Femtosecond pulses in the mid-infrared generated by downconversion of a traveling-wave dye laser.

Amplified pulses from a colliding-pulse mode-locked dye laser at 620 nm and the output of a femtosecond traveling-wave dye cell are mixed in a LiIO(3) crystal to generate femtosecond pulses in the wavelength range near 5 microm. As much as 10% of the photons from the traveling-wave laser are converted to stable infrared pulses of less than 400-fs duration, 10-nJ energy, and 8-kHz repetition rate.

Time-domain images.

We demonstrate that, using a hologram, we can convert a spatial x-y image into an x-t image, where one spatial dimension is now transformed into the time domain. In particular, we generate the temporal equivalent of our corporate logo, where every pixel of the image is now represented by a short optical pulse.

Stable operation of a synchronously pumped colliding-pulse mode-locked ring dye laser.

Pulses of 100-fsec duration are obtained by synchronous pumping of a colliding-pulse ring dye laser with a mode-locked Ar(+)-ion laser. Stable operation of the synchronously pumped colliding-pulse mode-locked laser over hours was obtained by a suitable choice of the distance between the gain and the absorber in combination with an appropriate pump-pulse sequence.

Chemical recognition of gases and gas mixtures with terahertz waves.

A time-domain chemical-recognition system for classifying gases and analyzing gas mixtures is presented. We analyze the free induction decay exhibited by gases excited by far-infrared (terahertz) pulses in the time domain, using digital signal-processing techniques. A simple geometric picture is used for the classif ication of the waveforms measured for unknown gas species. We demonstrate how the recognition system can be used to determine the partial pressures of an ammonia-water gas mixture.

Amplitude noise reduction of 50 dB in colliding-pulse mode-locking dye lasers.

Spurious sidebands observed in the noise spectrum of colliding-pulse mode-locking dye lasers result from coupling of the longitudinal mode beating modulations of the (argon-ion) pump laser with the dye laser. We achieve a 50-dB amplitude noise reduction at the mixing frequencies by inserting a single-frequency étalon in the argon-laser cavity. Furthermore, we find the amplitude noise of the colliding-pulse mode-locking laser to be entirely dominated by the (plasma discharge) noise of the argon laser.

Time-to-space mapping of femtosecond pulses.

We report time-to-space mapping of femtosecond light pulses in a temporal holography setup. By reading out a temporal hologram of a short optical pulse with a continuous-wave diode laser, we accurately convert temporal pulse-shape information into a spatial pattern that can be viewed with a camera. We demonstrate real-time acquisition of electric-field autocorrelation and cross correlation of femtosecond pulses with this technique.

T-ray tomography.

We demonstrate tomographic T-ray imaging, using the timing information present in terahertz (THz) pulses in a reflection geometry. THz pulses are reflected from refractive-index discontinuities inside an object, and the time delays of these pulses are used to determine the positions of the discontinuities along the propagation direction. In this fashion a tomographic image can be constructed.

Wavelength-division multiplexing with femtosecond pulses.

We demonstrate wavelength-division multiplexing with a single broadband femtosecond source by slicing the 3.7-THz spectral bandwidth of 85-fs laser pulses into 16 channels that are modulated individually.

Terahertz emission from electric field singularities in biased semiconductors.

We use electric field singularities in biased metal semiconductor microstructures to enhance the generation of terahertz (THz) radiation from semiconductors. We find that, regardless of the mechanism that is responsible for enhanced THz emission near the anode, singular electric fields near sharp anode features will enhance this emission by as much as an order of magnitude. We show scanning THz measurements of several of these structures and discuss the physical mechanism responsible for this enhanced emission. A new family of more efficient terahertz emitters based on these effects can be designed that will improve the dynamic range of THz imaging and spectroscopy systems.

Millimeter wave-induced vibrational modes in DNA as a possible alternative to animal tests to probe for carcinogenic mutations.

Developing methods for alternative testing is increasingly important due to dwindling funding resources and increasing costs associated with animal testing and legislation. We propose to test the feasibility of a new and novel method for detecting DNA mutagenesis using millimeter wave spectroscopy. Although millimeter wave spectroscopy has been known since the 1950s, the cost was prohibitive and studies did not extend to large biological proteins such as DNA. Recent advances have made this technology feasible for developing laboratory and field equipment. We present preliminary findings for lesion-induced vibrational modes in DNA observed from 80 to 1000 gigahertz (GHz). These findings suggest that there are vibrational modes that can be used as identification resonances. These modes are associated with localized defects of the DNA polymers. They are unique for each defect/lesion, and should be easy to detect. We described a field-detecting detector based on the local modes.