266
O. Tzang et al.
the nonlinear emission response of plasmonic nanoparticle labels, has been reported
(see Chap. 8 by Shi-Wei Chu in this book). Note also that in laser-induced ablation,
the nonlinear response of matter was used to achieve SR ablation with a diffractionlimited focused beam [24].
11.2.3 Timescales of the Experiment
Several timescales are relevant to the optimization of NPMR:
• A short pump pulse, in the femtosecond to picosecond range, is desirable so that
the photo-excited spatial distribution of physical properties will not diffuse and
blur the initial instantaneous profile [22].
• The delay between the pump and the probe should be optimized according to the
monitored physical property. For example, to monitor the change in temperature,
the desired delay should be about 1–2 ps to allow the transfer of energy to phonon
degrees of freedom. With longer delays, heat diffusion will quickly induce spatial
smearing and decay of intensity. On the other hand, a short pump–probe delay
allows monitoring of charge–carrier excitation that may take 100 fs.
• Each P&P interaction lasts a few picoseconds, and within this timeframe, the
temperature of the sample can reach up to a few hundreds of degrees. It is desirable
that the next pair of P&P pulses in the train will reach the system after its complete
electronic and thermal relaxation. Within this constraint, a high repetition rate
laser is desired for high signal-to-noise ratio (SNR) and speed. The 12.5 ns pulse
spacing in a ps/fs laser, with repetition rate of 80 MHz, is adequate to minimize
the steady-state heating of a thermally conducting sample (~7 K), and also allows
high-frequency (~1.1 MHz) modulation of the pulse train.
11.2.4 The Relation Between High Harmonics Detection
and SR
The origin of resolution enhancement can be rationalized by representing the nonlinear change in reflectivity, R, as a Taylor series of the pump intensity I pu :
R
R
α, I pu , τ, λ pr
= a 1
α, τ, λ pr
I pu + a 2
α, τ, λ pr
I
2
pu
+ a 3
α, τ, λ pr
I
3
pu + · · ·
(11.2)
where α denotes the material monitored, λ pr denotes the wavelength of the probe (e.g.
the coefficient of thermo-reflectance is wavelength-dependent), and τ is the delay
between the pump and the probe ultra-short pulses. The excitation of the sample
O. Tzang et al.
the nonlinear emission response of plasmonic nanoparticle labels, has been reported
(see Chap. 8 by Shi-Wei Chu in this book). Note also that in laser-induced ablation,
the nonlinear response of matter was used to achieve SR ablation with a diffractionlimited focused beam [24].
11.2.3 Timescales of the Experiment
Several timescales are relevant to the optimization of NPMR:
• A short pump pulse, in the femtosecond to picosecond range, is desirable so that
the photo-excited spatial distribution of physical properties will not diffuse and
blur the initial instantaneous profile [22].
• The delay between the pump and the probe should be optimized according to the
monitored physical property. For example, to monitor the change in temperature,
the desired delay should be about 1–2 ps to allow the transfer of energy to phonon
degrees of freedom. With longer delays, heat diffusion will quickly induce spatial
smearing and decay of intensity. On the other hand, a short pump–probe delay
allows monitoring of charge–carrier excitation that may take 100 fs.
• Each P&P interaction lasts a few picoseconds, and within this timeframe, the
temperature of the sample can reach up to a few hundreds of degrees. It is desirable
that the next pair of P&P pulses in the train will reach the system after its complete
electronic and thermal relaxation. Within this constraint, a high repetition rate
laser is desired for high signal-to-noise ratio (SNR) and speed. The 12.5 ns pulse
spacing in a ps/fs laser, with repetition rate of 80 MHz, is adequate to minimize
the steady-state heating of a thermally conducting sample (~7 K), and also allows
high-frequency (~1.1 MHz) modulation of the pulse train.
11.2.4 The Relation Between High Harmonics Detection
and SR
The origin of resolution enhancement can be rationalized by representing the nonlinear change in reflectivity, R, as a Taylor series of the pump intensity I pu :
R
R
α, I pu , τ, λ pr
= a 1
α, τ, λ pr
I pu + a 2
α, τ, λ pr
I
2
pu
+ a 3
α, τ, λ pr
I
3
pu + · · ·
(11.2)
where α denotes the material monitored, λ pr denotes the wavelength of the probe (e.g.
the coefficient of thermo-reflectance is wavelength-dependent), and τ is the delay
between the pump and the probe ultra-short pulses. The excitation of the sample