S. Williamson

375‐GHz‐bandwidth photoconductive detector

We report the development of a new, integrable photoconductive detector, based on low‐temperature‐grown GaAs, that has a response time of 1.2 ps and a 3‐dB bandwidth of 375 GHz. The responsivity is 0.1 A/W. Signal amplitudes up to 6 V can be produced with virtually no degradation in response time.

Photoconductive sampling probe with 2.3‐ps temporal resolution and 4‐μV sensitivity

We report on a novel probe technology which is applied to the measurement of high‐speed guided electrical signals. The probe consists of a high‐impedance gate fabricated using an interdigitated electrode structure on semi‐insulating, low‐temperature‐grown GaAs, and its operation is based on the optoelectronic technique of photoconductive sampling. The probe has a dynamic range of ≳106, permitting the linear measurement of short‐duration signals with amplitudes ranging from microvolts up to several volts. Its resistance is 100 MΩ, and its capacitance is less than 0.1 fF, making this probe attractive for noninvasive, external circuit testing of ultrahigh‐speed devices and circuits.

1.4 ps rise‐time high‐voltage photoconductive switching

We report on the generation of 825 V electrical pulses with 1.4 ps rise time and 4.0 ps duration using a pulse‐biased low‐temperature‐grown GaAs photoconductive switch triggered by an amplified femtosecond dye laser. Dependence of the pulse shape on both electric field and optical energy is observed and discussed.

Technology and Applications of Terahertz Imaging Non‐Destructive Examination: Inspection of Space Shuttle Sprayed On Foam Insulation

The implementation of terahertz (THz) imaging for non‐destructive evaluation shows great promise in 2 and 3 dimensional non‐contact inspection of non‐conductive materials such as plastics, foam, composites, ceramics, paper, wood and glass. THz imaging employs safe low power non‐ionizing electromagnetic pulses, which produce images with lateral resolution <200 microns, and depth resolution <50 microns. We demonstrate the detection of voids and disbonds intentionally incorporated within the sprayed on foam insulation of a space shuttle external tank mock‐up segment using time domain THz imaging. Recently, highly integrated turn‐key THz imaging systems have been introduced commercially. An industrially hardened THz scanning system which has been deployed to scan the space shuttle tank with small remote THz transceiver on a 30 meter fiber optic umbilical, is described.

High-speed metal-semiconductor-metal photodiodes with Er-doped GaAs

Very high-speed MSM photodiodes have been fabricated on Er-doped GaAs over a doping range of 10/sup 18/-10/sup 20/ cm/sup -3/. The impulse response (characterized by photoconductive sampling) of these diodes, with finger widths/spacings of 2 /spl mu/m, has been found to be tunable over a range of about 3 ps-22 ps. Electro-optic sampling was used to characterize MSM diodes with finger widths/spacings of 0.5 /spl mu/m and 1 /spl mu/m on a sample with [Er]=10/sup 19/ cm/sup -3/, resulting in 3-dB bandwidths of 160 GHz and 140 GHz, respectively, corresponding to pulse widths of 2.7 ps and 3.3 ps. Correlation measurements were also done on the GaAs:Er samples, using an all-electronic Sampling Optical Temporal Analyzer (SOTA) structure.<<ETX>>

TIME DOMAIN TERAHERTZ IMAGING OF THREATS IN LUGGAGE AND PERSONNEL

We demonstrate a large area time domain terahertz (THz) imaging system capable of scanning 1 meter square area in less than 20-100 minutes for several security applications. The detection of concealed explosives; metallic and non-metallic weapons (such as ceramic, plastic or composite guns and knives); and flammables in luggage, packages and personnel has been demonstrated. Transmission mode images of luggage containing threat items are discussed. Reflection mode images of luggage and personnel are discussed. Time domain THz images can be analyzed for 3 dimensional and volumetric information. Time domain THz images have advantages over coherent narrow band imaging methods, with freedom from interference artifacts and with greater ability to discard irrelevant or intervening reflections through time discrimination.

Time domain terahertz detection of concealed threats in luggage and personnel

We demonstrate a large area time domain terahertz (THz) imaging system capable of scanning 1 meter square area in less than 20-100 minutes for several security applications. The detection of concealed explosives; metallic and non-metallic weapons (such as ceramic, plastic or composite guns and knives); and flammables in luggage, packages and personnel has been demonstrated. Transmission mode images of luggage containing threat items are discussed. Reflection mode images of luggage and personnel are discussed. Time domain THz images can be analyzed for 3 dimensional and volumetric information. Time domain THz images have advantages over coherent narrow band imaging methods, with freedom from standing wave artifacts and with greater ability to discard irrelevant or intervening reflections through time discrimination.

Security and non destructive evaluation application of high speed time domain terahertz imaging

We report on the application of a transmission and reflection reconfigurable large area time domain terahertz imager capable of imaging 1 square meter in Lt 100 minutes at 100 pixels/sec to homeland security and non-destructive evaluation.

Time-domain network analysis of MM-wave circuits based on a photoconductive probe sampling technique

A photoconductive probe sampling technique with 2-ps temporal resolution and microvolt sensitivity has been developed. The photoconductive probe sampling technique combines the ultrafast optical technology of the 120-fs Ti-sapphire short pulse laser and the microfabrication technology of the silicon-on-sapphire photoconductive sampling probe, which consists of a high-impedance interdigitated photoconductive switch. This technique can be applied to the measurement of the S-parameters of millimeter-wave circuit components with a 120-GHz measurement bandwidth. The probe technology was applied to the characterization of millimeter-wave band-block filters used in InP-based heterostructure MMICs (monolithic microwave integrated circuits) for 90-GHz to 180-GHz frequency doubling. Millimeter-wave delay lines have also been characterized, and the properties of transmission lines on thin semiconductor substrates have been studied.<<ETX>>

Terahertz time-domain reflectometry applied to the investigation of hidden mural paintings

Terahertz time-domain reflectometry has been used to investigate hidden layers of mural paintings. Images of graphite underdrawings and dielectric paint patterns have been resolved through plaster. The reflectivity of several historical paint pigments were compared.