• Current Optics and Photonics
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• 제5권3호
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• pp.289-297
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• 2021

We describe a comb-mode resolving spectroscopic technique by direct time-domain phase correction of unstable interferograms obtained from loosely locked two femtosecond lasers. A low-cost continuous wave laser and conventional repetition rate stabilization method were exploited for locking carrier and envelope phase of interferograms, respectively. We intentionally set the servo control at low bandwidth, resulting in severe interferograms' fluctuation to demonstrate the capability of the proposed correction method. The envelope phase of each interferogram was estimated by a quadratic fit of carrier peaks to correct timing fluctuation of interferograms in the time domain. After envelope phase correction on individual interferograms, we successfully demonstrated 1 Hz linewidth of RF comb-mode over 200 GHz optical spectral-bandwidth with 10-times signal-to-noise ratio (SNR) enhancement compared to the spectrum without correction. Besides, the group delay difference between two femtosecond pulses is successfully estimated through a linear slope of phase information.

• 한국초전도ㆍ저온공학회논문지
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• 제21권1호
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• pp.15-17
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• 2019

We present terahertz optical properties of $GdBa_2Cu_3O_{7-x}$ (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a $LaAlO_3$ (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K.

• Current Optics and Photonics
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• 제7권5호
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• pp.569-573
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• 2023

We utilized terahertz time-domain spectroscopy (THz-TDS) to measure the thickness and electrical properties of nickel-chromium (Ni-Cr) films. This technique not only aligns well with traditional methods, such as haze-meter and transmission-densitometer measurements, but it also reveals the electrical properties and thickness of films down to a few tens of nanometers. The complex conductivity of the Ni-Cr thin films was extracted using the Tinkham formula. The experimental values closely aligned with the Drude model, indicating the reliability of our Ni-Cr film's electrical and optical constants. The thickness of Ni-Cr was estimated using the complex conductivity. These findings emphasize the potential of THz-TDS in quality control of metallic nanofilms, pointing toward an efficient and nondestructive test (NDT) for such analyses.

• 한국초전도ㆍ저온공학회논문지
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• 제15권1호
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• pp.11-13
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• 2013

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for transmission and reflection measurements of metallic thin films. Using our THz-TDS system, we studied the conventional superconductor niobium (Nb) in the normal state in the spectral range from 5 to $50cm^{-1}$. Both the real and imaginary parts of the conductivity are acquired without Kramers-Kronig analysis. Nb exhibits a nearly frequency independent real conductivity spectrum in the terahertz range, with a very small imaginary part.

• 비파괴검사학회지
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• 제28권2호
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• pp.119-124
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• 2008

최근 비파괴 검사를 위한 테라헤르츠 전자기파 기술에 대한 관심이 높아지고 있으며 특히 비파괴검사에서 많은 응용이 기대된다. 테라헤르츠 시간 영역 분광법은 이러한 테라헤르츠 기술에 핵심이 되는 기술로 많은 실험이 이루어지고 있다. 본 논문에서는 테라헤르츠 시간 영역 분광에서 비선형 전광물질을 이용하는 광정류 방식을 통해 발생된 테라헤르츠 전자기파 스펙트럼이 비선형 맥스웰 방정식의 해와 실험에 의해 결정되는 흡수, 회절, 표면간섭 효과 등을 고려한 본문의 모델을 통하여 예측가능하며 실제 티타늄:사파이어 레이저 펄스를 $400{\mu}m$ CdTe에 조사하여 발생된 테라헤르츠파 주파수 스펙트럼의 측정 결과와 비교하여 매우 유사하다는 사실을 보여준다. 이를 통하여 본문에서 소개된 모델은 다른 전광물질을 통해 발생된 테라헤르츠 스펙트럼에도 확장되어 적용 될 수 있다.

• 한국광학회:학술대회논문집
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• 한국광학회 2002년도 하계학술발표회
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• pp.90-91
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• 2002

강한 펄스 레이저가 렌즈에 의해 투명한 매질 속에 집광되면 가시광선 영역에서부터 적외선 영역에 걸친 넓은 파장대의 빛이 발생하게 되는데 이것을 White-Light Continuum이라고 한다. 비선형 결정인 $\beta$-BBO에 의해서도 넓은 영역의 백색광을 발생시킬 수 있다. 이러한 펄스 백색광은 Time-resolved spectroscopy에서의 Probe beam과 다양한 파장대의 펄스 레이저 발생 및 증폭을 위한 Seed pulse 등으로 응용되고 있다. (중략)

• 한국광학회지
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• 제25권2호
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• pp.72-77
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• 2014

본 논문에서는 초소형의 광섬유 결합형 테라헤르츠 모듈을 이용하여 테라헤르츠 시간영역 분광 및 영상 시스템을 구성하였다. 구성된 THz 분광 시스템을 이용하여 3 THz 이상의 자유공간에 분포되어 있는 수분의 고유 흡수 스펙트럼을 관측하였고 테라헤르츠 대역에서의 Si, $Al_2O_3$, GaAs기판에 대한 굴절률을 측정하였으며 측정을 위해 다중 반사를 고려한 전달 함수를 이용한 굴절률 측정 방법을 이용하였다. 또한, 테라헤르츠 영상 시스템을 이용하여 의료용 칼과 클립을 측정하여 250 ${\mu}m$ 간격으로 $192{\times}89$ 픽셀의 영상을 얻었다.

• 센서학회지
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• 제27권5호
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• pp.328-334
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• 2018

The paper reviews an improved continuous-wave (CW) terahertz (THz) imaging system developed for nondestructive inspection, such as CW-THz quasi-time-domain spectroscopy (QTDS) and interferometry. First, a comparison between CW and pulsed THz imaging systems is reported. The CW-THz imaging system is a simple, fast, compact, and relatively low-cost system. However, it only provides intensity data, without depth and frequency- or time-domain information. The pulsed THz imaging system yields a broader range of information, but it is expensive because of the femtosecond laser. Recently, to overcome the drawbacks of CW-THz imaging systems, many studies have been conducted, including a study on the QTDS system. In this system, an optical delay line is added to the optical arm leading to the detector. Another system studied is a CW-THz interferometric imaging system, which combines the CW-THz imaging system and far-infrared interferometer system. These systems commonly obtain depth information despite the CW-THz system. Reportedly, these systems can be successfully applied to fields where pulsed THz is used. Lastly, the applicability of these systems for nondestructive inspection was confirmed.

• Bulletin of the Korean Chemical Society
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• 제30권10호
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• pp.2265-2268
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• 2009

Terahertz time domain spectroscopy (THz-TDS) is applied in transmission to identify the five forms of modifications of furosemide and one commercial product from 0.3 THz to 1.6 THz at room temperature. The different absorption spectra of the different forms are sensitive to crystal structures. Density function theory (DFT) calculation was used to understand the vibrational modes of furosemide in the THz region. X-ray powder diffractometry (XRPD) was applied to confirm the different forms of modifications. The results demonstrate that THz-TDS is a potential analytical technique in investigating polymorphic forms in the pharmaceutical fields.

• Journal of the Optical Society of Korea
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• 제20권5호
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• pp.628-632
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• 2016

We studied experimentally and theoretically the vertical range of the confined electric field in the gap area of metamaterials, which was analyzed for various gap widths using terahertz time-domain spectroscopy. We measured the resonant frequency as a function of the thickness of poly(methyl methacrylate) in the range 0 to 3.2 μm to quantify the effective detection volumes. We found that the effective vertical range of the metamaterial is determined by the size of the gap width. The vertical range was found to decrease as the gap width of the metamaterial decreases, whereas the sensitivity is enhanced as the gap width decreases due to the highly concentrated electric field. Our experimental findings are in good agreement with the finite-difference time-domain simulation results. Finally, a numerical expression was obtained for the vertical range as a function of the gap width. This expression is expected to be very useful for optimizing the sensing efficiency.