arXiv subjects

James Greenberg

Publications and source records attributed to James Greenberg.

18 recordsLinked to original sources

arXiv2026

Low noise resonant amplification by optical injection-locking and residual phase noise cancellation

We demonstrate a low noise, high-gain, resonant optical amplifier that combines injection locking with feed-forward cancellation of residual phase noise. The wavelength-agnostic architecture uses a commercial semiconductor diode laser as a power amplifier while preserving the spectral purity of a weak reference. Although injection locking enforces phase coherence, finite residual phase noise within the locking regime limits high-fidelity transfer of low phase noise from the reference laser to the injection-locked laser, particularly at large gain. Here, the residual phase error is measured via optical heterodyne detection and canceled using feed-forward phase correction. Compared to injection locking alone, the amplifier achieves up to 38 dB phase-noise reduction at Fourier frequencies above 1 kHz for injection ratios down to -57 dB. This approach enables ASE-free amplification of low-power, low-noise optical references, including individual lines from optical frequency combs.

physics.optics↗

arXiv2026

Dual-Wavelength Cancellation of Dispersion-Induced Phase Noise in Opto-Terahertz Fiber Links

Stable dissemination of terahertz (THz) signals over long distances is important for next-generation synchronization networks, radio astronomy, and high-capacity wireless systems. Optical fiber provides a low-loss platform for coherent frequency transfer; however, when a THz carrier is encoded as the difference between two optical wavelengths, chromatic dispersion introduces differential phase noise that degrades spectral purity. Here, we demonstrate phase-coherent distribution of opto-THz carriers over 38 km of standard single-mode fiber using a dual-wavelength Brillouin laser (DWBL) combined with a dual-channel round-trip noise-cancellation architecture. By extracting the differential phase noise between the two optical lines via a dual-channel round-trip measurement, dispersion-mediated phase fluctuations are compensated, and the intrinsic stability of the source is effectively preserved at the remote end within the measurement sensitivity. Opto-THz carriers at 150, 300, and 600 GHz exhibit sub-femtosecond timing stability and fractional frequency instabilities below 1e-17 at 10,000 seconds of averaging.

physics.optics↗

arXiv2026

Tunable microwave frequency synthesis with optically-derived spectral purity

Microwave synthesizers are central to test and measurement systems across applications including wireless communications, radar, spectroscopy, and time and frequency metrology. State-of-the-art microwave sources, however, are fundamentally constrained by trade-offs between frequency tunability and spectral purity. Electro-optic frequency division (eOFD) is an emerging technique for dividing down the purity of optical sources to the microwave domain. Previously reported eOFD-based synthesizers generally have limited tunability due to feedback stabilization requirements. Here we demonstrate a feed-forward eOFD architecture in which the frequency tunability of a microwave source is preserved while optical spectral purity is divided through feed-forward cancellation, without any downstream electronic frequency synthesis. By canceling the phase noise of the microwave source without feedback, this eOFD approach removes loop bandwidth and source noise constraints observed in prior eOFD architectures. We achieve octave-spanning tunability, including the entire X-band, with phase noise below -140 dBc/Hz at kilohertz offsets and a high-frequency noise floor between -155 dBc/Hz and -145 dBc/Hz for carrier frequencies from 8 to 16 GHz. This performance corresponds to single-femtosecond integrated timing jitter, enabling, to our knowledge, the first demonstration of coherent, optically referenced microwave synthesis under wide tuning with this level of spectral purity.

physics.optics↗

arXiv2026

Attosecond-timing millimeter waves via Kerr optical frequency division

Millimeter-wave oscillators underpin key applications in communication, spectroscopy, radar, and astronomy, yet their achievable spectral purity remains limited. Approaches that directly generate millimeter-wave carriers are fundamentally limited by quantum and thermal phase-noise processes. Here we show that these limits can be overcome by combining Kerr-induced optical frequency division in a chip-scale microresonator with a large-spacing dual-wavelength Brillouin laser. This 3.3 THz optical reference injection-locks a Kerr soliton microcomb, with a repetition rate that becomes a coherently divided 300 GHz carrier with phase noise below the quantum limit of a corresponding 300 GHz dual-wavelength Brillouin laser and far below the thermo-refractive noise of a microring resonator. Cross-correlation phase-noise measurements were developed to show that the resulting oscillator reaches a phase-noise floor of -152 dBc/Hz at 1 MHz offset, consistent with photodetection shot noise. Integration of the measured spectrum yields an RMS timing jitter of 135 as from 1 kHz to 1 MHz. These results establish optical frequency division as a generic method for generation of sub-terahertz carriers with coherence no longer constrained by direct-generation limits.

physics.optics↗

arXiv2025

Terahertz amplification by injection locking of waveguide resonant tunneling diode

High power and low phase noise oscillators at terahertz frequencies are required for several burgeoning scientific and technological applications, including radioastronomy, imaging, molecular spectroscopy, radar, and wireless communications. Operating at terahertz oscillation frequencies presents unique challenges based on the method of generation. Electronic oscillators can produce ample power but suffer from relatively high phase noise due to the nonlinear multiplication of microwave sources. Meanwhile, photomixing of optical sources provides superior spectral purity but low usable power, due to the limited bandwidth of the photomixer. We propose a hybrid solution involving injection locking of an electronic oscillator, a resonant tunneling diode, by a low phase noise photomixed source, a dual-wavelength Brillouin laser. In this study, we demonstrate a proof-of-concept injection-locking amplifier at 260 GHz, achieving up to 40 dB gain for nanowatt-level input signals. For the first time, we characterize the residual phase noise of an injection-locked waveguide RTD, showing quantitative consistency with theoretical predictions based on detailed analysis of its free-running noise. This architecture has the potential to scale to frequencies of 1 THz and beyond, which would provide a clear path to realize a terahertz oscillator with high power and low phase noise.

physics.optics↗

arXiv2025

Dual-Wavelength Brillouin Lasers as compact Opto-Terahertz References for Low-Noise Microwave Synthesis

Compact, ultra-low phase noise 10 GHz signals are essential for modern radar, coherent communications, and time-frequency metrology, especially with rising demands for additional spectral purity and portability. Optical frequency division (OFD) of ultra-stable optical references produce the lowest noise microwaves, but typically rely on ultra-low-expansion cavities, self-referenced frequency combs, pulse interleaving, and high-end photodetectors. In contrast, electro-optic frequency division (eOFD) offers a streamlined alternative in which an electro-optic (EO) comb is generated from a microwave source and stabilized to an optically carried terahertz (opto-terahertz) reference. eOFD has already demonstrated comparable phase noise to OFD at 10, 20, and 40 GHz when using division ratios from references spanning over 1 THz, requiring broad EO comb spectra to bridge between optical signals. Generating such broad spectra often demands complex techniques such as cascaded modulators, pulse compression, and nonlinear fiber. We demonstrate a compact, low-noise 300 GHz opto-terahertz reference utilizing a dual-wavelength Brillouin laser within a total system volume of 20 liters. The 300 GHz phase noise of this system is transferred to a 10 GHz dielectric resonant oscillator via eOFD using a simple architecture that could be miniaturized. The resulting microwave signal achieves phase noise levels of -130 dBc/Hz at 1 kHz, -150 dBc/Hz at 10 kHz, and -170 dBc/Hz at 10 MHz. This architecture drastically simplifies eOFD while maintaining state-of-the-art phase noise.

physics.optics↗

arXiv2025

Terahertz molecular frequency references: theoretical analysis of optimal instability

We report a comprehensive theoretical analysis of the instability achievable by using phase modulation spectroscopy to lock a terahertz local oscillator to an absorptive reference consisting of the rotational transition of molecules at room temperature. We find that the signal-to-noise ratio of the THz detector provides the limitation to the instability that can be achieved and analyze a number of viable candidate molecules, identifying several as being of particular interest, including OCS and HI. We find that a one-second instability in the $10^{-13}$ decade is achievable for molecules confined to waveguide, while instability at the $10^{-14}$ level can be reached for molecules in free space. We also present calculations of the intermodulation effect for spectroscopy taking place far outside the quasistatic regime and find that this source of noise presents constraints on which THz local oscillators are appropriate to be used for such frequency references.

physics.atom-ph↗

arXiv2024

Dual wavelength brillouin laser terahertz source stabilized to carbonyl sulfide rotational transition

Optical-based terahertz sources are important for many burgeoning scientific and technological applications. Among such applications is precision spectroscopy of molecules, which exhibit rotational transitions at terahertz frequencies. Stemming from precision spectroscopy is frequency discrimination and stabilization of terahertz sources. Because many molecular species exist in the gas phase at room temperature, their transitions are prime candidates for practical terahertz frequency references. We demonstrate the stabilization of a low phase-noise, dual-wavelength Brillouin laser (DWBL) terahertz oscillator to a rotational transition of carbonyl sulfide (\ce{OCS}). We achieve an instability of $1.2\times10^{-12}/\sqrt{\tau}$, where $\tau$ is the averaging time in seconds. The signal-to-noise ratio and intermodulation limitations of the experiment are also discussed. We thus demonstrate a highly stable and spectrally pure terahertz frequency source. Our presented architecture will likely benefit metrology, spectroscopy, precision terahertz studies, and beyond.

physics.optics↗

arXiv2023

Brillouin laser-driven terahertz oscillator up to 3 THz with femtosecond-level timing jitter

The terahertz (THz) frequency range, spanning 0.1 to 10 THz, is a field ripe for innovation with vast, developing potential in areas like wireless communication and molecular spectroscopy. Our work introduces a dual-wavelength laser design that utilizes stimulated Brillouin scattering in an optical fiber cavity to effectively generate two highly coherent optical Stokes waves with differential phase noise inherently mitigated. To guarantee robust operation, the Stokes waves are optically injected into their respective pump lasers, which also serves to greatly improve the resulting coherence. The frequency difference between the two wavelengths is converted into THz waves through a uni-traveling-carrier photodiode. This innovative design facilitates the generation of THz waves with phase noise levels of less than -100 dBc/Hz, translating to timing noise below 10~$\mathrm{as} / \sqrt{\mathrm{Hz}}$ at 10 kHz Fourier frequency, over a carrier frequency range from 300 GHz to 3 THz. This development in phase noise reduction establishes a new benchmark in the spectral purity of tunable THz sources. Such advances are pivotal for applications to move beyond oscillator constraints.

physics.optics↗

arXiv2023

60 Gbps real-time wireless communications at 300 GHz carrier using a Kerr microcomb

Future wireless communication infrastructure will rely on terahertz systems that can support an increasing demand for large-bandwidth, ultra-fast wireless data transfer. In order to satisfy this demand, compact, low-power, and low noise sources of terahertz radiation are being developed. A promising route to achieving this goal is combining photonic-integrated optical frequency combs with fast photodiodes for difference frequency generation in the THz. Here, we demonstrate wireless communications using a 300 GHz carrier wave generated via photomixing of two optical tones originating from diode lasers that are injection locked to a dissipative Kerr soliton frequency microcomb. We achieve transfer rates of 80 Gbps using homodyne detection and 60 Gbps transmitting simultaneously both data and clock signals in a dual-path wireless link. This experimental demonstration paves a path towards low-noise and integrated photonic millimeter-wave transceivers for future wireless communication systems.

physics.optics↗

arXiv2022

Dissipative Kerr soliton photonic terahertz oscillator referenced to a molecule

Controlling the coherence between light and matter has enabled the radiation of electromagnetic waves with spectral purity and stability that defines the Système International (SI) second. While transitions between hyperfine levels in atoms are accessible in the microwave and optical domains, faithfully transferring such stability to other frequency ranges of interest is not trivial. Such stability is specifically sought after for the terahertz domain to improve the resolution in very long baseline interferometry and molecular spectroscopy, and advance the technological development of high-speed, high data rate wireless communications. However, there is an evident lack of native frequency references in this spectral range, essential for the consistency of measurements and traceability. To mitigate the frequency drift encompassed in such waves, we experimentally demonstrate that using rotational spectroscopy of nitrous oxide N2O can lead to linewidth reduction up to a thousandfold. A pair of diode lasers, optically injected with a low-noise, chip-based dissipative Kerr soliton, were incident upon a uni-travelling-carrier photodiode. We frequency-locked the emitted terahertz wave to the center of a rotational transition of N2O through phase modulation spectroscopy. A terahertz wave with a 6 Hz linewidth was achieved (fractional frequency stability of $2 \times 10^{-11}$ at 1 second averaging time) while circumventing the need of frequency multiplication or division of frequency standards.

physics.optics↗

arXiv2022

Exceeding octave tunable Terahertz waves with zepto-second level timing noise

Spectral purity of any millimeter wave (mmW) source is of the utmost interest in low-noise applications. Optical synthesis via photomixing is an attractive source for such mmWs, which usually involves expensive spectrally pure lasers with narrow linewidths approaching monochromaticity due to their inherent fabrication costs or specifications. Here, we report an alternative option for enhancing the spectral purity of inexpensive semiconductor diode lasers via a self-injection locking technique through corresponding Stokes waves from a fiber Brillouin cavity exhibiting greatly improved phase noise levels and large wavelength tunability of ~1.8 nm. We implement a system with two self-injected diode lasers on a common Brillouin cavity aimed at difference frequency generation in the mmW and THz region. We generate tunable sub-mmW (0.3 and 0.5 THz) waves by beating the self-injected two wavelength Stokes light on a uni-travelling carrier photodiode and characterize the noise performance. The sub-mmW features miniscule timing noise levels in the zepto-second (zs.Hz^-0.5) scale outperforming the state of the art dissipative Kerr soliton based micro-resonator setups while offering broader frequency tunability. These results suggest a viable inexpensive alternative for mmW sources aimed at low-noise applications featuring lab-scale footprints and rack-mounted portability while paving the way for chip-scale photonic integration.

physics.optics↗

arXiv2022

Photonic Generation of Millimeter-Waves Disciplined by Molecular Rotational Spectroscopy

Optical generation of millimeter-waves (mm-wave) is made possible by an optical heterodyne of two diode lasers on a uni-traveling-carrier photodiode (UTC-PD). We utilized this technique to produce a mm-wave oscillator with desirable phase-noise characteristics, which were inherited from a pair of narrow-linewidth diode lasers. We present the long-term stabilization of our oscillator, achieved by referencing it to a rotational transition of gaseous nitrous oxide (N2O). Direct frequency modulation spectroscopy at 301.442 GHz (J=11) generated an error signal that disciplined the frequency difference between the diode lasers and thus, locked the millimeter-wave radiation to the molecular rotational line. The mm-wave frequency was down-converted using an electro-optic (EO) comb, and recorded by a frequency counter referenced to a Rubidium (Rb) clock. This resulted in short-term fractional frequency stability of $1.5 \times 10^{-11}/\sqrtτ$ and a long term-stability of $4\times 10^{-12}$ at 10,000 s averaging time.

physics.optics↗

arXiv2020

Isotope-specific reactions of acetonitrile (CH3CN) with trapped, translationally cold CCl+

The gas-phase reaction of CCl+ with acetonitrile (CH3CN) is studied using a linear Paul ion trap coupled to a time-of-flight mass spectrometer. This work builds on a previous study of the reaction of CCl+ with acetylene and further explores the reactivity of CCl+ with organic neutral molecules. Both of the reactant species are relevant in observations and models of chemistry in the interstellar medium (ISM). Nitriles, in particular, are noted for their relevance in prebiotic chemistry, such as is found in the atmosphere of Titan, one of Saturn's moons. This work represents one of the first studied reactions of a halogenated carbocation with a nitrile, and the first exploration of CCl+ with a nitrile. Reactant isotopologues are used to unambiguously assign ionic primary products from this reaction: HNCCl+ and C2H3+. Branching ratios are measured and both primary products are determined to be equally probable. Quantum chemical and statistical reaction rate theory calculations illuminate pertinent information for interpreting the reaction data, including: reaction thermodynamics, a potential energy surface for the reaction, as well as rate constants and branching ratios for the observed products. In particular, the reaction products and potential energy surface stimulate questions regarding the strength and role of the nitrile functional group, which can be further explored with more reactions of this class.

physics.chem-ph↗

arXiv2020

Translationally cold trapped CCl+ reactions with acetylene (C2H2)

Ion-neutral chemical reactions are important in several areas of chemistry, including in some regions of the interstellar medium, planetary atmospheres, and comets. Reactions of CCl+ with C2H2 are measured and the main products include c-C3H2+ and l-C3H+, both relevant in extraterrestrial environments. Accurate branching ratios are obtained, which favor formation of c-C3H2+ over l-C3H+ by a factor of four. Measured rate constants are on the order of Langevin and complementary electronic structure calculations are used to aid in the interpretation of experimental results.

physics.chem-ph↗

arXiv2019

Quantum-State-Specific Reaction Rate Measurements for the Photo-induced Reaction Ca$^+$ + O$_2$ $\rightarrow$ CaO$^+$ + O

Atoms and molecules often react at different rates depending on their internal quantum states. Thus, controlling which internal states are populated can be used to manipulate the reactivity and can lead to a more detailed understanding of reaction mechanisms. We demonstrate this control of reactions by studying the excited state reaction reaction Ca$^+$ + O$_2$ $\rightarrow$ CaO$^+$ + O. This reaction is exothermic only if Ca$^+$ is in one of its excited electronic states. Using laser-cooling and electrodynamic trapping, we cool and trap Ca$^+$ at millikevin temperatures for several minutes. We can then change the fraction of time they spend in each of the two excited states by adjusting the detunings of the cooling lasers. This allows us to disentangle the reactions that begin with Ca$^+$ in the $^2$P$_{1/2}$-state from the ones where Ca$^+$ is in the $^2$D$_{3/2}$-state. Using time-of-flight mass spectrometry, we determine independent reaction rate constants for Ca$^+$ in both electronically excited quantum states.

physics.atom-ph↗

arXiv2017

High resolution ion trap time-of-flight mass spectrometer for cold trapped ion experiments

Trapping molecular ions that have been sympathetically cooled with laser-cooled atomic ions is a useful platform for exploring cold ion chemistry. We designed and characterized a new experimental apparatus for probing chemical reaction dynamics between molecular cations and neutral radicals at temperatures below 1 K. The ions are trapped in a linear quadrupole radio-frequency trap and sympathetically cooled by co-trapped, laser-cooled, atomic ions. The ion trap is coupled to a time-of-flight mass spectrometer to readily identify product ion species, as well as to accurately determine trapped ion numbers. We discuss, and present in detail, the design of this ion trap time-of-flight mass spectrometer, as well as the electronics required for driving the trap and mass spectrometer. Furthermore, we measure the performance of this system, which yields mass resolutions of $m/Δm \geq 1100$ over a wide mass range, and discuss its relevance for future measurements in chemical reaction kinetics and dynamics.

physics.ins-det↗

arXiv2015

Atom Interferometer Gyroscope with Spin-Dependent Phase Shifts Induced by Light near a Tune-Out Wavelength

Tune-out wavelengths measured with an atom interferometer are sensitive to laboratory rotation rates because of the Sagnac effect, vector polarizability, and dispersion compensation. We observed shifts in measured tune-out wavelengths as large as 213 pm with a potassium atom beam interferometer, and we explore how these shifts can be used for an atom interferometer gyroscope.

physics.atom-ph↗