The terahertz region of the electromagnetic spectrum, spanning from 100 GHz through 10 THz, is of increasing importance for a wide range of scientific, military and commercial applications. This interest is spurred by the unique properties of this spectral band and the very recent development of convenient terahertz sources and detectors. However, the terahertz band is also extremely challenging, in large part because it spans the transition from traditional electronics to photonics. This paper reviews the importance of this frequency band and summarizes the efforts of scientists and engineers to span the "terahertz technology gap." The emphasis is on solid-state circuits that use nonlinear diodes to translate the functionality of microwave technology to much higher frequencies.

The spectral performance of freestanding resonant metal-mesh bandpass filters operating with center frequencies ranging from 585 GHz to 2.1 THz is presented. These filters are made up of a 12-µm-thick copper film with an array of cross-shaped apertures that fill a circular area with a 50-mm diameter. The filters exhibit power transmission in the range 97-100% at their respective center frequencies and stop-band rejection in excess of 18 dB. The theoretically predicted nondiffracting properties of the meshes are experimentally verified through high-resolution beam mapping. Scalability of the filter spectra with mesh dimensions is demonstrated over a wide spectral range. Several modeling methods are considered, and results from the models are shown.

No abstract

The terahertz frequency absorption spectraof DNA molecules reflect low-frequencyinternal helical vibrations involvingrigidly bound subgroups that are connectedby the weakest bonds, including thehydrogen bonds of the DNA base pairs,and/or non-bonded interactions. Althoughnumerous difficulties make the directidentification of terahertz phonon modes inbiological materials very challenging, ourresearch has shown that such measurementsare both possible and fruitful. Spectra ofdifferent DNA samples reveal a large numberof modes and a reasonable level ofsequence-specific uniqueness. In an attemptto show that the long wavelength absorptionfeatures are intrinsic properties ofbiological materials determined by phononmodes, a normal mode analysis has been usedto predict the absorption spectra ofpolynucleotide RNA Poly[G]-Poly[C]. Directcomparison demonstrated a correlationbetween calculated and experimentallyobserved spectra of the RNA polymers, thusconfirming that the fundamental physicalnature of the observed resonance structureis caused by the internal vibration modesin the macromolecules.In this work we demonstrate results fromFourier-Transform Infrared (FTIR)spectroscopy of DNA macromolecules andrelated biological materials in theterahertz frequency range. Carefulattention was paid to the possibility ofinterference or etalon effects in thesamples, and phenomena were clearlydifferentiated from the actual phononmodes. In addition, we studied thedependence of transmission spectra ofaligned DNA and polynucleotide film sampleson molecule orientation relative to theelectromagnetic field, showing the expectedchange in mode strength as a function ofsample orientation. Further, the absorptioncharacteristics were extracted from thetransmission data using the interferencespectroscopy technique, and a stronganisotropy of terahertz characteristics wasdemonstrated.

No abstract

A detailed investigation of phonon modes in DNA macromolecules is presented. This work presents experimental evidence to confirm the presence of multiple dielectric resonances in the submillimeter-wave spectra (i.e., approximately 0.01-10 THz) obtained from DNA samples. These long-wave (i.e., approximately 1-30 cm(-1)) absorption features are shown to be intrinsic properties of the particular DNA sequence under study. Most importantly, a direct comparison of spectra between different DNA samples reveals a large number of modes and a reasonable level of sequence-specific uniqueness. This work establishes the initial foundation for the future use of submillimeter-wave spectroscopy in the identification and characterization of DNA macromolecules.

Significant progress has been achieved during the last several years relating to experimental and theoretical aspects of terahertz (or submillimetre wave) Fourier transform spectroscopy of biological macromolecules. However, previous research in this spectral range has been focused on bio-materials in solid state since it was common opinion that high water absorption will obscure the spectral signatures of the bio-molecules in solutions. At the same time, the biological functions of DNA and proteins take place in water solutions. In this work, the spectra of DNA samples have been measured in liquid phase (gel) over the spectral range 10–25 cm−1 and compared with spectra obtained from solid films. The results demonstrate that there is very little interference between the spectral features of the material under test and the water background except for the band around 18.6 cm−1. Multiple resonances due to low frequency vibrational modes within biological macromolecules in solutions are unambiguously demonstrated. Higher level of sensitivity and higher sharpness of vibrational modes are observed in the liquid environment in comparison with the solid phase, with the width of spectral lines 0.3–0.5 cm−1. Gel sample spectra are found to be polarization-dependent. The ability of THz spectroscopy to characterize samples in liquid phase could be very important since it permits examination of DNA interactions in real (wet) samples. One demonstrated example of practical importance is the ability to discriminate between spectral patterns for native and denaturated DNA.

No abstract