Hua Cao

Resonantly enhanced transmission of terahertz radiation through a periodic array of subwavelength apertures

We demonstrate resonantly enhanced transmission of freely propagating coherent terahertz radiation through free-standing metal foils perforated with periodic arrays of sub-wavelength apertures. These arrays consist of 400 microm diameter apertures periodically spaced by 1 mm and 600 microm diameter apertures periodically spaced by 1.5 mm. We measure absolute amplitude transmission coefficients of ~0.6 at the resonance wavelength. Correspondingly, the ratio of the absolute amplitude transmission coefficient to the fractional aperture area at these resonance frequencies is ~5. This value at terahertz frequencies is significantly larger than equivalent values measured at optical frequencies.

Influence of aperture shape on the transmission properties of a periodic array of subwavelength apertures

We demonstrate that the resonantly enhanced transmission spectrum associated with a periodic array of subwavelength apertures is dependent upon the shape of the apertures. This is demonstrated using coherent terahertz radiation and aperture arrays fabricated in 75 microm thick stainless steel foils. We examine rectangular apertures with different aspect ratios as well as circular apertures. In the absence of periodicity in the arrays, no resonance features are present. For periodic arrays, we show that the ratio of the transmission coefficients for the two lowest order resonances can be directly related to the ratio of the appropriate aperture dimensions. From the time-domain waveforms, we find two independent, yet phase-coherent, transmission processes: non-resonant transmission related to the simple transmission through subwavelength apertures and a time-delayed resonant transmission related to the interaction of the THz pulse with the periodic aperture array. In these waveforms, we also observe a sign inversion for the primary bipolar pulse relative to the reference. This is shown to be a simple consequence of diffraction.

Time-domain analysis of enhanced transmission through a single subwavelength aperture.

We have measured the enhanced transmission properties of a single subwavelength aperture surrounded by periodically spaced annular grooves using time-domain techniques. While the present measurements utilize terahertz time-domain approaches, with appropriately scaled device parameters, the general observations should be applicable to other spectral ranges. In contrast to measurements that rely on continuous wave excitation and frequency domain measurements, we are able to determine the contribution of each individual groove to the transmitted terahertz waveform. Using structures containing only a single annular groove surrounding the aperture, we find that each groove can couple a large fraction of the incident terahertz bandwidth in the form of a surface wave pulse. When multiple annular grooves surround the aperture, we observe oscillations in the time-domain waveform that are temporally delayed from the initial bipolar waveform in direct relation to the distance of the groove from the aperture. This is further demonstrated by using structures containing defects (absence of annular grooves).

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.

Broadband generation of terahertz radiation in a waveguide

We report a novel geometry that allows for the phase-matched generation of broadband terahertz radiation in a polymer-based parallel-plate metal waveguide by means of optical rectification. Both the optical pump beam and the generated terahertz radiation propagate in the fundamental mode of the waveguide. This allows for noncritical phase matching over a broad range of terahertz frequencies. We demonstrate guided-wave interaction lengths of up to 3 mm.

Excitation and scattering of surface plasmon-polaritons on structured metal films and their application to pulse shaping and enhanced transmission

Using terahertz (THz) time-domain techniques, we measure the relative coupling and scattering characteristics of surface plasmon-polariton waves for rectangular cross-section grooves on a thick metal foil. This is accomplished by using a single subwavelength aperture surrounded by one or more annular grooves. A unique aspect of the measurement technique is that it allows one to discriminate between the contributions of each groove in the transmitted THz waveform. The measurements are obtained by using structures that contain either a single annular groove, for coupling measurements, or two concentric annular grooves, for scattering measurements. The basic approach should be extendable to other spectral ranges and applicable to different forms of surface electromagnetic waves. Using the aforementioned measurements, we demonstrate the ability to alter the pulse shape of the transmitted waveform using a single aperture surrounded by four annular grooves of varying dimensions. In addition to the value of this capability for pulse shaping applications, the basic technique allows for greater control of the transmission resonance lineshape related to enhanced transmission through a single subwavelength aperture.

Controlling the transmission resonance lineshape of a single subwavelength aperture

Recent demonstrations have shown that by creating a periodic surface corrugation about a single subwavelength aperture, the transmission can be enhanced at wavelengths related to the periodicity. We demonstrate that by varying the phase of the surface corrugation relative to the single subwavelength aperture, dramatic variations can be made in the transmission resonance lineshape at THz frequencies, ranging from transmission enhancement to transmission suppression. This finding is particularly surprising, since the nearly exclusive focus of published work has been on understanding and optimizing the enhancement process in these and associated structures. We present a simple model that qualitatively explains our observations. This represents, we believe, a first step in fully controlling the spectral transmission properties of these structures.

Coherent detection of pulsed narrowband terahertz radiation

We demonstrate the generation and coherent detection of narrowband terahertz radiation using a Q-switched laser pumped optical parametric oscillator as the optical source. Narrowband terahertz radiation is produced using conventional difference frequency mixing and coherently detected via a frequency domain technique that relies on coherent upconversion of the terahertz field combined with optical homodyning to suppress background noise. GaSe crystals are used for both generation and coherent detection processes. Although we provide a proof-of-principle demonstration at 2THz, the infrared radiation may be tuned from the far-infrared through the midinfrared by simply tuning the idler wavelength of the optical parametric oscillator and the orientation of the two crystals.

Shaping terahertz pulses using structured metal films

Using a single subwavelength aperture surrounded by annular grooves, we demonstrate the ability to alter the pulse shape of an incident single cycle THz pulse. Arbitrarily complex time-domain waveforms may be generated, in principle.

The influence of aperture shape on the enhanced transmission properties of a periodic array of subwavelength apertures

We demonstrate that the enhanced transmission properties of metal foils perforated with periodic arrays of subwavelength apertures depend critically upon the aperture shape. We demonstrate this dependence using rectangular apertures of varying aspect ratios. The variation is explained in terms of the geometrical properties of the apertures.