Electro-optic Detection Research Articles

Study of broadband THz time-domain spectroscopy at different relative humidity levels

Two detection techniques of broadband terahertz (THz) time-domain spectroscopy-THz air-biased coherent detection (THz-ABCD; from 0.3 to 14 THz) and electro-optical (EO) detection (from 0.3 to 7 THz) - are both performed at several different relative humidity levels. The THz power exponentially decays with the increase in relative humidity. The dynamic range of the main pulse in the time domain linearly decreases as the relative humidity increases from 0% to 40%, and linear fittings show that the slopes are -0.017 and -0.019 for THz-ABCD and EO detection, respectively. Because of the multiple reflections caused by the crystal in the common EO detection, THz-ABCD has better spectral resolution (17 GHz) than that of EO detection (170 GHz). The spectrum of water vapor absorption measured by THz-ABCD is also compared with that measured by the Fourier transform infrared spectroscopy (FTIR).

Nuclear reactor pulse calibration using a CdZnTe electro-optic radiation detector

A CdZnTe electro-optic radiation detector was used to calibrate nuclear reactor pulses. The standard configuration of the Pockels cell has collimated light passing through an optically transparent CdZnTe crystal located between crossed polarizers. The transmitted light was focused onto an IR sensitive photodiode. Calibrations of reactor pulses were performed using the CdZnTe Pockels cell by measuring the change in the photodiode current, repeated 10 times for each set of reactor pulses, set between 1.00 and 2.50 dollars in 0.50 increments of reactivity.

Preliminary Demonstration of an IonCCD as an Alternative Pixelated Anode for Direct MCP Readout in a Compact MS-Based Detector

We report on the preliminary testing of a new position-sensitive detector (PSD) by combining a microchannel plate (MCP) and a charge-sensitive pixilated anode with a direct readout based on charge-coupled detector (CCD) technology, which will be referred to as IonCCD (Hadjar et al. J Am Soc Mass Spectrom 22(4):612-623, 2011; Johnson et al. J Am Soc Mass Spectrom 22(8):1388-1394, 2011; Hadjar et al. J Am Soc Mass Spectrom 22(10):1872-1884, 2011). This work exploits the recently discovered electron detection capability of the IonCCD (Hadjar et al. J Am Soc Mass Spectrom 22(4):612-623, 2011), allowing it to be used directly behind an MC. This MCP-IonCCD configuration potentially obviates the need for electro-optical ion detector systems (EOIDs), which typically feature a relatively difficult-to-implement 5-kV power source as well as a phosphorus screen behind the MCP for conversion of electrons to photons prior to signal generation in a photosensitive CCD. Thus, the new system (MCP-IonCCD) has the potential to be smaller, simpler, more robust, and more cost efficient than EOID-based technologies in many applications. The use of the IonCCD as direct MCP readout anode, as opposed to its direct use as an ion detector, will benefit from the instant three-to-four-order-of-magnitude gain of the MCP with virtually no additional noise. The signal/noise gain can be used for either sensitivity or speed enhancement of the detector. The speed enhancement may motivate the development of faster IonCCD readout speeds (currently at 2.7 ms) to achieve the 2 kHz frame rate for which the IonCCD chip was designed, a must for transient signal applications. The presented detector exhibits clear potential not only as a trace analysis detector in scan-free mass spectrometry and electron spectroscopy but also as a compact detector for photon and particle imaging applications.

Two alternative approaches to electro-optical detection of terahertz pulses

The electro-optical detection of terahertz radiation is analyzed theoretically. While a conventional detection scheme of this type is based on the measurement of the phase shift of an optical pulse arising from its interaction with the terahertz field in a nonlinear medium, it is shown that both the phase and amplitude of the optical pulse vary because of this interaction. The amplitude modulation carries information on the dynamics of the amplitude and phase of the terahertz field and can be used to measure these parameters. With this energy-sensitive detection scheme, avoiding all restrictions on the symmetry type of the nonlinear medium is much simpler than with the phase scheme. Variants of the electro-optical detection technique using periodically poled crystals with the lithium-niobate symmetry and crystals with the zincblende structure are considered. It is shown that spectral sensitivities of the methods based on measurements of the phase and amplitude modulations are related as the frequencies of the laser and terahertz fields.

THz Active Imaging Systems With Real-Time Capabilities

This paper presents a survey of the status of five active THz imaging modalities which we have developed and investigated during the last few years with the goal to explore their potential for real-time imaging. We start out by introducing a novel waveguide-based all-electronic imaging system which operates at 812 GHz. Its salient feature is a 32-pixel linear detector array heterodyne-operated at the eighth subharmonic. This array in combination with a telescope optics for object distances of 2-6 m reaches a data acquisition speed suited for real-time imaging. The second system described then is again an all-electronic scanner (now for around 300 GHz ), designed for object distances of ≥ 8 m , which combines mechanical scanning in vertical direction, synthetic-aperture image generation in horizontal direction, and frequency-modulated continuous-wave sweeping for the depth information. The third and fourth systems follow an optoelectronic approach by relying on several- to multi-pixel parallel electrooptic detection. One imager is based on a pulsed THz-OPO and homodyne detection with a CCD camera, the other on either continuous-wave electronic or femtosecond optoelectronic THz sources and a photonic-mixing device (PMD) camera. The article concludes with a description of the state of the art of imaging with focal-plane arrays based on CMOS field-effect transistors.

Nonorthodox heterodyne electro-optic detection for terahertz optical systems

An electro-optic detector of terahertz electromagnetic waves with boosted performance is presented. The detector utilizes a heterodyne detection technique to achieve orders of magnitude better responsivity compared to a standard detector setup. We prove theoretically as well as demonstrate experimentally the feasibility of this technique for electro-optic detection.

Real-time terahertz near-field microscope

We report a terahertz near-field microscope with a high dynamic range that can capture images of a 370 x 740 μm2 area at 35 frames per second. We achieve high spatial resolution (14 μm corresponding to λ/30 for a center frequency at 0.7 THz) on a large area by combining two novel techniques: terahertz generation by tilted-pulse-front excitation and electro-optic balanced imaging detection using a thin crystal. To demonstrate the microscope capability, we reveal the field enhancement at the gap position of a dipole antenna after the irradiation of a terahertz pulse.

Simultaneous measurement of orthogonal components of polarization in a free-space propagating terahertz signal using electro-optic detection

We report a polarization-sensitive terahertz time-domain spectroscopy system, which allows the simultaneous measurement of orthogonal components of the polarization of a free-space propagating terahertz beam using a dual electro-optic detection scheme. We demonstrate the operation of our system by measuring the birefringence of lithium niobate, simultaneously obtaining terahertz spectra from two orthogonal crystallographic directions.

Pulse-shaper-assisted coherent control of shift currents

We investigate the dynamics of the ultrafast photocurrent process in bulk GaAs. The coherent-control scheme features a pulse shaper and a spatial light modulator. We use the versatility of the setup to create two orthogonally polarized phase-stable femtosecond pulses and manage the shift current density through manipulation of the combined pulse envelope. The behavior of the current induces characteristic terahertz emission patterns that are measured in a standard electro-optical detection scheme. The experimental results are corroborated by a theoretical model that links the behavior of the current density to the pulse envelope.

A novel distributed optic fiber transduser for landslides monitoring

Unstable slopes have been monitored since the beginning of the last century. Current electro-optic detection technology can achieve automatic monitoring remotely with high safety and includes such methods as time domain reflectometry, optical time domain reflectometry and Brillouin optical time domain reflectometry. However, these technologies cannot simultaneously meet the requirements of distributed sensing, high initial measurement accuracy, large sliding distance and high dynamic range. Based on the space frame theory of reinforced concrete beams, this study presents an innovative design for a distributed optic fiber sensor: a novel transduser with a bowknot. Using the optic fiber microbending loss mechanism and optical time domain reflectometry technology, bending and shear tests based on the combined fiber sensor are conducted, and the vertical displacement of midspan, optical fiber sliding distance and loss data under three different spans are collected. Feasibility study and economic analysis of the transduser used for landslide monitoring are also presented. The results show that the maximum sliding distance of our transduser is 21.8, 26.5 and 30.6 mm with corresponding initial accuracies of 1.2, 2.3 and 3.3 mm, and the dynamic ranges are 0–20.6, 0–23.2 and 0–27.3 mm. The cost of the transduser is economical at $0.15/m, which demonstrates promising economic application, high monitoring effectiveness and stability in monitoring civil works, such as slope, dam and tunnel construction and measurement.

Tapered and Tip-Grounded Waveguide Electrooptical Microsensors

A tip-grounded waveguide microsensor was proposed to overcome the difficulty of quantitative voltage calibration in electrooptical detection for integrated circuit (IC) test. On this basis, we optimized the thickness of the electrooptical material of the sensor to eliminate the influence of the circuit layout on the measured signals. The improved sensor in return made it possible to calibrate the voltage with known reference electric signals quantitatively. This method circumvented the uncertainty of the probe conditions of each measurement point. Finally, a calibration accuracy of better than 6% was obtained, which satisfied broad applications in the IC industry.

Time-domain spectroscopy of mid-infrared quantum cascade lasers

In this work we present phase-resolved measurements of the stimulated emission process in a quantum cascade laser emitting at 11.7 µm. In particular, we are focusing on the temperature performance of the intersubband gain in a spectrally broad region. We find that the gain turns into strong absorption at higher temperatures. We show that the phase information is mandatory to describe the correct relation between stimulated emission and absorption. Due to the importance of the correct phase estimation, we present an extended noise analysis of our optimized electro-optic sampling detection scheme. The dynamic range is >80 dB with signal components between 6 and 67 THz.

Terahertz polarization real-time imaging based on balanced electro-optic detection

A terahertz (THz) polarization real-time imaging system that can effectively reduce experimental time consumption for acquiring a sample's polarization information is achieved. An alternative THz polarization measurement method is proposed. In this method, a <110> zinc-blende crystal is used as the sensor, and the probe polarization is adjusted to detect THz electric fields on the two orthogonal polarization components. The relative sensitivity of the imaging system to the THz polarization angle is estimated to be less than 0.5°. To illustrate the ability of the system, two samples are designed and measured by using the system. From their THz polarization real-time images, each region of these samples can be precisely presented. Experimental results clearly show the special influences of different materials on the THz polarization. This work effectively extends the information content obtained by THz real-time imaging and improves the feasibility of the imaging technique.

Optical phase detection in a 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate crystal for terahertz time domain spectroscopy system at 1.55 μm wavelength

We report the electro-optic detection of terahertz (THz) pulses in a 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate crystal (DAST) using optical phase detection technique and femtosecond optical pulses at telecom wavelength. A THz time-domain spectroscopy system based on 1.55 μm wavelength fiber laser pulses that integrates an ion-irradiated In0.53Ga0.47As photoconductive antenna as the emitter and a DAST electro-optic crystal as the detector is shown to provide a detection bandwidth of 5 THz.

On the feasibility of electro-optical detection of dust-impact ionization in tokamaks

The feasibility of the optical and electrical detection of dust-impact ionization events in the scrape-off layers of tokamak plasmas is evaluated. It is shown that the expected light emission and the charge released during a dust impact on a biased target can be measured above the light emission and the charge collected due to the background plasma. A scheme of an electro-optical probe for diagnostics of fast dust particles is proposed.

Terahertz real-time imaging with balanced electro-optic detection

We propose a method for two-dimensional terahertz (THz) real-time imaging which systemically integrates the THz balanced electro-optic sampling technique and the dynamic subtraction technique. The proposed method can effectively improve the signal to noise ratio and the spectrum measurement accuracy of the imaging system. Experiment results demonstrate the advantage of the method.

Coherent electro-optical detection of terahertz radiation from an optical parametric oscillator

We report the realization of coherent electro-optical detection of nanosecond terahertz (THz) pulses from an optical parametric oscillator, which is pumped by a Q-switched nanosecond Nd:YVO4 laser at 1064 nm and emits at approximately 1.5 THz. The beam profile and wavefront of the THz beam at focus are electro-optically characterized toward the realization of a real-time THz camera. A peak dynamic range of approximately 37 dB/radical Hz is achieved with single-pixel detection.

Terahertz emission from metal-organic chemical vapor deposition grown Fe:InGaAs using 830 nm to 1.55 μm excitation

We demonstrate the generation of broadband terahertz (THz) frequency radiation from photoconductive emitters formed from Fe-doped InGaAs (Fe:InGaAs), grown by metal-organic chemical vapor deposition, following pulsed (femtosecond) laser excitation at wavelengths ranging from 830 nm to 1.55 μm. The Fe is incorporated epitaxially during growth, giving precise control over the doping level. Using both single-crystal ZnTe and GaP electro-optic detectors over the same wavelength range, the emission spectra from several Fe:InGaAs wafers with different Fe content were measured, with THz emission from all wafers showing bandwidths in excess of 2.0 THz. The THz output power was found to be strongly dependant on the Fe content, the thickness of the Fe:InGaAs layer, and the excitation wavelength.

Phase-locking of the beat signal of two distributed-feedback diode lasers to oscillators working in the MHz to THz range

We present difference-frequency stabilization of free-running distributed-feedback (DFB) diode lasers, maintaining a stable phase-lock to a local oscillator (LO) signal. The technique has been applied to coherent hybrid THz imaging which employs a high-power electronic radiation source emitting at 0.62 THz and electro-optic detectors. The THz radiation of the narrow-band emitter is mixed with the difference frequency of the DFB diode laser pair. The resulting intermediate frequency is phase-locked to the LO signal from a radio-frequency generator using a fast laser-current control loop. The stabilization scheme can be adapted readily to a wide range of applications which require stabilized laser beat-notes.

Electron Bunch Timing with Femtosecond Precision in a Superconducting Free-Electron Laser

High-gain free-electron lasers (FELs) are capable of generating femtosecond x-ray pulses with peak brilliances many orders of magnitude higher than at other existing x-ray sources. In order to fully exploit the opportunities offered by these femtosecond light pulses in time-resolved experiments, an unprecedented synchronization accuracy is required. In this Letter, we distributed the pulse train of a mode-locked fiber laser with femtosecond stability to different locations in the linear accelerator of the soft x-ray FEL FLASH. A novel electro-optic detection scheme was applied to measure the electron bunch arrival time with an as yet unrivaled precision of 6 fs (rms). With two beam-based feedback systems we succeeded in stabilizing both the arrival time and the electron bunch compression process within two magnetic chicanes, yielding a significant reduction of the FEL pulse energy jitter.