Near-Infrared Single-Cycle Pulses

• Near-infrared single-cycle pulses are ultrashort optical transients (~2–4 fs) defined by a single optical cycle envelope and extreme CEP sensitivity.
• They are generated through ultrabroadband spectral broadening in gas-filled fibers and advanced dispersion compression techniques that manage higher-order dispersions.
• These pulses enable breakthrough applications in attosecond science, strong-field physics, ultrafast spectroscopy, and relativistic plasma optics by precisely controlling electron dynamics.

Near-infrared (NIR) single-cycle pulses are intense, ultrashort optical transients whose temporal envelope contains only a single period of the carrier frequency, typically in the spectral region spanning 0.7–2.5 μm. These pulses, characterized by durations on the order of a few femtoseconds or less, enable field-resolved control of matter, time-domain studies of ultrafast electronic and nuclear motion, and novel nonlinear light–matter interactions. Achieving and characterizing such pulses in the NIR requires precisely engineered spectral broadening, compression, and compensation of higher-order dispersion, as well as advanced spatiotemporal metrology sensitive to space–time couplings on attosecond timescales.

1. Definition and Key Physical Properties

A single-cycle pulse in the NIR is an electromagnetic field

$E(t) = A(t) \cos(\omega_0 t + \phi),$

where the envelope $A(t)$ and the carrier frequency $\omega_0$ are such that the full width at half maximum (FWHM) of $A(t)$ is close to $T = 2\pi/\omega_0$, i.e., one optical cycle. For example, a transform-limited pulse at $\lambda_0 = 800~\text{nm}$ has $T \approx 2.67$ fs; achieving NIR pulse durations in the range 2–4 fs at sufficient pulse energy requires supporting a relative spectral bandwidth $\Delta\lambda/\lambda_0$ on the order of unity. The extreme confinement of optical energy in time enhances peak electric field strength, raises the ponderomotive potential $U_p \propto I\lambda^2$, and magnifies carrier-envelope phase (CEP) sensitivity, making deterministic CEP control and diagnostics essential.

2. Techniques for Generation: Spectral Broadening and Compression

A universal theme in single-cycle NIR pulse generation is ultrabroadband spectral broadening followed by careful dispersion compensation. Several experimental architectures have achieved this:

• Gas-filled Hollow-Core Fibers (HCFs): Self-phase modulation (SPM) in noble-gas-pressurized HCFs enables efficient spectral broadening. The optimal regime for HCF post-compression is achieved by adjusting gas pressure and input pulse energy such that the accumulated nonlinear B-integral

$B = \frac{2\pi}{\lambda_0} \int_0^{L_F} n_2 I(z)dz$

is $\sim 4$ at the point of optimal pulse quality (Jarque et al., 2017). Dispersion-scan (d-scan) techniques using chirped mirrors and glass wedges are employed to compress the broadened spectrum to durations below 4 fs, revealing the presence of residual third-order dispersion (TOD) from self-steepening, which sets a practical limit for achievable bandwidth and pulse quality.

• Single-Stage HCF Compression: Yb:KGW amplifier systems combined with a single neon-filled HCF stage can efficiently generate multi-octave supercontinua. In such systems, ~90 fs, 1 mJ pulses are broadened to span 250–1400 nm (2.4 octaves) before compression with chirped mirrors and fine wedge tuning to reach 3.2 fs (1.1 cycles at 880 nm) (Pi et al., 16 Apr 2025). This single-stage approach simplifies pulse generation compared to multi-stage Ti:Sapphire-based front ends.
• Fiber-Integrated Approaches: Cascaded self-compression in polarization-maintaining (PM) and highly nonlinear fibers (HNLF), pumped by Tm:fiber or Er:fiber seed sources, achieves pulse durations near one optical cycle at high repetition rate (100 MHz–1 GHz) and watt-level average power (Xing et al., 2021, Zhang et al., 30 May 2024). Precise management of higher-order dispersion (TOD, FOD) in short HNLF segments and advanced amplifier stages is essential to avoid energy breakup into pedestals and maximize the fraction of energy in the central cycle.
• Light-Field Synthesis: Extreme spectral broadening via cascaded HCFs (to >3.5 octaves), followed by channel-resolved compression in a light-field synthesizer, enables the recombination of "color" channels with independent dispersion compensation to synthesize single-cycle transients from broadband amplifier sources such as Yb:KGW (Pi et al., 10 Oct 2024).
• Kerr Instability Amplification (KIA): High-gain parametric amplification of a broadband seed pulse via Kerr four-wave mixing under intensity-tuned phase matching allows direct amplification of few-cycle NIR transients, especially around the pump wavelength, without introducing spatial chirp (Ghosh et al., 2023).

3. Characterization of Spatiotemporal Structure and Space–Time Couplings

Due to the broad bandwidth and short duration of single-cycle pulses, spatiotemporal couplings (STC) such as wavefront rotation (WFR) and pulse front tilt (PFT) can have pronounced effects:

• SEA-F-SPIDER Metrology: The Spatially Encoded Arrangement Filter–based SPIDER (SEA-F-SPIDER) technique enables single-shot characterization of STCs in near-single-cycle pulses (Witting et al., 2016). The recorded interferogram

$$S(x, \omega) = |E(x, \omega)|^2 + |E(x, \omega-\Omega)|^2 + 2|E(x, \omega)||E(x, \omega-\Omega)| \cos([\varphi(x, \omega)-\varphi(x, \omega-\Omega) + \Delta k \cdot x + (\theta\Omega x/c)])$$

reveals spatial chirp and phase distortions. WFR, resulting from spatially dependent carrier frequency, is quantified via the spatial rate of change of the optical period $dT_0/dx$, yielding rotation speeds up to $2.8 \times 10^{11}$ rev/s (1.38 mrad per half-cycle). PFT, quantifying envelope tilt, is measured as the parameter $p$ in $E(x,y,t) = E(x,y)E(t-px)$ ($p$ from –0.33 fs/μm to –3.03 fs/μm). Accurate removal of correction terms (e.g., $\theta\Omega x/c$) is required for precise phase retrieval.

• Soliton Self-Compression in High-Spatial HCF Modes: Direct single-cycle self-compression using higher-order spatial modes (e.g., HE13) in gas-filled HCF leverages their stronger anomalous dispersion to balance SPM, achieving 2.6–2.9 fs durations at 800 nm without any external compressor. Optimization requires both minimal loss and tailored excitation, allowing the far-field beam pattern to serve as a diagnostic (Lopez-Zubieta et al., 2017).

4. Impact of Carrier-Envelope Phase, Temporal Resolution, and Control

Single-cycle pulses exhibit electric field waveforms highly sensitive to the carrier-envelope phase (CEP), making deterministic CEP stabilization and measurement core requirements for attosecond and strong-field experiments:

• CEP Effects in Strong-Field and Plasma Physics: In laser wakefield acceleration (LWFA), NIR pulses with durations $\lesssim1.5$ cycles demonstrate that the ponderomotive approximation fails, and both plasma bubble symmetry and electron injection conditions become CEP-dependent (Huijts et al., 2020, Ouillé et al., 2019). CEP control enables sub-cycle precision in electron beam pointing, energy spectra, and betatron X-ray emission, critical for reproducible relativistic particle and radiation sources.
• Streaking and Waveform Characterization: Homochromatic Attosecond Streaking (HAS) directly samples the electric field waveform of single-cycle pulses, enabling attosecond-level metrology and deterministic control of the instantaneous field at the interaction point (Pi et al., 16 Apr 2025, Pi et al., 10 Oct 2024).
• CEP-Dependent Lightwave STM: Illumination of an ultrafast STM junction with CEP-stable NIR single-cycle pulses enables the generation of sub-optical cycle tunneling currents, with the CEP controlling both magnitude and sign of the induced current. A modulation scheme involving direct CEP dithering and lock-in detection isolates the coherent (CEP-dependent) component of the photocurrent, suppressing thermal artifacts and allowing STM imaging with atomic spatial and attosecond temporal resolution (Rossetti et al., 22 Jul 2025).

5. Applications and Scientific Significance

The availability of NIR single-cycle pulses with deterministic electric field control has transformative impact:

Application Area	Benefit from Single-Cycle NIR Pulses	Distinguishing Feature
Attosecond Science	Isolated XUV/soft X-ray pulses, direct electron imaging	Sub-cycle field gating, waveform control
Strong-Field Physics	NSDI, tunneling, high-harmonic generation (HHG)	CEP dependence, high peak field
Ultrafast Spectroscopy	Time-domain measurement of vibrational/electronic states	Broad spectral coverage, <5 fs duration
Nonlinear Optical Metrology	Low-noise frequency combs, CEO control	Multi-octave supercontinuum, GHz rate
Relativistic Plasma Optics	LWFA, CEP-tagged betatron emission	High field, deterministic injection
Nanoscale Electronic Dynamics	CEP-driven ultrafast STM, localized emission	Atomic–attosecond resolution

A striking result is the extension of the high-harmonic generation (HHG) cutoff and the appearance of high-energy plateaus in two-color (NIR + single-cycle IR) field schemes. Here, the single-cycle IR field imparts a rapid directional momentum "kick" to electrons, allowing recollision energies and harmonic spectra unattainable with NIR pulses alone. The cutoff energy extension scales as $$E_\text{max} \sim 3.17 U_p^\text{NIR} + \alpha I_\text{IR}/\omega_\text{IR}^2$$ (Taoutioui et al., 2020).

6. Experimental Implementations and Technological Advances

Notable technical achievements include:

• GHz-Rate Single-Cycle Fiber Lasers: Fiber-based soliton self-compression, with suppression of higher-order dispersion, enables 7.1 fs (1.1-cycle) pulses at 1.8 W average power and 1 GHz repetition rate (110 kW peak) with high CEO beat SNR (43 dB) (Zhang et al., 30 May 2024). This supports frequency comb stabilization at single-cycle duration.
• Ultra-Broadband Yb:KGW Platforms: Dual HCF modules combined with light-field synthesizers achieve the synthesis and compression of >3.5 octave supercontinua from a Yb:KGW amplifier to single-cycle pulses over the 380–1000 nm region, with temporal structures as short as 2.14 fs FWHM (0.85 cycles at 736 nm), verified by attosecond streaking (Pi et al., 10 Oct 2024).
• Mid-Infrared and Relativistic Sources: OPCPA combined with HCF and SPM in krypton at 4 μm delivers passively CEP-stable, 1.6-cycle, 21.5 fs, 2.6 mJ pulses, supporting XUV soft X-ray generation in HHG (Wang et al., 2017). LWIR single-cycle pulses (5–14 μm) have been generated by frequency downshifting in tailored plasma density wakes, yielding 1.7% conversion efficiency and CEP transfer from the drive laser (Nie et al., 2017).
• All-Silica Fiber Frequency Combs: Turn-key, all-fiber configurations combining cascaded self-compression and high-doped Tm:fiber amplification enable the generation of multi-octave spanning, 6.8 fs (1.05 cycle) pulses at 100 MHz, used for comb spectroscopy and supercontinuum generation in both the NIR and MIR (Xing et al., 2021).

7. Limitations, Trade-Offs, and Future Directions

Single-cycle pulse generation in the NIR is fundamentally constrained by the interplay between nonlinearity (for spectral broadening), dispersion (for compression), and higher-order distortions such as residual TOD. The maximum fraction of energy in the central burst (typically ~60%) is set by dispersion management and the suppression of satellite formation (Zhang et al., 30 May 2024). Trade-offs exist between pulse energy, repetition rate, and the achievable spectral bandwidth. Advances in fiber and waveguide engineering, tailored plasma nonlinearities, and integration of dispersion-engineered optical components are expected to further compress pulse durations, stabilize CEO frequencies, and scale average power.

Looking ahead, integration of waveform-controlled, high-energy, single-cycle pulses with time-resolved microscopy, multidimensional spectroscopy, and field-sensitive nanoscience will drive new regimes of attosecond and sub-femtosecond science. Precision characterization tools such as SEA-F-SPIDER and homochromatic streaking remain central to the reliable deployment of ultrashort pulses for both table-top and large-scale facilities.

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This article is based entirely on information present in (Witting et al., 2016, Chen et al., 2017, Jarque et al., 2017, Wang et al., 2017, Nie et al., 2017, Lopez-Zubieta et al., 2017, Fourcade-Dutin et al., 2018, Ouillé et al., 2019, Huijts et al., 2020, Taoutioui et al., 2020, Xing et al., 2021, Ghosh et al., 2023, Zhang et al., 30 May 2024, Pi et al., 10 Oct 2024, Meier et al., 14 Oct 2024, Pi et al., 16 Apr 2025), and (Rossetti et al., 22 Jul 2025).