Oleg Mitrofanov

Silver/polystyrene-coated hollow glass waveguides for the transmission of terahertz radiation.

We have applied techniques developed for IR waveguides to fabricate Ag/polystyrene (PS) -coated hollow glass waveguides (HGWs) for transmission of terahertz radiation. A loss of 0.95 dB/m at 119 microm (2.5 THz) was obtained for a 2 mm bore, 90 cm long Ag/PS HGW. We found that TE modes are supported in HGWs with thin PS films, while hybrid (HE) modes dominate when PS film thickness increases. The lowest losses are obtained for the thicker PS films and the propagation of the HE modes.

Terahertz pulse propagation through small apertures

Transmission of single-cycle terahertz pulses through subwavelength apertures is experimentally studied in the near-field zone. Measurements of throughput for aperture sizes d as small as λ/300 show that the theoretical d3 law requires a correction term which takes into account the physical thickness of the aperture screen. Frequency dependent transmission of the broad band pulses explains changes in their spectral and temporal characteristics. Practical application of small apertures in near-field microscope probes allows achievement of spatial resolution of 7 μm for pulses with a broad spectral content of λ=120–1500 μm.

Reducing Transmission Losses in Hollow THz Waveguides

Research on reducing material absorption in Terahertz (THz) waveguides has lead to development of guiding structures with transmission losses as low as 1 dB/m. Among waveguides that exhibit low loss at THz frequencies are the dielectric-lined hollow cylindrical metallic waveguides. Loss reduction in this waveguide is attributed to an ideal profile of the dominant hybrid HE11 mode. This mode profile also results in relatively low dispersion and very high coupling efficiency. In this contribution we overview properties of dielectric-lined hollow cylindrical metallic waveguides for THz waves, their design principles and the fabrication process. The impact of the mode profile on losses and dispersion at THz frequencies is confirmed experimentally by THz near-field imaging and THz time-domain spectroscopy and numerically by the finite element method.

Terahertz near-field microscopy based on a collection mode detector

We report on the development of a collection mode near-field probe for the terahertz spectral range. The near-field detector is based on an aperture type probe with dimensions of 30×30 μm2. The collection mode technique provides higher sensitivity and higher resolution than the similar illumination mode approach. Spatial resolution better than 40 μm is demonstrated for a broad spectrum of 300–600 μm, which equals to λ/15 for the longest wavelength. The observed resolution is determined by the size of the probe aperture.

Low-loss modes in hollow metallic terahertz waveguides with dielectric coatings

Cylindrical hollow-core metallic waveguides with an inner coating of polystyrene (PS) deposited over silver have losses less than 1dB/m for terahertz waves propagating in the HE11 mode. The thickness of the PS film determines whether the hybrid HE11 mode or the transverse TE01 mode exhibits the lowest loss. The mode selection is confirmed by studying mode profiles and transmission losses at 2.5 THz in waveguides with the dielectric coating thickness ranging from 0 to the optimum value of approximately 10 μm.

Dielectric-lined cylindrical metallic THz waveguides: mode structure and dispersion

Thin dielectric layers deposited on the inner surface of hollow cylindrical metallic waveguides for Terahertz (THz) waves reduce transmission losses below 1 dB/m. Impact of the dielectric layer on the waveguide dispersion is experimentally investigated by near-field mapping of guided short THz pulses at the input and the output of the waveguide. We obtain dispersion characteristics for the low-loss waveguide modes, the linearly-polarized HE(11) mode and the TE(01) mode, and compare the experimental results to the metallic waveguide dispersion. The additional dispersion due to the dielectric layer is found to be small for the HE(11) mode and the phase velocity is primarily determined by the waveguide radius.

Impact of Si doping on radio frequency dispersion in unpassivated GaN/AlGaN/GaN high-electron-mobility transistors grown by plasma-assisted molecular-beam epitaxy

We report on the effect of Si doping on the transient behavior of unpassivated high-power GaN/AlGaN/GaN high-electron-mobility transistors grown by plasma-assisted molecular-beam epitaxy on 6H–SiC. The incorporation of Si into the heterostructure barrier is found to reduce the level of radio frequency dispersion as compared to undoped structures. In some devices which incorporate Si doping of the barrier, the pulsed and steady-state current–voltage characteristics coincide, and gate lag is found to be insignificant. More typically, ∼90% of the dc value of drain current is restored at 1 μs after pulsing the gate from pinch off to VGS=0 V. Significant gate lag is observed in devices that are not doped with Si. In the undoped structure, the drain current reaches only ∼70% of the dc value within 1 μs. The transient behavior in the two designs is attributed to the same defect state with activation energy of 0.22 eV. Dispersion reduction is correlated with an increase of gate leakage current in Si-doped devices.

Near-field microscope probe for far infrared time domain measurements

A near-field probe fabrication technique for far-infrared frequencies based on photoconducting antennas is developed. A subwavelength-size field source is accomplished by means of an aperture and protruding high refractive index tip. The near-field probe is tested by using free space traveling electromagnetic pulses with a broadband spectrum in the range of 0.3‐1.5 THz. A spatial resolution of 60 mm is achieved for a 50 mm aperture. The described probe may be used for near-field transmission microscopy in illumination and collection modes. Resolution may be further improved by means of a smaller aperture.

Resonant optical nonlinearities from intersubband transitions in GaN/AlN quantum wells

We measured the resonant nonlinear optical response of the intersubband transitions in GaN/AlGaN multiple quantum well structures. The measured value for the nonlinear susceptibility is found to be much smaller than previous theoretical predictions. This is attributed to a large electron dephasing due to material imperfections. We show that at higher incident intensities, absorption saturation is possible, and measure the saturation intensity for various wavelengths around the transition resonance. We also discuss the prospects of using such structures as building blocks for all-optical nonlinear switches, in light of our experimental findings.

Waveguide mode imaging and dispersion analysis with terahertz near-field microscopy

Propagation of terahertz waves in hollow metallic waveguides depends on the waveguide mode. Near-field scanning probe terahertz microscopy is applied to identify the mode structure and composition in dielectric-lined hollow metallic waveguides. Spatial profiles, relative amplitudes, and group velocities of three main waveguide modes are experimentally measured and matched to the HE11, HE12, and TE11 modes. The combination of near-field microscopy with terahertz time-resolved spectroscopy opens the possibility of waveguide mode characterization in the terahertz band.