Terahertz Generation in 3-Nitroaniline Single Crystals
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3882–3884
V. Krishnakumar* and R. Nagalakshmi Department of Physics, Periyar UniVersity, Salem-636 011, India ReceiVed June 16, 2007; ReVised Manuscript ReceiVed July 25, 2008
ABSTRACT: Currently, there is substantial activity of research in the development of the new sources in the terahertz (1 THz ) 1 × 1012 Hz) region of the electromagnetic spectrum. Coherent tunable terahertz waves have great potential for frequency domain spectroscopy and THz imaging applications. We report for the first time the terahertz radiation generated in the renowned organic nonlinear optical crystal, 3-nitroaniline also called as m-nitroaniline (m-NA) using ultra short femtosecond laser. The terahertz (1 THz ) 1 × 1012 Hz) frequency range, usually defined as 100 GHz to 10 THz, represents a significant portion of the electromagnetic spectrum that bridges the gap between optical waves and microwaves for which compact, solid-state, roomtemperature sources are still not available. Traditionally speaking, this region of the electromagnetic spectrum (wavelengths in the range 30-300 µm or photon energies between 4 and 40 meV) lies between those frequencies referred to as the microwave and infrared regions. The spectral range of the terahertz electromagnetic wave is shown in Figure 1. The electromagnetic radiation in the THz band is useful for different kind of nondestructive imaging1 and medical or technical diagnostics,2 the emerging so-called T-ray imaging. On the other hand, the domain of terahertz radiation is also important from the basic research point of view since many excitations observed in condensed matter, liquids, gases, and biological substances correspond to the terahertz range of frequencies. Spectral analysis in the terahertz band can be used for studies of these excitations, including phonons and cyclotron or spin resonance, as well as for investigations of molecular (rotational and vibrational) excitations.Because of the development of ultra short pulsed laser techniques, one can readily generate and detect terahertz radiation if good nonlinear optical crystals having enough macroscopic second-order nonlinearity are available.3 For efficient THz wave generation, the NLO crystal is required to have large nonlinear and low absorption coefficients. So, organic crystals with a large nonlinearity are promising candidates for wideband THz generation. Thus the THz system relies on nonlinear optical crystals for the design of THz emitter and receiver for sensing from mid to long-range applications. m-nitroaniline is a prospective material and has been the subject of many studies because of its relatively simple molecular structure4,5 and large electro-optic effect6-8 and nonlinear optic effects.9-20 Furthermore, mNA has been found recently to show piezoelectric21 and ferroelectric22 behaviors. The terahertz radiation that is generated from this title NLO crystal is the first of its kind. The single crystals of this molecule were grown by the solution method. The synthesis and X-ray crystallography have already been reported.23,24 Semiempirical calculations were used for the vibrational analysis of the low-frequency phonon modes as well as to determine the frequency-dependent first-order hyperpolarizibility of the title nonlinear optical crystal. Before calculating the hyperpolarizibility for the investigated compound, the geometry taken from the starting structures based on its crystallographic data was optimized in the UHF (unrestricted open-shell Hartree-Fock) level. Molecular geometries were fully optimized by Berny’s optimization algorithm using redundant internal coordinates. All optimized structures were confirmed to be minimum energy conformations. At the optimized structure, no imaginary frequency modes were obtained, proving that a true minimum on the potential * Corresponding author. E-mail:
[email protected]. Tel.: 04272345766, ext 214. Fax: 0427-2345565.
Figure 1. Spectral range of terahertz in the electromagnetic spectrum.
energy surface was found. All the calculations were carried out by the density functional triply parameter hybrid model DFT/B3LYP using the GAUSSIAN 98W Package.25 The 6-31G(d,p) basis set has been employed. Figure 2 is a schematic illustration of the terahertz experimental setup. Optical pulses with a time width of 200 fs, repetition rate of 42 MHz, and center of wavelength 800 nm from a mode locked Ti: Sapphire laser is used to irradiate the photoconductive antenna which emits THz radiation into free space. As a terahertz emitter, a photoconductive antenna made on a low-temperature grown gallium arsenide is used. A material with very low carrier life times is needed that allows the signal to modulate its carrier concentration. GaAs can provide these low carrier lifetimes with sufficient mobility when grown under special conditions. The THz radiation collimated by an off axis paraboloidal reflector is focused on to another photoacoustic detector by a second paraboloidal reflector. The photoacoustic detector is triggered by gate pulses separated by optical pump pulses delivered by the Ti:sapphire laser after passing through the time delay circuit. The waveform of the electric field of the THz wave is measured by scanning the optical delay stage. A sample of 3-nitroaniline is placed in the terahertz optical path and the waveforms are measured. Fourier transformation obtains the complex amplitudes obtained as a function of frequency. Thus the temporal waveforms were acquired by varying the time delay terahertz of the probe light. Because we could detect the THz beam with horizontal polarization, the emitter crystals were rotated to get the largest THz signal. The fast Fourier transform (FFT) amplitude spectra of the THz output signals were calculated from the temporal waveforms. Panels a and b in Figure 3 show the temporal waveform and FFT amplitude spectra of the 3-nitroaniline crystal. Sources in this region have found many applications such as THz spectroscopy for studies of carrier dynamics and intermolecular dynamics in light. The production of THz pulses by exciting the molecular organic crystals is due to electron transfer with in the constituent molecules. By varying the crystal orientation, the polarization of exciting light and the detected polarization of the emitted THz field, it is possible to determine the absolute orientation in space of the current associated with electron transfer process. When a pulsed light beam containing a broad frequency spectrum determined by the shape and duration of the pulse is incident upon a nonlinear optical sample, the nonlinear interaction between any
10.1021/cg070548i CCC: $40.75 2008 American Chemical Society Published on Web 09/24/2008
Communications
Crystal Growth & Design, Vol. 8, No. 11, 2008 3883
Figure 2. Schematic illustration of experimental terahertz generation set up.
the terahertz waves at the interfaces of the Si lens. In addition to that, a common dip at 1.8 THz was also observed because of absorption by water vapor. The peaks above 3 THz in the 3-nitroaniline crystals in the FFT amplitude spectrum are attributed to noise. The β(2ω;ω -ω) values were predicted by quantum chemical calculations. In the present experimental set up of optical rectification we excite the (001) plane of the 3-nitroaniline crystal. The light wave propagates along the Z direction and the nonlinear polarization is induced in the XY plane of the crystal. Hence, the b-tensor component |bxxx| is effective as regards the terahertz radiation. According to the geometrical considerations for the molecular arrangements in the crystal, the b-tensor components can be explicitly described using the first order molecular hyperpolarizabilities (β).26 The largest β tensor component is found for |βxxx| (196.76), which is equal to |bxxx| in the title crystal. This is the indication of the nonlinear polarization induced by the component and this particular tensor is responsible for THz generation in the crystal. In the crystalline m-NA molecule, the intermolecular hydrogen bonding plays an important role in the significant increase of β value responsible for NLO activity. It is also noticed that in the |βxxx| (which is the principal dipole moment axis and is parallel to the charge transfer axis) direction, the biggest values of hyperpolarizability are noticed and subsequently delocalization of electron cloud is more in that direction. The terahertz intensity is proportional to d-constant that has a close relation with bcomponent. Ultimately we discuss the spectral dependence on the terahertz radiation intensity. From Figure 3b, it is found that the spread of spectral band shape for the title NLO crystal is restricted to 2.3 THz. The remarkable variations in band shape are due to absorption by low frequency phonons modes. Thus the spread of terahertz bandwidth of the investigated compound is of moderate range (0.9-2.3 THz). This material can be a better entrant for terahertz imaging or the other applications. Figure 3. (a)Temporal waveform of the terahertz generation from 3-nitroaniline. (b) FFT amplitude spectra of the terahertz generation from 3-nitroaniline.
two frequency components will induce a polarization and radiate electromagnetic waves at their frequency. The radiation has a continuous spectrum with a frequency as high as several terahertz and a special waveform. From panels a and b in Figure 3, it was found that the terahertz radiation emitted from the title crystal was enough to detect. Some small dips were found in the FFT amplitude spectra of the title compound may be caused by the reflection of
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