Alfred Leitenstorfer

Nanoscale imaging magnetometry with diamond spins under ambient conditions

Magnetic resonance imaging and optical microscopy are key technologies in the life sciences. For microbiological studies, especially of the inner workings of single cells, optical microscopy is normally used because it easily achieves resolution close to the optical wavelength. But in conventional microscopy, diffraction limits the resolution to about half the wavelength. Recently, it was shown that this limit can be partly overcome by nonlinear imaging techniques, but there is still a barrier to reaching the molecular scale. In contrast, in magnetic resonance imaging the spatial resolution is not determined by diffraction; rather, it is limited by magnetic field sensitivity, and so can in principle go well below the optical wavelength. The sensitivity of magnetic resonance imaging has recently been improved enough to image single cells, and magnetic resonance force microscopy has succeeded in detecting single electrons and small nuclear spin ensembles. However, this technique currently requires cryogenic temperatures, which limit most potential biological applications. Alternatively, single-electron spin states can be detected optically, even at room temperature in some systems. Here we show how magneto-optical spin detection can be used to determine the location of a spin associated with a single nitrogen-vacancy centre in diamond with nanometre resolution under ambient conditions. By placing these nitrogen-vacancy spins in functionalized diamond nanocrystals, biologically specific magnetofluorescent spin markers can be produced. Significantly, we show that this nanometre-scale resolution can be achieved without any probes located closer than typical cell dimensions. Furthermore, we demonstrate the use of a single diamond spin as a scanning probe magnetometer to map nanoscale magnetic field variations. The potential impact of single-spin imaging at room temperature is far-reaching. It could lead to the capability to probe biologically relevant spins in living cells.

How many-particle interactions develop after ultrafast excitation of an electron-hole plasma

Electrostatic coupling between particles is important in many microscopic phenomena found in nature. The interaction between two isolated point charges is described by the bare Coulomb potential, but in many-body systems this interaction is modified as a result of the collective response of the screening cloud surrounding each charge carrier. One such system involves ultrafast interactions between quasi-free electrons in semiconductors—which are central to high-speed and future quantum electronic devices. The femtosecond kinetics of nonequilibrium Coulomb systems has been calculated using static and dynamical screening models that assume the instantaneous formation of interparticle correlations. However, some quantum kinetic theories suggest that a regime of unscreened bare Coulomb collisions might exist on ultrashort timescales. Here we monitor directly the temporal evolution of the charge–charge interactions after ultrafast excitation of an electron–hole plasma in GaAs. We show that the onset of collective behaviour such as Coulomb screening and plasmon scattering exhibits a distinct time delay of the order of the inverse plasma frequency, that is, several 10-14 seconds.

Generation and field-resolved detection of femtosecond electromagnetic pulses tunable up to 41 THz

Bandwidth-limited infrared pulses as short as 50 fs are generated in thin GaSe crystals by phase-matched optical rectification of 10 fs laser pulses. The central frequency of the transients is continuously tunable over a wide interval extending from 41 THz (λ=7 μm) to the far-infrared. The electric field of the THz transients is directly monitored via ultrabroadband free-space electro-optic sampling. A simulation of the spectra based on a plane-wave model shows excellent agreement with the experiment.

Phase-locked generation and field-resolved detection of widely tunable terahertz pulses with amplitudes exceeding 100 MV/cm

Phase-locked terahertz transients with peak electric fields of 108 MV/cm and center frequencies continuously tunable from 10 to 72 THz are generated via difference-frequency mixing of two parametrically amplified pulse trains from a single white-light seed. Free space electro-optic sampling with 8 fs gating pulses from a two-branch Er:fiber laser allows us to monitor all transients directly in the time domain. We identify extreme terahertz nonlinearities in the detector crystal with subcycle resolution.

Detectors and sources for ultrabroadband electro-optic sampling: Experiment and theory

A detailed calculation of the amplitude and phase response of ultrathin ZnTe and GaP electro-optic sensors is presented. We demonstrate that the inclusion of the dispersion of the second-order nonlinearity is essential. Significant structures in experimental data can be explained by the theoretical response function. Correcting for the detector characteristics, we determine the precise shape of electromagnetic transients with a time resolution of 20 fs. In addition, we show that ultrafast transport of photocarriers in semiconductors can act as an efficient source for coherent electromagnetic radiation covering the entire far-to-mid-infrared regime.

Sub-cycle switch-on of ultrastrong light-matter interaction

Controlling the way light interacts with material excitations is at the heart of cavity quantum electrodynamics (QED). In the strong-coupling regime, quantum emitters in a microresonator absorb and spontaneously re-emit a photon many times before dissipation becomes effective, giving rise to mixed light–matter eigenmodes. Recent experiments in semiconductor microcavities reached a new limit of ultrastrong coupling, where photon exchange occurs on timescales comparable to the oscillation period of light. In this limit, ultrafast modulation of the coupling strength has been suggested to lead to unconventional QED phenomena. Although sophisticated light–matter coupling has been achieved in all three spatial dimensions, control in the fourth dimension, time, is little developed. Here we use a quantum-well waveguide structure to optically tune light–matter interaction from weak to ultrastrong and turn on maximum coupling within less than one cycle of light. In this regime, a class of extremely non-adiabatic phenomena becomes observable. In particular, we directly monitor how a coherent photon population converts to cavity polaritons during abrupt switching. This system forms a promising laboratory in which to study novel sub-cycle QED effects and represents an efficient room-temperature switching device operating at unprecedented speed.

Amplified femtosecond pulses from an Er:fiber system: Nonlinear pulse shortening and selfreferencing detection of the carrier-envelope phase evolution.

We present an improved design of an amplified Er:fiber laser system for the generation of intense femtosecond pulses. By properly controlling the influence of optical nonlinearities inside a stretched pulse amplifier, the spectrum is broadened to over 100 nm. The pulses are recompressed to 65 fs. A linearly polarized output of 110 mW is obtained at 67 MHz repetition rate. As a first application, we report the generation of an octave-spanning supercontinuum inside a short piece of a highly nonlinear fiber. Self-referencing detection of the carrier-envelope phase evolution with an f-to-2f interferometer is demonstrated.

Mid-infrared difference-frequency generation of ultrashort pulses tunable between 3.2 and 4.8 μm from a compact fiber source

We report single-pass difference-frequency generation of mid-infrared femtosecond pulses tunable in the 3.2-4.8 microm range from a two-branch mode-locked erbium-doped fiber source. Average power levels of up to 1.1 mW at a repetition rate of 82 MHz are obtained in the mid infrared. This is achieved via nonlinear mixing of 170 mW, 65 fs pump pulses at a fixed wavelength of 1.58 microm, with 11.5 mW, 40 fs pulses tunable in the near-infrared range between 1.05 and 1.18 microm. These values indicate that the tunable near-infrared input component is downconverted with a quantum efficiency that exceeds 30%.

Ultrabroadband detection of multi-terahertz field transients with GaSe electro-optic sensors: Approaching the near infrared

Field-resolved detection of ultrabroadband infrared pulses is implemented with GaSe crystals. Via phase matching, we extend the detectable frequency range toward the near infrared (λ=2.5μm) and directly record transform-limited pulses as short as 28fs with a 3dB bandwidth of 41THz. The continuous tunability of the center frequency over a wide interval ranging from 31THz to the far-infrared is demonstrated using thick sensors.

Long term comparison of two fiber based frequency comb systems

Mode-locked erbium-doped fiber lasers are ideal comb generators for optical frequency metrology. We compare two fiber frequency combs by measuring an optical frequency independently with both combs and comparing their results. The two frequency measurements agree within 6x10-16. This is to our knowledge the first direct comparison between two fiber based frequency combs.