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Abstract
Wavelength-tunable spiral-phase-contrast (SPC) imaging was experimentally accomplished in the visible wavelengths spanning a broad bandwidth of ∼200 nm based on a single off-axis spiral phase mirror (OSPM). By the rotation of an OSPM, which was designed with an integer orbital angular momentum (OAM) of l = 1 at a wavelength of 561 nm and incidence angle of 45°, high-quality SPC imaging was obtained at different wavelengths. For the comparison with wavelength-tunable SPC imaging using an OSPM, SPC imaging using a spiral phase plate (manufactured to generate an OAM of l = 1 at 561 nm) was performed at three wavelengths (473, 561, and 660 nm), resulting in clear differences. Theoretically, based on field tracing simulations, high-quality wavelength-tunable SPC imaging could be demonstrated in a very broad bandwidth of ∼400 nm, which is beyond the bandwidth of ∼200 nm obtained experimentally. This technique contribute to developing high-performance wavelength-tunable SPC imaging by simply integrating an OSPM into the current optical imaging technologies.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the field of optics and photonics, wavelength tuning—the manipulation of the output wavelength of an optical source—is one of the most basic and significant optical technologies [1,2]. Because its application is very versatile, various technological advances have been accomplished for a long time. Given that the areas of interest of wavelength tuning are not limited to a particular wavelength range, beam shape, and light source operation, active studies are currently in progress as one of the cutting-edge optics and photonics technologies [3–7].
Among them, the light sources in the visible and near-infrared wavelengths are utilized in the fields of typical optical imaging and microscopy [8–10], and various types of imaging objects, including biological and material samples, are imaged in real time at a high-resolution. If the light source generates light at multiple or broadband wavelength ranges, the wavelength tunability is crucial because high-quality, high-resolution optical imaging should be implemented regardless of a particular wavelength condition.
An effort to observe the biological samples with faint optical contrast led to the development of phase contrast imaging, which sensitively visualizes the variation of the refractive index or the thickness of the target [11]. Phase contrast was first realized by Zernike in the 1930s, and it has yielded various appearances of the imaging targets by utilizing various types of phase-shifting masks [12,13]. Spiral-phase-contrast (SPC) imaging is one of the approaches applying a spiral phase filter at the Fourier plane (focal plane) of the optical imaging system. The vortex beams formed by the spiral phase element are responsible for the enhanced edge contrast defining the shapes of imaging objects and the high resolution beyond the usual diffraction limit [14,15]. During the past decades, SPC imaging has been studied and developed to maximize the intensity of edge contrast regardless of the amplitude (mainly material samples made up of different absorbing parts) or phase objects (mainly biological samples that are transparent to visible light with different refractive indices) [16–19].
Recent studies on SPC imaging mainly focus on the miniaturization and compatibility with conventional optical configurations for easy access to applications such as imaging and microscopy [20–22]. These studies utilize a metasurface spatial filter or SPP to generate vortex beams rather than the SLM based on holography, which results in a complex and bulky configuration and the issue on the reflection efficiency. In addition, expanding the accessible wavelength range (corresponding to the infrared range) of SPC imaging over the visible range has become crucial for developing SPC imaging [19,23,24]. Nondestructive biomedical microscopy, astronomical observations, and military imaging might be improved by SPC imaging in the infrared spectral range. However, SPC imaging in the infrared range relies on a complicated configuration due to the wavelength conversion from the visible to infrared range.
If the wavelength of a light source changes, difficulties may arise in achieving high-quality SPC imaging because it is impossible to maintain the integer orbital angular momentum (OAM, l) with l = 1, which is the critical parameter for the realization of well-defined vortex beams [25–28]. Several complicated methods have been proposed for generating wavelength-tunable vortex beams [29–31]. However, these studies remained at the level of partially improving the characteristics of wavelength-tunability of the beam itself, and using this, there were no approaches applied to wavelength-tunable SPC imaging. We recently demonstrated wavelength-tunable vortex beams by simply applying a single off-axis spiral phase mirror (OSPM) [32]. The vortex beam quality improved over broad bandwidth wavelengths implies that the approach can be directly applied to the study of wavelength-tunable SPC imaging by tuning non-integer OAM into integer OAM (l = 1) with only the rotation of a single OSPM.
In this study, for the first time, we successfully accomplished wavelength-tunable SPC imaging by utilizing a single OSPM for a line grating target using visible-light wavelengths spanning a broad bandwidth of ∼200 nm. Theoretically, based on field tracing simulations [33], high-quality wavelength-tunable SPC imaging spanning a very broad bandwidth (∼400 nm) could be realized, which is beyond the wavelength-tunable SPC imaging with a bandwidth (∼200 nm) obtained experimentally.
2. Experiments
The OSPM, which was designed with an OAM of l = 1 at the wavelength of 561 nm and incidence angle of 45°, was manufactured using mechanical processes [34]. Accordingly, wavelength-tunable SPC imaging was experimentally verified. We compared the SPC imaging outcomes in two cases: in one case, imaging was performed at two different wavelengths (473 nm and 660 nm) at a fixed incidence angle of 45°, whereas the other was performed at incidence angles of 53.4° and 33.7°, which satisfy the OAM of l = 1 at 473 nm and 660 nm, respectively. The former corresponds to vortex beams with OAM values of l = 1.19 and 0.85, respectively, and the latter corresponds to both vortex beams converted to an OAM of l = 1 by rotating the OSPM [32]. In addition, we applied three wavelengths (473, 561, and 660 nm) to the spiral phase plate (SPP) (manufactured to generate an OAM of l = 1 at 561 nm), which is essentially available only for a single wavelength. By performing SPC imaging with vortex beams with OAM values of l = 1.19, 1, and 0.85, the differences were more apparent compared with the results of wavelength-tunable SPC imaging using an OSPM.
The experimental configuration for the wavelength-tunable SPC imaging is shown in Fig. 1. A laser system (Model: C-Flex, HÜBNER Photonic) equipped with three wavelengths (λ = 473, 561, and 660 nm) is utilized. After light is reflected by mirror 1, enlarged collimated beams are formed by the combination of objective lens 1 and convex lens 1. After reflecting from mirror 2, it is divided into two beams by a beam splitter. One is the transmitted beamline intended for the wavelength-tunable SPC imaging through the OSPM, and the other is the reflected beamline intended for conventional SPC imaging through the SPP. The variable line grating target (Model: R1L3S6P, Thorlabs) is used as a specimen for each beamline to quantify the quality of the edge contrast in SPC imaging.
Fig. 1. Experimental configuration for wavelength-tunable spiral-phase-contrast (SPC) imaging based on an off-axis spiral phase mirror (OSPM) (transmitted beamline after beam splitter). For comparison, the conventional SPC imaging outcomes based on a spiral-phase plate (SPP) (reflected beamline after beam splitter) are considered. In the transmitted beamline, the OSPM is located at the Fourier plane in a typical 4f imaging system. For the wavelength tuning, OSPM rotates along the curve in the graph, thus yielding the depicted plot showing the variation of the incidence angle (θ) as a function of the wavelength (λ).
For the transmitted beamline, the beam is focused by convex lens 2 after it passes through the specimen. Herein, the front surface (spiral structure is engraved) of the OSPM (manufactured with an OAM of l = 1 at the wavelength of 561 nm and incidence angle of 45°) is placed at the Fourier plane. After the reflection by flat mirror 4, the beam is restored to the original beam size by convex lens 3, located at the right side of the Fourier plane. This two-lens configuration, which is symmetrical to the Fourier plane, is a typical 4f imaging system [16,35]. The reduced collimated beams formed by the combination of convex lens 4 and objective lens 2 are imaged by a beam profiler 1 (Model: BGS-USB3-LT665, Ophir Optronics). The essential operation for the wavelength-tunable SPC imaging by the OSPM is presented schematically (red-dotted line) in Fig. 1. As the laser beams at wavelengths other than 561 nm, which is the design wavelength of the OSPM at an incidence angle of 45°, are incident to the OSPM, the variation of the incidence angle (θ) as a function of the wavelength (λ) can be plotted (as shown) to maintain the OAM of l = 1. From the formula hr = l · λ/(2cosθ), which determines the final spiral height of the OSPM, the incidence angle (θ) is given by θ = cos-1(λ/2hr) (considering an OAM of l = 1), where hr is 396 nm. For the wavelengths 473 nm and 660 nm, the incidence angles that fulfill the condition of OAM l = 1 are 53.4° (blue solid circle) and 33.7° (red solid circle), respectively. They are corrected by the rotation of the OSPM and a flat mirror. Specifically, for the flat mirror, a linear translation is also incorporated with the rotation to fix all the other components in this configuration.
Similarly, for the reflected beamline, the beam is focused by convex lens 5 after it passes through the specimen. Herein, the front surface of the SPP (customized model V-561-20-1 from Vortex photonics; manufactured with an OAM of l = 1 at 561 nm wavelength) is placed at the Fourier plane. The beam is restored to the original beam size by convex lens 6, which constitutes the 4f imaging system. Finally, the collimated beams formed by the combination of convex lens 7 and objective lens 3 are imaged by beam profiler 2.
3. Results and discussion
The experimental results used to compare the difference in SPC imaging before and after wavelength tuning (in conjunction with the use of an OSPM) are shown in Fig. 2. The target used is a variable line grating, as mentioned above. The imaging area applied to SPC imaging is a periodic grating structure with a regular interval spanning seven vertical bars. Figures 2(a) and 2(b) present the results of two-dimensional (2D) SPC imaging before and after wavelength tuning at three wavelengths (660, 561, and 473 nm). Because the OSPM was designed with an OAM of l = 1 at a wavelength of 561 nm and incidence angle of 45°, the imaging results at 561 nm before and after wavelength tuning are identical. The results for 660 nm and 473 nm cases with OAM values of l = 0.85 and l = 1.19 at the 45° incidence angle satisfy the OAM value of l = 1 after rotating the OSPM to the angles of 33.7° and 53.4°, respectively, thus demonstrating the effects of wavelength tuning. Because the 2D SPC images observed at 660 nm and 473 nm before wavelength tuning deviate from l = 1, the edge intensity of each bar is inclined to the right or left sides, and the direction of this asymmetry also changes depending on whether the OAM value increases or decreases with respect to l = 1. However, by converting to l = 1 based on the wavelength tuning process, as shown in Fig. 2(b), the intensity asymmetry of the edge contrast at both wavelengths is corrected such that it is almost identical to that in the 561 nm wavelength case. These results are quantitatively compared more clearly by analyzing the line profiles before and after the wavelength tuning process (see Figs. 2(c) and 2(d)). Before the wavelength tuning process, the edge intensity of each bar at 660 nm (l = 0.85) is inclined to the left side. Conversely, that at 473 nm (l = 1.19) is inclined to the right side compared with that at 561 nm. These outcomes clearly differ from the result at 561 nm (l = 1), wherein a minor intensity difference is noted between the right and left sides. However, because the OAM values of 660 nm and 473 nm after wavelength tuning are both equal to one, the imaging result of the line profile with enhanced edge contrast yields almost an equivalent level compared with that attained at 561 nm, which is uniform and symmetrical overall.
Fig. 2. (a) Observed two-dimensional (2D) SPC images of line gratings before the wavelength tuning process of the OSPM. Three wavelengths, namely, 660 nm (top), 561 nm (middle), and 473 nm (bottom) are incident on the OSPM at an incidence angle of 45°. (b) Observed 2D SPC images of line gratings after the wavelength tuning of the OSPM. Three wavelengths, namely, 660 nm (top), 561 nm (middle) and 473 nm (bottom) are incident on the OSPM at the incidence angles of 33.7°, 45°, and 53.4°, respectively. (c) and (d) are the line profiles of (a) and (b), respectively, with intensities integrated in the vertical direction. Herein, the intensities of the line profiles in (c) and (d) are normalized based on the maximum intensity obtained after wavelength tuning (l = 1) at each wavelength. The intensities of line profiles of grating structure were integrated except for the bottom area of the line grating bars.
SPC imaging was conducted (see the reflected beamline in Fig. 1) using the commonly utilized SPP (as mentioned before, it was manufactured based on an OAM of l = 1 at the wavelength of 561 nm) as a cross-validation of image asymmetry, and its directionality was observed from the OSPM-based SPC imaging before wavelength tuning. Figures 3(a) and 3(b) show the 2D SPC images and their line profiles obtained at three wavelengths. For SPP, the OAM values at the three wavelengths are l = 0.85, 1, and 1.19, which are the same as those obtained in the OSPM case before wavelength tuning. Compared with the case of 561 nm in Fig. 3(a), in the case of 660 nm, the edge intensity for the left side of each bar is higher than that of the right side; however, in the case of 473 nm, the opposite tendency is evident. Moreover, the asymmetrical direction of the edge contrast is different based on OAM of l = 1 and its direction is consistent with the result of SPC imaging (in conjunction with the use of the OSPM) before the wavelength tuning process. Comparing the three line profiles in Fig. 3(b), the edge intensities for the left and right sides of each bar for the 660 nm and 473 nm cases show a large difference of approximately two times unlike the 561 nm case. Therefore, the asymmetrical directions for the two cases (660 nm at l = 0.85 and 473 nm at l = 1.19) are verifiably opposite to each other.
Fig. 3. (a) Observed 2D SPC images of line gratings using SPP. Three wavelengths, namely, 660 nm (top), 561 nm (middle), and 473 nm (bottom) are incident on the SPP. (b) Line profiles of (a) with intensities integrated in the vertical direction. The line profiles are normalized based on the maximum value of 561 nm.
Here, new resolution parameters that can be utilized in the OSPM based wavelength-tunable SPC imaging can be presented. Considering that the resolution of the rotation angle of OSPM is 0.01° (the mechanical resolution of a motorized rotation stage), the resolutions of the wavelength and OAM can be given as a performance value of the OSPM based wavelength tuning. The wavelength and OAM resolutions of wavelength-tunable SPC imaging are defined by the equations Δλ = |2hrcos(θref + Δθ)/l – λ| and Δl = |2hrcos(θref + Δθ)/λ – l| (where, hr is the final spiral height of the OSPM, θref is the reference incidence angle satisfying OAM l = 1 at a given wavelength, Δθ is the rotation angle resolution of the OSPM (Δθ = 0.01°), l is the OAM value (l = 1) at the wavelength-tuned condition, and λ is the wavelength.), respectively. Δλ = |2hrcos(θref + Δθ)/l – λ| is given by the difference between the initial (or designed) wavelength (with OAM l = 1) and the wavelength deviated from OAM l = 1 by the rotational error (0.01°) after wavelength tuning. Δl = |2hrcos(θref + Δθ)/λ – l| is given by the difference between the initial (or designed) OAM l = 1 and the OAM away from OAM l = 1 by the rotational error (0.01°) after wavelength tuning.
Figure 4 shows the resolution of the wavelength and OAM of the OSPM with the angle resolution of 0.01° in the visible wavelength range (from 400 nm to 700 nm). Both wavelength and OAM present very high resolutions of Δλ ∼ 0.12 nm and Δl ∼ 3 × 10−4, respectively, ensuring a high resolution of the wavelength tuning.
Fig. 4. The resolution of wavelength (a) and OAM (b) for the OSPM based wavelength tuning with the accuracy of rotation angle 0.01° at a visible wavelength range.
The experimental results of SPC imaging (with the utilization of an OSPM and SPP) were verified based on field tracing simulations using VirtualLab Fusion, which is a fast physical-optics software program [34]. In this simulation, the geometry of the simulation adhered to the experimental configuration of Fig. 1, which included the beam splitter, grating target, 4f imaging system, and OSPM (or SPP). The whole system is divided into subdomains for each optical component. For each subdomain, Maxwell’s equations are solved by switching between fields such as time, space, frequency, and k-field to minimize the computational complexity. In this process, to minimize the sampling effort, advanced Fourier Transform techniques such as Fast Fourier Transform (FFT), Semi-Analytical Fourier Transform (SFT), and Pointwise Fourier Transform (PFT) were applied to the domain-to-domain switching [36]. In the diffractive optics modeling of OSPM and SPP, which apply a sub-micro spiral structure, a parabasal thin-element approximation (PTEA) approach was employed for the stack analysis [37].
The simulation results that reproduce the experimental results (Fig. 2) of the SPC imaging are shown in Fig. 5. Figures 5(a) and 5(b) are the results of 2D SPC imaging before and after wavelength tuning at the three wavelengths, and Figs. 5(c) and 5(d) show their respective line profiles. In Fig. 5(a), for the case of 660 nm (l = 0.85), the intensity of each bar is inclined to the left side and becomes stronger compared to the wavelength of 561 nm with OAM of l = 1. However, in the case of 473 nm (l = 1.19), the overall intensity of the edge contrast is weakened, and the intensity of each bar is inclined to the right side. This tendency becomes clearer in the line profiles of Fig. 5(c). Overall, the simulation results are in good agreement with the asymmetry, and its direction of SPC imaging deviated from OAM of l = 1 as shown in Figs. 2(a) and 2(c). In the case of the 2D images of Fig. 5(b) and the line profiles of Fig. 5(d), which are the wavelength-tuned results (such that the OAM value becomes l = 1), the results obtained at 660 nm and 473 nm are almost indistinguishable from the symmetrical results at 561 nm. These results reproduce the experimental results of Figs. 2(b) and 2(d) well. Theoretically, Fig. 5 validates the reliability and accuracy on the experimental results of OSPM-based wavelength-tunable SPC imaging. Thus, based on both the experimental and simulation results, well-defined wavelength-tunable SPC imaging that spanned a bandwidth of ∼200 nm was realized, and did not depend on a particular singular wavelength.
Fig. 5. (a) Simulated 2D SPC images of line gratings before wavelength tuning of OSPM. (b) Simulated 2D SPC images of line gratings after wavelength tuning of OSPM. The simulation conditions of (a) and (b) are the same as the experimental conditions of Fig. 2. (c) and (d) are the line profiles of (a) and (b), respectively, with intensities integrated in the vertical direction. Herein, the intensities for line profiles of (c) and (d) are normalized based on the maximum intensity obtained after wavelength tuning (l = 1) at each wavelength.
The simulation results that reproduce the experimental results (Fig. 3) of SPC imaging with the use of the SPP are shown in Fig. 6. Figures 6(a) and 6(b) show the results of 2D SPC imaging and the line profiles obtained at the three wavelengths, respectively. Overall, the simulation results of SPP-based SPC imaging are consistent with the asymmetry and its direction of SPC imaging deviated from OAM of l = 1, as shown in Figs. 3(a) and 3(b). In addition, these results are almost similar to the results of OSPM-based SPC imaging before the wavelength tuning process (see Figs. 2(a) and 2(b) along with Figs. 5(a) and 5(b), which are the experimental and simulation results, respectively). Thus, only OSPM-based SPC imaging has the advantage of wavelength tuning, unlike SPP-based SPC imaging.
Fig. 6. (a) Simulated 2D SPC images of line gratings using SPP. The simulation conditions of (a) are the same as the experimental conditions of Fig. 3. (b) Line profiles of (a) with intensities integrated in the vertical direction. The line profiles are normalized based on the maximum value of 561 nm.
Among the previous studies on the SPC imaging using conventional SLM, there have been studies analyzing the fractional OAMs and the unbalanced edge enhancement [27,28], but they were all mainly focused on the OAM values in the range of 0 to 1. This study presents, for the first time, the change in the direction of nonuniform edge intensity in cases where the OAM values are larger or less than l = 1, beyond the simply unbalanced enhancement of edge contrast by fractional OAMs. According to G. Situ et al. [28], it is mentioned that the directionality of nonuniform edge intensity can be changed depending on the orientation of edge discontinuity (the point where the final spiral height is located) on the surface of the SPP with the fractional OAM value between 0 and 1. Thus, in this study, if the direction (right) of the edge discontinuity of the OSPM at the condition of the fractional OAM is changed to the opposite direction (π rotation with respect to the right), it is thought that edge intensity opposite to the current results may be observed.
The experimental and simulation results presented above verify that high-quality wavelength-tunable SPC imaging using a single OSPM is possible in the wavelength bandwidth of ∼187 nm (from 473 nm to 660 nm). However, the availability of OSPM-based wavelength-tunable SPC imaging is not limited to the wavelength bandwidth of ∼200 nm. Even if the wavelength difference becomes larger, the principle of wavelength-tunable SPC is still valid; the wavelengths of 473 nm and 660 nm presented are merely the light sources with the largest wavelength difference prepared experimentally in this study. To verify this, theoretically, a field tracing simulation was carried out on wavelength-tunable SPC imaging at 760 nm and 360 nm with a difference of ±200 nm with respect to 561 nm with a 400 nm bandwidth, which is more than twice compared to ∼187 nm (see Fig. 7). Figures 7(a) and 7(b) are the results that incorporate the wavelength-tunable SPC imaging outcomes at 760 nm and 360 nm to Figs. 5(a) and 5(b); Figs. 7(c) and 7(d) show their corresponding line profiles. Before the onset of the wavelength tuning process (all the incidence angles were equal to 45°), the OAM values are equal to l = 0.74, 0.85, 1, 1.19, and 1.56, at the five respective wavelengths, i.e., 760, 660, 561, 473, and 360 nm. Conversely, the incident angles are respectively converted to 16.7°, 33.7°, 45°, 53.4°, and 63° such that the OAM values after the wavelength tuning process for all five wavelengths become equal to l = 1 (see the graph featured within the red-dotted line in Fig. 1). Herein, the incidence angles of 63° and 16.7° are the available incidence angles experimentally. In the case of 760 nm (before wavelength tuning), the intensity of each bar is more inclined to the left side compared with the case of 660 nm (l = 0.85), and its intensity increases because the OAM becomes equal to l = 0.74. Conversely, in the case of 360 nm (before wavelength tuning), the intensity of each bar is reduced considerably compared with the case of 473 nm (l = 1.19) because the OAM becomes equal to l = 1.56. This intensity is approximately half the intensity obtained at OAM l = 1. However, with the wavelength tuning process (corrected by the conversion of the incidence angle by the rotation of OSPM at each wavelength such that the OAM is l = 1), it is possible to obtain uniform and symmetrical high-quality SPC imaging outcomes that are almost indistinguishable at all five wavelengths (see Figs. 7(b) and 7(d)). Therefore, the wavelength-tunable SPC imaging was verified theoretically within the broad bandwidth of 400 nm, which is considerably broader than the experimental bandwidth of ∼200 nm.
Fig. 7. (a) Simulated 2D SPC images of line gratings before the wavelength tuning of OSPM at five wavelengths (760, 660, 561, 473, and 360 nm). (b) Simulated 2D SPC images of line gratings after the wavelength tuning of OSPM. (c) and (d) are the line profiles of (a) and (b), respectively, with intensities integrated in the vertical direction. Herein, the intensities for line profiles of (c) and (d) are normalized (same as in Fig. 5).
In the OSPM-based wavelength-tunable SPC imaging presented in this study, the wavelength and incidence angle, which are the key parameters designed to achieve an OAM of l = 1 in a single OSPM, are not limited to specific values. These two design parameters that determine a single OSPM can also be available in an infrared or ultraviolet range at any arbitrary incidence angle (from 0° to 90°), other than the wavelength of 561 nm and incidence angle of 45° presented herein. Additionally, the aforementioned 400 nm bandwidth may not be the maximum value that is possible for OSPM-based wavelength-tunable SPC imaging. For example, if two incident angles are used (10° and 80°) after wavelength tuning (in the experiments these angles are still acceptable), the wavelengths that satisfy the OAM of l = 1 are 781 nm and 137 nm, respectively. Accordingly, wavelength-tunable SPC imaging may still be effective over the significantly broad bandwidth of more than 600 nm. However, for a single OSPM with l = 1 that is designed based on the initial wavelength (designed wavelength), we need to consider quantitatively the difference between the spiral height curve as a function of the 2π rotation (azimuthal surface rotation of OSPM), which determines the OAM of l = 1 at the initial wavelength and that at a different wavelength after wavelength tuning if a different wavelength incident on this OSPM is far away from the initial wavelength on the light spectrum. Therefore, an additional study ought to be conducted in the near future to address these issues. Consequently, because the OSPM-based wavelength-tunable SPC imaging technique presented in this study is a universal and intuitive approach with superior scalability, it is expected to assist the development of high-performance technologies, including the enhancement of edge contrast, by simply combining it with various tunable (or broad bandwidth) optical imaging configurations in the future.
4. Conclusions
In this study, we successfully achieved wavelength-tunable SPC imaging that covers a broad bandwidth (∼200 nm) in the visible wavelength range based on a single OSPM. Using an OSPM (which was designed with an OAM value of l = 1 at the 561 nm wavelength and incidence angle of 45°), SPC imaging experiments were conducted, which corresponded to the OAM values of l = 1.19, 1, and 0.85 at the three wavelengths of 473, 561, and 660 nm. For OAM of l = 1.19 and l = 0.85, the results of SPC imaging showed that the overall shapes of edge contrast were not symmetrical but inclined to one side. Additionally, the direction of this asymmetry was different depending on whether the OAM value increased or decreased with respect to l = 1. To obtain the vortex beams for satisfying the OAM of l = 1 at the three wavelengths, the incidence angles were changed to 53.4° (473 nm) and 33.7° (660 nm), and correspondingly high-quality wavelength-tunable SPC imaging with uniformly improved edge contrast was achieved, which yielded outcomes identical to those obtained at 561 nm. In addition, by performing SPC imaging (using vortex beams with OAM of l = 1.19 (473 nm), 1 (561 nm), and 0.85 (660 nm)) at the three wavelengths using the currently extensively used SPP (manufactured to have OAM of l = 1 at 561 nm), comparisons were possible with wavelength-tunable SPC imaging that used the OSPM. Theoretically, for a single OSPM, even in the much wider bandwidth of ∼400 nm (considerably broader than the experimentally verified bandwidth of ∼200 nm), high-quality wavelength-tunable SPC imaging corresponding to OAM l = 1 was demonstrated. Consequently, by improving the uniformity for the intensity of edge contrasts in various materials (mainly amplitude objects) and biological samples (mainly phase objects)—the research for which is ongoing—this study will be able to contribute to the development of high-performance wavelength-tunable SPC imaging by simply integrating an OSPM into currently existing technologies.
Funding
Korea Basic Science Institute (D210300); National Research Foundation of Korea (NRF2021R1F1A104940712); National Research Foundation of Korea (NRF2021M3D1A204643712); Institute for Information and Communications Technology Promotion (1711126171).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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