Intensity-to-Phase Coupling in High-Repetition-Rate CEP Measurements

Abstract

This study addresses the challenge of precise, high-repetition-rate, single-shot measurements of the carrier–envelope phase in ultrafast optics. We constructed a Mach–Zehnder interferometer based on dual-arm white-light interference, and systematically investigated the intensity-to-phase coupling that arises during white-light generation. In the experiment, an acousto-optic modulator introduced small, periodic perturbations to the laser intensity in one arm of the interferometer; both arms then generated white light, which was recombined to produce interference fringes. Spectral analysis of these fringes enabled extraction of CEP-related phase information. The measurements demonstrate that the output phase of the white light responds significantly to perturbations in the incident laser pulse energy, exhibiting a clear intensity-to-phase coupling relationship across different wavelengths. By performing measurements under varying input conditions, we quantitatively determined the magnitude of the intensity-to-phase coupling coefficient and its spectral dependence. This work deepens our understanding of the origins of phase noise in white-light generation, provides guidance for optimizing the stability of supercontinuum sources, and offers essential technical support for implementing single-shot CEP measurements in high-repetition-rate strong-field and attosecond science experiments.

Popular Abstract

To film electrons in action, scientists use `flashes' of light lasting mere attoseconds. But these `camera flashes' can `wobble', blurring our view. My project helps deepen the understand of the measurement of this jitter for clearer electron movies. This `wobble', technically known as the carrier-envelope phase (CEP), is crucial. If we can detect and control it, we can make even sharper `electron movies' and unlock new secrets of the quantum world. My research delved into a key noise source of the measurement of this jitter, paving the way for more stable and precise measurements.

Imagine trying to take a sharp photo of a hummingbird's wings, you need an incredibly fast camera flash. Scientists face a similar challenge when studying the lightning-fast dance of atoms and electrons. They use laser pulses lasting just attoseconds (billionths of a billionth of a second!) as their `flash'. For context, Anne L’Huillier won the Nobel Prize for capturing instantaneous changes in electron dynamics in matter, essentially pioneering the field of ``attosecond physics”.

However, for the most advanced experiments, just having a short flash isn't enough. The exact shape and timing of the light wave within each pulse, its CEP, is critically important. Think of it as the precise rhythm of the light wave. If this rhythm, or CEP, isn't consistent or known for each flash, it's like trying to film a high-speed race with a shaky camera, the details get blurred, and we might miss crucial moments.

The challenge my project tackled is quantitatively analyzing the noise introduced into the detection of this `wobble'. A promising technique to measure the CEP of every single laser pulse involves converting the laser light into a broad spectrum of colors, called `white light' (supercontinuum generation). However, this very conversion process can ironically introduce its own jitter, making precise measurements tricky. Existing methods to measure CEP are often too low in efficiency, average over many pulses, or are too complex for the advanced lasers.
To carry on experiments, I built a special kind of optical setup called an interferometer. I split a laser pulse into two. Both beams were then converted into `white light'. Crucially, before one of the beams underwent this white-light generation, I used a device to subtly change its brightness, or intensity. After both beams generated their white light, they were recombined, and I analyzed how they interfered with each other – the pattern of this interference tells us about their jitter, or relative phase.

The exciting discovery was a direct and quantified link: even tiny changes in the input laser's brightness (intensity) caused predictable shifts in the jitter (phase) of the white light produced. We could actually measure how much the phase would change for a given change in intensity, and this relationship varied for different colors within the white light. It’s like finding out that if you slightly change the fuel flow to an engine, the engine's hum changes pitch in a predictable way. This ``intensity-to-phase coupling" means that fluctuations in laser power, which are almost always present, directly translate into phase noise.

Why is this important? Understanding this intensity-to-phase coupling is like finding a hidden knob to fine-tune our ultrafast `cameras'. This means sharper `electron movies': it helps us understand and potentially compensate for a significant source of noise in CEP measurements. This means scientists can get more reliable data from each laser pulse, leading to clearer insights into ultrafast phenomena like electron movements in new materials; better light sources: it provides guidance for designing more stable white-light sources, which are vital tools in many areas of science and technology; advancing ultrafast science: ultimately, this project supports the development of more precise single-shot CEP measurement techniques. This is crucial for experiments in strong-field physics and attosecond science, opening up new windows into the quantum realm.

My work helps to systematically understand a piece in dealing with these incredibly fast light pulses, bringing us one step closer to perfectly controlled `flashes' for observing the universe at its most fundamental timescales.

• author: Qin, Zijie
• supervisor: Cord Arnold; Chen Guo
• organization: Atomic Physics; Department of Physics
• course: PHYM03 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: ultrafast and nonlinear optics, white-light generation, intensity-to-phase relation, interferometry, carrier-envelope phase
• publication/series: Lund Reports on Atomic Physics (LRAP)
• report number: LRAP612
• language: English
• id: 9191090
• date added to LUP: 2025-05-28 10:02:02
• date last changed: 2025-05-28 10:02:02