Mid-infrared (mid-IR) spectroscopy provides a way to study structures and dynamics of complicated molecules in condensed phases. Therefore, developing compact and broadband mid-IR spectrometer has been a long-standing challenge. Here, we show that a highly coherent and broadband mid-IR frequency comb can be generated by using an intra-pulse difference-frequency-generation with a train of pulses from a few-cycle Ti:Sapphire oscillator. By tightly focusing the oscillator output beam into a single-pass fan-out-type periodically-poled lithium niobate crystal and tilting the orientation of the crystal with respect to incident beam, it is shown that mid-IR frequency comb with more than an octave spectral bandwidth from 1550 cm-1 (46 THz) to 3650 cm-1 (110 THz) and vanishing carrier-envelop offset phase can be generated. Then, using two coherent mid-IR frequency combs, we demonstrate that ultrabroad mid-IR dual frequency comb spectroscopy of both aromatic compounds and amino acids in solutions is experimentally feasible. We thus anticipate that our mid-IR frequency combs could be used to further develop ultrafast and broadband IR spectroscopy of chemically reactive and biological molecules in condensed phases.
Nonlinear propagation of intense femtosecond laser pulses in bulk transparent media leads to a specific propagation regime, termed femtosecond filamentation, which in turn produces dramatic spectral broadening, or superbroadening, termed supercontinuum generation. Femtosecond supercontinuum generation in transparent solids represents a compact, efficient and alignment-insensitive technique for generation of coherent broadband radiation at various parts of the optical spectrum, which finds numerous applications in diverse fields of modern ultrafast science. During recent years, this research fi…
We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Due to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of $^{14}\text{CH}_4$ is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon met…
We propose a new type of broadband and low repetition rate frequency comb generator which has the shape of an elongated and nanoscale-shallow optical bottle microresonator created at the surface of an optical fiber. The free spectral range (FSR) of the broadband azimuthal eigenfrequency series of this resonator is the exact multiple of the FSR of the dense and narrowband axial series. The effective radius variation of the microresonator is close to a parabola with a nanoscale height which is greater or equal to lambda/2pi*n0 (here lambda is the characteristic radiation wavelength and n0 is the…
We present a monolithic solution to extract efficiently light from terahertz quantum cascade lasers with metal-metal waveguides suitable for broadband frequency comb applications. The design is optimized for a bandwidth of 500 GHz around a center frequency of 2.6 THz. A five-fold increase in total output power is observed compared to standard metal-metal waveguides. The extractor features a single-lobed far-field pattern and increases the frequency comb dynamical range to cover more than 50 % of the laser dynamic range. Frequency comb operation up to a spectral bandwidth of 670 GHz is achieved…
Frequency comb assisted diode laser spectroscopy, employing both the accuracy of an optical frequency comb and the broad wavelength tuning range of a tunable diode laser, has been widely used in many applications. In this letter we present a novel method using cascaded frequency agile diode lasers, which allows extending the measurement bandwidth to 37.4 THz (1355 to 1630 nm) at MHz resolution with scanning speeds above 1 THz/s. It is demonstrated as a useful tool to characterize a broadband spectrum for molecular spectroscopy and in particular it enables to characterize the dispersion of inte…
Historically, there has been a trade-off in spectroscopic measurements between high spectral resolution, broadband coverage, and acquisition time. Optical frequency combs, initially envisioned for precision spectroscopy of the hydrogen atom in the ultraviolet region, are now commonly used for probing molecular rotational-vibrational transitions throughout broad spectral bands in the mid-infrared with superior accuracy, resolution and speed. Here we demonstrate acquisition of 2.5 million spectral points over the continuous wavelength range of 3.2 to 5.1 microns (frequency span 1200 cm^{-1}, res…
We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83 THz ($Δf/f=50\%$) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low …
We propose an approach to actively tune the scattering pattern of a Mie-type spherical antenna. The scheme is based on separate control over the induced electric dipole and induced magnetic dipole using two coherent focused beams of radial polarization and azimuthal polarization. By carefully tuning the amplitude and phase relation of the two beams, a broadband unidirectional scattering can be achieved, even at the wavelength where the antenna scatters most efficiently. By moving the focus of one beam, a drastic switch of the unidirectional scattering can be observed. Such scheme enables the d…
We demonstrate fiber mode-locked dual frequency comb spectroscopy for broadband, high resolution measurements in a rapid compression machine (RCM). We apply an apodization technique to improve the short-term signal-to-noise-ratio (SNR), which enables broadband spectroscopy at combustion-relevant timescales. We measure the absorption on 24345 individual wavelength elements (comb teeth) between 5967 and 6133 cm-1 at 704 microsecond time resolution during a 12-ms compression of a CH4-N2 mixture. We discuss the effect of the apodization technique on the absorption spectra, and apply an identical e…
Development of chip-scale optical frequency comb with the coverage from ultra-violet (UV) to mid-infrared (MIR) wavelength is of great significance. To expand the comb spectrum into the challenging UV region, a material platform with high UV transparency is crucial. In this paper, crystalline aluminum nitride (AlN)-onsapphire film is demonstrated for efficient Kerr frequency comb generation. Near-infrared (NIR) comb with nearly octave-spanning coverage and low parametric threshold is achieved in continuous-wave pumped high-quality-factor AlN microring resonators. The competition between stimul…
We suggest a technique for using off-resonance spectral comb generation to produce broadband frequency modulated, and therefore, amplitude quieted light. Results include closed-form formulae for the amplitudes and phases of all of the spectral components.
We have recently introduced the Vernier-based Direct Frequency Comb Cavity-Enhanced Spectroscopy technique and we present the corresponding formalism for quantitative broadband spectroscopy. We achieve high sensitivity and broadband performance by acquiring spectra covering more than 2000 cm$^{-1}$ around 12600 cm$^{-1}$ (800 nm), resolving the 3$ν$+$δ$ band of water vapor and the entire A-band of oxygen in ambient air. 31 300 independent spectral elements are acquired at the second time scale with an absorption baseline noise of 2$\times$10$^{-8}$ cm$^{-1}$ providing a merit figure of 1,1.10$…
We report a 92 channel RF channelizer based on a 48.9 GHz integrated micro-comb that operates via soliton crystals, together with a passive high-Q ring resonator that acts as a periodic filter with an optical 3dB bandwidth of 121.4 MHz. We obtain an instant RF bandwidth of 8.08 GHz and 17.55 GHz achieved through temperature tuning. These results represent a major advance to achieving fully integrated photonic RF spectrum channelizers with reduced low complexity, size, and high performance for digital-compatible signal detection and broadband analog signal processing.
We report on a heterogeneous active region design for terahertz quantum cascade laser based frequency combs. Dynamic range, spectral bandwidth as well as output power have been significantly improved with respect to previous designs. When operating individually the lasers act as a frequency comb up to a spectral bandwidth of 1.1 THz, while in a dispersed regime a bandwidth up to 1.94 THz at a center frequency of 3 THz can be reached. A self-detected dual-comb setup has been used to verify the frequency comb nature of the lasers.
A high-Q silica whispering-gallery mode microresonator is an attractive platform on which to demonstrate a broad and phase-locked Raman comb in various wavelength regimes. Raman combs can be used for applications such as compact pulse laser sources, sensors, optical clocks, and coherence tomography. However, the formation dynamics of a Raman comb has not been well exploited. Here we study the dynamics of the Raman comb formation in silica rod microresonators, which have cavity free spectral ranges in microwave rates. We generated a Raman comb with a smooth spectral envelope and also observed t…
Programmable ultra-broadband photonic chaos platform enabled by microwave-chaos-driven electro-optic frequency combs
State Key Laboratory of Optical Field Manipulation Science and Technology, Institute of Optics and Electronics, Chinese Academy of Sciences（中国科学院光电技术研究所光场操控科学与技术重点实验室）；Research Center on Vector Optical Fields, Institute of Optics and Electronics, Chinese Academy of Sciences（中国科学院光电技术研究所矢量光学场研究中心）；Tianfu Xinglong Lake Laboratory（天府兴隆湖实验室）；College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences（中国科学院大学材料科学与光电技术学院）
We have carried out dual-comb spectroscopy and observed in a simultaneous acquisition a 140-THz-wide spectrum from 1.0 to 1.9 μm using two fiber-based frequency combs phase-locked to each other. This ultra-broad wavelength bandwidth is realized by setting the difference between the repetition rates of the two combs to 7.6 Hz using the sub-Hz-linewidth fiber combs. The recorded spectrum contains five vibration-rotation bands of C${_2}$H${_2}$, CH${_4}$, and H${_2}$O at different wavelengths across the whole spectrum. The determined transition frequencies of C${_2}$H${_2}$ agree with those from …
We present a chip-scale scanning dual-comb spectroscopy (SDCS) approach for broadband ultrahigh-resolution spectral acquisition. SDCS uses Si3N4 microring resonators that generate two single soliton micro-combs spanning 37 THz (300 nm) on the same chip from a single 1550-nm laser, forming a high-mutual-coherence dual-comb. We realize continuous tuning of the dual-comb system over the entire optical span of 37.5 THz with high precision using integrated microheater-based wavelength trackers. This continuous wavelength tuning is enabled by simultaneous tuning of the laser frequency and the two si…
Broadband dual-comb spectroscopy has attracted increasing interests due to its unique advantages in high spectral resolution, fast detection, and so on. Although the dual-comb technique is relatively mature in the infrared wavelengths, it is, currently, not commercially capable of practical applications in the terahertz regime due to the lack of high performance broadband terahertz dual-comb sources. In the terahertz frequency range, the electrically pumped quantum cascade laser (QCL) is a suitable candidate for the dual-comb operation. However, free running terahertz QCL dual-comb sources nor…
We report molecular detection using dual frequency-comb spectroscopy with highly coherent broadband (6.6-11.4 {\m}m) long-wavelength infrared (LWIR) combs. The combs were produced via intra-pulse difference frequency generation (IDFG) in ZGP crystals using sub-three-cycle (20 fs) driving pulses from mode-locked Cr:ZnS lasers at the central wavelength of 2.4 {\m}m). Real-time and up to video rate (0.1-12 s per spectrum) acquisition of molecular spectra with some 240,000 comb-mode-resolved data points spaced by 80 MHz and referenced to a Rb clock with a signal-to-noise ratio (SNR) >300 has been …
We report and characterize low-temperature, plasma-deposited deuterated silicon nitride thin films for nonlinear integrated photonics. With a peak processing temperature less than 300$^\circ$C, it is back-end compatible with pre-processed CMOS substrates. We achieve microresonators with a quality factor of up to $1.6\times 10^6 $ at 1552 nm, and $>1.2\times 10^6$ throughout $λ$ = 1510 -- 1600 nm, without annealing or stress management. We then demonstrate the immediate utility of this platform in nonlinear photonics by generating a 1 THz free spectral range, 900-nm-bandwidth modulation-instabi…
We report a broadband microwave frequency converter based on a coherent Kerr optical micro-comb generated by an integrated micro-ring resonator. The coherent micro-comb displays features that are consistent with soliton crystal dynamics with an FSR of 48.9GHz. We use this to demonstrate a high-performance millimeter-wave local oscillator at 48.9GHz in the Q-band for microwave frequency conversion. We experimentally verify the microwave performance up to 40 GHz, achieving a ratio of 6.8 dB between output RF power and IF power and a spurious suppression ratio of > 43.5 dB. The experimental resul…
Broadband terahertz comb with sub-Hz comb linewidth
Jinan Institute of Quantum Technology（济南量子技术研究院）；Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences（中国科学院上海微系统与信息研究所）
Developments in integrated photonics have led to stable, compact, and broadband comb generators that support a wide range of applications. Current on-chip comb generators, however, are still limited by low optical pump-to-comb conversion efficiencies. Here, we demonstrate an integrated electro-optic frequency comb with a conversion efficiency of 30% and an optical bandwidth of 132 nm, featuring a 100-times higher conversion efficiency and 2.2-times broader optical bandwidth compared with previous state-of-the-art integrated electro-optic combs. We further show that, enabled by the high efficie…
Spectroscopy is the basic tool for studying molecular physics and realizing bio-chemical sensing. However, it is challenging to realize sub-femtometer resolution spectroscopy over broad bandwidth. In this paper, broadband and high-resolution spectroscopy with calibrated optical frequency is demonstrated by bridging the fields of speckle patterns and electro-optic frequency comb (EOFC). A novel wavemeter based on whispering-gallery-mode (WGM) speckles (or WGM barcodes) is proposed to link the frequency of a tunable continuous-wave (CW) laser to an optical reference provided by an ultra-stable l…
Nanophotonic waveguides with sub-wavelength mode confinement and engineered dispersion profiles are an excellent platform for application-tailored nonlinear optical interactions at low pulse energies. Here, we present fully air clad suspended-silicon waveguides for infrared frequency comb generation with optical bandwidth limited only by the silicon transparency. The achieved spectra are lithographically tailored to span 2.1 octaves in the mid-infrared (2.0-8.5 um or 1170--5000 cm-1) when pumped at 3.10 um with 100 pJ pulses. Novel fork-shaped couplers provide efficient input coupling with onl…
Dissipative Kerr-microresonator soliton combs (hereafter called soliton combs) are promising to realize chip scale integration of full soliton comb systems providing high precision, broad spectral coverage and a coherent link to the micro/mm/THz domain with diverse applications coming on line all the time. However, the large soliton comb spacing hampers some applications. For example, for spectroscopic applications, there are simply not enough comb lines available to sufficiently cover almost any relevant absorption features. Here, we overcome this limitation by scanning the comb mode spacing …
We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23°x19°) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser's emission spectrum and current-voltage characteristics are not affected, as well as frequency co…
The quest for extrasolar planets and their characterisation as well as studies of fundamental physics on cosmological scales rely on capabilities of high-resolution astronomical spectroscopy. A central requirement is a precise wavelength calibration of astronomical spectrographs allowing for extraction of subtle wavelength shifts from the spectra of stars and quasars. Here, we present an all-fibre, 400 nm wide near-infrared frequency comb based on electro-optic modulation with 14.5 GHz comb line spacing. Tests on the high-resolution, near-infrared spectrometer GIANO-B show a photon-noise limit…
We report the coherent generation and detection of terahertz (THz) pulses covering the bandwidth of 0.1-9 THz in a high repetition rate, low pulse energy laser system. In this work we demonstrate the application and evaluation of solid-state biased coherent detection in combination with a spintronic emitter. This combination was used to generate and detect THz pulses in a time-domain spectroscopy (TDS) setup and tested on bulk nonlinear crystals. These results establish a new promising candidate to extend the possibilities for compact, broadband THz TDS systems driven by high repetition rate l…
High-harmonic generation (HHG) provides short-wavelength light that is useful for precision spectroscopy and probing ultrafast dynamics. We report efficient, phase-coherent harmonic generation up to 9th-order (333 nm) in chirped periodically poled lithium niobate waveguides driven by phase-stable $\leq$12-nJ, 100 fs pulses at 3 $μ$m with 100 MHz repetition rate. A mid-infrared to ultraviolet-visible conversion efficiency as high as 10% is observed, amongst an overall 23% conversion of the fundamental to all harmonics. We verify the coherence of the harmonic frequency combs despite the complex …
Microresonator frequency combs, or microcombs, have gained wide appeal for their rich nonlinear physics and wide range of applications. Stoichiometric silicon nitride films grown via low-pressure chemical vapor deposition (LPCVD), in particular, are widely used in chip-integrated Kerr microcombs. Critical to such devices is the ability to control the microresonator dispersion, which has contributions from both material refractive index dispersion and geometric confinement. Here, we show that modifications to the LPCVD growth conditions, specifically the ratio of the gaseous precursors, has a s…
Low propagation loss in high confinement waveguides is critical for chip-based nonlinear photonics applications. Sophisticated fabrication processes which yield sub-nm roughness are generally needed to reduce scattering points at the waveguide interfaces in order to achieve ultralow propagation loss. Here, we show ultralow propagation loss by shaping the mode using a highly multimode structure to reduce its overlap with the waveguide interfaces, thus relaxing the fabrication processing requirements. Microresonators with intrinsic quality factors (Q) of 31.8 $\pm$ 4.4 million are experimentally…
