Abram Young

Terahertz characterization of multi-walled carbon nanotube films

Multi-walled carbon nanotube films are characterized using terahertz time-domain spectroscopy. Both transmission and reflection experiments are performed in order to measure both the complex refractive index and the wave impedance. This method allows simultaneous extraction of both the permittivity (e=e′−ie″) and permeability (μ=μ′−iμ″) without any assumptions. Experimental results are obtained from 50 to 370 GHz and compared well with the microwave data (8–50 GHz) of the same sample measured using a vector network analyzer. The measured complex permittivity can be fitted with a Drude–Lorentz model in the 8–370 GHz frequency range.

THz Thermal Radiation Enhancement Using an Electromagnetic Crystal

Thermal radiation in the terahertz (THz) range only occupies a tiny portion of the whole blackbody power spectrum at room temperature. We demonstrate that a thermal radiator, which is constructed from an electromagnetic (EM) crystal, can be designed so that its photon density of states (DOS) is enhanced in the THz frequency range. We also demonstrate, as a consequence, that this source may lead to large enhancements of the radiated power over the values associated with normal blackbody radiation at those frequencies. The THz thermal radiation enhancement effects of various EM crystals, including both silicon and tungsten woodpile structures and a cubic photonic cavity (CPC) array, are explored. The DOS of the woodpile structures and the CPC array are calculated, and their thermal radiation intensities are predicted numerically. These simulations show that the radiated power can be enhanced by a factor of 11.8 around 364 GHz and 2.6 around 406 GHz, respectively, for the silicon and tungsten woodpile structures in comparison to the normal blackbody radiation values at those frequencies. It is also shown that an enhancement factor of more than 100 may be obtained by using the CPC array. A silicon woodpile EM crystal with a band gap around 200 GHz was designed and fabricated. The transmission property of this woodpile structure was verified using the THz time-domain spectroscopy (TDS). Thermal emissions from the fabricated silicon woodpile and a control blackbody sample were measured. Enhancements of the woodpile source radiation over the blackbody were observed at several frequencies which are consistent with the theoretical predictions.

Heterodyne Detection of Intracavity Generated Terahertz Radiation

Heterodyne detection is used to characterize the terahertz (THz) emission of a novel room-temperature continuous wave source based on difference frequency generation within the cavity of a dual-color vertical external cavity surface emitting laser. Employing the high intracavity intensities allows for the generation of mW powers in a wide frequency range within the terahertz spectrum. Experimental results of heterodyne detection are presented for the emission frequencies of 820 GHz and 1.9 THz using Schottky and hot electron bolometer mixers. Simultaneous emission of multiple narrow-line THz frequency components is observed.

Terahertz generation by difference frequency conversion of two single-frequency VECSELs in an external resonance cavity

We demonstrate a continuous wave, single frequency terahertz (THz) source based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency VECSELs with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and are used as pump sources for the nonlinear down conversion. The emitting THz radiation is centered at 1.9 THz and has a linewidth of less than 100 kHz. The output power of the source exceeds 100 μW. We show that the THz source can be used as local oscillator to drive a receiver used in astronomy applications.

THz thermal radiation enhancement using electromagnetic crystals

In this paper, a new idea on thermal radiation based THz source utilizing EM crystals (also known as electromagnetic band gap structures, or EBG) is introduced. Preliminary theoretical and experimental results are reported and give promising indications that carefully designed EM crystal thermal radiators may be useful in thermal imaging and identification applications and enable a new type of low cost and high performance THz source.

Terahertz traveling wave tube amplifiers as high-power local oscillators for large heterodyne receiver arrays

The size of existing and projected submillimeter heterodyne receiver arrays is rapidly increasing. As receiver arrays grow ever larger, the local oscillator power they require increases as well. We have developed Terahertz (THz) Traveling Wave Tube Amplifiers (TWTA) that promise to provide more than enough power in the 200 to 700 GHz frequency range to pump the largest arrays being planned for submillimeter telescopes. This technology combines revolutionary carbon nanotube cathodes and electron gun design with unique software modeling and micro-fabrication capabilities. We review key enabling technologies that make this breakthrough possible, present the design, realization, computer models and preliminary results of the THz TWT we have fabricated at 220 and 350 GHz

Investigation of Terahertz (THz) electromagnetic band gap structures

Three important EBG structures with low to high dimensions have been demonstrated at THz. Designed and fabricated samples are characterized by different methods. Agreements between experiments and EM simulations are good. The success of this work proves the feasibility of high-quality THz EBG structure fabrications and accurate characterizations.

IF system design for the Galactic/Extragalactic ULDB Spectroscopic Terahertz Observatory (GUSTO)

We present the design, and prototype phases of the intermediate frequency (IF) system for the upcoming balloon borne observatory, Galactic/Extragalactic Ultra-Long Duration Balloon (ULDB) Spectroscopic Terahertz Ob- servatory (GUSTO). GUSTO is a multi-organizational project whose goal is to address several key unanswered questions concerning all of the phases of the stellar life cycle within the Interstellar Medium (ISM). Using the NASA ULDB system for its platform, GUSTO will employ on-the-fly mapping techniques to scan a total of 124 square degrees of the Milky Way and Large Magellanic Cloud (LMC). GUSTO will survey the three brightest cooling lines in the Milky Way and the LMC. These lines are [CII], [OI], and [NII] corresponding to the three wavelengths of 158, 63, and 205 micron respectively. The completed survey will provide higher angular, and velocity resolution than that of previous surveys of [CII], [OI], and [NII]. These lines will be measured using three 8-pixel heterodyne arrays, each one dedicated to an individual cooling line, and all working together to make a 24-pixel focal plane. The GUSTO IF system is being designed to operate at low power consumption and high sensitivity all in a compact and lightweight package. The IF system will include a wideband 0.3 - 5 GHz, cryogenic, low noise amplifier (LNA), which will boost the IF output of a superconducting hot electron bolometer (HEB) mixer. The LNA was designed with commercial, off the shelf SiGe heterojunction bipolar transistors, and surface mount passive components. The LNA design has been optimized for low power consumption, and for sensitivity. The input impedance of the LNA is matched to the output impedance of the mixer over a wide range of frequencies to reduce reflections, and standing waves. Warm IF electronics have also been designed using commercial, off the shelf, surface mount SiGe transistors in order to achieve a high, and at gain (>50dB) over the entire bandwidth. These components provide variable gain and deliver an optimum signal level to the analog to digital converter of the backend spectrometer. The warm IF components were optimized for wide bandwidth, low power consumption, as well as reliability, and fit in a compact package. Commercially fabricated custom flexible printed circuit boards are being used for multi-channel stripline-based transmission lines, instead of the traditional stainless steel cryogenic semi-rigid coaxial cables. Replacing coaxial cables with the flexible printed circuit boards allows us to transmit through up to 16 lines on a single flex circuit, without losing performance, and furthering the design goal of providing a compact/lightweight solution. Each of the components used in this IF system will undergo rigorous qualification testing, and documentation in accordance with a NASA Class-D balloon mission. We discuss the design challenges in adapting cryogenic, and warm IF electronics to operate for an ultra long duration balloon mission.

Cryogenics on the stratospheric terahertz observatory (STO)

The Stratospheric TeraHertz Observatory (STO) is a NASA funded, Long Duration Balloon experiment designed to address a key problem in modern astrophysics: understanding the Life Cycle of the Interstellar Medium. STO surveys a section of the Galactic plane in the dominant interstellar cooling line at 1.9 THz and the important star formation tracer at 1.46 THz, at ~1 arc minute angular resolution, sufficient to spatially resolve atomic, ionic, and molecular clouds at 10 kpc. The STO instrument package uses a liquid helium cryostat to maintain the THz receiver at < 9 K and to cool the low noise amplifiers to < 20 K. The first STO mission (STO-1) flew in January of 2012 and the second mission (STO-2) is planned for December 2015. For the STO-2 flight a cryocooler will be added to extend the mission lifetime. This paper discusses the integration of the STO instrument into an existing cryostat and the cryogenic aspects of the launch and operation of the STO balloon mission in the challenging Antarctic environment.

Intracavity generation of continuous wave terahertz radiation

We present a terahertz source based on difference frequency generation within a laser cavity. Combining the high intracavity intensities of a dual-color vertical external cavity surface emitting laser (VECSEL) with the high nonlinear coefficient of a periodically poled lithium niobate crystal enables the generation of milliwatts of continuous wave terahertz radiation. As the frequency spacing between the two simultaneously oscillating laser lines can be adjusted freely, the entire range of the terahertz gap can be covered. We discuss different approaches for the wavelength control of the dual-color laser sources as well as emission characteristics of the nonlinear crystal. Exemplarily, we chose the frequencies 1.9 THz to characterize the source in term of the beam shape, the linewidth and power scalability. To investigate the emitted THz spectrum, heterodyne detection is employed.