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NPL’s quantum capabilities for industry

Quantum Optoelectronic Metrology on the Nanoscale

Advanced probing of light-matter interaction at nanoscale for quantum optoelectronics

Scattering-type scanning near-field optical microscopy (s-SNOM) delivers nanoscale, non-destructive materials characterisation of the sample surface and subsurface at the spatial resolution of an atomic force microscope, typically ~ 20 nm. It allows direct imaging and spectroscopy of optical, optoelectronic, chemical, and structural properties (such as dielectric functions, photocurrent, local electric field distribution, chemical composition and defects) of quantum materials and nanostructures, beyond the far-field resolution limit (diffraction) by exploiting the properties of evanescent waves of tunable illumination and excitation of tip-sample interaction.

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Facilities

Our quantum nano-optoelectronic metrology suite consists of s-SNOM systems both centrally at NPL (Teddington) and regionally at University of Manchester and Henry Royce Institute. The combined capabilities allow correlative or hyperspectral optoelectronic characterisation at nanoscale for quantum materials across visible-NIR-MIR(-THz via collaboration) at room-cryogenic temperatures.
 

s-SNOM facilities at NPL (Teddington)

Our s-SNOM facilities at NPL (Teddington) are equipped with a visible source at 532 nm and two mid-IR sources – a quantum cascade laser (QCL) tunable in the range 950-2000 cm-1 (5-10.5 μm) and an optical parametric oscillator (OPO) in the range 950-4000 cm-1 (2.5-10.5 μm), as well as customised electronics for optoelectronic measurements at room temperature.

Mid-IR and visible s-SNOM facilities at NPL (Teddington site).                       Internal optics and components of the NPL s-SNOM

s-SNOM facilities at University of Manchester and Henry Royce Institute

Our recently established s-SNOM facilities at University of Manchester and Henry Royce Institute include a room temperature s-SNOM operating at 633, 785, 1064 and 1550 nm, as well as 532 nm laser that is coupled to the existing cryogenic s-SNOM within the CUSTOM facility based at the Photon Science Institute. These systems are capable of simultaneous 2D mapping of amplitude and phase of the scattered light (pseudo-heterodyne detection) at visible and NIR ranges at room temperature and at a visible wavelength of 532 nm down to 10 K. Additional excitation sources such as broadband MIR and THz are accessible via the CUSTOM facility at University of Manchester and Henry Royce Institute, which is led by Dr Jessica Boland. 

 

Regional visible and NIR s-SNOM at the University of Manchester.                            Visible cryogenic s-SNOM at the University of Manchester, operating at 532 nm and 10 K.

Techniques

AFM

As a basic function of s-SNOM, AFM measures the topography of the sample surface to gain structural and mechanical information at nanometre resolution. Such measurement is independent of light excitation and can be used to correlate with s-SNOM measurements.

s-SNOM

 As an optical detection scheme, s-SNOM utilises asymmetric interferometry in which the scattered light from the AFM tip-sample arm is recombined with a light beam from the reference arm and measured at the detector. We use either homodyne or pseudo-heterodyne interferometric scheme to vary the reference phase, allowing for separate/simultaneous measurement of phase and amplitude of the tip-scattered light.

Higher harmonic modulation of the signal detection is essential to eliminate the far-field background and retain pure near-field maps. Demodulation order and tip tapping amplitude can be modified to tune the probing depth. We can also accommodate extra polarisation optics to excite and detect in a geometry suitable for a wide variety of samples and devices.

s-SNOM can be used to measure and image phonon-, plasmon- and exciton-polaritons in quantum materials, absorption and reflection (dielectric functions) of quantum dots, semiconductor thin films, 2D and topological materials nanostructures, as well as local field of nano-antennas and nanophotonic metasurfaces.

Comparison of diffraction limited and SNOM imaging of phonon-polaritons in 2D materials.

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Strongly Absorbing Nanoscale Infrared Domains within Strained Bubbles at hBN–Graphene Interfaces

QuantIC Enhanced Imaging Hub

Photocurrent nanoscopy

By adding electrical read-out, our s-SNOM is modified to enable scanning nearfield photocurrent nanoscopy and nano-spectroscopy. This capability measures lateral photocurrent or photovoltage at higher harmonics of the tip-light modulation, instead of measuring purely the scattered light of a typical s-SNOM.

Photo-electrical response originated from nanoscale features (such as quantum-confined structures, defects and grain boundaries) can be mapped at lateral resolution of ~ 20 nm. Both visible and IR sources are available for this measurement mode.

Such measurements are suitable for characterising a wide range of optoelectronic devices (such as photothermoelectric detectors, photovoltaic light sensors and photo-FETs), especially providing valuable information on device quality and uniformity by assessing nanoscale variations.

Topology and photocurrent nanoscopy of a CVD graphene device.

Photothermal AFM-IR

This technique measures materials IR absorption at nanoscale. Upon absorbing IR radiation, local thermal expansion of the sample is mechanically detected by the AFM tip. Measurement of point IR absorption spectra and 2D mapping can be achieved in a variety of configurations including contact, resonance-enhanced and tapping modes with the QCL or OPO as sources of monochromatic, tuneable illumination.

This technique is suitable for chemical identification of a sample surface at nanoscale, such as investigating contaminations, mapping mixtures of organic blends, or characterising biochemical functionalisation of advanced materials.

Tapping AFM-IR spectra and mapping of PBASE functionalise graphene.