Scanning Resonator Microscopy (SRM)

These projects are developing a new form of scanning probe microscopy using whispering gallery mode resonators for high resolution optical contrast.

Scanning Resonator Microscopy Integrating Phase Sensitive Detection

(Applied Optics 2017, 56(35), 9716-9723)

Scanning resonator microscopy (SRM) is a scanning probe technique that uses a small, optical resonator attached to the end of a conventional atomic force microscopy (AFM) cantilever to simultaneously measure optical and topography properties of sample surfaces. In SRM, whispering gallery mode (WGM) resonances excited in the attached optical resonator shift in response to changes in surface refractive index (RI), providing a mechanism for mapping RI with high spatial resolution. In our initial report, the SRM tip was excited with a fixed excitation wavelength during sample scanning, which limits the approach. An improved method based on a wavelength modulation coupled with phase sensitive detection is reported here. This results in real-time characterization of WGM spectral shifts while eliminating complications arising from measurements based solely on signal intensity. This improved approach, combined with a modified tip design enabling integration of smaller resonators, is shown to enhance signal-to-noise and lead to sub-100 nm spatial resolution in the SRM optical image. The improved capabilities are demonstrated through measurements on thin dielectric and polymer films.

SRM Tip and Scheme

(A) and (B) A 10 μm diameter barium titanate microsphere is attached to the end of a conventional AFM cantilever to form the SRM probe. (C) Contact-mode feedback is used to position and hold the SRM tip near the sample surface while a separate x-y piezo stage scans the sample under the tip during imaging. Excitation light from a tunable diode laser is sent into a Dove prism to evanescently excite WGM resonances in the SRM tip as it nears the surface. Evanescently scattered light from the SRM tip is collected from below and detected with either an APD or PMT. (D) WGM spectrum of the SRM tip shown in panels A and B.

SRM Topography and Optical Images

40 μm x 40 μm surface (A) topography and (B) SRM optical signals measured on a test sample consisting of a thin MgF2 film (RI = 1.3770 at 633 nm) evaporated onto a glass substrate. A mask used during the coating process created the observed 60 nm step from the glass substrate to MgF2 film. This step corresponds to a large change in the RI as measured in the SRM phase image. Smaller MgF2 islands are observed on the glass substrate side. A magnified view from the boxed region in panel A is shown below in the 15 μm x 15 μm (C) topography and (D) SRM optical images. Note that the images in C and D were taken slightly out of the field of view of A, so the boxed region does not entirely encompass the imaged area.

SRM topography and Optical Images

Simultaneously collected 30 μm x 30 μm (A) topography and (B) SRM optical images of an amorphous fluoropolymer spin coated onto a glass substrate. From the topography image, defects in the film exposing the glass substrate indicate a film thickness of 10-20 nm. These defects correspond with changes in the SRM optical signal shown in B. 15 μm x 15 μm (C) topography and (D) SRM optical measurements made in the boxed region shown in A. (E) 7.5 μm x 7.5 μm topography and (F) SRM optical measurements made in the boxed region (partially) shown in C. The smallest optical features in F have a FWHM less than 100 nm.

Scanning Resonator Microscopy: Integrating Whispering Gallery Mode Sensing with Atomic Force Microscopy

(ACS Photonics 2015, 2(6), 699-706)

Scanning resonator microscopy (SRM) is developed to integrate whispering gallery mode (WGM) sensing with atomic force microscopy (AFM). The hybrid technique combines the exquisite refractive index sensing of whispering gallery mode resonators with the topography mapping capabilities of AFM. A 45 μm diameter barium titanate microsphere is attached to the end of a conventional AFM cantilever and acts as both a WGM resonator and stylus for mapping surface topography. Calibration plots, taken in contact-mode feedback, show that the WGM spectrum responds to changes in both solution and substrate refractive index. SRM imaging of a glass substrate reveals changes in surface refractive index that correspond to a small, 36 nm high feature measured simultaneously in the contact-mode topography image. Spectral measurements confirm that the contrast arises from refractive index changes and not coupling with sample topography, thus validating the approach. Additional measurements on thin polymer films and protein coated surfaces are presented and discussed in terms of possible areas of application for SRM.

SRM Tip and Apparatus

(A) Magnified view of AFM cantilever with 45 μm diameter barium titanate microsphere attached on the side. (B) Schematic of SRM platform that combines whispering gallery mode sensing with AFM. The microresonator tip is held in an AFM head that adjusts the tip vertically in contact-mode feedback. The sample below is mounted on a Dove prism which is held in x-y piezo scanner to scan the sample under the tip. Excitation light from a tunable diode laser is directed into the Dove prism creating an evanescent wave at the sample surface to excite WGMs in the microresonator tip. Evanescently scattered excitation from the tip is collected from below and detected on an avalanche photodiode (APD). (C) WGM spectrum of the microresonator tip shown in (A) measured by tuning the diode laser while detecting the scattered excitation. The spectrum was collected with the tip held in contact-mode at a glass surface under aqueous conditions.

SRM Images and Spectral Shifts

SRM topography (A) and optical (B) images of a cleaned glass substrate under aqueous conditions. The excitation wavelength is held constant at 634.842 nm, which corresponds to the WGM resonance of the tip on glass. The topography image reveals a small 36 nm high feature in the center of the image that corresponds to a large decrease in scattered intensity from the microresonator tip. (C) Spectra collected on and off the feature confirm the intensity decrease observed in (B) arises from a shift of the WGM resonance of the tip. SRM topography (D) and optical (E) images of the same sample region with the excitation now held at 634.846 nm which corresponds to the WGM resonance of the tip on the feature. An increase in intensity is observed as the tip comes into resonance while scanning across the feature.