Rapid-scanning terahertz precision spectrometer with more than 6 THz spectral coverage

Controlling intracavity dual-comb soliton motion in a single-fiber laser

Ultrafast science builds on dynamic compositions of precisely timed light pulses, and evolving groups of pulses are observed in almost every mode-locked laser. However, the underlying physics has rarely been controlled or used until now. Here, we demonstrate a general approach to control soliton motion inside a dual-comb laser and the programmable synthesis of ultrashort pulse patterns. Introducing single-pulse modulation inside an Er:fiber laser, we rapidly shift the timing between two temporally separated soliton combs. Their superposition outside the cavity yields ultrashort soliton sequences. On the basis of real-time spectral interferometry, we observe the deterministic switching of intersoliton separation arising from the interplay of attracting and repulsing forces via ultrafast nonlinearity and laser gain dynamics. Harnessing these insights, we demonstrate the high-speed all-optical synthesis of nano- to picosecond pump-probe delays and programmable free-form soliton trajectories. This concept may pave the way to a new class of all-optical delay generators for ultrafast measurements at unprecedented high tuning, cycling, and acquisition speeds.

Optimizing pump-probe reflectivity measurements of ultrafast photoacoustics with modulated asynchronous optical sampling

Time-resolved optical pump-probe experiments enable the study of complex light-matter interactions on ultrafast timescales, provided that they reach sufficient sensitivity. For instance, with pump-induced ultrafast photoacoustics, probing the typically small changes in optical properties requires a high signal-to-noise ratio. Asynchronous optical sampling (ASOPS), using two separate pulsed lasers at slightly different repetition rates, can be effective at removing noise by averaging many rapidly acquired traces. However, the pump-probe delay scan with ASOPS is always as long as the pump pulse interval, which is inefficient if the delay-time range of interest is shorter. Here, we demonstrate two modified ASOPS schemes that optimize measurement efficiency by only scanning the range of interest. The modification based on frequency modulated ASOPS (MASOPS) is most efficient, especially in the presence of low-frequency flicker noise. We provide a proof-of-concept measurement of ultrafast photoacoustics in which we use MASOPS to scan a time delay of 1/20 of the pump pulse interval. The resulting noise floor is 20 times lower compared to conventional ASOPS, allowing for 20 times faster measurements. Furthermore, we show that by taking experimental noise characteristics into account, more traditional pump-probe methods can also be optimized.

Jitter correction for asynchronous optical sampling terahertz spectroscopy using free-running pulsed lasers

We demonstrate a jitter correction method for asynchronous optical sampling (ASOPS) terahertz (THz) time-domain spectroscopy using two free-running oscillators. This method simultaneously records the THz waveform and a harmonic of the laser repetition rate difference, Δ f _r, to monitor the jitter information for software jitter correction. By suppressing the residual jitter below 0.1 ps, the accumulation of the THz waveform is achieved without losing the measurement bandwidth. Our measurement of water vapor successfully resolves the absorption linewidths below 1 GHz, demonstrating a robust ASOPS with a flexible, simple, and compact setup without any feedback control or additional continuous-wave THz source.

Analysis of sugars and sweeteners via terahertz time-domain spectroscopy

A THz-TDS spectrometer (terahertz time-domain spectroscopy) with ASOPS technology (asynchronous optical sampling) was employed in this study, aiming to explore the potential of the technique and to develop analytical applications for the identification of some saccharides. Analytical curves with linear responses were obtained in the saccharide concentration range from 1.7 to 11.7 (w/w). The characteristic peaks used for each sugar for univariate calibrations were fructose (1.704 THz), sucrose (1.827 THz), glucose (1.435 THz), lactose (1.373 THz), saccharin (2.664 THz), and sucralose (2.219 THz). Limits of detection around 1.0% (m/m) were obtained. Gaussian peak fitting was also employed as a tool to aid in the identification of saccharide components in mixtures and it was observed that the region between 0.5 and 2.0 THz is more adequate for such analysis since scattering is less evident. It was observed that in a mixture of sucrose, glucose and lactose (5% or 10% (m/m) each) in the range of 1.31 to 1.51 THz some sub-peaks of pure lactose and glucose can be identified.

Influence of delay stage positioning error on signal-to-noise ratio, dynamic range, and bandwidth of terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) employs a mechanical stage to introduce the time delay between pump and probe optical pulses during the sampling process of the time-domain electric field of a terahertz pulse. The positioning error and limited resolution of a mechanical delay stage cause an inaccurate sampling of a terahertz pulse. In this study, we numerically and experimentally investigated the influence of delay stage positioning error on the major characteristics of THz-TDS, such as bandwidth and dynamic range of the system, along with the signal-to-noise ratio (SNR) of the intensity spectra. We demonstrate that the positioning error in the delay stage not only reduces bandwidth and dynamic range of the measurement system but also decreases the SNR of the intensity spectra.

Compact terahertz spectrometer based on sequential modulation of disordered rough surfaces

We present herein a compact terahertz (THz) spectrometer in which transmission intensity distribution associated with dispersive interference effects in disordered random surfaces are used for reconstructing the frequency contents of an incoming THz beam. The device sweeps the frequency-dependent parameter of a roughened transmission plate through lateral displacement or electro-optic modulation. 2D transmission intensities are sequentially captured by a single detector for a range of modulation depths. With a calibration data set as the reference, one can reconstruct the spectra of the probe THz beam by solving a system of simultaneous linear equations. A smoothing Tikhonov regularization approach has been implemented to improve the accuracy of the spectral reconstruction. The reported compact, broadband, high-resolution THz spectrometer is well suited for portable THz spectroscopy applications.

High-speed THz spectroscopic imaging at ten kilohertz pixel rate with amplitude and phase contrast

By combining the advantages of the high-speed ASOPS technology and efficient THz generation, we have realized a high-speed laser-based spectroscopic THz imaging system with more than 10,000 pixels per second acquisition speed and an excellent signal-to-noise ratio of more than 100. Unlike THz line cameras or mm-wave intensity detectors, the present device allows for a much higher spatial resolution and attributes each imaging pixel with phase and amplitude information up to several THz while simultaneously maintaining a very high scanning speed unmatched by any other technique presented so far. The high-speed acquisition allows for samples to be scanned even at sample velocities of 5 m/s or higher while preserving the fundamental resolution limit of the THz radiation, which is on the order of 500 µm in the present case.

Electro-optic THz dual-comb architecture for high-resolution, absolute spectroscopy

An absolute-frequency terahertz (THz) dual-frequency comb spectrometer based on electro-optic modulators for tunable, high-resolution, and real-time rapid acquisition is presented. An optical line of a master frequency comb (filtered via optical injection locking) serves as the seed to electro-optically generate a pair of new frequency combs (probe and local oscillator). Photomixing both combs with another coherent line from the same original master comb generates a narrow linewidth THz dual-comb with teeth frequencies that can be referenced to a radio-frequency standard. The system is validated with a proof-of-principle measurement of a microwave filter in the W-band.

Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1-2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.

High-Speed Terahertz Waveform Measurement for Intense Terahertz Light Using 100-kHz Yb-Doped Fiber Laser

We demonstrate a high-speed terahertz (THz) waveform measurement system for intense THz light with a scan rate of 100 Hz. To realize the high scan rate, a loudspeaker vibrating at 50 Hz is employed to scan the delay time between THz light and electro-optic sampling light. Because the fast scan system requires a high data sampling rate, we develop an Yb-doped fiber laser with a repetition rate of 100 kHz optimized for effective THz light generation with the output electric field of 1 kV/cm. The present system drastically reduces the measurement time of the THz waveform from several minutes to 10 ms.

Ultrafast time-domain spectroscopy system using 10 GHz asynchronous optical sampling with 100 kHz scan rate

An ultrafast time-domain spectroscopy system employing asynchronous optical sampling at a repetition rate of 10 GHz is presented. Two ultra-compact Ti:sapphire femtosecond ring lasers allow to achieve scan rates as high as 100 kHz for a 100 ps long time window and a time-delay resolution of 100 fs. The feasibility of this high-speed ASOPS system is evaluated by performing THz time domain spectroscopy on molecular gases where signal-to-noise ratios exceeding 30 dB for averaging times in the millisecond range have been obtained. In order to demonstrate the benefits of this system for ultrafast pump-probe spectroscopy we demonstrate the high-sensitivity detection of coherent acoustic phonons with dephasing times in the range of the 100 ps time window.

Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy

Terahertz (THz) spectroscopy is a promising method for analysing polar gas molecules mixed with unwanted aerosols due to its ability to obtain spectral fingerprints of rotational transition and immunity to aerosol scattering. In this article, dynamic THz spectroscopy of acetonitrile (CH₃CN) gas was performed in the presence of smoke under the atmospheric pressure using a fibre-based, asynchronous-optical-sampling THz time-domain spectrometer. To match THz spectral signatures of gas molecules at atmospheric pressure, the spectral resolution was optimized to 1 GHz with a measurement rate of 1 Hz. The spectral overlapping of closely packed absorption lines significantly boosted the detection limit to 200 ppm when considering all the spectral contributions of the numerous absorption lines from 0.2 THz to 1 THz. Temporal changes of the CH₃CN gas concentration were monitored under the smoky condition at the atmospheric pressure during volatilization of CH₃CN droplets and the following diffusion of the volatilized CH₃CN gas without the influence of scattering or absorption by the smoke. This system will be a powerful tool for real-time monitoring of target gases in practical applications of gas analysis in the atmospheric pressure, such as combustion processes or fire accident.

Dual-comb spectroscopy

Dual-comb spectroscopy is an emerging new spectroscopic tool that exploits the frequency resolution, frequency accuracy, broad bandwidth, and brightness of frequency combs for ultrahigh-resolution, high-sensitivity broadband spectroscopy. By using two coherent frequency combs, dual-comb spectroscopy allows a sample's spectral response to be measured on a comb tooth-by-tooth basis rapidly and without the size constraints or instrument response limitations of conventional spectrometers. This review describes dual-comb spectroscopy and summarizes the current state of the art. As frequency comb technology progresses, dual-comb spectroscopy will continue to mature and could surpass conventional broadband spectroscopy for a wide range of laboratory and field applications.

A decade-spanning high-resolution asynchronous optical sampling terahertz time-domain and frequency comb spectrometer

We present the design and capabilities of a high-resolution, decade-spanning ASynchronous OPtical Sampling (ASOPS)-based TeraHertz Time-Domain Spectroscopy (THz-TDS) instrument. Our system employs dual mode-locked femtosecond Ti:Sapphire oscillators with repetition rates offset locked at 100 Hz via a Phase-Locked Loop (PLL) operating at the 60th harmonic of the ∼80 MHz oscillator repetition rates. The respective time delays of the individual laser pulses are scanned across a 12.5 ns window in a laboratory scan time of 10 ms, supporting a time delay resolution as fine as 15.6 fs. The repetition rate of the pump oscillator is synchronized to a Rb frequency standard via a PLL operating at the 12th harmonic of the oscillator repetition rate, achieving milliHertz (mHz) stability. We characterize the timing jitter of the system using an air-spaced etalon, an optical cross correlator, and the phase noise spectrum of the PLL. Spectroscopic applications of ASOPS-THz-TDS are demonstrated by measuring water vapor absorption lines from 0.55 to 3.35 THz and acetonitrile absorption lines from 0.13 to 1.39 THz in a short pathlength gas cell. With 70 min of data acquisition, a 50 dB signal-to-noise ratio is achieved. The achieved root-mean-square deviation is 14.6 MHz, with a mean deviation of 11.6 MHz, for the measured water line center frequencies as compared to the JPL molecular spectroscopy database. Further, with the same instrument and data acquisition hardware, we use the ability to control the repetition rate of the pump oscillator to enable THz frequency comb spectroscopy (THz-FCS). Here, a frequency comb with a tooth width of 5 MHz is generated and used to fully resolve the pure rotational spectrum of acetonitrile with Doppler-limited precision. The oscillator repetition rate stability achieved by our PLL lock circuits enables sub-MHz tooth width generation, if desired. This instrument provides unprecedented decade-spanning, tunable resolution, from 80 MHz down to sub-MHz, and heralds a new generation of gas-phase spectroscopic tools in the THz region.

Adaptive sampling dual terahertz comb spectroscopy using dual free-running femtosecond lasers

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.

Fiber-coupled high-speed asynchronous optical sampling with sub-50 fs time resolution

We present a fiber-coupled pump-probe system with a sub-50 fs time resolution and a nanosecond time window, based on high-speed asynchronous optical sampling. By use of a transmission grism pulse compressor, we achieve pump pulses with a pulse duration of 42 fs, an average power of 300 mW and a peak power exceeding 5 kW at a pulse repetition rate of 1 GHz after 6 m of optical fiber. With this system we demonstrate thickness mapping of soft X-ray mirrors at a sub-nm thickness resolution on a cm(2) scan area. In addition, terahertz field generation with resolved spectral components of up to 3.5 THz at a GHz frequency resolution is demonstrated.

All fiber-coupled THz-TDS system with kHz measurement rate based on electronically controlled optical sampling

We demonstrate a completely fiber-coupled terahertz (THz) time-domain spectrometer (TDS) system based on electronically controlled optical sampling with two erbium-doped femtosecond fiber lasers at a central wavelength of 1560 nm. The system employs optimized InGaAs/InAlAs photoconductive antennas for THz generation and detection. With this system, we achieve measurement rates of up to 8 kHz and up to 180 ps scan range. We further achieve 2 THz spectral bandwidth and a dynamic range of 76 dB at only 500 ms measurement time.

Spectrally interleaved, comb-mode-resolved spectroscopy using swept dual terahertz combs

Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5 MHz and enhanced spectral accuracy of 8.39 × 10⁻⁷ in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

Non-degenerated photoluminescence excitation correlation spectroscopy using an optical sampling technique

We have developed a highly time-resolved photoluminescence spectroscopy based on the excitation correlation method. Successive irradiation of a pair of ultrashort light pulses with different wavelength combinations taken from two sub-picosecond lasers has exposed both temporal and energetic correlation in photoluminescence intensity associated with a nonlinear response of a sample. An optical sampling technique has been introduced successfully in order to avoid consideration of the synchronization control of ultrashort light pulses. We have demonstrated the abilities of this technique by applying to the nonlinear photoluminescence dynamics of organic dye molecules in solution.

Low-temperature THz time domain waveguide spectrometer with butt-coupled emitter and detector crystal

A compact high-resolution THz time-domain waveguide spectrometer that is operated inside a cryostat is demonstrated. A THz photo-Dember emitter and a ZnTe electro-optic detection crystal are directly attached to a parallel copper-plate waveguide. This allows the THz beam to be excited and detected entirely inside the cryostat, obviating the need for THz-transparent windows or external THz mirrors. Since no external bias for the emitter is required, no electric feed-through into the cryostat is necessary. Using asynchronous optical sampling, high resolution THz spectra are obtained in the frequency range from 0.2 to 2.0 THz. The THz emission from the photo-Dember emitter and the absorption spectrum of 1,2-dicyanobenzene film are measured as a function of temperature. An absorption peak around 750 GHz of 1,2-dicyanobenzene displays a blue shift with increasing temperature.

High-performance fiber-laser-based terahertz spectrometer

We have developed a rapid scanning terahertz (THz) spectrometer based on a synchronized two-fiber-laser system. When the system is set to the asynchronous optical sampling mode, THz spectra extending to 3 THz can be acquired within 1 μs at a signal-to-noise ratio of the electric field of better than 20. Signal averaging results in a dynamic range of more than 60 dB, and frequency components of more than 4 THz can be detected. When the lasers are set to the same repetition rate, electronically controlled optical sampling at a rate of 2.5 kHz is demonstrated, making the system versatile for different spectroscopic applications. Finally, we compare the THz emission spectra of a photoconductive switch that is pumped at 780 nm and a nonlinear DAST crystal excited at 1550 nm. We find that the spectral range of the spectrometer is significantly enhanced at higher frequencies, while the dynamic range remains constant.

High-speed terahertz time-domain spectroscopy based on electronically controlled optical sampling

We demonstrate high-speed terahertz (THz) time-domain spectroscopy based on electronically controlled optical sampling (ECOPS). The ECOPS system utilizes two synchronized Ti:sapphire femtosecond lasers with a 100 MHz repetition frequency. The time delay between the two laser pulses is demonstrated to be rapidly swept at a scan rate of 1 kHz on a time delay window of 77 ps by using an external offset voltage applied to a locking electronics. It is shown that a THz pulse can be exactly measured by ECOPS, as is done by asynchronous optical sampling (ASOPS), and the measurement time is shortened by a factor of 50 by using ECOPS compared with ASOPS in the case of employing 100 MHz repetition-rate lasers.

Subwavelength film sensing based on terahertz anti-resonant reflecting hollow waveguides

A simple dielectric hollow-tube has been experimentally demonstrated at terahertz range for bio-molecular layer sensing based on the anti-resonant reflecting wave-guidance mechanism. We experimentally study the dependence of thin-film detection sensitivity on the optical geometrical parameters of tubes, different thicknesses and tube wall refractive indices, and on different resonant frequencies. A polypropylene hollow-tube with optimized sensitivity of 0.003 mm/μm is used to sense a subwavelength-thick (λ/225) carboxypolymethylene molecular overlayer on the tube's inner surface, and the minimum detectable quantity of molecules could be down to 1.22 picomole/mm(2). A double-layered Fabry-Pérot model is proposed for calculating the overlayer thicknesses, which agrees well with the experimental results.

High-speed asynchronous optical sampling with sub-50fs time resolution

We report an ultrafast time-domain spectroscopy system based on high-speed asynchronous optical sampling operating without mechanical scanner. The system uses two 1 GHz femtosecond oscillators that are offset-stabilized using high-bandwidth feedback electronics operating at the tenth repetition rate harmonics. Definition of the offset frequency, i.e. the time-delay scan rate, in the range of a few kilohertz is accomplished using direct-digital-synthesis electronics for the first time. The time-resolution of the system over the full available 1 ns time-delay window is determined by the laser pulse duration and is 45 fs. This represents a three-fold improvement compared to previous approaches where timing jitter was the limiting factor. Two showcase experiments are presented to verify the high time-resolution and sensitivity of the system.

Terahertz emission from lateral photo-Dember currents

The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In(0.53)Ga(0.47)As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.