Narrow Results

Differential terahertz (THz) time-domain spectroscopy (TDS) is a technique for decreasing noise levels in THz thin film characterization experiments. Characterizing thin films in the GHz to THz range is critical for the development of fast integrated circuits and photonic systems, and is potentially applicable to biosensors and proteomics. This paper shows how the differential technique, combined with double modulation, enables the study of thin films with noise reduction over normal TDS that improves at the film gets thinner. Double modulated differential THz-TDS has enabled the characterization of films with less than 1-μm thickness.

Quantum simulations and experiments show that photomixing in laser-assisted field emission has promise as a new method for wide-band tunable sources of radiation at terahertz frequencies. The tunable bandwidth is only limited by the means for coupling power from the oscillations in the field emission current from photomixing, and not by the processes that generate this current. Photomixing is simulated as a stationary stochastic process in which the frequencies and phases of the incident optical radiation are random variables. The waveform of the current is determined by solving the Schrödinger equation at discrete time steps for which the effective potential barrier is a superposition of the incident radiation field and the static barrier. These samples satisfy the criteria of a Poisson process to allow for the discrete emission of electrons at a specified total current. The one-sided power spectral density for this current is calculated with the FFT to produce periodogram estimates. The simulations show that the signal-to-noise ratio may be increased by (1) raising the power flux density of each laser, (2) raising the DC static current, (3) reducing the linewidth of each laser, and (4) using a static current density of no more than 10¹⁰ A/m.

Terahertz (THz) plasma oscillation in n⁺nn⁺ InGaAs vertical diodes is studied by using a numerical approach based on the hydrodynamic (HD) equations. The 1D HD model is coupled to 1D Poisson equation. We simulate the diode response to the optical excitation of plasma waves at room temperature. Our results clearly show the presence of 3D plasma resonances in the THz frequency domain. The selection of appropriately shaped optical excitation pulses helps in the realization of the detected frequency. The investigation is completed by introducing stochastic perturbations according to the Langevin equation.