2.32 THz quantum cascade laser frequency-locked to the harmonic of a microwave synthesizer source.

Frequency stabilization of a THz quantum cascade laser (QCL) to the harmonic of a microwave source has been accomplished using a Schottky diode waveguide mixer designed for harmonic mixing. The 2.32 THz, 1.0 milliwatt CW QCL is coupled into the signal port of the mixer and a 110 GHz signal, derived from a harmonic of a microwave synthesizer, is coupled into the IF port. The difference frequency between the 21st harmonic of 110 GHz and the QCL is used in a discriminator to adjust the QCL bias current to stabilize the frequency. The short-term frequency jitter is reduced from 550 kHz to 4.5 kHz (FWHM) and the long-term frequency drift is eliminated. This performance is compared to that of several other THz QCL frequency stabilization techniques.

Terahertz inverse synthetic aperture radar (ISAR) imaging with a quantum cascade laser transmitter.

A coherent transceiver using a THz quantum cascade (TQCL) laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented.

Optical-limiter MEMS dynamic range compression deconvolution.

We propose dynamic range compression deconvolution by a new nonlinear optical-limiter microelectromechanical system (NOLMEMS) device. The NOLMEMS uses aperturized, reflected coherent light from optically addressed, parabolically deformable mirrors. The light is collimated by an array of microlenses. The reflected light saturates as a function of optical drive intensity. In this scheme, a joint image of the blurred input information and the blur impulse response is captured and sent to a spatial light modulator (SLM). The joint information on the SLM is read through a laser beam and is Fourier transformed by a lens to the back of the NOLMEMS device. The output from the NOLMEMS is Fourier transformed to produce the restored image. We derived the input-output nonlinear transfer function of our NOLMEMS device, which relates the transmitted light from the pinhole to the light intensity incident on the back side of the device, and exhibits saturation. We also analyzed the deconvolution orders for this device, using a nonlinear transform method. Computer simulation of image deconvolution by the NOLMEMS device is also presented.

Frequency stabilization of a single mode terahertz quantum cascade laser to the kilohertz level.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH(2)DOH THz CO(2) optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t approximately sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers.

MEMS-based optical limiter.

We propose the design of an optical limiter based on a microelectromechanical systems deformable mirror. The design is based on aperturing focused light reflected out of an optically driven deformable mirror, deformed in a parabolic form. We derive an expression for the reflected light intensity, and we show that the reflected light saturates as a function of back illumination light intensity.

Terahertz sideband-tuned quantum cascade laser radiation.

A compact, tunable, narrowband terahertz source was demonstrated by mixing a single longitudinal mode 2.408 THz, free running quantum cascade laser with a 2-20 GHz microwave sweeper in a conventional corner-cube-mounted Schottky diode. The sideband spectra were characterized with a Fourier transform spectrometer, and the radiation was tuned through several D(2)O rotational transitions to estimate the longer term (t > or = several sec) bandwidth of the source. A spectral resolution of 2 MHz in CW regime was observed.

Spectrally variable two-beam coupling nonlinear deconvolution.

In previous work, we introduced a dynamic range compression-based technique for image correction using nonlinear deconvolution; the impulse response of the distortion function and the distorted image are jointly transformed to pump a clean reference beam in a photorefractive two-beam coupling arrangement. The Fourier transform of the pumped reference beam contains the deconvolved image and its conjugate. Here we extend our work to spectrally variable dynamic range compression. This approach allows the retrieval of distorted signals embedded in a very high noise environment and does not require one to work with a very high beam ratio as in our previous work. Resolution recovery of blurred noisy images is demonstrated for several different types of image blur.

Fabrication of a novel micron scale Y-structure-based chiral metamaterial: Simulation and experimental analysis of its chiral and negative index properties in the terahertz and microwave regimes.

In this report, we describe the fabrication of a chiral metamaterial based on a periodic array of Y-shaped Al structures on a dielectric Mylar substrate. The unit cell dimensions of the Y-structure are approximately 100 microm on a side with 8 microm linewidths. The fabricated Y-structure elements are characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Quantitative elemental analyses were carried out on both the Y-structure, comprised of Al and its oxide, as well as adjacent regions of the underlying mylar substrate using the energy dispersive X-ray spectroscopy (EDS) capability of the SEM. Finite-Difference Time-Domain (FDTD) calculations of the negative index of refraction for a 3D wedge of multiple layers of the 2D metamaterials showed that these metamaterials possess double negative (-mu,-epsilon) electromagnetic bulk properties at THz frequencies. The same negative index of refraction was determined for a wedge comprised of appropriately scaled larger Y-structures simulated in the microwave region. This double negative property was confirmed experimentally by microwave measurements on a 3D wedge comprised of stacked and registered Y-structure sheets.

Optically addressed microelectromechanical systems driven with high-frequency modulated light.

We propose and analyze a new mode of operation for an optically addressed deformable mirror device. The device consists of an array of metallized membrane mirrors supported above an optically addressed photoconductive substrate. A conductive transparent electrode is deposited on the backside of the substrate. A variable polarity voltage is applied between the membrane and the back electrode of the device accompanied with high-frequency modulated light. The membrane is deformed when a modulated light illuminates the backside of the device. This occurs due to impedance and bias redistribution between the two cascaded impedances. This operating mechanism of a microelectromechanical systems device is suitable for detecting moving targets.

Wet-etch optimization of free-standing terahertz frequency-selective structures.

Free-standing frequency-selective surfaces consisting of approximately 10-microm-thick copper films with cross-aperture arrays are found to be tunable toward lower frequencies by means of wet chemical etching. Center frequencies were tuned from 1.57 to 1.53 THz while maintaining high transmittance. Wet etching also adjusts bandwidth, peak transmittance, and sidelobe transmittance. The advantage of the wet-etch technique is demonstrated by employment of these devices as bandpass filters for difluoromethane-based terahertz lasers. Adjustment in aperture dimensions because of etching results in suppression of a competing laser line (133.93 microm) by 15 dB while maintaining high transmittance at the operating wavelength of 192.06 microm.