Ex) Article Title, Author, Keywords
New Phys.: Sae Mulli 2023; 73: 156-159
Published online February 28, 2023 https://doi.org/10.3938/NPSM.73.156
Copyright © New Physics: Sae Mulli.
Kyoung-Ho Kim*, Seungwoo Chae, Evan S. H. Kang, Jeongho Lee, Sunjin Eom
Department of Physics, Research Institute for Nanoscale Science and Technology, Chungbuk National University, Cheongju 28644, Korea
Correspondence to:*E-mail: email@example.com
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Light scattering by small particles enables the characterization of their size, density, and refractive index. For example, a collimated laser beam can be used to detect micrometer-scale particles, such as fine dusts in air or microplastics in water, by measuring the intensity of the light scattered by them. However, millimeter-size laser beams cannot be used to detect particles smaller than 1 μm because the size of these particles is comparable to the wavelength of the laser beam, resulting in low light scattering. In this study, we demonstrated the measurement of light scattering by small particles with diameters of hundreds of nanometers by using a home-built laser microscope. We used two identical objective lenses to achieve optimized light illumination and collection in the measurements. The signal-to-noise ratio was minimized by achieving balanced detection with the lock-in amplifiers. Single particles were clearly revealed in the scanning scattering-intensity map. We expect that our proposed method will be useful for detecting and characterizing nanoscale particles in environment-monitoring systems.
Keywords: Laser microscope, Optical scattering
Small particles in the environment, such as the fine dusts in air and microplastics in water, pose threats to public health by increasing the risks of respiratory and cardiovascular diseases[1, 2]. Furthermore, such small particles can reduce yields of manufacturing processes in industries such as semiconductor industries by causing surface contamination via undesirable particle deposition[3, 4]. It is important to identify and characterize such small particles to understand their source and path of spread in the environment and production lines[5, 6].
A laser, which is a coherent light source, can be used to detect particles by measuring their scattering intensity. Indeed, scattered light carries information of the small particles. Hence, light scattering measurement systems have been developed for detecting and identifying target particles. However, the conventional light scattering instruments are developed for many-particle measurements to characterize the size and density of the particles. These instruments have low detection reliability when the particle size is comparable to the wavelength of the incident light. In this study, we demonstrated the detection of single particles by using a laser microscope for the light scattering measurement with balanced detection accompanied by lock-in amplification . The scanning scattering-intensity map with an enhanced signal-to-noise ratio clearly visualizes single particles of size comparable to the incident light wavelength. This method can improve the detection capability of small particles in environment-monitoring systems.
We first designed a laser microscope (Fig. 1) for light scattering measurement. It has three components: a visible-light microscope, laser illumination component, and detection component (Fig. 1). The visible microscope allows a wide-field view for particle positioning for laser illumination (yellow light path in Fig. 1(a)). In our microscope, a monochrome camera (Basler ace acA1300-200um, Germany) was used to view the sample surface, and Köhler illumination with a broadband white light source from a halogen lamp was employed for the wide-field view. We used a 20× objective lens (Olympus MPLFN 20X) for white light illumination.
Next, laser illumination was implemented in such a way that the beam path was shared with the visible microscope by using a beam expander and an objective–condenser lens set (green colored light path in Fig. 1(a)). The laser beam with a wavelength of 532 nm was obtained from the condenser lens, which was placed on opposite side of the objective lens. The condenser lens was selected to be identical to the objective lens to achieve optimized laser illumination and collection during the scattering measurements. The collimated laser beam was incident on the condenser lens, resulting in a focused laser spot at the focal plane, which coincided with the sample surface of the target particle. To minimize the focal spot, a beam expander (Thorlab GBE02-A) was used to increase the laser beam diameter (full width at half maximum; FWHM) to 2 mm, which is a quarter of size of the condenser lens pupil. The focal plane of the objective lens was set to coincide with the sample surface to collect the light scattered by the particle. The focused laser beam was imaged by the monochrome camera to search for the target particle and place the laser spot near the particle.
For detecting the particles with an enhanced signal-to-noise ratio, we employed balanced detection with lock-in amplifiers. After passing through the beam expander, the incident laser beam was divided into two—a reference beam and a signal beam—by using a pellicle beam splitter. Two Si photodetectors (Thorlabs DET36A2) were used to measure the reference and signal laser beam intensities. The signal laser beam passed through the condenser–objective lens after light scattering by a particle, whereas the reference laser beam reached the detector without light scattering. The electrical signals from the two detectors were input into the lock-in amplifier (EG&G 5210), and the difference between the two beam intensities was estimated. The zero amplitude of difference measurement was defined when the signal beam passed a blank area without particles. An optical chopper (SR540) was used to provide the laser modulation frequency to the lock-in amplifier. A photograph of the setup is shown in Fig. 1(b).
To examine the scattering measurement of nanoscale particles, we used polystyrene (PS) beads of different diameters (~700 and ~565 nm; ThermoFisher Scientific, 4000 series monosized particles) (Fig. 2). PS beads suspended in water were dropped on a glass surface, and subsequently, the surface was dried in a convection oven. To examine the detection of a single-particle, an isolated particle on the sample surface was selected, and the laser spot was placed near the particle to measure its scattering-intensity. By scanning the stage in a two-dimensional area near the particle, a scattering map of a single PS bead was obtained.
Figure 3 shows the scattering maps of a single PS bead with a diameter of ~700 nm obtained by the proposed method (Fig. 3(a)) and the simple difference method (Fig. 3(b)). The scattering-intensity in the simple difference method was obtained by subtracting the reference signal from the sample signal. For both methods, single-particle scattering is clearly visible (the white dashed line). The peak scattering was analyzed by using a two-dimensional Gaussian function. The FWHM in the radial direction is shown as 770 and 760 nm in Fig. 3(a) and 3(b), respectively. These results indicate that the scattering signal is sufficiently high to be detected when the particle size is larger than the wavelength of the incident light.
In contrast, the scattering changes drastically when the particle size is comparable to the laser wavelength. Figure 4 shows the scattering maps of a single PS bead with a diameter of ~565 nm obtained using our proposed method (Fig. 4(a)) and the simple difference method (Fig. 4(b)). The white dashed lines indicate the PS bead. Although both methods could visualize the scattering particle, the single-particle was not clearly identified by the simple difference method as noisy false particles were detected. Indeed, the low scattering-intensity of the small particle makes difficult to detect the particle when the particle size is comparable to the wavelength of the incident light. The peak scattering analysis with two-dimensional Gaussian function showed that FWHM in the lateral direction was 800 and 950 nm (see Fig. 4(a) and 4(b), respectively). In addition, the vertical direction FWHM determined by our proposed method was 400 nm, possibly because of the scattering of two laterally connected particles rather than a single-particle. These results indicate that noise reduction is essential for the scattering measurement for a small particle. We note that the single detector method for measuring only the sample signal provided much lower quality particle-scattering results compared to the simple difference method. Therefore, our proposed method is feasible for detecting single or composite particles of sizes comparable to the incident light wavelength.
In conclusion, we experimentally measured light scattering by small particles by employing balanced detection with lock-in amplifiers for reducing the noise in scattering measurements. Single-particle scattering was visualized in a scanning scattering-intensity map. The results revealed that our proposed method can detect single particles of sizes comparable to incident light wavelength. We believe that our method will be useful in environment-monitoring systems to detect numerous nanoscale particles.
This research was supported by Chungbuk National University Korea National University Development Project (2021).