金膠體溶液中雷射誘發微氣泡與奈米氣泡之光聲效應

Abstract

本論文探討在金奈米桿(gold nanorod; GNR)膠體溶液中脈衝雷射誘發生成的氣泡特性，依其生成機制分為光崩潰(optical breakdown)效應造成的微氣泡(microbubble)以及表面電漿子共振(surface plasmon resonance; SPR)效應造成的奈米氣泡(nanobubble)。 首先發現水中加入GNR確實可以有效降低微氣泡的閾值，尤其在長波長脈衝雷射的降幅更明顯。當雷射波長接近GNR的SPR共振波長與GNR濃度增加時，可以加強此效應，但同時也會受到更多的GNR之強散射效應法拉第-廷得耳效應(Faraday-Tyndall effect)，因此在SPR共振波峰附近，選擇適當的雷射波長，對於產生微氣泡可有較佳的效果。本研究以兩種具不同SPR (718 nm, 966 nm)的GNR水溶液為實驗樣品。 使用不同波長的奈秒脈衝雷射並以一20倍物鏡聚焦，再以超音波探頭量測微氣泡的光聲(photoacoustic)瞬時訊號，以及利用氦氖雷射結合光偵測器同步量測微氣泡成形過程造成的動態變化；穿透試管的光強度因光遮斷大小而隨時間之變化曲線。依據量測結果將微氣泡分成三種型態: 單氣泡、合成氣泡、分裂氣泡。調整脈衝雷射的能量由低到高，產生微氣泡型態的機率由大到小，分別為單氣泡、合成氣泡、分裂氣泡。單氣泡和分裂氣泡的壽命(lifetime)皆隨脈衝能量增加而增加，但增長行為逐漸趨緩。另一方面，因為GNR的法拉第-廷得耳效應，單氣泡和分裂氣泡的壽命與GNR濃度則呈負相關。此外，合成氣泡的壽命則與相鄰氣泡的間距及接觸時間有關，且只發生於長波長雷射激發的情況。 另外，當GNR膠體溶液被脈衝雷射光照射時，每個金奈米粒子都會因SPR效應，產生光熱(photothermal)效應，導致在其周圍產生一奈米氣泡，並伴隨衝擊波(shockwave)，即光聲(photoacoustic)訊號，若吸收的能量相同，則奈米氣泡群將一起成長及消滅。我們以較高濃度的GNR膠體溶液(例如200 ppm)為例，在能量為80 mJ的脈衝雷射(波長532 nm)照射瞬間，由光遮斷量測的時域訊號前期，可發現一約55 nsec的光遮斷時段，即奈米氣泡群的平均壽命，以此估算其平均的最大半徑約為195 nm，並且這些奈米氣泡呈現多周期的消長現象。此外將超音波訊號經快速傅立葉轉換(Fast Fourier Transform; FFT)，可分析頻率域的特性，藉由特定截止頻率的濾波，可以將微氣泡和奈米氣泡的訊號分離。

This thesis aims to study the characteristics of laser-induced bubbles in gold nanorod (GNR) colloid. Two kinds of bubble are investigated: the microbubble due to the optical breakdown, and the nanobubble due to the surface plasmon resonance (SPR). First of all, we find that the energy threshold of pulsed laser to induce microbubble in water can be reduced by adding gold nanorods (GNRs). In particular, the threshold becomes lower as a long-wavelength laser is used. When the wavelength is near the SPR peak and the concentration of GNR is raised, the effect of reducing threshold by GNR becomes more significant. However, the Faraday-Tyndall effect of GNR colloid, which is a strong light scattering, also becomes more severe. Therefore, it is a trade-off for selecting a proper wavelength to obtain an optimal performance for inducing microbubble. Two types of GNR with different SPR (718 nm, 966 nm) are prepared for experiment. A 12-nsec pulsed laser with tunable wavelength combined with an objective lens with a magnification of 20 is used to irradiate GNR colloid for generating microbubble. For the measurement, we utilize an ultrasound transducer to measure the transient photoacoustic signal, and a probing laser (He-Ne laser) combined with a photodetector to detect the light’s intensity related to the dynamic formation of microbubbles/nanobubbles. According to the features of these signals, we can divide the microbubbles into three types: single bubble, coalesced bubble and split bubble. As we increase the energy of pulsed laser, the probability of generating a single bubble is higher than those of coalesced and split bubbles; the split one always needs higher energy to be induced. The lifetimes of single bubble and split one are proportional to the pulse energy, while they approach a constant. However, they are inversely proportional to the concentration of GNR due to the Faraday-Tyndall effect of GNR. For the coalesced bubble, which is found only for the cases irradiated by a long-wavelength laser, the lifetime is related to the contact time between the adjacent bubbles. When GNR colloid is irradiated by a pulsed laser beam without an objective, nanobubble around each GNR is generated at the beginning of irradiation due to the plasmonic photothermal effect. A shockwave is also induced, accompanying with these nanobubbles; this is the photoacoustic effect. If the absorbed energy by each NGR is the same, the lifetimes of nanobubbles are the same. For example, a GNR colloid of high concentration (e.g. 200 ppm) is irradiated by a pulsed laser of 532 nm with an energy of 80 mJ. At the initial state of photodetector’s transient signal, a 55-nsec period of light blocking is observed, indicating the average lifetime of nanobubbles. Based on the lifetime, an estimated maximum of radius of nanobubble is about 195 nm. In addition, the multi-cycle process of nanobubble growth was observed. Alternatively, we characterize the nanobubble by using the spectrum analysis of the ultrasonic signal, through the fast Fourier transform. We can use a filter with a specific cut-off frequency for ultrasonic signal in frequency domain to separate the component of nanobubble from that of microbubble.