Terahertz Spectroscopy

The terahertz region is of scientific interest because it probes the low frequency spectrum, which is generally poorly understood for two reasons. The first difficulty arises in prediction and interpretation of the spectrum because of the complications of large amplitude vibrational motion involving the entire molecule or molecular system. The second is the historical lack of THz emitters and detectors.  As technology has exploded in this area in the past 20 years, these limitations are beginning to disappear. The information in the THz spectrum is very rich, since the spectrum, much like the fingerprint region of the mid-infrared, is unique to virtually every species and changes depending on chemical environment or temperature. Because the vibrations studied are large amplitude, they extend to not only intramolecular motions, but also intermolecular motions. Thus, chemical reactions can be characterized as well, since such large amplitude motions are necessarily involved in the often very large changes to molecular structures that take place in such a reaction. Crystals and well-ordered structures can also be probed in the terahertz region of the spectrum, because the lowest phonon modes are often present in such regions. Lastly, terahertz radiation is very important in the study of semiconductors or metals due to absorption of the radiation by free electrons or excitons. This makes the time-resolved pump-probe THz spectrum of particular interest because of the large signal and also direct probe of the electrons.  Frequency dependent conductivity and permittivity can be directly measured.

In relation to other types of terahertz or far-infrared spectroscopy, THz-TDS has some significant advantages.  It is perhaps most similar to Fourier-transform infrared (FTIR) spectroscopy since the data is collected in the time-domain. THz-TDS is also a broadband technique, potentially covering many octaves of frequency detection at once.  It therefore shares the multiplex advantage over single wavelength absorption spectroscopies. Unlike FTIR, however, a coherent, laser-like source of radiation is used, giving the possibility of time-domain spectroscopy. In fact, the phase sensitive detection that is common to THz-TDS gives a result which is directly proportional to the electric field strength of the THz radiation. This allows for direct calculation of both the real index of refraction (n) and imaginary index of refraction (k), which is related to the absorption coefficient. This can be performed without any sort of Kramers-Kronig analysis.

The distinguishing feature of THz-TDS is the detection method. The linear electro-optic (EO) effect (Pockels effect) is most commonly used as a detection method. A small portion of the laser fundamental is split and is variably delayed, in order to gate the detection of the time-varying THz electric field. The THz electric field causes partial optical rotation of the gate pulse. This is sensed by a polarization analyzer and a pair of detectors. Similar to other ultrafast techniques involving condensed phase nonlinear interactions, the THz light and probe light must be phase matched, which limits the gate wavelength and material that can be used. With a Ti:sapphire laser, zinc telluride (ZnTe) or gallium phosphide (GaP) are most commonly used.

The THz source can be any of many coherent THz generation sources. Common sources use optical rectification, the inverse of electro-optic detection, for generation.  Inorganic or organic eletro-optic (EO) crystals can be used with a variety of input wavelength excitation pulses. Tilted-pulse-front generation can be used for phase matching in high-nonlinearity materials, such as lithium niobate, in order to increase the field strength. The standard method used in Newport's THz-TDS is generation via two-color laser-induced plasma formation. In this technique, the air or other non-dispersive gas is used for easy phase matching. The generation method may be phenomenologically similar to a process described by four-wave rectification, however scientists believe that detailed description of the process involves acceleration of electrons and ions in the two-color field, which gives rise to strong, and broadband, THz emission.

The THz-TDS allows for the THz spectra of materials to be measured via two-color laser-induced plasma emission and electro-optic detection. Additionally, a second optical laser pulse can excite the sample of interest and the THz light can probe the sample as a function of time after photoexcitation. In this way, the time-dependent conductivity and permittivity can be derived, as well as the time-dependent THz absorption spectrum.

Flexible and Upgradable

The THz-TDS is designed with flexibility in mind.  The basic system comes with the essential components required for acquiring a THz time-domain waveform and also for optical pump-THz probe ultrafast spectroscopy; however, almost every system that is shipped is specially built and tailored to meet the customers’ needs.  Some things that can be customized or upgraded are:

  • Addition of internal second harmonic generation for pump
  • Support for external pump source such as an OPA, NOPA, or harmonics generator
  • Chopper integration – reduces noise
  • Delay range (up to 8.5 ns)
  • Automated system alignment/realignment using patented GuideStar™ technology
  • Reference channels for probe
  • Sample holder – moving stage
  • Automated pump power control – shutter or motorized filter wheel
  • Pump reference – useful for very noisy OPAs
  • Extended warranty

Most features are offered as upgrades and can be added in the future as they are required, or as budget allows.

Newport scientists and engineers are always working to support new additions and types of pump-probe spectroscopy, so, if you have an idea that you don’t see listed, feel free to enquire.

High Quality Newport Components

Newport’s Terahertz Time-Domain Spectrometer is assembled and thoroughly tested and calibrated by a dedicated team of scientists and engineers.  THz-TDS is constructed from Newport’s highest quality components, for stable performance as well as increased durability and lifetime.  Some examples are:

  • Precision Grade (PG) series optical breadboards with patented integrated modal damping to lower vibrations.

  • SUPREMA™ series optical mounts for reduced thermal fluctuations and increased stiffness.

  • Forkless stainless steel optical pedestals for extra space and elimination of bending stress induced by clamping forks, while still maintaining flexibility in positioning.

The use of high quality components improves the resistance to vibration induced by the moving components, such as chopper or motion stages, as well as reduced drift due to temperature changes. 

More importantly, support and training is provided either locally or remotely, before, during and after installation.  THz-TDS features free software upgrades for life, with support for new, customer inspired, features as they are added.   

Each system includes installation and training by a Newport scientist, engineer, or representative and includes high quality beam routing for steering the ultrafast amplified laser beams to the THz-TDS system.

Many Pump Options

Many samples require a pump with wavelength different from that of the fundamental wavelength of the ultrafast laser. For this reason, THz-TDS is often sold with an external optical parametric amplifier or harmonics generator. THz-TDS is also offered with an addition of a second harmonic generation (SHG) option for the pump beam.

The energy of the pump beam can be controlled internally by using a variable reflective neutral density filter imprinted on a thin glass substrate. This reduces the GVD introduced to the beam, as compared to a wave plate and polarizer, while allowing power control over a range of 0-4 OD. As an option, computer control of the pump power can be added for measurement repeatability and increased sample lifetime. A computer controlled shutter can be added to automatically protect samples when not in use.  Additionally, a motorized filter wheel, with 8 indexed neutral density filters, can be added for automated and fixed attenuation.

Optimized Delay Stages for Spectroscopy

THz-TDS benefits from the newly released DL series delay line stages, which are ideal for ultrafast spectroscopy due to the fast speed, acceleration, and repeatable positioning. The standard maximum optical delay between pump and probe in THz-TDS is 4.3 ns. This is limited by the length of the stage (325 mm) and the number of optical passes that the probe beam makes to and from the high quality retroreflector (<1 arc sec deviation of return beam) used on the stage. In standard configurations, there are four passes (to and from the retroreflector twice). As an option, an additional retroreflector and associated optics can be added to double the maximum pump-probe delay to 8.6 ns.

The THz gate pulse for electro-optic detection is delayed by a 125 mm stage with a single entry and exit to the retroreflector. This sets the maximum range to 800 ps, which is far more than is necessary for THz probing. 

The resolution changes depending on the mode of operation. In the best case, the resolution is set by the minimal incremental motion (MIM) of the delay stage. This is applicable when performing a single measurement where the stage is incremented linearly in only one direction. In the standard THz-TDS configuration, this is ±1.0 fs. If many scans are to be averaged, the repeatability of positioning must be considered. If performing the same measurement, the uni-directional repeatability must be accounted for, which is 1 fs in the standard THz-TDS configuration. Lastly, if the set of time points is acquired randomly, as is often the case to minimize the effect of laser and sample changes to the measured dynamics, the bi-directional repeatability is the most important factor.  For the standard THz-TDS, this has a value of ±2 fs. In a fast experiment, delay stage settling time is also a consideration, and the positioning may suffer if the settling time is low. The THz-TDS software also supports a constant velocity scanning mode for rapid acquisition of the THz waveform.

Easy-to-use Software

The THz-TDS comes with easy-to-use LabVIEW based software designed to speed up and simplify data collection with free updates for the lifetime of the product. The software seamlessly integrates the many devices used in THz-TDS, such as the motion stage(s), detectors, and optical chopper. The software features a Setup tab, where you can optimize the signal level, delay line zero, chopper phase, and more on a real-time basis. After this is complete, data acquisition can be started on the Measure tab. There are options for setting up the scan range and step size for both stages, as well as number of averages and selection of random or linear scan. Data can be acquired in a stepped or continuously swept mode in order to decrease scan time. The estimated time remaining for the scan is estimated initially and updates as the data is acquired.  Every scan is saved to disk, so that in case of a stopped scan, data is not lost. Saved data can also be reloaded for later viewing and a copy of the software can easily be run in data viewing mode on an office or home computer. The standard output file type is .csv with headers.

THz-TDS Specifications

 Specification  Standard  Maximum Upgrade
 THz detection range1 0.25-3.0 THz (8-117 cm-1) (1200 - 85 μm) 0.25 – 4 THz (8-133 cm-1) (1200 – 75 μm)
 Best THz resolution2 1.25 GHz 0.230 GHz
 Maximum Optical Pump-THz Probe Delay Range 4.3 ns 8.6 ns
 Minimum Optical Pump-THz Probe Delay Step Size 1 fs
 Maximum THz Gate Delay Range 0.8 ns 2 ns
 Minimum THz Gate Step Size 0.25 fs
 Delay Stage Bidirectional Repeatability <4 fs pump, <2 fs gate
 Delay Stage Maximum Speed3 500 mm/s (6.67 ns/s) 500 mm/s (13.3 ns/s)
 Delay Stage Maximum Acceleration3 5 m/s2 (66.7 ns/s2) 4 m/s2 (106.7 ns/s2)
 Supported Data Acquisition Rate 10 Hz - 2 kHz @ 16-bit 10 Hz – 500 kHz
 Supported Laser Rate 0.1 - 2 kHz 0.1 – 1 MHz

1)  The THz detection range is reported for a 50 fs pulse width at 800 nm with 1 mm or 0.2 mm thick ZnTe wafers serving as EO crystal. The range is limited by pulse duration, averaging time, EO crystal , EO crystal thickness,  and laser noise.  User can replace EO crystal to increase bandwidth at the cost of signal intensity.

2) The THz resolution is limited by the total time scanned, which is set by the maximum THz gate delay. Commonly, detection of the THz waveform after >150 ps is often limited by noise.

3) Derived units are for the pump delay stage.  Recommended maximum acceleration is reduced with added mass for 8-pass option.