Detection of picosecond strain pulses generated in VO2 films upon ultrafast phase transition

Picosecond acoustics utilizes short strain pulses generated by pulsed lasers to determine and affect properties of various media, from solids to biological cells. This represents a challenge in the field - to generate strong enough picosecond strain pulses with reduced heat dissipation. We show that a medium with a first-order ultrafast insulator-metal transition, such as vanadium dioxide VO2, may facilitate addressing this challenge. We used a 35 nm and 100 nm VO2 films grown epitaxially by PLD on a r-cut Al2O3 substrate in [100]M1 orientation ('XRD' data). On the other side of the substrate, a 30 nm Cr layer was deposited allowing photoelastic detection of generated pulses. We further characterized the phase transition in the samples. Static transition was characterized by heating the sample and monitoring reflectance at 1028 nm. The ultrafast trasition was characterized using femtosecond pump-probe by varying pump fluence and monitoring reflectivity change vs. pump-probe delay (See 'Characterization of phase transition' data). We also measured optical reflectivity and transmission from the samples on 1028 nm and derived optical absorption in them at temperatures below and above the static phase transition (See 'Static optic measurements' data). The laser source used for all but XRD experiments is a 170-fs Yb:KGd(WO4)2 regenerative amplifier with 5 kHz repetition rate and a central photon energy of 1.2 eV. A conventional picosecond acoustics setup was used with two beams taken from the same laser source. One beam was incident on the VO2 film acting as a pump to generate a strain pulse, the other one passing via a motorized delay line was directed on the Cr film and served as a probe. We monitored the intensity of a reflected probe pulse as a function of the time delay at 295 K and 350 K (See 'Acoustics pump-probe' data). Due to non-linear nature of strain pulse propagation in sapphire, longer pulse duration is one of the main markers for strong initial amplitude of the generated pulse. We derived photo-generated strain amplitude from the pulse duration. Plotting it versus light energy absorbed in VO2 film (See 'Photogenerated strain vs absorbed energy' data), we obtained a distinct deviation of the monotonous increase between the PIPT threshold and saturation which cannot be described by thermoelastic or deformation potential strain-generation mechanisms. We ascribe this to the contribution from the photo-induced phase transition. We also calculated the temperature rise in VO2 film associated with generation of the strain pulse for two initial phases of VO2 demonstrating effectiveness of utilization of the phase transition (See 'Calculated temperature rise vs absorbed energy' data).皮秒声学（Picosecond acoustics）利用脉冲激光产生的短应变脉冲，来表征并调控从固体到生物细胞等各类介质的物性。该领域面临一项核心挑战：在降低热耗散的前提下，生成强度足够高的皮秒应变脉冲。本研究表明，具备一阶超快绝缘体-金属相变的介质（例如二氧化钒VO₂）可助力解决这一难题。 本研究采用通过脉冲激光沉积（Pulsed Laser Deposition, PLD）在R面切割的Al₂O₃衬底上外延生长的35 nm与100 nm VO₂薄膜，取向为[100]M1（相关数据见“X射线衍射（XRD）”数据集）。在衬底的另一侧沉积了30 nm的铬（Cr）层，用于对生成的应变脉冲进行光弹性检测。我们进一步对样品的相变行为进行了表征：静态相变通过加热样品并监测1028 nm波长处的反射率来表征；超快相变则采用飞秒泵浦-探测技术，通过改变泵浦通量并监测反射率变化随泵浦-探测延迟时间的关系来表征（相关数据见“相变表征”数据集）。此外，我们还测量了样品在1028 nm波长下的光反射率与透射率，并推导了静态相变温度上下的光吸收系数（相关数据见“静态光学测量”数据集）。 除XRD实验外，其余所有实验均采用中心光子能量为1.2 eV、重复频率5 kHz、脉宽170 fs的掺镱铒钨酸钆（Yb:KGd(WO₄)₂）再生放大器作为激光源。实验采用传统皮秒声学测试平台，使用同一激光源输出的两束激光：一束入射至VO₂薄膜作为泵浦光以生成应变脉冲，另一束通过电动延迟线后入射至Cr薄膜，作为探测光。我们分别在295 K与350 K下，监测了反射探测光的强度随延迟时间的变化关系（相关数据见“声学泵浦-探测”数据集）。 由于应变脉冲在蓝宝石（sapphire）中的传播具有非线性特性，脉冲持续时间更长是生成脉冲初始振幅较高的核心标志之一。我们通过脉冲持续时间推导了光致应变振幅。将该振幅与VO₂薄膜吸收的光能进行关联绘图（相关数据见“光致应变与吸收能量的关系”数据集），结果显示在光诱导相变（Photoinduced Phase Transition, PIPT）阈值与饱和区间内，单调递增关系出现显著偏离，该现象无法通过热弹性或形变势应变生成机制来解释。我们将其归因于光诱导相变的贡献。此外，我们还针对VO₂的两种初始相态，计算了与应变脉冲生成相关的VO₂薄膜温升，验证了利用相变提升性能的有效性（相关数据见“计算温升与吸收能量的关系”数据集）。