Full-Field Subwavelength Imaging Using a Scattering Superlens

Light-matter interaction gives optical microscopes tremendous versatility compared with other imaging methods such as electron microscopes, scanning probe microscopes, or x-ray scattering where there are various limitations on sample preparation and where the methods are inapplicable to bioimaging with live cells. However, this comes at the expense of a limited resolution due to the diffraction limit. Here, we demonstrate a novel method utilizing elastic scattering from disordered nanoparticles to achieve subdiffraction limited imaging. The measured far-field speckle fields can be used to reconstruct the subwavelength details of the target by time reversal, which allows full-field dynamic super-resolution imaging. The fabrication of the scattering superlens is extremely simple and the method has no restrictions on the wavelength of light that is used.

Figure 1

Schematic diagram of experimental concept. (a) Measurement of the TM with a pointlike source generated by a near-field aperture as the input basis. The TM is composed of speckle fields propagating through the turbid medium from the near-field aperture. $|a⟩$, $|b⟩$, $|c⟩$ are corresponding speckle fields after the turbid medium as a response to the pointlike illuminating fields $|A⟩$, $|B⟩$, $|C⟩$ on the turbid medium, respectively. (b) Recovery of the original field on the turbid medium from the speckle field generated by an arbitrary sample. The speckle field $|x⟩$, which is generated by the sample field $|x⟩$. is multiplied by the time-reversed $\mathrm{TM}({\mathrm{TM}}^{†}),$ which is equivalent to decomposing $|x⟩$ into a linear combination of $|a⟩$, $|b⟩$, $|c⟩$ with the appropriate complex coefficients.

Figure 2

Experimental setup and reconstructed point source. (a) A Mach-Zehnder interferometer is incorporated with a commercial NSOM system. BS: beam splitter, $P$: polarizer, $F$: optical fiber, HWP: half-wave plate, PD: quadrant photodiode, OL: objective lens. (b) SEM image of the NSOM tip aperture. (c) Far-field image of light radiating from the tip obtained with an objective lens of 0.8 NA. The diffraction-limited beam spot is about 420 nm. (d) Measured amplitude and phase of the far-field speckle pattern after light from the NSOM aperture has scattered through the turbid medium and has propagated through over 1 m of far-field optics. The color represents the phase while brightness represents the normalized amplitude. (e) Reconstructed image of the same light source using the TM and the principle of time reversal. (f) FWHM of the reconstructed beam spot is about 167 nm. Thick black line represents the average of 150 experimental sets (thick green stripe represents maximum and minimum of the experimental data, respectively).

Figure 3

Dynamic 2D image reconstruction. (a) Actual loci of the movement of the tip aperture relative to the turbid medium to generate a pseudosample displaying the letters NANO. (b) Diffraction-limited image of NANO, constructed by convolution of the tip movement with the system’s point spread function shown in Fig. 2. (c) Reconstructed image using the near-field TM. (d)–(f) Same as in (a)–(c) for a different pattern displaying a triangle. (g) Dynamic full-field imaging where the speckle field is measured dynamically rather than accumulating for the entire pattern generating process. Each full-field image is obtained with a single measurement with a time resolution decided solely by the sensitivity of the EMCCD. Each image shows a higher signal to noise ratio compared with (c),(f) due to effective management of the dynamic range.

Figure 4

Real image reconstruction. (a) SEM image of the rectangular aperture object fabricated by truncating a NSOM probe, (b) measured propagating speckle field with light illuminating the object, and (c) reconstructed image using the near-field TM.