherseifer, M. Nagel, P. H. Bolivar, and H. Kurz, “Labelfree
probing of the binding state of DNA by timedomain
sensing,” Appl. Phy. Lett. 77(24), 4049 (2000).
3. A. J. Fitzgerald, V. P. Wallace, M. JimenezLinan,
L. Bobrow, R. J. Pye, A. D. Purushotham, and D. D. Arnone, “Terahertz Pulsed
Imaging of Human Breast Tumors,” Radiology 239(2), 533 (2006).
4. P. C. Asworth, E. PickwellMachPherson,
E. Provenzano, S. E. Pinder, A. D. Purushotham, M. Pepper, and V. P. Wallace, “Terahertz
pulsed spectroscopy of freshly excised human breast cancer,” Opt. Express 17(15), 12444 (2009).
5. A. Krotkus, “Semiconductors for terahertz photonics applications,” J. Phys. D: Appl. Phys. 43(27), 273001 (2010).
6. B. Ferguson and X. Zhang, “Materials for terahertz science and technology,” Nat. Mater. 1(1), 26 (2002).
7. L. Li, L. Chen, J. Zhu, J. Freeman, P. Dean, A. Valavanis, A. G. Davies, and E. H. Linfield, “Terahertz quantum cascade lasers with
>1 W output powers,” Electron. Lett. 50(4), 309 (2014).
8. K. Peng, P. Parkinson, L. Fu, Q. Gao, N. Jiang, Y.H.
Guo, F. Wang, H. J. Joyce, J. L. Boland, H. H. Tan, C. Jagadish, and M. B.
Johnston, “Single Nanowire Photoconductive Terahertz Detectors,” Nano Lett. 15(1), 206 (2015).
9. N. V. Vvendenski, A. I. Korytin, V. A. Kostin, A. A. Murzanev, A. A. Silaev, and A. N. Stepanov, “TwoColor
Gneration of Terahertz Radiation Using a FrequencyTunable
Half Harmonic of a Femtosecond Pulse,” Phys. Rev. Lett. 112(5),