Ajay Nahata

A wideband coherent terahertz spectroscopy system using optical rectification and electro‐optic sampling

We present a scheme for exploiting the nonresonant second‐order nonlinearities in electro‐optic media to extend the bandwidth of coherent spectroscopy in the far‐infrared using ultrafast laser pulses. Using optical rectification and electro‐optic sampling in 〈110〉 ZnTe for the generation and coherent detection of freely propagating THz radiation, respectively, we have demonstrated spectral sensitivity beyond 3 THz. This was accomplished by achieving phase matching for both optical rectification and electro‐optic sampling over a broad range of THz frequencies.

Coherent detection of freely propagating terahertz radiation by electro‐optic sampling

We report the demonstration of an electro‐optic sampling technique that allows for the detection of freely propagating terahertz radiation. Coherent sampling is performed in a poled polymer device that is physically separated from the emitter. The poling electrodes in the sampling element are found to have an integrating effect on the incident terahertz field. The shot noise limited minimum detectable field in the polymer is 100 (mV/cm)/√Hz. We discuss methods by which the sensitivity may be significantly enhanced.

Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling

We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.

FREE-SPACE ELECTRO-OPTIC DETECTION OF CONTINUOUS-WAVE TERAHERTZ RADIATION

We present a scheme for the coherent detection of freely propagating continuous-wave terahertz radiation using electro-optic detection. The terahertz radiation is generated by photomixing two single-mode laser diodes in an antenna fabricated on low-temperature-grown GaAs. This radiation is detected using the electro-optic effect in 〈110〉 ZnTe. In contrast to typical terahertz detection techniques, this is a frequency-domain measurement that relies on coherent up-conversion of the terahertz field combined with optical homodyning to suppress background noise.

Detection of freely propagating terahertz radiation by use of optical second-harmonic generation

We report the application of electric-field-induced optical second-harmonic generation as a new technique for measuring the field of freely propagating terahertz radiation. Using silicon as the nonlinear medium, we demonstrate subpicosecond time resolution and a sampling signal that varies linearly with the terahertz electric field. This approach, which is attractive for centrosymmetric media, permits a significantly broadened class of materials to be exploited for free-space sampling measurements.

Electro-optic determination of the nonlinear-optical properties of a covalently functionalized Disperse Red 1 copolymer

We report the development of a methodology for the comprehensive characterization of the linear- and nonlinear-optical properties of a model copolymer, Disperse Red 1, covalently functionalized to a methyl methacrylate backbone. From refractive-index and electro-optic measurements, we have evaluated both fundamental macroscopic and microscopic nonlinear-optical properties. We have measured the refractive indices of the copolymer as a function of chromophore concentration, wavelength, and poling field. For the linear-optical properties we find that the wavelength dependence is well described by a single-oscillator Sellmeier equation and that the poling-field dependence is well described by a simple theory related to the order parameter. From the observed refractive-index anisotropy as a function of poling field, we have obtained an effective dipole moment for the Disperse Red chromophore. We have measured the linear electro-optic effect in these systems as a function of poling temperature, film thickness, poling-field strength, wavelength, and concentration. We have found that the electro-optic coefficients may be modeled by an independent-response two-level dispersion model, from which an independent determination of the effective dipole moment, nonresonant second-order susceptibility χ(2), and nonresonant molecular hyperpolarizability β have been obtained.

Generation of Terahertz Radiation From a Poled Polymer

We report the generation of terahertz radiation via optical rectification from a poled polymer using femtosecond optical pulses. We have measured the refractive index and power absorption in the terahertz frequency range. The corresponding difference frequency mixing coherence length in the poled polymer (1 mm at 1 THz) is ∼20 times greater than that of LiNbO3 (50 μm at 1 THz). The observed far‐infrared electric field radiated from a 16 μm thick polymer sample is only 4 times smaller than that from a 1 mm thick y‐cut LiNbO3 crystal. We discuss conversion efficiencies for thicker polymer samples.

Electro-optic detection of femtosecond electromagnetic pulses by use of poled polymers

We report the generation and coherent detection of freely propagating ultrashort baseband electromagnetic pulses. Using optical rectification in ?110? GaAs for wideband emission and electro-optic sampling in a poled polymer for wideband detection, we demonstrate spectral sensitivity that extends from the far infrared (lambda~100 mum) to ~33 THz(lambda = 9 mum) . Over a band of nearly 20 THz, a relatively flat frequency response is observed. We discuss issues that limit the response bandwidth.

Robust photopolymers for MCM, board, and backplane optical interconnects

A robust polymeric waveguide technology is proposed for affordable optoelectronic interconnects in massively parallel processing applications. We have developed high-performance organic polymeric materials that can be readily made into both multimode and single-mode optical waveguide structures of controlled numerical aperture and geometry. These materials are formed from highly-crosslinked acrylate monomers with specific linkages that determine properties such as flexibility, toughness, loss, and stability against yellowing. These monomers are intermiscible, providing for precise adjustment of the refractive index from 1.30 to 1.60. Waveguides are formed lithographically, with the liquid monomer mixture polymerizing upon illumination in the UV via either mask exposure or laser direct-writing. A wide range of rigid and flexible substrates can be used, including glass, quartz, oxidized silicon, glass-filled epoxy printed circuit board substrate, and flexible polyimide film. Our waveguides are low loss (0.02 dB/cm at 840 nm) as well as temperature resistant (over 65 years at 100 degree(s)C) and humidity resistant (no effect on unpackaged guides after 600 hours at 85 degree(s)C 85% RH), enabling use in a variety of demanding applications. We discuss the use of these materials on multi-chip modules, boards, and backplanes. Waveguiding structures measuring tens of inches in length can be produced on backplanes, and guides that are meters long can be laser-written on rolls of plastic. We also discuss the fabrication of symmetrically-clad flexible strips of waveguide arrays that are compatible with MT- type connectors.

Generation of subpicosecond electrical pulses by optical rectification

We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.