Ultrafast solid-state quantum optics Department of Physics and Center for Applied Photonics (CAP) Rudolf Bratschitsch University of Konstanz, Germany
Outline Solid-state systems for quantum optics Semiconductor quantum dots dielectric microcavities ultrafast & nano femtosecond dynamics in a fewfermion system Color centers in diamond ultrastable & nano & ambient conditions imaging magnetometry on the nanoscale Nature 455, 648 (2008) metal nanoantennas Nano Lett. 7, 2897 (2007) Opt. Expr. 16, 9791 (2008) Nature Photonics 2, 230 (2008) Nature Physics 5, 352 (2009)
100 years of the photon 1905: quanta of light introduced by Albert Einstein Since then: highly successful quantum optical experiments with this elusive particle! entangled photons: GHZ states A. Zeilinger et al., Happy centenary, photon, Nature 433, 230 (2005)
A solid-state system for quantum optics? - Miniaturization (nanoscale) and scalability e.g. semiconductor industry - Robustness (in air, in water, in space, ) - Uniqueness (new quantum optical experiments)
Solid-state quantum optical systems Superconducting circuits: circuit QED niobium films microwave photons T = mk Yale Semiconductor quantum dots Color centers in diamond U. Washington Photon emission in the visible!
Semiconductor quantum dots (QDs) What are QDs? Special nanocrystals! 1 cm 10 nm Evident Electron motion is restricted in QD! De Broglie wavelength of an electron bulk semiconductor quantum dot λ DB = h p = h 2m eff E = 25 nm m = 0. 1 eff m e De Broglie wavelength of the electron extensions of nanocrystal E 300 K therm = 25 mev quantum confinement : particle in a box
QDs: excitons and optical transitions molecule Quantum dots = artificial atoms quantum dot bulk semiconductor Electrical and optical properties different from bulk semiconductor material quantum confinement Energy conduction band Exciton (bound electron-hole pair) modeled as the electron and proton in a hydrogen atom size valence band Exciton recombination (optical transition) photon emission 2 Electron (+) Hole (-) E = hν 12 Semiconductor quantum dot = nanoscopic light emitter 1
Quantum dot applications Optoelectronic Devices sensors, LEDs, lasers, solar cells Biology and medicine J. Lee et al., Adv. Mater. 12, 1102 (2000) robust bio-markers (flourescence microscopy/tissue staining) www.qdots.com Emory University Mouse kidney section labeled with an anti-laminin primary antibody and visualized using greenfluorescent Qdot 565 IgG. Multicolor quantum dot probes injected into a mouse to detect and track multiple tumor targets
Semiconductor quantum dots: an on-demand quantum light source Deterministic single photon source: photon turnstile device optically driven: P. Michler et al., Science 290, 2282 (2000) C. Santori et al., Nature 419, 594 (2002) electrically driven: Z. Yuan et al., Science 295, 102 (2002) Source of triggered entangled photon pairs R. M. Stevenson at al., Nature 439, 179 (2006) N. Akopian et al., PRL 96, 130501 (2006) Quantum information processing and communications
Ultrafast quantum optics with semiconductor quantum dots Almost all QO experiments: stationary, narrowband lasers But: quantization and correlation energies of 10-100 mev Ultrafast excitation, readout and control: 100 fs pulse bandwidth of 15 mev femtosecond light pulses 1 fs = 10-15 s = 0.000000000000001 s ultrafast stroboscope light sheet d d = c t = 300 nm 1 fs wikipedia
Femtosecond pump-probe experiment high temporal and spectral resolution, limited only by uncertainty principle time delay probe pump QD - - + probe measurement of the probe transmission 10 nm
Question Is it possible to measure the transient transmission due to a single electron through a 10 nm object with femtosecond time resolution? Yes, and more!
Self-assembled CdSe/ZnSe quantum dot Photoluminescence (PL): trion ground state (s-shell emission) photoluminescence excitation (PLE): excited states (higher shell absorption) E 600 mev ZnSe CdSe E g = 2.8 ev E g = 1.9 ev 300 mev - + s z = ± 1/2 j z = ± 3/2 PL Intensity x QD light emission (stationary) pump: trion = t p = 700 fs charged exciton X - probe: t p = 170 fs 2.10 2.15 2.20 2.25 Photon Energy (ev) PLE Intensity
Tunable two-color femtosecond Er:fiber laser system fs Er:fiber oscillator fs Er:fiber amplifier wavelength conversion F. Tauser et al., Opt. Express 11, 594 (2003); K. Moutzouris et al., Opt. Lett. 31, 1148 (2006); F. Adler et al., Opt. Lett. 32, 3504 (2007)
Resonant excitation of single quantum dot: first femtosecond transient transmission data Transmission Change ΔT/T x 104 pump: p-shell, P = 200 μw, tp = 700 fs probe: fundamental X- resonance, P = 50 μw, tp = 170 fs F. Sotier et al., Nature Physics 5, 352 (2009)
t D = 2 ps: Bleaching of X - due to instantaneous Coulomb renormalization absolute probe transmission T 0 : without pump T exc : with pump Transmission X - Probe Photon Energy Transmission Probe Photon Energy pump - + F ee - probe F eh s z = ± 1/2 j z = ± 3/2 differential transmission: ΔT/T = (T exc -T 0 )/T 0 ΔT/T X - Probe Photon Energy ΔT/T x 10 4 6 4 2 0 Pump: 557 nm, 200µW, Probe: 587 nm, 50 µw, T = 4.4 K t D = 2 ps F. Sotier et al., Nature Physics 5, 352 (2009) 2.108 2.112 2.116 Probe Photon Energy (ev)
t D = -2 ps: probe pulse precedes pump pulse perturbed free induction decay of X - probe field and probe induced polarization T 0 : without pump T exc : with pump - - s z = ± 1/2 pump + probe j z = ± 3/2 ΔT/T @ t d = -2 ps: experimental data and fit based on optical Bloch equations ΔT/T x 10 4 6 4 2 0 t D = -2 ps 2.108 2.112 2.116 Probe Photon Energy (ev)
t D = 20 ps: hot carrier relaxation inversion and gain at fundamental X - res. absolute probe transmission T 0 : without pump T exc : with pump Transmission X - Transmission X - probe - - - s z = ± 1/2 inversion! Probe Photon Energy Probe Photon Energy differential transmission signals: + + j z = ± 3/2 ΔT/T x 10 4 6 4 2 t D = 2 ps ΔT/T x 10 4 6 4 2 t D = 20 ps ΔT/T x 10 4 6 4 2 t D = 50 ps 0 0 0 2.108 2.112 2.116 Probe Photon Energy (ev) 2.108 2.112 2.116 Probe Photon Energy (ev) 2.108 2.112 2.116 Probe Photon Energy (ev)
Linear probing Nonlinear probing linear regime: weak probe pulse Pump + t D = 20 ps: system in inverted state: X - > X-> e> Field Amplitude Theory (OBE) 0.0 1.0 Time (ps) ΔT/T x 10 4 envelopes of probe field and probe induced polarization Differential Transmission 2 Pprobe = 75 μw 1 0 2.112 2.115 Photon Energy (ev)
Nonlinear probing: deterministic absorption and emission of single photons Nonlinear regime: Field Amplitude no polarization tail 400 μw 1 one photon added to pulse by narrow resonance no change of temporal and spectral 0 envelope of probe pulse! towards 0.0 single-photon 1.0 amplifier 2.112 2.115 0.0 1.0 Time (ps) ΔT/T x 10 4 ΔT/T x 10 4 F. Sotier et al., Nature Physics 5, 352 (2009) 2 Pprobe = 2 Pprobe = 800 μw 1 0 2.112 2.115 Photon Energy (ev)
Towards femtosecond single-photon technology goal: optimization of the coupling nanoobject light field two concepts I. microcavity II. metal nanoantenna 1-2 µm AFM laser focus 10 nm metalnanoantenna semiconductor nanocrystal Multiple reflections at cavity mirrors / electric field enhancement Electric field enhancement in antenna feedgap
Dielectric optical cavity: Bragg mirrors TiO 2 SiO 2 no absorption high reflectivity due to multipath interferences high flexibility in target wavelength design d i = λ/4n i Substrate (SiO 2 ) 100 80 Bragg mirror transmission Fabrication via Radio Frequency (RF) sputtering Reflectivity depends on the number of layers and the refractive index contrast (n 1 /n 2 ) Width of the stopband depends on the refractive index contrast transmittance (%) 60 40 20 15 layer pairs 0 400 500 600 700 800 900 1000 wavelength (nm)
Emission from colloidal quantum dots embedded in a planar cavity Planar resonator (two dielectric mirrors with 14 layer pairs each) width of resonance: Δλ = 0,074 nm resonator quality: Q factor : Q = Δλ/λ = 8200 substrate (SiO 2 ) colloidal CdSe/ZnS quantum dots embedded in liquid glass (polysilazane) luminescence intensity (a.u.) λ/2 cavity 10 8 6 4 2 cavity with two mirrors 14 mirror layers each cw = 616.3 nm FWHM = 0.074 nm int. time 300 s exitation: 532 nm (cw) Photoluminescence lunimescence fit (Lorentzian) Q = 8217 0 615.0 615.5 616.0 616.5 617.0 617.5 wavelenght (nm) Q-factor 50x higher than previously reported values.
Cavity with 3D light confinement goal: confinement of the light field in all three dimensions planar resonator: light confinement only perpendicular to the mirrors mode volume V mod not optimized Dielectric mirror Dielectric mirror Substrate no lateral confiement solution: cylindrical waveguiding structure cut into the planar cavity lateral mode guiding by total internal reflection at the walls cavity Dielectric mirror
Fabrication of pillar microresonators with the focussed ion beam (FIB) Milling of a pillar resonator into a planar dielectric cavity circular pillar resonator SEM image M. Kahl et al., Nano Lett. 7, 2897 (2007) elliptical pillar resonator
Pillar cavity modes HE12 HE31, EH31 HE01,HE21 HE11 Theory space energy (ev) 80 1.91 1.92 1.93 1.94 1.95 PL Intensity 70 λ exc : 532 nm wavelength T int = 1500 s PL intensity (a.u.) I = 400 W /cm 2 60 50 40 30 d=5.1µm HE11 HE21 HE31 λ exp (nm) 648.2 nm 646.5 nm 644.2 nm 644.1 nm PL emission CCD image λ theo (nm) 648.4 nm 646.7 nm 644.6 nm 20 634 636 638 640 642 644 646 648 650 HE12 HE41 643,5 nm 641.4 nm 643.8 nm 641.9 nm wavelength (nm) M. Kahl et al., Nano Lett. 7, 2897 (2007)
Pillar diameters energy (ev) 2.08 2.06 2.04 2.02 2 1.98 μpl (arb. u.) 60000 30000 1.51 µm 0 922 nm 600 605 610 615 620 625 wavelength (nm) 3D light confinement: blueshift with decreasing pillar diameter
Metal nanoantennas radioantenna optical nanoantenna 1-2 µm laser focus 10 nm metalnanoantenna semiconductor nanocrystal USW: 87.5 108 MHz wavelength ~ 3 meters! Light: 100s of THz nanometers!
Fabrication of optical nanoantennas colloidal chemistry electron-beam lithography focused ion beam (FIB) milling colloidal lithography 100 nm
Fabrication of a gold nanotriangle with a colloidal mask side view gold glass colloid top view colloid AFM image glass gold nanotriangle: triangle length: 128 nm triangle height: 110 nm thickness: 32 nm
Gold bowtie nanoantenna antenna length: 300 nm Antenna feedgap: 85 nm AFM image scattering intensity (a.u.) scattering intensity (a.u.) 1.00 0.75 0.50 0.25 Darkfield scattering spectrum feedgap: 85 nm 100 nm 0 0.00 500 600 700 800 900 wavelength (nm) 100 nm Scattering spectrum = sum of two single nanotriangles
Bowtie antenna with variable feedgap nanomanipulation + nanooptics
Nanoantenna with variable feedgap: optical response 100 nm AFM image scattering intensity (a.u.) scattering intensity (a.u.) (a.u.) 1.00 1.00 0.75 0.75 0.50 0.50 0.25 0.25 Mode Splitting Peak intensity decreases Redshift 30 nm 0.00 0 500 600 700 800 900 wavelength (nm) (nm) 100 nm Splitting of dipole plasmon mode (nanotriangles couple!) Redshift of longer wavelength mode Intensity of shorter wavelength mode decreases Drastic change when antenna arms in contact J. Merlein et al., Nature Photonics 2, 230 (2008)
Plasmon resonances: gap-dependent splitting and redshift triangular prisms 3D shape cross section truncated tetrahedra scattering intensity (a.u.) scattering intensity (a.u.) antenna gap 85 nm 40 nm 25 nm 15 nm 5 nm 0 nm -16 nm calculation discrete dipole approximation (DDA) scattering intensity (a.u.) scattering intensity (a.u.) antenna gaps 85 nm 40 nm 25 nm 15 nm 5 nm 0 nm -16 nm 500 600 700 800 900 1000 wavelength (nm) wavelength (nm) Only one resonance! Redshift with smaller gap 500 600 700 800 900 1000 wavelength (nm) Two resonances! Redshift of longer wavelength mode Decreasing short wavelength mode 3D shape plasmon resonances J. Merlein et al., Nature Photonics 2, 230 (2008)
Diamond nanophotonics Goal: robust solid-state quantum optical system at room temperature Color centers in diamond 637 nm Nitrogen vacancy (NV - ) center: molecule in solid-state matrix
Optically detected magnetic resonance (ODMR) with single spin in NV center ODMR of single spin! diamond nanocrystal with single NV center nanoscale resolution magnetic field sensor G. Balasubramanian et al., Nature 455, 648 (2008)
Nanoscale imaging magnetometry measuring magnetic fields with high precision on the nansocale at ambient conditions biophysics G. Balasubramanian et al., Nature 455, 648 (2008)
Conclusions Semiconductor quantum dots: - femtosecond initialization, manipulation and readout of single fermions in a semiconductor quantum dot - nonlinear probing: adding of a single photon to a fs laser pulse - towards femtosecond single-photon technology: optical microcavities metallic nanoantennas Color centers in diamond: - magnetic field sensor with nanoscopic resolution at ambient conditions