Electro-optic Detection Research Articles

Generation of terahertz radiation via optical rectification and electro-optic detection in ZnTe crystals

We report on the generation of terahertz (THz) radiation via optical rectification and the detection using electro-optic sampling in ZnTe crystal by the pump-probe technique. The pulsewidth of the observed THz radiation is about 0.2 ps, resulting in the wideband THz spectra with a spectral sensitivity beyond 4 THz and the full width at half-maximum of about 2.4 THz. The Jones matrix method has been employed successfully for the calculation of the THz radiation. We study the effects of the optical pulse wavelength, pulsewidth and the ZnTe crystal axis on the generation of THz radiation in the ZnTe crystal. The dependence of the polarization-modulation on the THz detection has also been investigated both experimentally and theoretically by changing the polarization of the probe light.

Near-infrared fluorescence optical imaging and tomography.

The advent of recent advances in near-infrared laser diodes and fast electro-optic detection has spawned a new research field of diagnostic spectroscopy and imaging based on targeting and reporting exogenous fluorescent agents. This review seeks to concisely address the physics, instrumentation, advancements in tomography, and near-infrared fluorescent contrast agent development that promises selective and specific molecular targeting of diseased tissues. As an example of one area of the field, recent work focusing on pharmacokinetic analysis of fluorophores targeting the epidermal growth factor receptor (EGFR) is presented in a human breast cancer xenograft mouse model to demonstrate specificity of molecularly targeted contrast agents. Finally, a critical evaluation of the limitations and the opportunities for future translation of fluorescence-enhanced optical imaging of deep tissues is presented.

Laser pointing angle and time of measurement verification of the ICESat laser altimeter using a ground-based electro-optical detection system

The Ice, Cloud, and land Elevation Satellite (ICESat) will begin science operations in 2003 with an emphasis on determination of the ice sheet temporal variations in the Arctic and Antarctic regions. The ICESat bus will serve as the transport for an instrument called the Geoscience Laser Altimeter System (GLAS). GLAS will provide altimetry and lidar measurements with a high level of accuracy. For altimetry, the GLAS data will enable determination of the laser pointing angle to within 1.5 arcsec and the laser pulse time of arrival on the ground to within 100 sec. Both of these data products contribute to the determination of the measured altitude vector from the spacecraft to the ice surface. Verification of both the laser pointing angle and the timing can be achieved by using a unique experimental technique designed to ‘capture’ an altimeter pulse on the surface of the Earth. The capture of the laser pulse is accomplished by covering the illuminated area with devices designed to detect the arrival of energy within the altimeter footprint. This ground-based technique will supply an independent, unambiguous determination of the laser footprint geolocation and the epoch time associated with the arrival of the pulse on the surface. Knowledge of the laser footprint centroid on the ground will infer the laser pointing direction in the geocentric reference frame. This in situ measurement of the footprint geolocation and time of arrival will be compared to the corresponding data products provided by GLAS. The comparison of the GLAS laser pointing and the timing data with an independent measurement will verify the accuracy and/or will indicate the existence of any biases or errors in the generation of the GLAS altimetry data products. The detectors have been designed and tested in the laboratory and analyzed for energy level thresholds, system stability, temperature response and overall performance. Timing hardware has been tested and software has been written to achieve event detection within the desired accuracy.

Electro-optic sampling of single electron beam bunches of ultrashort duration

The effect of ultrafast electron beam bunch dynamics on single shot electro-optic sampling detection schemes is examined. It is shown that ultrashort electron bunch fields of adequate magnitude can dynamically impose additional bandwidth on laser probe pulses. The significance of this effect is evaluated by comparing the dynamics of the laser probe to that of the nonradiative field of a single electron bunch for a given crystal material. Dynamic effects can be distinguished with ultrafast temporal resolution of the transmitted probe spectrum. Furthermore, velocity matching of probe and bunch fields in a co-propagation scheme is less restrictive. Such time resolved spectra then can noninvasively determine single bunch dynamics and represent a new type of electro-optic sampling.

Detectors: Electro-Optic Detection of Continuous-Wave Mid-Infrared Radiation

Detectors: Electro-Optic Detection of Continuous-Wave Mid-Infrared Radiation

A terahertz system using semi-large emitters: noise and performance characteristics

We have built a relatively simple, highly efficient, terahertz (THz) emission and detection system centred around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused on a 120 μm diameter spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1 mm thick (110) oriented ZnTe crystal. The maximum THz-induced differential signal that we observe is ΔP/P = 7 × 10−3, corresponding to a THz peak amplitude of 95 V cm−1. The THz average power was measured to be about 40 μW, to our knowledge the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.

Electro-optic detection of continuous-wave mid-infrared radiation

We demonstrate coherent detection of continuous-wave mid-infrared radiation. This radiation is produced by use of conventional difference-frequency mixing and detected via the linear electro-optic effect. The detection process allows for the simultaneous measurement of the amplitude and phase properties of the infrared field. Both processes require an amplitude-modulated optical beam that is derived from the superimposed output of two single-frequency lasers. With appropriate choice of lasers and nonlinear optical crystals, the technique may be applied to any wavelength throughout the far and mid infrared.

Electro-optic detection of subwavelength terahertz spot sizes in the near field of a metal tip

We report on a method to obtain a subwavelength resolution in terahertz time-domain imaging. In our method, a sharp copper tip is used to locally distort and concentrate the THz electric field. The distorted electric field, present mainly in the near field of the tip, is electro-optically measured in an (100) oriented GaP crystal. By raster scanning the tip along the surface of the crystal, we find the smallest THz spot size of 18 μm for frequencies from 0.1 to 2.5 THz. For our peak frequency of 0.15 THz, this corresponds to a resolution of λ/110. Our setup has the potential to reach a resolution down to a few μm.

Two-dimensional imaging of continuous-wave terahertz radiation using electro-optic detection

We report the demonstration of two-dimensional electro-optic imaging using continuous-wave THz radiation. Two single-frequency laser diodes serve as the optical source for both the generation and detection processes. The generation process occurs by mixing in a photoconductive emitter, while the detection process utilizes an electro-optic crystal and charge coupled device camera. The method permits far-IR imaging using only conventional optical components held at room temperature. In this demonstration, we image the two-dimensional properties of a THz beam focused into a 〈110〉 ZnTe electro-optic detection crystal.

Indium–tin–oxide-coated glass as dichroic mirror for far-infrared electromagnetic radiation

We propose to utilize glass plates coated with indium tin oxide (ITO) as far-infrared dichroic mirrors, e.g., in optoelectronic terahertz spectroscopy for purposes such as electro-optic detection of terahertz radiation or in time-resolved experiments with pump pulses in the visible or near-infrared spectral range and probe pulses at terahertz frequencies. Measurements of the complex reflection and transmission coefficients in the spectral range from 0.1 to 2.8 THz indicate that commercially available ITO glass is suitable for this purpose.

Tunable terahertz generation using femtosecond pulse shaping

Femtosecond pulse shaping and photomixing methods are combined to make a tunable terahertz source. Grating-pair pulse shaper selects two main frequency components to produce pulses that are modulated with a period which is inversely proportional to the frequency separation. The shaped pulse is then photomixed in semi-insulating GaAs under external bias. Terahertz (THz) emission frequency follows the modulation frequency of the excitation pulses, from 0.5 to 3 THz. Contrary to cw photomixing, this method can easily be combined with electro-optic and photoconductive sampling detection methods.

Coherent detection of mid-infrared radiation up to 60 THz with an LT-GaAs photoconductive antenna

The shaker modulation technique is demonstrated for ultrabroadband detection of terahertz (THz) radiation with a low-temperature-grown GaAs photoconductive (PC) antenna gated with 15 fs laser pulses. The differentiation of the time-resolved waveforms with the shaker modulation is advantageous to enhance the high-frequency components of the detected radiation without degrading the signal-to-noise ratio (SNR) because the PC response function determined by the temporal profile of the gating carrier number rapidly decreases with the increase in frequency. With the quantitative comparison of SNR between PC and electro-optic detection, the PC detection of radiation with the shaker modulation technique is promising for further spectroscopic application. The detection of radiation up to 60 THz is demonstrated.

Deeper into the (infra)red

Generation and coherent electro-optic detection of far-infrared and mid-infrared radiation enable new capabilities compared to photoconductive detection techniques.

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.

Design and performance of a THz emission and detection setup based on a semi-insulating GaAs emitter

We have built a relatively simple, highly efficient, THz emission and detection system centered around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused to a 120 μm diam spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1-mm-thick (110) oriented ZnTe crystal or a (110) oriented 0.1-mm-thick GaP crystal. The maximum THz-induced differential signal that we observe is ΔI/I=7×10−3, corresponding to a THz peak amplitude of 95 V/cm. The THz average power was measured to be about 40 μW, to our knowledge, the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.

Real-time single-shot electron bunch length measurements

Linear accelerators employed as drivers for X-ray free electron lasers (FELs) require relativistic electron bunch with sub-picosecond bunch length. Precise bunch length measurements are important for the tuning and operation of the FELs. Previously, we have demonstrated that electro-optic detection is a powerful technique for sub-picosecond electron bunch length measurements. In those experiments, the measured bunch length was the average of all electron bunches within a macropulse. Here, for the first time, we present the measurement of the length of individual electron bunches using a development of our previous technique. In this experiment, the longitudinal electron bunch shape is encoded electro-optically on to the frequency spectrum of a chirped laser pulse. Subsequently, the laser pulse is dispersed by a grating and the spectrum is imaged with a CCD camera. Single bunch measurements are achieved by using a nanosecond gated camera, and synchronizing the gate with both the electron bunch and the laser pulse repetition rates. The electron bunch length is determined by measuring the laser pulse spectra with and without the presence of an electron bunch. We demonstrate that this method enables a real-time diagnostic for the bunch length of single electron bunches with a time resolution of 370 femtoseconds and a high signal-noise-ratio.

State and Future Outlook for the Development of Hybrid Optical Image Converters

An analytic survey of hybrid optical image converters constructed on the basis of electrooptic converters and solid-state multi-element radiation detectors is presented. Current trends in the development of this type of optical radiation converter are formulated.

Application of electro-optic sampling in FEL diagnostics

The electro-optic sampling technique has been used for the full characterization (both amplitude and phase) of freely propagating pulsed electromagnetic radiation (such as FEL pulses, transition radiation) and for the quasistatic electric field of relativistic electron bunches. Measurements of the electron bunch length and shape have been performed with both non-intercepting and intercepting methods. Sub-picosecond time resolution has been obtained with this electro-optic detection technique.

80 to 10 Gbit/s clock recovery using phase detectionwith Mach–Zehnder modulator

A Mach–Zehnder modulator, driven by a harmonic of a local oscillator, is used as an electro-optic phase detector to lock the oscillator to the 10 GHz base rate of an 80 Gbit/s time-division multiplexed data stream with a timing jitter of 341 fs. Clock recovery is robust and may be scaled to higher data rates.

Environmental biosensors for organochlorines, cyanobacterial toxins and endocrine disrupting chemicals

Environmental biosensors and related techniques for monitoring organochlorines, endocrine disrupting chemicals and cyanobacterial toxins are described. The practical requirements for an ideal environmental biosensor are analyzed. Specific case studies for environmental applications are reported for triazines chlorinated phenols, PCBs, microcystins, and endocrine disrupting chemicals. A new promising approach is reported for microcystins and alkylphenols that utilize electrooptical detection.