Femtosecond coherent control of terahertz radiation by transient nanophotonic structures

频率从0.3到30太赫兹的相干辐射最近已经可以通过飞秒激光技术获得。太赫兹（THz）波有许多应用，特别是低频激发（如分子振动/旋转模式，声子和固体中的自旋波）的光谱学，用于成像以及以太赫兹比特率进行短距离无线通信。太赫兹光可以通过诸如透镜、偏光镜和滤光片等静态元件来操纵。然而，也需要对太赫兹辐射进行超快调制，例如在超快光开关或移频器中，它们的发展才刚刚开始。在这个提议中，我们将实现一种非常灵活的方法来实现瞬态纳米光子结构对太赫兹辐射的超快操纵。为此，我们用可见飞秒激光束照射半导体板，其空间轮廓可以通过光调制器任意塑造。通过半导体中的光吸收，光束模式被转换成平板中电子空穴等离子体的密度模式。受辐照的区域变成金属，而黑暗的区域仍然是电介质。由于光辐射可以实现比太赫兹波长小得多的特征（大约300微米），我们可以在飞秒时间尺度上为太赫兹辐射打开定制的纳米光学结构。利用这种方法，我们将实现和研究瞬态周期结构，即超材料和光子晶体，有三个目标：(1)我们将通过产生具有高度各向异性的超材料来实现瞬态偏振片和波片，这带来了所需的双折射。我们还将实现瞬态光子晶体，并研究我们可以在多大程度上调整它们的色散关系来实现大带隙和慢光。本文还将讨论自由载流子对太赫兹辐射的欧姆损耗。(2)通过改变瞬态结构中的载流子密度，我们将测试基本光学概念的有效性。例如，在哪种载流子密度下，太赫兹频率下的结构会变成金属？一个瞬态圆盘阵列的金属性必须达到何种程度，才能与太赫兹波的磁场分量表现出明显的耦合？(3)最后，我们将在飞行中相干地操纵太赫兹辐射，即我们将在太赫兹脉冲传播的同时超快地修饰光学纳米结构。这种超快纳米结构的一个重要应用是太赫兹脉冲的频移，这将使我们能够进入尚未开发的5到8太赫兹的频率窗口。此外，我们计划绝热压缩太赫兹脉冲的频谱（光谱透镜）。

Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible by femtosecond laser technology. Terahertz (THz) waves have many applications, in particular for spectroscopy of low-frequency excitations (such as molecular vibrational/rotational modes, phonons and spin waves in solids), for imaging as well as short-range wireless communication at THz bit rates. THz light can be manipulated by static elements such as lenses, polarizers, and filters. However, ultrafast modulation of THz radiation is required as well, for instance in ultrafast optical switches or frequency shifters, whose development is just at the beginning. In this proposal, we will realize an extremely flexible approach towards the ultrafast manipulation of THz radiation by transient nanophotonic structures. For this purpose, we irradiate a semiconducting slab with a visible femtosecond laser beam whose spatial profile can be shaped arbitrarily by a light modulator. Through light absorption in the semiconductor, the beam pattern is translated into a density pattern of an electron-hole plasma in the slab. Irradiated areas become metallic whereas dark areas remain dielectric. Since features much smaller than the THz wavelength (roughly 300 micrometers) can be realized with optical radiation, we can switch on a tailored nanooptical structure for THz radiation on a femtosecond timescale.Using this approach, we will realize and study transient periodic structures, namely metamaterials and photonic crystals with three objectives:(1) We will realize transient polarizers and waveplates by generating metamaterials with a highly anisotropic unit cell, which brings about the required birefringence. We will also realize transient photonic crystals and investigate the extent to which we can tune their dispersion relation to realize large band gaps and slow light. Ohmic loss of THz radiation by free charge carriers will also be addressed.(2) By varying the charge-carrier density in the transient structures, we will test the validity of basic optical concepts. For example, at which carrier density does a structure become metallic at THz frequencies? How metallic must an array of transient disks be such that it exhibits an appreciable coupling to the magnetic-field component of the THz wave?(3) Finally, we will coherently manipulate THz radiation on-the-fly, that is, we will ultrafast modify an optical nanostructure whilst a THz pulse propagates within. An important application of such ultrafast nanostructures is frequency shifting of THz pulses, which will allow us to access the underexplored frequency window from 5 to 8THz. In addition, we plan to adiabatically compress the spectrum of a THz pulse (spectral lensing).