Femtosecond Transient Absorption Spectrometer

Probe Spectral Range: UV-VIS-IR
Extendable Time Window
Microscopy Extensions

HELIOS is an automated femtosecond Transient Absorption Spectrometer. It is designed to work with a variety of amplified femtosecond lasers, including high-energy Ti:Sapphire amplifiers and high repetition rate Yb amplifiers. Together with our patented optical delay line, HELIOS delivers an unmatched level of performance and user-friendliness.

Features & Specs

Broad probe spectral range

with Ti:Sapphire lasers:

with Yb lasers:

280 - 380 nm

350 - 520 nm

320 - 650 nm

480 - 950 nm

420 - 820 nm*

800 - 1600 nm*

820 - 1600 nm

1400 - 2000 nm

1600 - 2400 nm

* Probing around the fundamental wavelength (~800 nm for Ti:Sapphire and ~1030 nm for Yb) requires manual filter adjustments.

Spectral resolution optimized for transient absorption

For transient absorption higher spectral resolution is not always better. It is important to map out all the spectral features, but it is also critical to deliver enough probe light to each pixel of the detector. These two parameters counteract - with not enough probe light the data can be noisy; and with not enough spectral resolution some important features can be missed. Therefore, we configure the spectral resolution to be sufficient for resolving what is practical in condensed-phase experiments, but not excessively high to allow for enough probe light on the detector.

Spectral Range

Intrinsic spectral resolution

Spectral resolution with a 200 µm slit (recommended)


2 nm

4 nm


5 nm

13 nm


5 nm

13 nm

8 ns time window. Extendable to ms.

The nanosecond window is achieved by using a direct-drive high-speed optical delay line. Custom-designed mounts are employed for the delay line optics to increase the beam alignment reproducibility and overall reliability. This delay line features high resolution as well as high speed. Scanning at high speeds is very important because it allows for pseudo-random stepping without a significant increase in the experiment time. This type of stepping is very useful for minimizing the effects of laser instability and sample degradation.

The standard 8 ns time window is extendable to milliseconds with the EOS add-on.

Optical delay line specifications:

  • Time window: 8 ns

  • Resolution: 14 fs

  • Minimum step size: 2.8 fs

  • Max. speed: >10 ns/s

  • Acceleration: > 260 ns/s^2

  • Automated alignment time: 3-5 min

  • Beam pointing drift: <10 µm over 8 ns delay range

Reflective probe light management

We use off-axis parabolic reflectors to collimate and focus the probe light in Helios. This results in a ~50 µm probe beam waist at the sample. Focusing the probe beam tightly allows to use low energy excitation, down to tens of nJ/pulse, without sacrificing signal amplitudes.

Also, using reflective optics in the probe path improves the time resolution of the setup.

Built-in automation

  • Automated optical delay line alignment (Smart Delay LineTM).

  • Automated switching between UV, VIS, NIR, and SWIR spectral ranges.

  • Automated Pump Beam Alignment.


All Helios detectors are fiber-coupled spectrographs with linear array detectors. Each spectrograph has an aberration-corrected concave grating for maximum light throughput (essential for high quality data). The ADC resolution is up to 16 bit. All detectors are mounted in a 19” electronics rack outside the optical bench.

UV-VIS. We have two detector options for this spectral range:

  • CMOS. This 1024 pixel CMOS sensor is ideal for higher speed data acquisition. Allows for individual laser pulse detection for up to 5 kHz. Spectral response: 200 – 1000 nm. Typical spectral range spans 600 nm (ie. 350 – 950 nm).

  • CCD. This 2048 pixel back-thinned CCD sensor is ideal for 1 – 2 kHz lasers and features very high sensitivity as well as the dynamic range. Spectral response: 200 – 1000 nm. Typical spectral range spans 600 nm (ie. 350 – 950 nm). Spectral acquisition rate – up to 2000 spectra/s.

NIR spectral range. This 256 pixel InGaAs sensor presents a great balance between spectral resolution and sensitivity. Spectral response: 800 – 1600 nm. Typical spectral range spans 800 nm (ie. 800 – 1600 nm). Spectral acquisition rate – up to 5000 spectra/s.

SWIR spectral range. 256 pixel InGaAs sensor (spectral response: 1000 – 2600 nm). Typical spectral range spans 800 nm (ie. 1600 – 2400 nm). Spectral acquisition rate – up to 5000 spectra/s.

Large sample area

The spacious (350 mm x 250 mm) sample compartment and the removable side panels allow for easy mounting of cryostats, translating sample holders and even coupling to external magnets. Also, simply having more space around the sample makes working with your samples easier.

Sample holder options

The magnetic stirrer allows for working with closed cuvettes (≥2 mm long) and can work with a simple cuvette holder. The translating sample holder can raster thinner cuvettes (which cannot be stirred easily), films, wafers, etc. The translating sample holder can work with transmissive as well as reflective samples.

Probe reference option

HELIOS has an option for second probe (reference) channel. In this case, the probe beam is split into two before passing through the sample. While one arm travels through the sample, the other is sent directly to the reference spectrometer that monitors the fluctuations in the probe beam intensity. The main advantage of this approach is that it allows the user to achieve the specified signal-to-noise ratio with a lower number of averaged laser pulses. This method is recommended for experiments with low repetition rate and/or easily photodegradable samples where the number of laser shots is strongly limited.

Optional computer-controlled filter wheel

for varying pump energy, etc.


The HELIOS data acquisition software has built-in support for the automated alignment of all critical optical elements for largely hands-off operation.

The software is also very user-friendly and versatile:

  • Automated alignment of the optical delay line.

  • Automated alignment of the pump beam.

  • Computer-controlled switching between UV, VIS, NIR, and SWIR modes.

  • Supports computer-controlled translating sample holder.

  • Support pump beam shutter.

  • Supports motorized filter wheel for automated pump intensity control.

  • Saves every individual kinetic scan, so if the experiment is aborted (due to laser fluctuations, power outages, etc.), all previous scans are not lost.

  • Threshold adjusted automatic continuum spike rejection- advanced setting which collects data points again if the continuum is not stable.

  • Automatic anisotropy calculation when appropriate optics are used and a reference channel is included.

  • Support for multiple choppers to facilitate customized experiments.

  • API (Application Programming Interface) for HELIOS is provided for further experiment customization and integration with external applications.


Helios IR can be used to monitor photoinduced species absorbing in the mid-infrared spectral region. For example, vibrationally excited states, charge carriers and electronically excited states in low band gap nanomaterials, etc.

Some research areas where HELIOS IR is useful are:


Materials science




Transient spectrometry

Cell biology

Many more areas

HELIOS owners are using the instrument in a variety of projects:

Photo-processes on single wall carbon nanotubes

Photoprocesses in triads of fullerene and phthalocyanine

Photophysical properties of two-photon chromophores

Plasmon damping in colloidal metallic nanoparticles

Non-linear absorbing platinum complexes

Surface plasmon resonance of metal nanoparticles

Blinking in silver nanodot fluorescence

Infrared photon harvesting using dye clusters

Acoustic vibrations in gold nanoparticles

Femtosecond spectrometry of lobster pigments

Material properties of metal nanoparticles

Geometric isomers of carotenoids

Photochemistry of cadmium selenide quantum dots

Quantum Confinement in Optically Excited Gold Clusters

Non-linear absorption of PbS nanoparticles

Optical Excitations in Supramolecular Metallocycles

Non-linear absorption and optical limiting in the near infrared

Ultrafast Polaron and Triplet Exciton Formation in Polythiophene Films

Methanofullerene cations on polymer solar cells

Electronic Properties of Oligoenes and Oligothiophenes

Supramolecular conglomerates of phthalocyanines and porphyrins

Photo-induced Electron Transfer in Ruthenium(II)/Tin(IV) Multiporphyrin Arrays

Photo-induced Processes in Metallo­-supramolecular Boxes

Photo-induced energy transfer in a rod-like dinuclear Ru(II) complex

Multilayers of Terpyridine-functionalized Perylene Bisimide Metal Complexes

Photo-induced Processes in Porphyrin-Perylenebisimide Symmetric Triads

Brochures & Examples
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