Zeptojoule detection of terahertz pulses by parametric frequency upconversion.

We combine parametric frequency upconversion with the single-photon counting technology to achieve terahertz (THz) detection sensitivity down to the zeptojoule (zJ) pulse energy level. Our detection scheme employs a near-infrared ultrafast source, a GaP nonlinear crystal, optical filters, and a single-photon avalanche diode. This configuration is able to resolve 1.4 zJ (1.4 × 10-21 J) THz pulse energy, corresponding to 1.5 photons per pulse, when the signal is averaged within only 1 s (or 50,000 pulses). A single THz pulse can also be detected when its energy is above 1185 zJ. These numbers correspond to the noise-equivalent power and THz-to-NIR photon detection efficiency of 1.3 × 10-16 W/Hz1/2 and 5.8 × 10-2%, respectively. To test our scheme, we perform spectroscopy of the water vapor between 1 and 3.7 THz and obtain results that are in agreement with those acquired with a standard electro-optic sampling (EOS) method. Our technique provides a 0.2 THz spectral resolution offering a fast alternative to EOS THz detection for monitoring specific spectral components in spectroscopy, imaging, and communication applications.

High-field THz source centered at 2.6 THz.

We demonstrate a table-top high-field terahertz (THz) source based on optical rectification of a collimated near-infrared pulse in gallium phosphide (GaP) to produce peak fields above 300 kV/cm with a spectrum centered at 2.6 THz. The experimental configuration, based on tilted-pulse-front phase matching, is implemented with a phase grating etched directly onto the front surface of the GaP crystal. Although the THz generation efficiency starts showing a saturation onset as the near-infrared pulse energy reaches 0.57 mJ, we can expect our configuration to yield THz peak fields up to 866 kV/cm when a 5 mJ generation NIR pulse is used. This work paves the way towards broadband, high-field THz sources able to access a new class of THz coherent control and nonlinear phenomena driven at frequencies above 2 THz.

Compact, low-cost, and broadband terahertz time-domain spectrometer.

Terahertz time-domain spectroscopy (THz-TDS) is a powerful technique that enables the characterization of a large range of bulk materials, devices, and products. Although this technique has been increasingly used in research and industry, the standard THz-TDS configuration relying on the use of a near-infrared (NIR) laser source remains experimentally complex and relatively costly, impeding its availability to those without the expertise to build a high-performance setup based on nonlinear optics or without the financial means to acquire a commercial unit. Broadband THz-TDS systems require an even larger financial investment, primarily because the generation and detection of spectral components exceeding 3 THz typically need an ultrafast NIR source delivering sub-100-fs pulses. Such an ultrafast source can be bulky and cost upwards of $100,000. Here, we present a broadband, compact, and portable THz-TDS system comprising three modules that allow for the implementation of a single low-cost ultrafast laser, hence significantly decreasing the overall cost of the system. In the first module, the output laser pulses are spectrally broadened through nonlinear propagation in a polarization-maintaining optical fiber and then temporally compressed to achieve a higher peak power. The other two modules utilize thick nonlinear crystals with periodically patterned surfaces that diffract NIR pulses and optimize the efficiency of THz generation and detection processes by enabling a noncollinear beam geometry. Phase-matching conditions in the nonlinear crystals are controlled by the period of the gratings to gain access to a large spectral THz bandwidth. The whole system, combining these three modules, provides access to a THz spectrum peaking at 3.5 THz and extending beyond 6 THz with a maximum dynamic range of 50 dB for time-resolved spectroscopy applications. We demonstrate the functionality of this configuration by performing THz spectroscopy measurements of water vapor contained within a closed cell. Our compact system design paves the way towards a high-performance, yet cost-effective, THz-TDS system that can be readily used in academia and industry.

Single-pulse terahertz spectroscopy monitoring sub-millisecond time dynamics at a rate of 50 kHz.

Slow motion movies allow us to see intricate details of the mechanical dynamics of complex phenomena. If the images in each frame are replaced by terahertz (THz) waves, such movies can monitor low-energy resonances and reveal fast structural or chemical transitions. Here, we combine THz spectroscopy as a non-invasive optical probe with a real-time monitoring technique to demonstrate the ability to resolve non-reproducible phenomena at 50k frames per second, extracting each of the generated THz waveforms every 20 µs. The concept, based on a photonic time-stretch technique to achieve unprecedented data acquisition speeds, is demonstrated by monitoring sub-millisecond dynamics of hot carriers injected in silicon by successive resonant pulses as a saturation density is established. Our experimental configuration will play a crucial role in revealing fast irreversible physical and chemical processes at THz frequencies with microsecond resolution to enable new applications in fundamental research as well as in industry.

Calibration and comparison of detection efficiency for free-space single-photon avalanche diodes at 850 nm.

A detection efficiency measurement system for free-space single-photon detectors has been established at the National Research Council (NRC) Canada. This measurement apparatus incorporates an 850 nm fiber laser source and utilizes a double-attenuation and substitution calibration technique. Detection efficiency calibrations of silicon single-photon avalanche photodiodes (SPADs) at incident photon rates in the range of 1.0×105 photon counts per second (Cts/s) (36 fW) to 2.1×106Cts/s (734 fW) are SI-traceable through the substitution configuration with a silicon transfer standard detector, calibrated directly using the NRC absolute cryogenic radiometer. The measurement approach taken by the NRC was compared with the SPAD calibration technique implemented at the National Institute of Standards and Technology (NIST) in the United States. The count-rate-dependent detection efficiency of a silicon SPAD was measured at NIST and compared with results from the same SPAD measured at NRC within the range of incident photon rates from 1×105Cts/s to 5×105Cts/s. Comparison of the calibration results shows agreement between the two laboratories within the combined measurement uncertainties.

Measurement of coherence decay in GaMnAs using femtosecond four-wave mixing.

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.  Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.  The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.  The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.  We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.   This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.