OPTICAL ATTOSECOND PULSES

迄今为止，阿秒脉冲的产生依赖于稀有气体中的高谐波产生（HHG）来产生阿秒脉冲序列，或者最近在XUV /软x射线区域产生孤立脉冲。在这里，我们提出了一种替代方法，通过在紫外到近红外区域的合适带宽上相干地组合飞秒脉冲，在真光学区域产生孤立的阿秒脉冲。阿秒脉冲的产生需要大约1 PHz的带宽，这相当于250 nm - 1000 nm的光学范围。利用激光介质产生阿秒脉冲是不可能的；即使是用于产生超短脉冲的材料——钛蓝宝石，其增益带宽也只足以支持~ 3fs的脉冲。相反，非线性晶体在整个透明区域都能获得增益；例如，KDP， BBO和LBO的带宽从1.4 - 1.8 PHz，强调HHG不是阿秒脉冲的唯一可能途径。我们将创造的光学阿秒脉冲将是电场的非振荡瞬态。我们提出的技术方法是由不同的母脉冲序列通过相干波形合成来构建这些脉冲。在我们早期工作的基础上，我们将使用一种新型的载波包络偏移相位控制飞秒光学参量振荡器（OPO）作为从紫外到近红外的母脉冲源。与HHG方法相比，这种全固态方法有望以高重复率高效产生，使用可获得的飞秒激光技术来产生从XUV /软x射线区域移除的阿秒脉冲，因此可以在空气中自由传播。光学阿秒脉冲为新的基础科学提供了重要的机会，例如：*可以操纵阿秒脉冲内相互相干模式的相位和强度来合成任何光场（与光强度包络整形相反）。来自OPO （200 - 7000 nm）的形状阿秒光脉冲可以实现相干控制，其中电子在0.2 - 6.0 eV分隔的多个状态之间被激发*宽带相相干脉冲可以用于探测复杂分子内的电子相干转移。例如，通过使用宽带二维光谱学，我们将集中精力开发和表征阿秒光脉冲的来源，这将成为研究化学和生命科学中复杂系统动态过程的专家的下一代超快工具。主要的光源开发工作将以赫瑞瓦特大学为基地，同时在牛津大学有一个平行的独立的阿秒光学特性工作包。随着源开发阶段接近完成，脉冲测量活动将主导项目，牛津大学开发的技术将在赫瑞瓦特系统上实施。

To date, attosecond pulse generation has relied on high-harmonic generation (HHG) in noble gases to produce either trains of attosecond pulses or, more recently, isolated pulses in the XUV / soft-X-ray region. Here, we put forward an alternative approach that will create isolated attosecond pulses in the truly optical region by coherently combining femtosecond pulses across a suitable bandwidth in the UV to near-IR region. The production of attosecond pulses requires a bandwidth of around 1 PHz, which is equivalent to the optical range from 250 nm - 1000 nm. The creation of attosecond pulses by using laser media is not possible; even Ti:sapphire, the material of choice for ultra-short pulse generation, has a gain bandwidth only sufficient to support ~3 fs pulses. By contrast, nonlinear crystals offer gain across their entire transparency region; for example, KDP, BBO and LBO have bandwidths from 1.4 - 1.8 PHz, emphasising the fact that HHG is not the only possible route to attosecond pulses.The optical attosecond pulses that we will create will be non-oscillating transients of electric field. Our proposed technical approach is to construct these pulses by coherent waveform synthesis from sequences of distinct parent pulses. Building on our earlier work, we will use a novel carrier-envelope offset phase controlled femtosecond optical parametric oscillator (OPO) as a source of the parent pulses from the UV to the near-IR.In contrast to HHG methods, this all-solid-state approach promises high-efficiency generation at high-repetition-rates using accessible femtosecond laser technology to create attosecond pulses removed from the XUV / soft-X-ray region and which therefore can freely propagate in air. Optical attosecond pulses present important opportunities for new fundamental science, for example: * the phases and intensities of the mutually-coherent modes within attosecond pulses could be manipulated to synthesise any optical field (in contrast to optical intensity envelope shaping).* shaped attosecond optical pulses from an OPO (200 - 7000 nm) could enable coherent control in which electrons are excited between multiple states separated by 0.2 - 6.0 eV* broadband phase-coherent pulses could be used to probe electronic coherence transfer within complex molecules, e.g. by using broadband 2D spectroscopy We will concentrate on developing and characterising the sources of attosecond optical pulses that will become the next generation of ultrafast tools for specialists studying the dynamical processes of complex systems in chemistry and the life-sciences.The principal source development work will be based at Heriot-Watt University, with a parallel self-contained work-package on attosecond optical characterisation based at Oxford. As the source development phase approaches completion the pulse measurement activity will dominate the project and techniques developed in Oxford will be implemented on the Heriot-Watt system.