National Physical Laboratory

Research

NPL's THz pulse spectrometer and monochromator system
NPL's THz pulse spectrometer and monochromator
system

New sources of Terahertz radiation have emerged as recently as 1990, using short-pulse laser systems. The 10 femtosecond (0.000 000 000 000 01 second) laser pulses have the ability to interact with certain materials in a way that creates Terahertz emissions into free-space. The materials generally fall into two categories, either non-linear crystal optics or fast-semiconducting materials. The utilisation of these techniques in THz measurement systems has advanced considerably in the past decade, finding applications in medical imaging, biohazard analysis, dentistry, non-destructive testing and security screening.

NPL has been active in Terahertz Metrology for more than 30 years, with a large amount of work undertaken on Dispersive Fourier Transform Spectroscopy (DFTS) and Far-infrared spectroscopy. More recently NPL has invested in Terahertz short-pulse capabilities to create a high-resolution, environmentally controlled pulse spectrometer. Our experience has included ZnTe, Dast and GaAs crystals and high-speed data collection for fast data acquisition and high-resolution spectra.

NPL's future research and application directions include:

  • Security and tracking
  • Non-destructive testing
  • Near field-microscopy techniques for bio and nano characterisation
  • Calibration techniques for THz penetration, power, wavelength, emissivity and scattering

Imaging and Spectroscopy

Terahertz pulsed imaging (TPI) is attracting much interest due to its unique features including its non-ionising nature, non-destructive imaging , and less scatter than that at visible and infrared wavelength. Examples of TPI applications include detection of skin cancer, identification of tooth abnormalities, and non-destructive analysis of tablet coating thicknesses.

With increasing applications for TPI, there is a strong need for a greater traceability to provide more confidence in the measurement and enable decision to be made based on the acquired imaging data. Traceable calibrated artefacts are being developed for calibration of spatial resolution for 2D and 3D imaging. The intention is to provide traceably calibrated artefacts to enable an efficient evaluation of the imaging uncertainty in different systems when using different acquisition schemes or data processing methods.

Instrumentation

There are a variety of activities in THz instrumentation aiming to cover various needs of THz measurements:

  • THz emitters and detectors using ZnTe, GaAs, etc
  • THz waveguide
  • Monochromator
  • Bolometer
  • Golay cell
  • THz pulsed system (TPS)
  • THz imaging
  • Dispersive Fourier Transform Spectrometer (DFTS)

Metrology

NPL is leading other measurement institutes by addressing THz metrology issues for:

  • Power
  • Wavelength
  • Penetration depth
  • Refractive index
  • Scattering
  • Emissivity of natural objects
  • Database and measurement system for analysis of absorption spectra of materials
  • Advice for future safety guidelines provided to standards bodies

Work on wavelength metrology has been particularly active. Traceable frequency measurement is well established in the optical and microwave regions and it is important to develop wavelength calibration standard at Terahertz frequencies to fill the gap.

An approach has been developed for validating the frequency scale of a Terahertz pulsed system to National Standards utilising a transfer measurement device, which is characterised within an independently traceable THz spectrometer. It allows traceable frequency measurement for THz frequencies in the range of 0.2 THz to 1.5 THz, which can be transferred to a TDS through a measurement artefact with an uncertainty of 30 GHz. Work is currently progressing to improve both the resolution and frequency range of the Monochromator device. Gas cell standards are being developed to address this need.

Microscopy

The spatial resolution of far-field THz imaging is limited by its wavelength to hundred micrometers according to Rayleigh's criterion. To achieve sub-wavelength resolution as required in biomedical imaging applications, near-field THz microscopy is promising, in which:

  • THz radiation can be highly localised by a metallic tip, achieving sub-wavelength resolution (100 nm possible)
  • Losses of water may be overcome
  • Small tips can be placed on selected molecules for direct measurement and label free detection

NPL is currently in collaboration with a UK university to investigate near field techniques for diagnosis and potentially therapy/stimulated reactions.

For more information, please contact: Richard Dudley

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