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Tiny terahertz spectrometer turns to VECSELs

06 Nov 2008

Bulky terahertz spectroscopy systems based on ultrafast lasers could be superseded by a miniature design based on a semiconductor source.

An all-semiconductor room-temperature terahertz time-domain spectrometer has been unveiled by researchers in the UK. The team believes that its spectrometer is the first to use an all-semiconductor construction, which gives it both cost and size advantages over traditional designs based on ultrafast lasers (Optics Letters 33 2125).

"Terahertz time-domain spectroscopy is traditionally based on Ti:sapphire lasers, which although have excellent performance, are cost- and size-expensive," Vasileios Apostolopoulos from the University of Southampton told optics.org. "Here we are using a miniature semiconductor laser source developed by Anne Tropper's group at Southampton. It has a very high repetition rate, and the potential to produce sub-200 fs pulses in the next 1–2 years."

In its all-semiconductor approach, the team starts by fabricating a vertical external-cavity surface-emitting laser (VECSEL) capable of emitting 480 fs pulses at a wavelength of 1044 nm. The cavity includes an optical Stark semiconductor saturable absorber mirror (SESAM), which acts as the modelocking element.

The VECSEL is then used in a conventional time-domain spectroscopy set-up where it illuminates a pair of photoconductive antennae. Apostolopoulos and his colleagues report that they can detect radiation over the bandwidth 0.1 to 0.8 THz, with adequate resolution to identify water absorption lines.

"The same technology is used to manufacture the terahertz photoconductive antennas and the VECSEL components, such as the quantum well gain sample and the SESAM, commented Apostolopoulos. "However, the true advantage comes from the low cost and compact form of these elements."

The next step is to create a set-up with 2 or 3 THz bandwidth. As a crucial first step, the team has just demonstrated a VECSEL that can produce 260 fs pulses.

"We also plan to study the effect of repetition rate on the signal to ratio as it is possible with VECSEL cavities to operate at up to 10 GHz," explained Apostolopoulos. "Our long-term target is to implement a compact spectrometer with a two-VECSEL pump-probe head that would lead to a miniature and very fast spectrometer, replacing both the traditional Ti:sapphire laser and the mechanical delay line."

Although Apostolopoulos and colleagues are currently focused on improving their design, they also have an eye on the potential commercial opportunity. "If we deliver the expected results on performance in the next two years there is certainly a demand for a fast commercial terahertz spectrometer with a very small footprint and we want to target this demand," he said.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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