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11.7 Conclusions
We have presented the essentials of nonlinear photo-modulated reflectivity (NPMR)
for far-field, label-free, super-resolution (SR) microscopy. Unlike some other labelfree SR techniques, no sample treatment or preparation is required here. NPMR is
suitable to characterize semiconductors and metals in vacuum, ambient, and liquid,
semi-transparent and opaque systems.
The improvement in resolution using NPMR scales like
√
n, where n is the nonlinearity order. While the intensities of high nonlinearity orders decrease exponentially,
the incremental improvement in resolution slows down with the nonlinearity order.
Therefore, in order to enhance resolution, it is desirable to combine NPMR with
other resolution enhancement modalities, as demonstrated by us using SPOM.
A major challenge in any SR technique is the declining sensitivity as the examined
objects decrease in size. One way to overcome this problem is to seek for resonance
sensitivities in NPMR, by tuning the pump/probe wavelengths to a plasmonic resonant wavelength. We foresee an intense enhancement in label-free SR with such an
approach.
Appendix: Pure Sinusoidal Photo-Modulation Using
an Acousto-optic Modulator
We have emphasized in Sect. 11.2.3 the need for pure sinusoidal light-intensity
modulation in NPMR. This is a desirable requirement in numerous scientific and
engineering applications. Here, we introduce our approach to achieve such a goal,
using acousto-optical modulators.
The methodology, presented here, for clean sine photo-modulation is based on
using an arbitrary wave generator (AWG) to drive the control voltage input of a
single AOM [53]. This voltage input controls the amplitude of the RF driver, which
determines the diffraction efficiency of the AOM. The diffracted intensity is not linear
with input voltage. Thus, one looks for an arbitrary waveform (WF) to generate the
required light intensity, having a pure sinusoidal time dependence at the modulation
frequency f m , while utilizing the full diffraction range of the AOM (90%).
The WF optimization consists of a two major stages. First, utilizing the static
response (fixed control voltage) of the AOM to estimate the required WF. Second,
using a feedback loop to optimize the WF to the dynamic frequency response of the
AOM.
In the first stage, the modulator is driven by a set of static control voltages. The
light output intensity at each voltage value is measured with a linear photo detector.
The data are interpolated to a smooth function and are used to calculate the initial
O. Tzang et al.
11.7 Conclusions
We have presented the essentials of nonlinear photo-modulated reflectivity (NPMR)
for far-field, label-free, super-resolution (SR) microscopy. Unlike some other labelfree SR techniques, no sample treatment or preparation is required here. NPMR is
suitable to characterize semiconductors and metals in vacuum, ambient, and liquid,
semi-transparent and opaque systems.
The improvement in resolution using NPMR scales like
√
n, where n is the nonlinearity order. While the intensities of high nonlinearity orders decrease exponentially,
the incremental improvement in resolution slows down with the nonlinearity order.
Therefore, in order to enhance resolution, it is desirable to combine NPMR with
other resolution enhancement modalities, as demonstrated by us using SPOM.
A major challenge in any SR technique is the declining sensitivity as the examined
objects decrease in size. One way to overcome this problem is to seek for resonance
sensitivities in NPMR, by tuning the pump/probe wavelengths to a plasmonic resonant wavelength. We foresee an intense enhancement in label-free SR with such an
approach.
Appendix: Pure Sinusoidal Photo-Modulation Using
an Acousto-optic Modulator
We have emphasized in Sect. 11.2.3 the need for pure sinusoidal light-intensity
modulation in NPMR. This is a desirable requirement in numerous scientific and
engineering applications. Here, we introduce our approach to achieve such a goal,
using acousto-optical modulators.
The methodology, presented here, for clean sine photo-modulation is based on
using an arbitrary wave generator (AWG) to drive the control voltage input of a
single AOM [53]. This voltage input controls the amplitude of the RF driver, which
determines the diffraction efficiency of the AOM. The diffracted intensity is not linear
with input voltage. Thus, one looks for an arbitrary waveform (WF) to generate the
required light intensity, having a pure sinusoidal time dependence at the modulation
frequency f m , while utilizing the full diffraction range of the AOM (90%).
The WF optimization consists of a two major stages. First, utilizing the static
response (fixed control voltage) of the AOM to estimate the required WF. Second,
using a feedback loop to optimize the WF to the dynamic frequency response of the
AOM.
In the first stage, the modulator is driven by a set of static control voltages. The
light output intensity at each voltage value is measured with a linear photo detector.
The data are interpolated to a smooth function and are used to calculate the initial