About

Sergio Carbajo is an associate professor at UCLA in the Electrical & Computer Engineering and Physics & Astronomy departments, and a visiting professor at Stanford University’s Photon Science Division at SLAC National Accelerator Laboratory. He is the founder and director of the Quantum Light-Matter Cooperative (QLMC), a scientific consortium dedicated to understanding, designing, and controlling light-driven physical processes to address interconnected socio-technological challenges. Additionally, Carbajo directs the Queered Science and Technology Center (QSTC) at UCLA, where he develops frameworks to address diversity and representation issues in STEM through queer, radical feminist, and black analyses of science and technology's societal impacts. His research focuses on ultrafast photon sciences and light-matter interactions, working on groundbreaking instruments that capture electronic, atomic, and molecular motion with unprecedented precision. Carbajo holds a BS in Telecom Engineering from Tecnun, Universidad de Navarra, an M.Sc. in Electrical and Computer Engineering from Colorado State University, and a Ph.D. in Physics from a joint program between MIT and the Deutsches Elektronen Synchrotron. He has received numerous awards recognizing his contributions to ultrafast photon sciences, including the 2024 Nature LSA Rising Star Award, the Humboldt Fellow Award, and the ONR Young Investigator Program award. Carbajo teaches photonics, ultrafast and quantum optics, and accelerator physics at UCLA and the U.S. Particle Accelerator School, and holds various patents with over 200 peer-reviewed publications and presentations at international conferences.

Research topics

• Photochemistry
• Chemistry
• Biochemistry
• Molecular physics
• Botany
• Crystallography
• Chemical physics
• Physics
• Atomic physics
• Physical chemistry
• Organic chemistry
• Materials science
• Optics
• Biology

Selected publications

• Charged-particle control via spatio-temporally tailored pulses from gas-based nonlinear optics

  ArXiv.org · 2025-11-14

  preprintOpen accessSenior author

  Gas-filled waveguides enable few-cycle, spatio-temporally coupled (STC) pulses with programmable structure, opening new routes to control charged particles with optical fields. This review maps the landscape of optical-field-driven photoemission, then surveys gas-based nonlinear drivers, photonic crystal fibers (PCFs) for low-energy, high-repetition operation and hollow-core capillaries (HCCs) for high-power, few-cycle synthesis. We highlight mechanisms for deterministic pulse shaping, including four-wave-mixing-based spectral-phase transfer in HCCs, and show how tailored STC waveforms steer emission dynamics from the multiphoton to tunneling regimes, enabling sub-cycle gating, momentum control, and brightness scaling. We conclude with open challenges: phase stability, mid-IR scalability, coupling to nanophotonic emitters, metrology of vectorial fields, and outline a path toward compact, ultrafast, phase-coherent electron sources and emerging quantum applications powered by nonlinear photonics.

  PublisherOA PDFDOI

• Femtosecond wavelength-tunable laser system using gain managed nonlinear amplifier

  Journal of Applied Physics · 2025-05-08

  articleOpen accessSenior author

  Optical parametric amplifiers require a seed energy at the spectral range of interest for amplification. In the case of pulsed laser systems, seed pulse characteristics such as spectral energy density and spectral uniformity influence laser system design and performance. In this letter, we present the design and modeling of a few micro-joules, wavelength-tunable, femtosecond parametric amplifier system. We employ a gain-managed nonlinear fiber amplifier output as the seed pulse. This seed pulse is relatively uniform across its bandwidth from 1010 to 1180 nm. Its spectral energy density averages 600 pJ/nm, 30 times higher than conventional counterparts. The overall amplification gain is 175, and the conversion efficiency is 17.5%. The inherent quasi-linear chirp of the seed pulse is utilized to alter the output wavelength. Our tunable femtosecond laser is an enabling technology for high-demand applications such as time-resolved spectroscopy, nonlinear and advanced material research, and biomedical imaging and microscopy modalities.

  PublisherDOI

• Laser Shaping for On-Demand High-Brightness Ultrashort Electron and X-ray Instrumentation

  2025-01-01

  articleSenior author

  We are investigating spatio-temporal shaping via spectral phase transfer through four-wave mixing in gas-filled hollow-core fibers targeting applications in high-power, high-brightness on-demand attosecond-level electron and X-ray beam generation and modulation.

  PublisherDOI

• Structured Light at the Extreme: Harnessing Spatiotemporal Control for High-Field Laser-Matter Interactions

  ArXiv.org · 2025-12-04 · 2 citations

  preprintOpen access1st authorCorresponding

  This review charts the emerging paradigm of intelligent structured light for high-field laser-matter interactions, where the precise spatiotemporal and vectorial control of light is a critical degree of freedom. We outline a transformative framework built upon three synergistic pillars. First, we survey the advanced electromagnetic toolkit, moving beyond conventional spatial light modulators to include robust static optics and the promising frontier of plasma light modulators. Second, we detail the optimization engine for this high-dimensional design space, focusing on physics-informed digital twins and AI-driven inverse design to automate the discovery of optimal light structures. Finally, we explore the groundbreaking applications enabled by this integrated approach, including programmable electron beams, orbital-angular-momentum-carrying γ-rays, compact THz accelerators, and robust communications. The path forward necessitates overcoming grand challenges in material science, real-time adaptive control at MHz rates, and the extension of these principles to the quantum realm. This review serves as a call to action for a coordinated, interdisciplinary effort to command, rather than merely observe, light-matter interactions at the extreme.

  PublisherOA PDFDOI

• Hybrid Deep Reconstruction for Vignetting-Free Upconversion Imaging through Scattering in ENZ Materials

  ArXiv.org · 2025-08-18

  preprintOpen accessSenior author

  Optical imaging through turbid or heterogeneous environments (collectively referred to as complex media) is fundamentally challenged by scattering, which scrambles structured spatial and phase information. To address this, we propose a hybrid-supervised deep learning framework to reconstruct high-fidelity images from nonlinear scattering measurements acquired with a time-gated epsilon-near-zero (ENZ) imaging system. The system leverages four-wave mixing (FWM) in subwavelength indium tin oxide (ITO) films to temporally isolate ballistic photons, thus rejecting multiply scattered light and enhancing contrast. To recover structured features from these signals, we introduce DeepTimeGate, a U-Net-based supervised model that performs initial reconstruction, followed by a Deep Image Prior (DIP) refinement stage using self-supervised learning. Our approach demonstrates strong performance across different imaging scenarios, including binary resolution patterns and complex vortex-phase masks, under varied scattering conditions. Compared to raw scattering inputs, it boosts average PSNR by 124%, SSIM by 231%, and achieves a 10 times improvement in intersection-over-union (IoU). Beyond enhancing fidelity, our method removes the vignetting effect and expands the effective field-of-view compared to the ENZ-based optical time gate output. These results suggest broad applicability in biomedical imaging, in-solution diagnostics, and other scenarios where conventional optical imaging fails due to scattering.

  PublisherOA PDFDOI

• The LCLS-II Photoinjector Laser System

  2025-01-01

  articleSenior author

  We present a comprehensive overview of the development of the LCLS-II photoinjector laser system, emphasizing its key components and advancements in producing high-quality, high-energy, attoseconds X-ray pulses for X-ray Free Electron Laser science.

  PublisherDOI

• Towards digital twins of high-power laser systems

  2025-01-27

  articleSenior author

  Optics and photonics are transforming rapidly as machine learning (ML) techniques advance in the field. These ML advancements require extensive datasets, often challenging to obtain experimentally. We present a start-to-end (S2E) software framework for high-power laser systems. Using this framework, we develop a digital twin for a complex laser system, including pulse shaping, amplification, and upconversion. We explore amplifier parameters to mimic experimental conditions and generate an ML model that achieves a 250-fold speed-up over traditional numerical simulations, paving the way for generalized digital twins and enhanced synergy between simulation and experiment.

  PublisherDOI

• Machine learning techniques for frequency comb optimization

  2025-03-19

  article

  We use supervised and unsupervised machine learning techniques to investigate how optical frequency combs may be used to identify gas molecules in the atmosphere. Dual comb heterodyne detection with GHz and THz repetition rates are often used as spectroscopic methods for probing ro-vibrational IR spectra of small molecules, proving to be promising in dispersive atmospheric sensing regimes. We simulate and recover an intensity spectrum from the molecule-specific IR absorption in the region spanned by the comb bandwidth. In practice, conventional frequency combs contain a narrow range of frequencies that may not span the spectral range associated with typical atmospheric target signals, prohibiting convenient molecule identification. We present several inexpensive, efficient machine learning methods to analyze optimal comb frequencies to maximize the ability to determine the molecular composition of a sample, based only on the interactions at those frequencies with respect to various channels of noise. Using a synthetic dataset, these methods are able to accurately identify molecules using simulated frequency combs, comparable to the ones used in practice. Furthermore, these methods withstand added noise (based on the imperfections of the experimental equipment) beyond what is expected in real applications. We also investigate the optimal comb generation parameters that provide the most informational value, and provide an analysis of the change in accuracy based on the number and size of allocated combs. The robustness of these methods suggests the techniques presented in this paper may be integrated into a laboratory setup and even in on-chip comb systems deployed in an atmospheric setting. This approach may be used as a detection framework for future quantum-enhanced systems via squeezed bi-photon comb inputs.

  PublisherDOI

• Nonlinear Shaping in the Picosecond Gap

  Ultrafast Science · 2025-01-01 · 2 citations

  articleOpen accessSenior author

  Lightwave pulse shaping in the picosecond regime has remained unaddressed because it resides beyond the limits of state-of-the-art techniques, due to either its inherently narrow spectral content or fundamental speed limitations in electronic devices. The so-called picosecond shaping gap hampers progress in all areas correlated with time-modulated light–matter interactions, such as photoelectronics, health and medical technologies, and energy and materials sciences. We report on a novel nonlinear method to simultaneously frequency-convert and adaptably shape the envelope of light wave packets in the picosecond regime by balancing spectral engineering and nonlinear conversion in solid-state nonlinear media, without requiring active devices. We capture computationally the versatility of this methodology across a diverse set of nonlinear conversion chains and initial conditions. We also provide experimental evidence of this framework producing picosecond-shaped, ultranarrowband, near-transform-limited light pulses from broadband, femtosecond input pulses, paving the way toward programmable lightwave shaping at gigahertz-to-terahertz frequencies.

  PublisherDOI

• Nurturing Deeper Ways of Knowing in Science

  Issues in Science and Technology · 2025-01-01

  article1st authorCorresponding

  Efforts to diversify representation in science and engineering require initiatives that increase diversity of thought as well.

  PublisherDOI

Frequent coauthors

• Franz X. Kärtner

  244 shared

• Randy Lemons

  198 shared

• Sébastien Boutet

  129 shared

• Koustuban Ravi

  82 shared

• Frederike Ahr

  Universität Hamburg

  78 shared

• Kensuke Tono

  Japan Synchrotron Radiation Research Institute

  76 shared

• Jack Hirschman

  76 shared

• Damian N. Schimpf

  Deutsches Elektronen-Synchrotron DESY

  74 shared

Awards & honors

• 2025 UCLA Inaugural HSI Fellow
• 2024 Nature Light Science and Applications Rising Star Award
• 2024 Humboldt Foundation Fellow (3 year term)
• 2024 ONR Young Investigator Program Award
• 2024 Blavatnik Young Investigator (Finalist)