7 Label-Free Pump–Probe Nanoscopy
177
onto a BBO crystal to detect the sum frequency generation (SFG) signal when the
pulses arrive on the crystal at the same time (Fig. 7.2c). The same principle can be
exploited using an autocorrelator. In order to optimize the temporal alignment at
the sample, the scattering signal coming from a stimulated Raman scattering (SRS)
pump–probe process [19] can be collected and maximized by moving the delay line.
By being a scattering process, this interaction can be considered instantaneous, and
the symmetric signal response as a function of the pump–probe delay gives a measure
of the system temporal resolution.
The pump–probe signal is recorded as the change in transmission (or in reflection)
T of the probe beam due to the pump absorption. Transmission experiments are
most appropriate when dealing with highly transparent samples, while reflection
detection is preferable with highly scattering specimens and in deep tissue imaging.
The probe beam is chromatically separated from the pump beam using appropriate
filters (F) in front of the detector, which is usually a photodiode.
Since the beam intensity variations T are usually very small, of the order of
<1% of the total signal T [20], a higher detection sensitivity is required to extract
the pump–probe signal from the background. This can be achieved introducing a
high-frequency modulation scheme, where the pump intensity is modulated at a high
frequency (typically >1 MHz [7]) and the detected probe is demodulated by a lockin amplifier (LIA) to extract its modulation amplitude at the same frequency. These
high-frequency intensity modulations cannot be achieved with mechanical choppers,
instead acousto- or electro-optic modulators (AOM, EOM) need to be used in the
pump optical path. With this approach, the low-frequency 1/f laser intensity noise is
circumvented and shot-noise limited detection sensitivity is achieved. In this limit,
the noise equals the square root of the detected signal, and detection sensitivities
down to T /T = 10
−7 were reported, much higher than the typical 10
−4 value
obtained with kHz modulation [8, 18].
The pump–probe detection scheme is sketched in Fig. 7.3a. Pump and probe pulse
trains before (left) and after (right) the interaction with the sample are presented.
The reference pump modulation is drawn as a dashed square wave, and the inset
shows the temporal alignment between the pulses. The input probe intensity level is
defined by a horizontal dashed line. T is the variation in the probe intensity with
respect to the input value, and its amplitude is extracted by the lock-in amplifier.
The different types of pump–probe interaction presented in Sect. 7.1.1 will generate
different output signals, equivalent to gain or loss in the probe intensity, as will be
fully explained in Sect. 7.2.2. In Fig. 7.3b an experimental example is shown, where
single-layer graphene (SLG) is imaged with the pump–probe technique without (left)
and with (right) the lock-in demodulation. Pump and probe beams are tuned to 800
and 1030 nm, respectively. Without the lock-in amplifier, a transmission image (TD)
is obtained. Switching the lock-in amplifier on, the small modulated fraction of the
transmitted probe beam can be extracted with high signal-to-noise ratio (SNR), and
a pump–probe image (PP) is obtained, showing high contrast and a high level of
sample details. For example, according to the line profiles shown, the central area
exhibits the highest signal in the TD image, while it results in being completely dark
in the PP image, confirming the presence of a hole in SLG sample.
177
onto a BBO crystal to detect the sum frequency generation (SFG) signal when the
pulses arrive on the crystal at the same time (Fig. 7.2c). The same principle can be
exploited using an autocorrelator. In order to optimize the temporal alignment at
the sample, the scattering signal coming from a stimulated Raman scattering (SRS)
pump–probe process [19] can be collected and maximized by moving the delay line.
By being a scattering process, this interaction can be considered instantaneous, and
the symmetric signal response as a function of the pump–probe delay gives a measure
of the system temporal resolution.
The pump–probe signal is recorded as the change in transmission (or in reflection)
T of the probe beam due to the pump absorption. Transmission experiments are
most appropriate when dealing with highly transparent samples, while reflection
detection is preferable with highly scattering specimens and in deep tissue imaging.
The probe beam is chromatically separated from the pump beam using appropriate
filters (F) in front of the detector, which is usually a photodiode.
Since the beam intensity variations T are usually very small, of the order of
<1% of the total signal T [20], a higher detection sensitivity is required to extract
the pump–probe signal from the background. This can be achieved introducing a
high-frequency modulation scheme, where the pump intensity is modulated at a high
frequency (typically >1 MHz [7]) and the detected probe is demodulated by a lockin amplifier (LIA) to extract its modulation amplitude at the same frequency. These
high-frequency intensity modulations cannot be achieved with mechanical choppers,
instead acousto- or electro-optic modulators (AOM, EOM) need to be used in the
pump optical path. With this approach, the low-frequency 1/f laser intensity noise is
circumvented and shot-noise limited detection sensitivity is achieved. In this limit,
the noise equals the square root of the detected signal, and detection sensitivities
down to T /T = 10
−7 were reported, much higher than the typical 10
−4 value
obtained with kHz modulation [8, 18].
The pump–probe detection scheme is sketched in Fig. 7.3a. Pump and probe pulse
trains before (left) and after (right) the interaction with the sample are presented.
The reference pump modulation is drawn as a dashed square wave, and the inset
shows the temporal alignment between the pulses. The input probe intensity level is
defined by a horizontal dashed line. T is the variation in the probe intensity with
respect to the input value, and its amplitude is extracted by the lock-in amplifier.
The different types of pump–probe interaction presented in Sect. 7.1.1 will generate
different output signals, equivalent to gain or loss in the probe intensity, as will be
fully explained in Sect. 7.2.2. In Fig. 7.3b an experimental example is shown, where
single-layer graphene (SLG) is imaged with the pump–probe technique without (left)
and with (right) the lock-in demodulation. Pump and probe beams are tuned to 800
and 1030 nm, respectively. Without the lock-in amplifier, a transmission image (TD)
is obtained. Switching the lock-in amplifier on, the small modulated fraction of the
transmitted probe beam can be extracted with high signal-to-noise ratio (SNR), and
a pump–probe image (PP) is obtained, showing high contrast and a high level of
sample details. For example, according to the line profiles shown, the central area
exhibits the highest signal in the TD image, while it results in being completely dark
in the PP image, confirming the presence of a hole in SLG sample.