Mitrofanov, Oleg

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UCL Discovery

Oleg MitrofanovUCL, UK1CLEO 2021 TutorialMay 13 | STh2C.5 | Terahertz Imaging and Detection THz near-field Imaging and Spectroscopy:Technology, Capabilities and Novel Applicationso.mitrofanov@ucl.ac.ukTHz Near-Field MicroscopyBroad range of scales and systems3Broad range of THz Instrumentationd / l10.10.011e-5    1e-4 1e-3300µm30µm3µm300nm30nmWaveguide modes, structured beamsPlasmons, metasurfaces, photonic crystals,  bio-tissueBiological cells, resonator modesnanowires, 2D materials Molecules, quantum dots, excitonsShort THz pulse sources and THz-TDSQCL and Self-mixing Detection Electro-optic SamplingFET and CMOS THz Detection PC DetectorsTutorial Scope and Aim4Approaches in THz near-field microscopy  THz near-field microscopy applications Right instrument for a given problemAppropriate set of problems for a given instrument? ? ? ?‘Right Instrument for the Job’… recorded with one of the first THz near-field systems ~20 years ago5Time-resolved (sub-ps) THz surface-plasmon waves on a THz antenna …OM et al. J. Select. Topics Quant. Electron. 103, 600 (2001)  ‘Right Instrument for the Job’6Excellent material contrast, but observation of THz surface-plasmon waves is not as straight forwardsee also:  CLEO 2021 Session SW2K.5-7 (T. Hannotte, A. Pizzuto, M. Liu)X. Chen et al. ACS Photonics 7, 687 (2020)  nm-scale resolution – imaging of subwavelength metallic structuresEarly Development of THz MicroscopesHunsche et al. OPTICS COMM (1998)7Early Development of THz MicroscopesVan der Valk et al., APPL. PHYS. LETT. (2002)Electro-optic near-field probes (THz-TDS)8Early Development of THz MicroscopesChen et al., APPL. PHYS. LETT. 83, 3009 (2003)Scattering Tip near-field microscopy (THz-TDS) 920 years of THz Microscopy DevelopmentNear-field probes with integrated THz detectorsSub-wavelength THz generationUse of patterns instead of aperturesEO materials/ultrathin crystalsHigh-E THz sourcesHigh Order DemodulationResonant Tip ProbesSelf-mixing DetectionHunsche et al., 1998 Van der Valk et al., 2002 Chen et al., 2003Stantchev (2016)Blanchard & Tanaka (2016)Grzyb (2016)Maissen (2019)11Siday (2020)OM (2016)OM (2017)Aperture-type THz MicroscopyPrinciple Practical LimitationsEnabling Technology Advances ApplicationsL. Hale et al., Laser. & Photon. Rev. 14 (4), 1900254 (2020) (illustration: T. Siday)SchematicIncident THz waveFar-field:Transmitted wave Diffracted waveSurface waves excited at edges (non-resonant)Resonant surface plasmonsNear-fieldSchematic – Collection ModeIncident THz waveLocalized fields are sampled and transferred to Far-field:Surface waves excited at edges (non-resonant)Resonant surface plasmonsNear-fieldSchematic – Illumination ModeIncident THz waveWave Transmission through Subwavelength Aperturesa3 l-2a3 l-1a1 l-1Evanescent waves Propagating waves H. A. Bethe  Phys. Rev. 66, 163 (1944)T = |Et|2/ |Einc|2 ~ a6Wave Transmission through Subwavelength Aperturesa3 l-2a3 l-1a1 l-1Evanescent waves Propagating waves H. A. Bethe  Phys. Rev. 66, 163 (1944)Improving resolution by factor of 2comes with power transmission penalty of almost 2 orders of magnitudenear-field Bethe: ~  a3OM et al.,  APL 79, 907 (2001)T = |Et|2/ |Einc|2 ~ a6Aperture-type Probes Limitation18d / lErnst Abbe (c. 1873)10.10.011e-5    1e-4 1e-3300µm30µm3µm300nm30nmWaveguide modes, structured beamsPlasmons, metasurfaces, photonic crystals,  bio-tissueBiological cells, resonator modesnanowires, 2D materials Molecules, quantum dots, excitonsNear-field DetectionOM et al., APL 77, 3496 (2000),  APL 79, 907 (2001)a3 l-2a3 l-1a1 l-1Evanescent waves Propagating waves T = |Et|2/ |Einc|2 ~ a6Aperture-type THz microscopyGold screen with5-50µm aperture (<< lTHz)LT GaAs PC antenna OM et al., APL. 77,  3496  (2000 )LT GaAs PC antenna sampling optical pulseintegratedprobeNear-field Detection270 nm500 nm4.0  µmOM et al. THz Sci. and Technology, IEEE Trans. (2016) Near-field Probes with Integrated DetectorsOM et al., ACS Photonics (2015)Electric field  (a.u.)Resolution Test:2 µm aperture22OM et al.THz Sci. and Technology, IEEE Trans. (2016) OM et al., Appl Phys Lett. 2017210-1-2Ed Nano-scale THz DetectorsFET-based 2D materials InAs nanowire detectorsOM et al. Sci. Rep. (2017)CMOS-based detectorsGrzyb et al. (2016)232DEG detectorsKawano et al. (2008)Ultrathin Photoconductive Metasurfaces (THz-TDS)BPOptica 5, 5, 651-657 (2018)Nano Lett. 19, 5, 2888 (2019)Coherent Aperture-type THz microscopy with QCLsOM et al., Scientific Reports 7, 44240 (2017)Phase Mapping and Sensitivity Gain Phase InformationSensitivity GainE (coher.)IntensityPhaseOptica 5, 5, 651-657 (2018)Consistent with THz-TDS NF microscopy:Optics Express 20(6), 6197 (2012)Local THz ProbesBhattacharya and Rivas, APL Photonics 1, 086103 (2016)26Commercial Tera-Spike probe from Protemicswww.protemics.com‘Optical Near-field Probes’ 27Chen et al. OPTICS LETTERS 25 (2000)Local (near-field) THz Wave Modulation, Generation and DetectionNear-field THz Modulation28Stantchev et al., Sci. Reports 8, 6924 (2018)Near-field Detection – Electro-Optic Imaging29Blanchard & Tanaka Optics Lett. 41, 4645 (2016)10 µm thick EO CrystalTHz Near-field EmissionK. Okada et al. Photonics, 8, 151 (2021)30Aperture-type THz Microscopy Application Range31d / lErnst Abbe (c. 1873)10.10.011e-5    1e-4 1e-3300µm30µm3µm300nm30nmWaveguide modes, structured beamsPlasmons, metasurfaces, photonic crystals,  bio-tissueBiological cells, resonator modesnanowires, 2D materials Molecules, quantum dots, excitonsScattering Tip THz Nanoscopy32Image: Tom SidayPrinciple Practical LimitationsEnabling Technology Advances ApplicationsScattering Tip THz Nanoscopy Challenges33Weak THz Scattering from Probe TipSolutions:ScatteringEnhancement Sensitive Detection Detailed Introduction: Invited talk by Mengkun Liu ( CLEO 2021 SW2K.7 )Challenges:Image InterpretationEnhancement of Scattering from the Probe34Two Main Factors affecting the Scattering efficiency:Tip apex sizeTip shaft lengthC. Maissen et al., ACS Photonics 6, 1279 (2019)Enhancement of Scattering from the Probe35T. Siday et al.ACS Photonics 7, 596 (2020) Two Main Factors affecting the Scattering efficiency:Tip apex sizeTip shaft lengthSelf-mixing effect in QCLs36see also works: Riccardo Degl’Innocenti (ACS Photonics 2017) – Tunning fork ProbeMiriam Vitiello (OpEx 2018) - NeaspecP. Dean et al. Appl. Phys. Lett. 108, 091113 (2016)THz s-SNOM Application Range37d / lErnst Abbe (c. 1873)10.10.011e-5    1e-4 1e-3300µm30µm3µm300nm30nmWaveguide modes, structured beamsPlasmons, metasurfaces, photonic crystals,  bio-tissueBiological cells, resonator modesnanowires, 2D materials Molecules, quantum dots, excitonsTHz sub-wavelength Imaging Applications38Surface waves / PlasmonsTHz Subwavelength ResonatorsTHz EmissionMetallic DielectricTHz Surface Plasmon Waves39Mitrofanov et al.J. STQE 103, 600 (2001)  THz Surface Plasmon Waves – Mapping with Aperture 40ETHz Surface Plasmon Waves – Mapping with Aperture 41Symmetry changes:from anti-symmetric to symmetric  Edet = dEz / dx  aEzTHz Surface Plasmon Waves – Mapping with Aperture 42Conductive carbon fibres: 6.5 µm diameter, 50-250 µm longTHz near-field ImagesTHz Surface Plasmon Waves – Mapping with Aperture 43OM et al.IEEE Trans. THz S&T 6, 382 (2016)  THz Surface Plasmon Waves - Graphene44Incident THz waveFar-field:Transmitted wave Diffracted waveSurface waves excited at edges (non-resonant)Resonant surface plasmonsNear-fieldTransmission contrast due to Graphene45x (µm)y(µm)E OM et al.,  Appl. Phys. Lett. 103, 111105 (2013)Imaging of Gr Ribbon ArraysHunsche et al. OPTICS COMM (1998)46C CB BA Avert. ribbonshoriz. ribbons grapheneEx (µm)x (µm)x (µm)time (ps)OM et al.,  Appl. Phys. Lett. 103, 111105 (2013)Surface Waves on Gr Bow-Tie Antenna47-1             0.8            -0.6t1-1            0.75           -0.5time (ps)E (a.u.)E E 100 µmt1             t2Epitaxial monolayer graphene - Gr on C-face SiCt2Optical image (enhanced contrast)OM et al. Solid State Comm. 224, 47-52 (2015) Surface Waves on Gr Bow-Tie Antenna48-1             0.8            -0.6t1-1            0.75           -0.5time (ps)E (a.u.)E E 100 µmt1             t2t2Optical image (enhanced contrast)Metallic Bow-tie antenna Mueckstein et al., J.of IRMMW 32, 1031 (2011)Epitaxial monolayer graphene - Gr on C-face SiCOM et al. Solid State Comm. 224, 47-52 (2015) THz s-SNOM Imaging of Graphene49C. Maissen et al., ACS Photonics 6, 1279 (2019)Z. Yao et al., OpEx 27, 13611 (2019)GInAsGInAsTHz Surface Plasmon Waves – Acoustic Modes50Alonso-Gonzalez et al. (2017)Subwavelength Metallic Resonators51Mitrofanov, Todorov, et al., Optics Express 26 (6), 7437 (2018)Subwavelength Metallic Resonators52A’AResonant oscillations EdetT1                                  T2y (µm)A’A Substrate reflectionIncidentPulsel/23l/2210-1-2Ed OM et al., Appl. Phys. Lett. 110 (6), 061109 (2017) Single Resonator Sensitivity53L. Hale et al., Laser. & Photon. Rev. 14 (4), 1900254 (2020)(illustration: T. Siday)Single Resonator Sensitivity54L. Hale et al., Laser. & Photon. Rev. 14, 1900254 (2020) Dielectric Resonators55TiO2 microsphere: ~20 µm diametere ~ 70 – 150d ~ l /10 eo , ee – ?Im(e ) – ?Can NF microscopy probe such resonators?fMD ~ 1 THzEffect on Sub-wavelength Aperture TransmissionHigh EM field confinement by a dielectric objectEnhanced transmission through aperture can be used to probe high-e resonators OM et al., Optics Express, 22, 23034 (2014) 56Mie resonances in isotropic TiO2 spheres57OM et al., Optics Express, 22, 23034 (2014)Anisotropic Dielectric THz Resonators: I. Khromova, et al., Laser & Photon. Rev. 10 (4), 681 (2016)5859Near-field Mapping of THz Emission60Engineering PhysicsNear-field Mapping of THz EmissionSci. Rep. 6, 38926 (2016)Generation of THz pulses at semiconductor surfacesIR THzTHzEcnezzBuilt-in surface fieldCarrier density gradiente –e –61Surface Optical RectificationBulk Optical RectificationnhTHz Emission for Normal Incidence Photoexcitation200 µmCarrier density gradientsSpatial and Temporal Mapping of THz EmissionTHz emission originates from two distinct points corresponding to the Slit Edges.200 µmTime (ps)x position (µm)E63Mueckstein et al.THz Sci. and Technology, IEEE Trans. 5 (2), 260 (2015) Emission due to Charge Density Gradient Time (ps)x position100 µm wide Slit 200 µm wide Slit 300 µm wide SlitTHz emission originates from two distinct points corresponding to the Slit Edges.No emission from uniformly illuminated region of the semiconductorEMueckstein et al.THz Sci. and Technology, IEEE Trans. 5 (2), 260 (2015) Nanoscale Mapping of THz EmissionP. Klarskov et al. ACS Photonics 4, 2676 (2017)65Concluding Remarks66As more THz near-field systems coming online, the growing library of THz near-field images will help identify appropriate instruments for phenomena of interest.Explore and look out for novel THz near-field imaging modalities and applications.After over 20 years, THz Near-Field Microscopy research  is just warming up, there is still so much to explore.Acknowledgement67L. Hale, T. Siday (U. Regensburg), R. Hermans (Industry), A. Macfaden (U. Cambridge), R. Mueckstein (Industry), M. Navarro-Cia (U. Birmingham), M. Natrella (Industry), and R. ThompsonUniversity College LondonJ. Reno, I. Brener, T. Harris, T.S. Luk and W. PanCINT, Sandia National LaboratoriesL. Viti, M. C. Giordano, E. Dardanis, and M. S. VitielloCNR-Nano, ItalyW. Yu, C. Berger, W. A. de Heer, and Z. Jiang Georgia Tech. Y. Todorov, D. Gacemi, A. Mottaghizadeh and C. SirtoriUniv. Paris Diderot, Paris, FranceZ. Han, F. Ding and S. I. BozhevolnyiChina Jiliang University and DTU, DenmarkJ. Keller, G. Scalari and J. FaistETH-Zurich I. Khromova (U. Navarra) P. Mounaix (CNRS, Bordeaux), P. Kuzel (Czech Acad. Sci.)

THz Near-field Imaging and Spectroscopy: Technology, Capabilities and Novel Applications

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THz Near-field Imaging and Spectroscopy: Technology, Capabilities and Novel Applications

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