Broadband THz Pulses Research Articles

Spatiotemporal Visualization of THz Near-Fields in Metamaterial Arrays

We present an experimental approach to record the spatiotemporal electric field distribution of coherent broadband THz pulses propagating through planar metamaterial arrays. The electric field can be measured with sub-wavelength precision within a volume that is several wavelengths in size, thus, having the potential to map the near-field to far-field transition of the resonant structures constituting the metamaterial. To demonstrate the potential we present measurements of THz pulses propagating through a planar array of double split-ring resonators and their inverse analogues.

An investigation of the lowest-order transverse-electric (TE_1) mode of the parallel-plate waveguide for THz pulse propagation

We experimentally and theoretically investigate the lowest-order transverse-electric (TE1) mode of the parallel-plate waveguide (PPWG) for the propagation of broadband THz pulses. We demonstrate undistorted THz pulse propagation via the single TE1 mode, solving the group-velocity-dispersion and spectral-filtering problems caused by the mode's low-frequency cutoff. We observe a remarkable counterintuitive property of the TE1 mode: its attenuation decreases with increasing frequency for all frequencies above cutoff. This phenomenon has not been observed with any other THz waveguide to date, and it can enable extremely low-loss propagation. We present a physical interpretation of this frequency-dependent behavior using a simple plane-wave description of the TE1 mode propagation. We also find that it is possible to achieve almost 100% coupling to the TE1 mode from a focused free-space Gaussian beam. In addition, using the above plane-wave analysis, we show how to mitigate the diffraction losses inherent to long path-length PPWGs via the use of transverse-concave plates.

A hydrogen-bonded organic nonlinear optical crystal for high-efficiency terahertz generation and detection

Broadband THz pulses have been generated in 2-[3-(4- hydroxystyryl)-5,5-dimethylcyclohex-2-enylidene]malononitrile (OH1) by optical rectification of sub-picosecond laser pulses. We show that OH1 crystals allow velocity-matched generation and detection of THz frequencies in the whole range between 0.3 and 2.5 THz for a pump laser wavelength range from 1200 to 1460 nm. OH1 crystals show a higher figure of merit for THz generation and detection in the optimized range compared to the benchmark inorganic semiconductor crystals ZnTe and GaAs and the organic ionic salt crystal 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate (DAST). The material shows a low THz absorption coefficient alpha3 in the range between 0.3 and 2.5 THz, reaching values lower than 0.2mm(-1) between 0.7 and 1.0 THz. This is similar as in ZnTe and GaAs, but much lower than in DAST in the respective optimum frequency range. A peak THz electric field of 100 kV/cm and a photon conversion efficiency of 11 percent have been achieved at a pump pulse energy of 45 microJ.

GaP waveguide emitters for high power broadband THz generation pumped by Yb-doped fiber lasers

We demonstrate the generation of broadband THz pulses by optical rectification in GaP waveguides pumped by high power Yb-doped fiber amplifiers. The dispersion of the GaP emitter can be controlled via the geometry of the waveguide; the peak frequency of the emitted THz radiation is tuned by varying the waveguide cross-section. Most importantly, the use of a waveguide for the THz emission increases the coherent buildup length of the THz pulses and offers scalability to higher power; this was investigated by pumping a GaP waveguide emitter with a high power Yb-doped fiber laser system. A 25-MHz-repetition-rate pulse train of THz radiation with 120 muW average power was achieved using 14 W optical power, which represents the highest average power for a broadband THz source pumped by fiber lasers to date.

Terahertz time-domain spectroscopy as a new tool for the characterization of dust forming plasmas

We report the application of terahertz time-domain spectroscopy as a new tool for plasma diagnostics. The short broadband THz pulses were radiated from a low temperature grown GaAs emitter by free charge carriers which were generated by focusing a 20 femtosecond TiSa-laser pulse onto the emitter. For sensitive signal recording a coherent detection scheme was applied. This allowed the measurement of the amplitude and sign of the electromagnetic field of the THz pulse after passing the plasma chamber. Fourier transformation allowed us to obtain the full spectrum in the frequency domain.We compared the transmitted THz intensities of a pure argon (Ar) and an acetylene (C2H2)/argon plasma. The presence of the ethynyl–radical (CCH) and cyclopropenylidene (c-C3H2) in the (C2H2)/argon plasma could be confirmed by the observations of rotational transitions in the region from 8 to 16 cm−1 corresponding to 0.3–0.5 THz.

Intrinsic complications in the analysis of optical-pump, terahertz probe experiments

A general formalism is presented for the interpretation of experiments in which a laser pulse (or pump pulse) creates charge carriers that are subsequently probed by a broadband THz pulse (or probe pulse). The time dependence of the sample properties is described in terms of the conductivity $\ensuremath{\sigma}(\ensuremath{\omega},\ensuremath{\tau})$, where the dependence on frequency $\ensuremath{\omega}$ describes the intrinsic properties of the charge carriers and the dependence on pump-probe delay $\ensuremath{\tau}$ describes their population dynamics. It is shown that there are significant complications in obtaining this quantity from experimental data for $\ensuremath{\tau}$ values comparable to the charge carrier response time. Also, in transient THz absorption experiments, fast dynamics in the measured signals may be observed that do not necessarily reflect sample dynamics.

Coupling of terahertz pulses onto a single metal wire waveguide using milled grooves

We demonstrate a novel approach for coupling freely propagating THz pulses onto a metal wire waveguide utilizing grooves fabricated directly into the metal. Using broadband THz pulses incident on the wire, we use metal segments containing zero, one, three, and eight uniformly spaced grooves to launch surface propagating multi-cycle pulses along the wire. We observe a one-to-one correspondence between the groove number and the number of oscillations in the THz waveform radiated from the end of the waveguide. We further demonstrate that this coupled radiation is radially polarized. Although the cross-sectional parameters of the grooves are identical in the present measurements, alteration of the individual grooves in a controlled manner should allow for arbitrarily shaped THz pulses to be launched on the waveguide.

Role of metal film thickness on the enhanced transmission properties of a periodic array of subwavelength apertures

We have measured the enhanced transmission properties of periodic arrays of subwavelength apertures fabricated in thin metal films as a function of the metal film thickness. In doing so, we determine the minimum metal film thickness for an array that exhibits resonantly enhanced transmission and observe the transmission properties as the metal film thickness is increased. The thickness range explored extends from delta/15, where delta is the skin depth, to approximately 2delta. Using terahertz time-domain spectroscopy, we measure both the amplitude and phase of the transmitted broadband THz pulses. Experimentally, we find that there is negligible transmission enhancement for metal films as thin as delta/15. As the film thickness increases, there is a sublinear increase in the enhancement until the film thickness is equal to the skin depth. For metal films thicker than one skin depth, there appears to be little additional enhancement at the resonant frequencies. We also observe that as the thickness of metal films increases, there is a corresponding increase in the resonance frequencies.

Distinguishing between materials in terahertz imaging using wide-band cross ambiguity functions

Terahertz pulsed imaging is a novel imaging technique whereby sub pico-second broadband THz pulses are used in the analysis of samples, through transmission and reflection modalities. This paper briefly introduces terahertz pulsed imaging and its potential medical applications, before exploring how wavelet analysis may be used in categorising terahertz imaged samples, specifically the use of the terahertz reference pulse as a mother function through wide-band cross ambiguity functions. We show that analysis using ambiguity functions can distinguish between a nylon and a resin step wedge phantom, and furthermore that the results obtained are consistent with those of traditional time domain and Fourier based techniques.

Audio signal transmission over THz communication channel using semiconductor modulator

The first demonstration has been made of the transmission of an audio signal via a THz communication channel using a recently developed room temperature semiconductor THz modulator which is based on the depletion of a two-dimensional electron gas. A standard THz time-domain spectroscopy setup is modified to transmit signals up to 25 kHz over a 75 MHz train of broadband THz pulses.