In the last decade, coherent anti-Stokes Raman scattering (CARS) microscopy has emerged as a powerful multiphoton imaging technique offering label-free chemical sensitivity and high three-dimensional resolution. However, its widespread application in the life sciences has been hampered by the use of costly pulsed lasers, the existence of a nonresonant background requiring involved technical solutions for its efficient suppression, and the limited acquisition speed of multiplex techniques addressing several vibrational resonances, if improved chemical specificity is needed. We have recently reported a differential CARS technique (D-CARS), which simultaneously measures two vibrational frequencies, enhancing the chemical selectivity and sensitivity without introducing costly hardware, while maintaining fast acquisition. In this study, we demonstrate a compact, fully automated, cost-effective module, which integrates on hardware and software level with a commercial multiphoton microscope based on a single 100 fs Ti:Sapphire oscillator and enables D-CARS microscopy in a user-friendly format for applications in the life sciences.
We provide direct evidence that the macroscopic response of the gain dynamics in electrically-pumped In-
GaAs/GaAs quantum dots is a superposition of intradot relaxation dynamics from microstates with multiple
discrete carrier numbers. The gain recovery in the presence of an optical pre-pump fully depleting the ground-state
gain is measured to be faster than without pre-pump. This effect, opposite to expectations from rate
equations with mean-field carrier distributions, is due to a conditional gain recovery in which microstates with
slow internal dynamics are suppressed by the pre-pump. The effect is evident at 15K and still observable at
300 K, beneficial for high-speed optical signal processing.
We have developed a novel multiphoton microscopy technique not relying on (and hence not limited by) fluorescence
emission, which exploits the third-order nonlinearity called four-wave mixing of gold nanoparticles
in resonance with their surface Plasmon. The coherent, transient and resonant nature of this signal allows its
detection free from backgrounds that limit other contrast methods for gold nanoparticles. We show detection
of single 10nm gold nanoparticles with low excitation intensities, corresponding to negligible average thermal
heating. Owing to the the third-order nonlinearity we measure a transversal and axial resolution of 140nm
and 470nm respectively, better than the one-photon diffraction limit. We also show high-contrast imaging of
gold-labels down to 5nm size in Golgi structures of HepG2 cells at useful imaging speeds (10 kHz pixel rate).
Thermal dissociation of gold nanoparticles from their bonding sites when varying the excitation intensity is also
We demonstrate frequency differential CARS (D-CARS) using femtosecond laser pulses linearly chirped by glass
elements of high group-velocity dispersion. By replicating the Pump-Stokes pair into a pulse train at twice the
laser repetition rate, and controlling the instantaneous frequency difference by glass dispersion, we adjust the
Raman frequency probed by each pair in an intrinsically stable way. The resulting CARS intensities are detected
simultaneously by a single photomultiplier as sum and difference using lock-in detection. We demonstrate
imaging of living cells with strongly suppressed non-resonant background. We also show D-CARS using a single
femtosecond laser source.
We demonstrate a novel multi-photon imaging modality based on the detection of four-wave mixing (FWM) from
colloidal nanoparticles. Four-wave mixing is a third-order signal which can be excited and detected in resonance
with the ground-state excitonic transition of CdSe/ZnS quantum dots. The coherent FWM signal is detected
interferometrically to reject incoherent backgrounds for improved image contrast compared to fluorescence methods.
We measure transversal and axial resolutions of 140nm and 590nm respectively, significantly beating the
one-photon diffraction limit. We also demonstrate optical imaging of quantum-dot-labeled Golgi structures of
We have compared the gain dynamics of the ground state excitonic
transition between undoped and p-doped electrically-pumped InGaAs
quantum-dot optical amplifiers, for temperatures from 300 K to
20 K. A pump-probe differential transmission technique in
heterodyne detection with sub-picosecond time resolution was used.
The comparison shows that in the gain regime at high temperatures
the recovery dynamics of the p-doped sample is slower than in the
undoped device operating at the same modal gain, due to a reduced
electron reservoir in the excited states. Conversely, at 20 K the
initial gain dynamics is faster in the p-doped device due to
We present a theoretical analysis of the response of whispering gallery modes for biosensing applications, studied
numerically in microcylinders and semi-analytically in microspheres. The effect of single and multiple particles
is calculated, simulating biological analytes of different sizes and polarizabilities attached to the microresonator
surface. Besides whispering-gallery-mode frequency shifts, we find that also broadenings and splittings (from
lifted rotational symmetry) appear due to particles attachment and/or the vicinity to a planar coupler. For a
single analyte, both particle size and refractive index can be determined from the broadening and shift, opening
the perspective to a new biosensing modality.
We have developed a home-built CARS microscope which exploits linearly-chirped ultrafast laser pulses. By using
glass of high group-velocity dispersion, Stokes and Pump pulses of 150 fs duration Fourier-limited are equally
chirped to pulse durations in the 0.5 ps-2.8 ps range. In this way we reduce the spectral width of the instantaneous
frequency difference to the Fourier limit of the chirped pulse duration (spectral focussing). As a proof of
principle, CARS spectroscopy with high spectral resolution is demonstrated on polystyrene beads. We also
show, both theoretically and experimentally, that for chirped pulse durations shorter than or comparable to the
Raman coherence time, maximum CARS signal occurs for a Pump arriving after the Stokes pulse. Furthermore,
we demonstrate the applicability of our CARS microscope to biological sciences by performing CARS microspectroscopy
on different live cells and fixed tissue samples.
The dephasing time in semiconductor quantum dots and quantum-dot molecules is measured using a sensitive four-wave mixing heterodyne technique. We find a dephasing time of several hundred picoseconds at low temperature in the ground-state transition of strongly-confined InGaAs quantum dots, approaching the radiative-lifetime limit. Between 7 K and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. On a series of InAs/GaAs quantum-dot molecules having different interdot barrier thicknesses a systematic dependence of the dephasing dynamics on the barrier thickness is observed. The results show how the quantum mechanical coupling of the electronic wavefunctions in the molecules affects both the exciton radiative lifetime and the exciton-acoustic phonon interaction.
We present temperature-dependent measurements of the dephasing time in the ground-state transition of strongly-confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. At low temperature we measure a dephasing time of several hundred picoseconds. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian lineshape with a sharp zero-phonon line and a broad band from elastic exciton-acoustic phonon interactions. We also explore the dephasing time beyond the one exciton occupation, by electrically injecting carriers. Electrical injection into the barrier region results in a dominantly pure dephasing of the excitonic ground-state transition. Once the injected carriers have filled the electronic ground state, additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation.
We have produced GaAs-based quantum-dot edge-emitting lasers operating at 1.16 μm with record-low transparency current, high output power, and high internal quantum efficiencies. We have also realized GaAs-based quantum-dot lasers emitting at 1.3 μm, both high-power edge emitters and low-power surface emitting VCSELs. We investigated the ultrafast dynamics of quantum-dot semiconductor optical amplifiers. The dephasing time at room temperature of the ground-state transition in semiconductor quantum dots is around 250 fs in an unbiased amplifier, decreasing to below 50 fs when the amplifier is biased to positive net gain. We have further measured gain recovery times in quantum dot amplifiers that are significantly lower than in bulk and quantum-well semiconductor optical amplifiers. This is promising for future demonstration of quantum dot devices with high modulation bandwidth.
Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.