The concept of pure optical photoacoustic microscopy(POPAM) was proposed based on optical rastering of a focused excitation beam and optically sensing the photoacoustic signal using a microring resonator fabricated by a nanoimprinting technique. After the refinements of the microring’s working wavelength and in the resonator structure and mold fabrication, an ultrahigh Q factor of 3.0×105 was achieved which provided high sensitivity with a noise equivalent detectable pressure(NEDP) value of 29Pa. This NEDP is much lower than the hundreds of Pascals achieved with existing optical resonant structures such as etalons, fiber gratings and dielectric multilayer interference filters available for acoustic measurement. The featured high sensitivity allowed the microring resonator to detect the weak photoacoustic signals from micro- or submicroscale objects. The inherent superbroad bandwidth of the optical microring resonator combined with an optically focused scanning beam provided POPAM with high resolution in the axial as well as both lateral directions while the axial resolution of conventional photoacoustic microscopy (PAM) suffers from the limited bandwidth of PZT detectors. Furthermore, the broadband microring resonator showed similar sensitivity to that of our most sensitive PZT detector. The current POPAM system provides a lateral resolution of 5 μm and an axial resolution of 8 μm, comparable to that achieved by optical microscopy while presenting the unique contrast of optical absorption and functional information complementing other optical modalities. The 3D structure of microvasculature, including capillary networks, and even individual red blood cells have been discerned successfully in the proof-of-concept experiments on mouse bladders ex vivo and mouse ears in vivo. The potential of approximately GHz bandwidth of the microring resonator also might allow much higher resolution than shown here in microscopy of optical absorption and acoustic propagation properties at depths in unfrozen tissue specimens or thicker tissue sections, which is not now imageable with current optical or acoustic microscopes of comparable resolution.

We demonstrate an ultrasonic detector with unprecedented broad bandwidth and high sensitivity, based on an imprinted polymer optical microring. It has an acoustic response of up to 350 MHz at −3 dB and noise-limited detectable pressure as low as 105 Pa in this frequency range. Application of such a detector in photoacoustic imaging leads to improved axial resolving ability compared with using the conventional ultrasound detector, and sub-3 μm axial resolution is achieved, which is more than a 2-fold improvement with respect to the reported record. The device's miniaturized cavity height guarantees its broadband response, and at the same time, its high optical quality factor ensures the detection sensitivity. Our work suggests that the polymer-based miniature microring resonator works as a high-performance ultrasound detector and has potential for acquiring volumetric photoacoustic images with cellular/subcellular resolution in three dimensions.

Smooth sidewall silicon micro-ring molds have been fabricated using resist reflow and thermal oxidation method. High Q factor polymer micro-ring resonators have been fabricated using these molds. Quality factors as high as 105 have been measured at telecommunication wavelength range. By carefully examining the different loss mechanisms in polymer micro-ring, we find that the surface scattering loss can be as low as 0.23 dB/cm, much smaller than the absorption loss of the polystyrene polymer used in our devices. When used as an ultrasound detector such a high Q polymer micro-ring device can achieve an acoustic sensitivity around 36.3 mV/kPa with 240 μW operating power. A noise equivalent pressure (NEP) is around 88 Pa over a bandwidth range of 1–75 MHz. We have improved the NEP by a factor of 3 compared to our previous best result.

We demonstrate carbon nanotube ͑CNT͒ composite-based optoacoustic transmitters that generate strong and high frequency ultrasound. The composite consists of CNTs grown on a substrate, which are embedded in elastomeric polymer used as an acoustic transfer medium. Under pulsed laser excitation, the composite generates very strong optoacoustic pressure: 18 times stronger than a Cr film reference and five times stronger than a gold nanoparticle composite with the same polymer. This enhancement persists over a broadband frequency range of up to 120 MHz and is confirmed by calculation. We suggest the CNT-polymer composites as highly efficient optoacoustic transmitters for high resolution ultrasound imaging. © 2010 American Institute of Physics. ͓doi:10.1063/1.3522833͔Laser-induced ultrasound generation is an effective way to make high frequency ultrasound transmitters by exploiting the high frequency spectra of laser pulses to achieve broad acoustic bandwidths. Typically, such transmitters are made of light-absorbing thin films containing specific structures designed to have high optical absorption that are capable of efficient optoacoustic conversion, for example, thin metal, 1 dye-doped polymer composites, 2,3 and two-dimensional ͑2D͒ gold nanoparticle ͑AuNP͒ arrays. 4 They are often integrated with optical interferometric detectors ͑e.g., FabryPérot etalon͒ 5 to make all-optical ultrasound transducers working over broadband and high frequency in 2D array platforms. In these arrays, the size of each element was determined by the spot size of the focused laser beam that is of the order of several microns.Thin metallic coatings on solid substrates are suitable as a common reference material for qualifying the performance of optoacoustic transmitters.1,2 While these thin films ͑Ͻ1 m͒ can be used as high frequency ultrasound sources their optoacoustic conversion efficiency is poor mainly because of the low thermal expansion. Also, a significant percentage of light is reflected back from the film surface.As large thermal expansion is desirable for strong pressure generation, an elastomeric polymer, polydimethylsiloxane ͑PDMS͒, has been used as an acoustic transfer medium to interface with light-absorbers.2-5 A composite film of PDMS with carbon black as a light-absorber has shown nearly 20 dB improvement in optoacoustic signal strength as compared to a reference Cr film alone.2 However, high frequency response was severely limited due to the composite film thickness ͑ϳ25 m͒ due to the acoustic attenuation. This is a serious issue because high frequency performance is vital for optoacoustic transmitters. Moreover, it is challenging to obtain uniform mixing and dispersion of carbon black particles in the PDMS matrix. Agglomeration of carbon black can cause uneven light absorption within the same film. Significant progress has been recently made using a planar array of AuNPs with an overlying PDMS layer of several microns. 4 High frequency output was improved by ϳ5 dB over 70-100 MHz as compared with those carbon black-PDMS...

Small size ultrahigh Q polymer microrings working at near visible wavelength have been experimentally demonstrated as ultralow noise ultrasound detectors with wide directivity at high frequencies ͑Ͼ20 MHz͒. By combining a resist reflow and a low bias continuous etching and passivation process in mold fabrication, imprinted polymer microrings with drastically improved sidewall smoothness were obtained. An ultralow noise-equivalent pressure of 21.4 Pa over 1-75 MHz range has been achieved using a fabricated detector of 60 m diameter. The device's wide acceptance angle with high sensitivity considerably benefits ultrasound-related imaging. © 2011 American Institute of Physics. ͓doi:10.1063/1.3589971͔Ultrasound-related medical imaging is a noninvasive modality and has become increasingly popular. Highresolution ultrasound and photoacoustic imaging, achieved by operating at high frequencies ͑Ͼ20 MHz͒, can provide accurate analysis and diagnosis in medical imaging. Such noninvasive modality is an excellent tool for small animal studies and intravascular imaging.1,2 To detect highfrequency ultrasound wave, a detector with both wideband response and small element size is needed. Small device size minimizes the spatial averaging effect for high-frequency waves, which is essential for high-resolution imaging. For example, phased-array imaging systems working at a center frequency of 30 MHz require / 2 element size and spacing on the order of 25 m, where is the acoustic wavelength. Another example is that the small device size for tomographic imaging provides high resolution and high contrast over a large imaging region.3,4 Although the piezoelectric material polyvinylidene fluoride ͑PVDF͒ based needle hydrophones can reach the requirement of wide bandwidth and small element size ͓e.g., 40 m ͑HPM04/01, Precision Acoustics, Dorchester, Dorset, UK͔͒, the device has poor sensitivity: the noise-equivalent pressure ͑NEP͒ is relatively high ϳ10 kPa, which seriously limits the imaging depth. Moreover, arrays with small element size and spacing and large element count, required for real-time imaging, are very difficult to realize using piezoelectric transducers because of the increased noise level, complexity of electrical interconnects, and fabrication challenges.Optical detection of ultrasound 5-9,15 could potentially address the above issues. It can achieve a low NEP ͑Ӷ1 kPa͒ with relative small element size and wideband response, 8 and would be easier to create dense arrays with small element size. 17 We have been exploiting polymer microring resonators as ultrasound detectors, and have demonstrated a low NEP of ϳ230 Pa over a wide bandwidth of 1-75MHz even with a relatively low cavity Q factor of 6000 in the past. 8 Besides, a flat frequency response from dc to over 90 MHz at Ϫ3 dB was calibrated by a wideband photoacoustic source. 8 With improved fabrication, further reduction in device's NEP ͑ϳ88 Pa͒ has been realized using a higher Q factor microring device.9 However the device diameter ͑D͒ was still around 100 m, limiting...