Nonlinear polariton phenomena in GaN-based slab waveguides at temperatures up to 300 K

由于光子和激子的混合(激子是凝聚态物质中氢的类似物)，可以在光学活性半导体材料中形成新的准粒子，即所谓的极化子。虽然在自由空间中碰撞的两个光子不会相互作用，但由于波函数中的激子成分，极化子会发生强烈的排斥。谢菲尔德小组已经证明，在任何其他超快光子材料中，极化子-极化子相互作用比有效光子-光子相互作用强几个数量级，在超快光子材料中，光与物质的耦合很弱。高效的偏振子-偏振子散射可用于开发新型超快光源、混频器和转换器，以及可扩展和紧凑的设备，以非常快的时间尺度和非常低的信号强度(潜在地在单光子水平上)执行光控制。潜在地，这可能对新型光子信号处理和量子技术硬件的发展产生强烈的影响。极化平台还非常适合于研究基本的多体现象，从玻色-爱因斯坦凝聚、超流和孤子到重要物理系统中的量子关联相位，例如拓扑绝缘体的光子模拟或量子霍尔系统。到目前为止，由于激子相互作用引起的现象在GaAs微谐振器中仅在4-50K被探索过，我们的建议利用了最近基于AlGaN/GaN材料的光波导中的紫外极化子的演示，其中由于激子结合能大和激子-光子耦合效率高，极化子在300K下是很强的，这为探索新的相互作用极化子的室温物理和使极化子的应用成为现实提供了机会。为了探索相互作用的GaN极化子的基本物理，我们将研究极化子孤子，即由非线性稳定的非扩展波包。这些相互作用依赖于许多因素，如激子玻尔半径、结合能和激子波函数中的激子分数。当不同的频率分量以不同的速度传播时，在偏振子波导中传播的弱光脉冲随时间展宽。相反，通过增加脉冲强度，极化子相互作用有望抵消导致孤子形成的扩散。考虑到极大的极化子非线性，孤子有望在极低的阈值和很短的长度尺度上形成，约为10微米。GaN基光波导的另一个优点是激子-光子杂化发生在很宽的频率范围内，使极化子-极化子的散射从光谱上很窄的脉冲到一个宽的准连续态。因此，预计将产生持续时间低至10‘S飞秒的非常短的UV极化孤子脉冲。这项研究的结果还将使新型的紫外光宽带脉冲源和频率转换器能够在非常低的阈值下工作，这对于生物光子学和分子光化学中的许多光谱应用是重要的。最后，我们将通过利用不同中心频率的极化脉冲在基于耦合波导的光子电路中传输的非线性相互作用来演示一个紧凑型超快全光极化开关的原型。这些相互作用在光信号中引起非线性相移，使得能够对超快光脉冲进行路由和切换。鉴于极化激元系统的响应速度非常快，这种开关有望以太赫兹频率工作。我们注意到GaN基纳米结构中的激子-极化子对热电子-空穴载流子的屏蔽也是非常健壮的，这使得研究在存在光学增益的情况下传播的极化子的放大成为可能。这对于极化器件的可扩展性是至关重要的。

Novel quasi-particles, so-called polaritons, can be formed in an optically active semiconductor material due to mixing of photons and excitons (an exciton is an analogue of hydrogen in condensed matter). While two photons colliding in free space do not interact, polaritons strongly repel due to the exciton component in their wavefunctions. The Sheffield group has demonstrated that polariton-polariton interactions are several orders of magnitude stronger than effective photon-photon interactions in any other ultrafast photonic materials, where light is weakly coupled to matter. The efficient polariton-polariton scattering can be utilised for development of novel ultrafast light sources, frequency mixers and converters as well as scalable and compact devices performing control of light by light on a very fast timescale and at very low signal intensities (potentially at a single photon level). Potentially, this may have a strong impact on development of novel photonic signal processing and quantum technology hardware. The polariton platform is also well suited to the study of fundamental many-body phenomena ranging from Bose-Einstein condensation, superfluidity and solitons to quantum correlated phases in important physical systems, such as photonic analogues of topological insulators or quantum Hall systems. So-far phenomena due to polariton interactions have been explored in GaAs microresonators only at 4-50 K. Our proposal capitalises on the recent demonstration of ultraviolet polaritons in waveguides based on AlGaN/GaN material, where polaritons are robust at 300 K given the large exciton binding energy and highly efficient exciton-photon coupling; this provides an opportunity to explore the novel room temperature physics of interacting polaritons and to bring polariton applications to reality. In order to explore the fundamental physics of interacting GaN polaritons we will address polariton solitons, i.e non-spreading wavepackets stabilised by the nonlinearity. These interactions depend on many factors, such as the exciton Bohr radius, binding energy and the exciton fraction in the polariton wavefunction. A weak light pulse propagating in a polariton waveguide broadens with time, as different frequency components propagate with different velocities. By contrast, by increasing the pulse intensity polariton interactions are expected to cancel the spreading leading to formation of a soliton. Given the giant polariton nonlinearities solitons are expected to form at ultra-low thresholds and on a very short length-scale of ~10 micrometers.Another advantage of GaN-based waveguides over GaAs counterparts is that exciton-photon hybridisation occurs over a broad range of frequencies enabling polariton-polariton scattering from a spectrally narrow pulse to a broad quasi-continuum of states. As a result very short UV polariton soliton pulses with a duration down to 10's femtoseconds are anticipated. The outcome of this research would also enable realisation of novel ultraviolet broadband pulsed sources and frequency converters operating at very low thresholds, which are important for many spectroscopy applications in biophotonics and molecular photochemistry. Finally, we will demonstrate a prototype of a compact ultrafast all-optical polariton switch by exploiting nonlinear interactions between the polariton pulses with different central frequencies propagating in a photonic circuit based on coupled waveguides. These interactions induce nonlinear phase shifts in the optical signals enabling routing and switching of the ultrafast optical pulses. Given the very fast response of polariton system such a switch is expected to operate at THz rates. We note that exciton-polaritons in GaN-based nanostructures are also very robust against screening by hot electron-hole carriers, enabling study of the amplification of propagating polaritons in the presence of optical gain. This is essential for scalability of polariton devices.