5.2 Microscopy and Spectroscopy
147
on the time binning of the time-correlator (TCSPC)
6 unit, which can be as good as
a few picoseconds.
Such single-photon counting method can be applied for instance to detect very
weak signals from single quantum emitters, 2D materials and indirect excitons (see
[7] for use with 2D materials). Comparably, a streak camera can be operated in a
single-photon counting mode (see for instance [34]). In addition, the streak camera’s
CCD-type camera can also enable the measurement of (1D) spatially-resolved PL
transients if the projection of the sample surface is directly imaged on the entrance
slit of the streak camera, i.e. without dispersive element before streak system.
For time-resolved PL, the resolution is limited by the excitation pulse duration
τ p and the temporal resolution of the detection τ d , which results in the overall timeresolution of t ≈
τ 2
p + τ
2
d . The “faster” the excitation source (in terms of pulse
duration), the sharper the events can be recorded around t = 0. However, the shorter
the excitation pulses in time, the broader their spectral bandwidth, whereby spectral
selectivity in resonant excitation schemes gets reduced due to the Fourier relationship γ ≥ 1/τ p . Similarly, temporal resolution comes at the cost of spectral resolution
through the uncertainty principle. A too strong grating dispersion in the monochromator before the streak camera affects the time trace quality. Remarkably, state-of-the
art streak cameras provide sub-200-fs resolution.
A very good temporal resolution for PL measurements can be obtained by a special
technique called PL up-conversion. Employing a nonlinear crystal and a reference
beam from the excitation laser, the sample signal is overlapped with the reference
for sum-frequency generation (SFG). This corresponds to cross correlation between
the probe laser pulse and the fluorescence signal. Thus, SFG signal is only generated
within the time period when the probe laser pulse is present in the crystal. Thereby,
the laser pulse exposure acts as an optical gating process [35]. By adjusting a temporal
delay between emitted pulse and reference pulse, the transient PL can be sampled
in time. While this technique provides temporal resolution on the order of the pulse
duration, recordings of transients from pulsed sample signal become particularly
challenging at longer delays when the emission signal becomes very weak owing to
the strong intensity dependence of SFG.
In fact, optical gating is the standard technique for the characterisation of ultrashort
pulses, using intensity autocorrelation in a nonlinear crystal for second-harmonic
generation (SHG). In this scheme, the pulse probes itself, and the temporal resolution
is given by the time duration of the pulse. Derivatives of the intensity autocorrelator
that provide a more complete characterisation of light pulses use spectral analysis of
the SHG signal for the extraction of phase information. FROG and GRENOUILLE
are two commonly used tools for this purpose with similar background but different
setup realisation. For a broader overview on pulse characterisation and commercial
tools, the interested reader is referred to the tutorials webpage of a prominent producer
of such pulse-analysis tools [36].
6 Time-correlated single-photon counting.
147
on the time binning of the time-correlator (TCSPC)
6 unit, which can be as good as
a few picoseconds.
Such single-photon counting method can be applied for instance to detect very
weak signals from single quantum emitters, 2D materials and indirect excitons (see
[7] for use with 2D materials). Comparably, a streak camera can be operated in a
single-photon counting mode (see for instance [34]). In addition, the streak camera’s
CCD-type camera can also enable the measurement of (1D) spatially-resolved PL
transients if the projection of the sample surface is directly imaged on the entrance
slit of the streak camera, i.e. without dispersive element before streak system.
For time-resolved PL, the resolution is limited by the excitation pulse duration
τ p and the temporal resolution of the detection τ d , which results in the overall timeresolution of t ≈
τ 2
p + τ
2
d . The “faster” the excitation source (in terms of pulse
duration), the sharper the events can be recorded around t = 0. However, the shorter
the excitation pulses in time, the broader their spectral bandwidth, whereby spectral
selectivity in resonant excitation schemes gets reduced due to the Fourier relationship γ ≥ 1/τ p . Similarly, temporal resolution comes at the cost of spectral resolution
through the uncertainty principle. A too strong grating dispersion in the monochromator before the streak camera affects the time trace quality. Remarkably, state-of-the
art streak cameras provide sub-200-fs resolution.
A very good temporal resolution for PL measurements can be obtained by a special
technique called PL up-conversion. Employing a nonlinear crystal and a reference
beam from the excitation laser, the sample signal is overlapped with the reference
for sum-frequency generation (SFG). This corresponds to cross correlation between
the probe laser pulse and the fluorescence signal. Thus, SFG signal is only generated
within the time period when the probe laser pulse is present in the crystal. Thereby,
the laser pulse exposure acts as an optical gating process [35]. By adjusting a temporal
delay between emitted pulse and reference pulse, the transient PL can be sampled
in time. While this technique provides temporal resolution on the order of the pulse
duration, recordings of transients from pulsed sample signal become particularly
challenging at longer delays when the emission signal becomes very weak owing to
the strong intensity dependence of SFG.
In fact, optical gating is the standard technique for the characterisation of ultrashort
pulses, using intensity autocorrelation in a nonlinear crystal for second-harmonic
generation (SHG). In this scheme, the pulse probes itself, and the temporal resolution
is given by the time duration of the pulse. Derivatives of the intensity autocorrelator
that provide a more complete characterisation of light pulses use spectral analysis of
the SHG signal for the extraction of phase information. FROG and GRENOUILLE
are two commonly used tools for this purpose with similar background but different
setup realisation. For a broader overview on pulse characterisation and commercial
tools, the interested reader is referred to the tutorials webpage of a prominent producer
of such pulse-analysis tools [36].
6 Time-correlated single-photon counting.